International VLBI Service for Geodesy and Astrometry 2014 Annual Report

NASA Technical Reports Server (NTRS)

Baver, Karen D. (Editor); Behrend, Dirk (Editor); Armstrong, Kyla L. (Editor)

2015-01-01

IVS is an international collaboration of organizations which operate or support Very Long Baseline Interferometry (VLBI) components. The goals are: 1. To provide a service to support geodetic, geophysical and astrometric research and operational activities. 2. To promote research and development activities in all aspects of the geodetic and astrometric VLBI technique. 3. To interact with the community of users of VLBI products and to integrate VLBI into a global Earth observing system.

IVS: Current Status and Future Plans

NASA Astrophysics Data System (ADS)

Behrend, D.; Nothnagel, A.; Petrachenko, W. T.; Tuccari, G.

2016-12-01

The International VLBI Service for Geodesy and Astrometry (IVS) is a globally operating service that coordinates and performs Very Long Baseline Interferometry (VLBI) activities through its constituent components. The VLBI activities are associated with the creation, provision, dissemination, and archiving of relevant VLBI data and products. The products mostly pertain to the determination of the celestial and terrestrial reference frames, the Earth orientation parameters (EOP), atmospheric parameters as well as other ancillary parameters. The IVS observational network currently consists of about 40 radio telescopes worldwide. Subsets of these telescopes (8-12 stations) participate in 24-hour observing sessions that are run several times per week and in 1-hour intensive sessions for UT1 determination every day. The current VLBI network was developed mainly in the 1970s and 1980s. A number of factors, including aging infrastructure and demanding new scientific requirements, started to challenge its future sustainability and relevance. In response, the IVS and other groups developed and started implementing the next generation VLBI system, called VGOS (VLBI Global Observing System), at existing and new sites. The VGOS network is expected to reach maturity in the early 2020s. We describe the current status, progress, and anticipated prospects of geodetic/astrometric VLBI and the IVS.

VizieR Online Data Catalog: Parallaxes of high mass star forming regions (Reid+, 2014)

NASA Astrophysics Data System (ADS)

Reid, M. J.; Menten, K. M.; Brunthaler, A.; Zheng, X. W.; Dame, T. M.; Xu, Y.; Wu, Y.; Zhang, B.; Sanna, A.; Sato, M.; Hachisuka, K.; Choi, Y. K.; Immer, K.; Moscadelli, L.; Rygl, K. L. J.; Bartkiewicz, A.

2016-04-01

Table1 lists the parallaxes and proper motions of 103 regions of high-mass star formation measured with Very Long Baseline Interferometry (VLBI) techniques, using the National Radio Astronomy Observatory's Very Long Baseline Array (VLBA), the Japanese VLBI Exploration of Radio Astrometry (VERA; http://veraserver.mtk.nao.ac.jp) project, and the European VLBI Network (EVN). We have include three red supergiants (NML Cyg, S Per, VY CMa) as indicative of high-mass star forming regions. (2 data files).

A Global Terrestrial Reference Frame from simulated VLBI and SLR data in view of GGOS

NASA Astrophysics Data System (ADS)

Glaser, Susanne; König, Rolf; Ampatzidis, Dimitrios; Nilsson, Tobias; Heinkelmann, Robert; Flechtner, Frank; Schuh, Harald

2017-07-01

In this study, we assess the impact of two combination strategies, namely local ties (LT) and global ties (GT), on the datum realization of Global Terrestrial Reference Frames in view of the Global Geodetic Observing System requiring 1 mm-accuracy. Simulated Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) data over a 7 year time span was used. The LT results show that the geodetic datum can be best transferred if the precision of the LT is at least 1 mm. Investigating different numbers of LT, the lack of co-located sites on the southern hemisphere is evidenced by differences of 9 mm in translation and rotation compared to the solution using all available LT. For the GT, the combination applying all Earth rotation parameters (ERP), such as pole coordinates and UT1-UTC, indicates that the rotation around the Z axis cannot be adequately transferred from VLBI to SLR within the combination. Applying exclusively the pole coordinates as GT, we show that the datum can be transferred with mm-accuracy within the combination. Furthermore, adding artificial stations in Tahiti and Nigeria to the current VLBI network results in an improvement in station positions by 13 and 12%, respectively, and in ERP by 17 and 11%, respectively. Extending to every day VLBI observations leads to 65% better ERP estimates compared to usual twice-weekly VLBI observations.

Global and regional kinematics with VLBI

NASA Technical Reports Server (NTRS)

Ma, Chopo

1994-01-01

Since a VLBI station cannot operate in isolation and since simultaneous operation of the entire VLBI network is impractical, it is necessary to design observing programs with periodic observing sessions using networks of 3-7 stations that, when treated together, will have the necessary interstation data and network overlaps to determine the desired rates of change. Thus, there has been a mix of global, intercontinental, transcontinental, and regional networks to make measurements ranging from plate motions to deformation over a few hundred km. Over time, even networks focusing on regional deformation using mobile VLBI included large stations removed by several thousand km to increase sensitivity, determine EOP more accurately, and provide better ties to the terrestrial reference frame (TRF). Analysis products have also evolved, beginning with baseline components, and then to full three-dimensional site velocities in a global TRF.

VLBI and GPS-based Time-Transfer Using CONT08 Data

NASA Technical Reports Server (NTRS)

Rieck, Carsten; Haas, Ruediger; Jaldehag, Kenneth; Jahansson, Jan

2010-01-01

One important prerequisite for geodetic Very Long Baseline Interferometry (VLBI) is the use of frequency standards with excellent short term stability. This makes VLBI stations, which are often co-located with Global Navigation Satellite System (GNSS) receiving stations, interesting for studies of time- and frequency-transfer techniques. We present an assessment of VLBI time-transfer based on the data of the two week long consecutive IVS CONT08 VLBI campaign by using GPS Carrier Phase (GPSCP). CONT08 was a 15 day long campaign in August 2008 that involved eleven VLBI stations on five continents. For CONT08 we estimated the worst case VLBI frequency link stability between the stations of Onsala and Wettzell to 1e-15 at one day. Comparisons with GPSCP confirm the VLBI results. We also identify time-transfer related challenges of the VLBI technique as used today.

Hartebeesthoek Radio Astronomy Observatory (HartRAO)

NASA Technical Reports Server (NTRS)

Nickola, Marisa; Gaylard, Mike; Quick, Jonathan; Combrinck, Ludwig

2013-01-01

HartRAO provides the only fiducial geodetic site in Africa, and it participates in global networks for VLBI, GNSS, SLR, and DORIS. This report provides an overview of geodetic VLBI activities at HartRAO during 2012, including the conversion of a 15-m alt-az radio telescope to an operational geodetic VLBI antenna.

Tropospheric Delay from VLBI and GNSS Measurements

NASA Astrophysics Data System (ADS)

Gubanov, V. S.

2018-02-01

Using an updated version of the QUASAR software package developed at the Institute of Applied Astronomy of the Russian Academy of Sciences, we have processed the VLBI observations within the international CONT14 program (May 6-20, 2014), in which a global network of 17 stations was involved (a total of 250 000 observations). The package update concerned the optimization of data structure and the refinement of stochastic models for the random variations in wet tropospheric delay and atomic clock difference. The main goal of this paper is to compare the VLBI determinations of the tropospheric delay with its independent determinations using global navigation satellite systems (GNSS). We show that both these determinations agree well between themselves only in the case of a global analysis of the VLBI observations, where the VLBI station coordinates are also refined, along with the tropospheric delay and the clock synchronization and Earth orientation parameters. If, alternatively, the station coordinates are insufficiently accurate and are not refined from VLBI observations, then it is appropriate not to determine the tropospheric delay from these observations, but to take it from the publicly accessible independent GNSS data. However, this requires that the VLBI and GNSS techniques operate simultaneously at a common observing site. We have established the shortcomings of the universally accepted method of stabilizing the global solution associated with the absence of a criterion for choosing reference stations and radio sources. Two ways of their elimination are proposed: (i) introducing a coordinated list of weight factors for the errors in the coordinates of such stations and sources into the stabilization algorithm and (ii) adopting a coordinated list of stations and sources the refinement of whose coordinates is not required at all for a certain time.

Simulation of Twin Telescopes at Onsala and Wettzell for the VLBI Global Observing System

NASA Astrophysics Data System (ADS)

Schönberger, Caroline; Gnilsen, Paul; Böhm, Johannes; Haas, Rüdiger

2015-04-01

The VLBI2010 committee of the International VLBI Service for Geodesy and Astrometry (IVS) developed a concept to achieve an improvement of the accuracy of geodetic Very Long Baseline Interferometry (VLBI) to 1 mm for station positions and 0.1 mm/yr for station velocities. This so-called VLBI2010 concept includes broadband observations with fast slewing telescopes and proposes twin telescopes to improve the handling of atmospheric turbulence that has been identified as a limiting factor for geodetic VLBI. There are several VLBI sites that have projects to install a Twin Telescope. The Wettzell Twin Telescope in Germany has already been constructed, and Twin Telescopes will be installed in the coming years at Onsala (Sweden), Ny-Ålesund (Spitsbergen, Norway) and Kazan (Russia). In this study, the Vienna VLBI Software (VieVS) is used to schedule and simulate a global VLBI network following the example of the CONT11 campaign, with and without the Twin Telescopes in Onsala and Wettzell. Different scheduling approaches (e.g., source-based scheduling, Twin Telescope observing in multidirectional mode, Twin Telescopes in continuous mode) were compared by evaluating the numbers of observations and scans as well as baseline length repeatabilities, station positions, Earth orientation parameters, atmospheric parameters and clock estimates. Comparison of the results show an improvement in estimated parameters with Twin Telescopes, especially with the Onsala Twin Telescope in a continuous observing mode and a strategy with four sources observed simultaneously.

Global VLBI Observations of Weak Extragalactic Radio Sources: Imaging Candidates to Align the VLBI and Gaia Frames

NASA Technical Reports Server (NTRS)

Bourda, Geraldine; Collioud, Arnaud; Charlot, Patrick; Porcas, Richard; Garrington, Simon

2010-01-01

The space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be important to align the Gaia and VLBI frames (International Celestial Reference Frame) with the highest accuracy. In this respect, it is found that only 10% of the ICRF sources are suitable to establish this link (70 sources), either because most of the ICRF sources are not bright enough at optical wavelengths or because they show extended radio emission which precludes reaching the highest astrometric accuracy. In order to improve the situation, we initiated a multi-step VLBI observational project, dedicated to finding additional suitable radio sources for aligning the two frames. The sample consists of about 450 optically-bright radio sources, typically 20 times weaker than the ICRF sources, which have been selected by cross-correlating optical and radio catalogs. The initial observations, aimed at checking whether these sources are detectable with VLBI, and conducted with the European VLBI Network (EVN) in 2007, showed an excellent 90% detection rate. This paper reports on global VLBI observations carried out in March 2008 to image 105 from the 398 previously detected sources. All sources were successfully imaged, revealing compact VLBI structure for about half of them, which is very promising for the future.

VLBI real-time analysis by Kalman Filtering

NASA Astrophysics Data System (ADS)

Karbon, M.; Nilsson, T.; Soja, B.; Heinkelmann, R.; Raposo-Pulido, V.; Schuh, H.

2013-12-01

Geodetic Very Long Baseline Interferometry (VLBI) is one of the primary space geodetic techniques providing the full set of Earth Orientation Parameter (EOP) and is unique for observing long term Universal Time (UT1) and precession/nutation. Accurate and continuous EOP obtained in near real-time are essential for satellite based navigation and positioning and for enabling the precise tracking of interplanetary spacecrafts. To meet this necessity the International VLBI Service for Geodesy and Astrometry (IVS) increased its efforts to reduce the time span between the VLBI observations and the availability of the final results. Currently the timeliness is about two weeks, but the goal is to reduce it to less than one day with the future VGOS (VLBI2010 Global Observing System) network. The FWF project VLBI-ART contributes to this new generation VLBI system by considerably accelerating the VLBI analysis procedure through the implementation of an elaborate Kalman filter. This true real-time Kalman filter will be embedded in the Vienna VLBI Software (VieVS) as a completely automated tool with no need of human interaction. This filter also allows the prediction and combination of EOP from various space geodetic techniques by implementing stochastic models to statistically account for unpredictable changes in EOP. Additionally, atmospheric angular momenta calculated from numerical weather prediction models are introduced to support the short-term EOP prediction. To optimize the performance of the new software various investigations with real as well as simulated data are foreseen. The results are compared to the ones obtained by conventional VLBI parameter estimation methods (e.g. least squares method) and to corresponding parameter series from other techniques, such as from the Global Navigation Satellite Systems (GNSS).

Dynamical Imaging with Interferometry

NASA Astrophysics Data System (ADS)

Johnson, Michael D.; Bouman, Katherine L.; Blackburn, Lindy; Chael, Andrew A.; Rosen, Julian; Shiokawa, Hotaka; Roelofs, Freek; Akiyama, Kazunori; Fish, Vincent L.; Doeleman, Sheperd S.

2017-12-01

By linking widely separated radio dishes, the technique of very long baseline interferometry (VLBI) can greatly enhance angular resolution in radio astronomy. However, at any given moment, a VLBI array only sparsely samples the information necessary to form an image. Conventional imaging techniques partially overcome this limitation by making the assumption that the observed cosmic source structure does not evolve over the duration of an observation, which enables VLBI networks to accumulate information as Earth rotates and changes the projected array geometry. Although this assumption is appropriate for nearly all VLBI, it is almost certainly violated for submillimeter observations of the Galactic center supermassive black hole, Sagittarius A* (Sgr A*), which has a gravitational timescale of only ∼ 20 s and exhibits intrahour variability. To address this challenge, we develop several techniques to reconstruct dynamical images (“movies”) from interferometric data. Our techniques are applicable to both single-epoch and multiepoch variability studies, and they are suitable for exploring many different physical processes including flaring regions, stable images with small time-dependent perturbations, steady accretion dynamics, or kinematics of relativistic jets. Moreover, dynamical imaging can be used to estimate time-averaged images from time-variable data, eliminating many spurious image artifacts that arise when using standard imaging methods. We demonstrate the effectiveness of our techniques using synthetic observations of simulated black hole systems and 7 mm Very Long Baseline Array observations of M87, and we show that dynamical imaging is feasible for Event Horizon Telescope observations of Sgr A*.

VLBI Observations of Geostationary Satellites

NASA Astrophysics Data System (ADS)

Artz, T.; Nothnagel, A.; La Porta, L.

2013-08-01

For a consistent realization of a Global Geodetic Observing System (GGOS), a proper tie between the individual global reference systems used in the analysis of space-geodetic observations is a prerequisite. For instance, the link between the terrestrial, the celestial and the dynamic reference system of artificial Earth orbiters may be realized by Very Long O Baseline Interferometry (VLBI) observations of one or several satellites. In the preparation phase for a dedicated satellite mission, one option to realize this is using a geostationary (GEO) satellite emitting a radio signal in X-Band and/or S-Band and, thus, imitating a quasar. In this way, the GEO satellite can be observed by VLBI together with nearby quasars and the GEO orbit can, thus, be determined in a celestial reference frame. If the GEO satellite is, e.g., also equipped with a GNSS-type transmitter, a further tie between GNSS and VLBI may be realized. In this paper, a concept for the generation of a radio signal is shown. Furthermore, simulation studies for estimating the GEO position are presented with a GEO satellite included in the VLBI schedule. VLBI group delay observations are then simulated for the quasars as well as for the GEO satellite. The analysis of the simulated observations shows that constant orbit changes are adequately absorbed by estimated orbit parameters. Furthermore, the post-fit residuals are comparable to those from real VLBI sessions.

An Improved Empirical Harmonic Model of the Celestial Intermediate Pole Offsets from a Global VLBI Solution

NASA Astrophysics Data System (ADS)

Belda, Santiago; Heinkelmann, Robert; Ferrándiz, José M.; Karbon, Maria; Nilsson, Tobias; Schuh, Harald

2017-10-01

Very Long Baseline Interferometry (VLBI) is the only space geodetic technique capable of measuring all the Earth orientation parameters (EOP) accurately and simultaneously. Modeling the Earth's rotational motion in space within the stringent consistency goals of the Global Geodetic Observing System (GGOS) makes VLBI observations essential for constraining the rotation theories. However, the inaccuracy of early VLBI data and the outdated products could cause non-compliance with these goals. In this paper, we perform a global VLBI analysis of sessions with different processing settings to determine a new set of empirical corrections to the precession offsets and rates, and to the amplitudes of a wide set of terms included in the IAU 2006/2000A precession-nutation theory. We discuss the results in terms of consistency, systematic errors, and physics of the Earth. We find that the largest improvements w.r.t. the values from IAU 2006/2000A precession-nutation theory are associated with the longest periods (e.g., 18.6-yr nutation). A statistical analysis of the residuals shows that the provided corrections attain an error reduction at the level of 15 μas. Additionally, including a Free Core Nutation (FCN) model into a priori Celestial Pole Offsets (CPOs) provides the lowest Weighted Root Mean Square (WRMS) of residuals. We show that the CPO estimates are quite insensitive to TRF choice, but slightly sensitive to the a priori EOP and the inclusion of different VLBI sessions. Finally, the remaining residuals reveal two apparent retrograde signals with periods of nearly 2069 and 1034 days.

The Water Maser in II Zw 96: Scientific Justification

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wiggins, Brandon Kerry

We propose a VLBI search to image and locate the water emission in II Zw 96. We propose 3 sites within II Zw 96 for VLBI followup (see the proposed target listing below). We request 2.5 hours of on-source integration time with the VLBA per source. The array will achieve ~ 65µJy sensitivity in K band in this time which will be sufficient to detect luminous water maser features.

Engineering processes for the African VLBI network

NASA Astrophysics Data System (ADS)

Thondikulam, Venkatasubramani L.; Loots, Anita; Gaylard, Michael

2013-04-01

The African VLBI Network (AVN) is an initiative by the SKA-SA and HartRAO, business units of the National Research Foundation (NRF), Department of Science and Technology (DST), South Africa. The aim is to fill the existing gap of Very Long Baseline Interferometry (VLBI)-capable radio telescopes in the African continent by a combination of new build as well as conversion of large redundant telecommunication antennas through an Inter-Governmental collaborative programme in Science and Technology. The issue of human capital development in the Continent in the techniques of radio astronomy engineering and science is a strong force to drive the project and is expected to contribute significantly to the success of Square Kilometer Array (SKA) in the Continent.

A highly magnetized twin-jet base pinpoints a supermassive black hole

NASA Astrophysics Data System (ADS)

Baczko, A.-K.; Schulz, R.; Kadler, M.; Ros, E.; Perucho, M.; Krichbaum, T. P.; Böck, M.; Bremer, M.; Grossberger, C.; Lindqvist, M.; Lobanov, A. P.; Mannheim, K.; Martí-Vidal, I.; Müller, C.; Wilms, J.; Zensus, J. A.

2016-09-01

Supermassive black holes (SMBH) are essential for the production of jets in radio-loud active galactic nuclei (AGN). Theoretical models based on (Blandford & Znajek 1977, MNRAS, 179, 433) extract the rotational energy from a Kerr black hole, which could be the case for NGC 1052, to launch these jets. This requires magnetic fields on the order of 103G to 104G. We imaged the vicinity of the SMBH of the AGN NGC 1052 with the Global Millimetre VLBI Array and found a bright and compact central feature that is smaller than 1.9 light days (100 Schwarzschild radii) in radius. Interpreting this as a blend of the unresolved jet bases, we derive the magnetic field at 1 Schwarzschild radius to lie between 200 G and ~ 8.3 × 104 G consistent with Blandford & Znajek models. The VLBI images shown in Figs. 3 and 4 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/593/A47

Next-generation Event Horizon Telescope developments: new stations for enhanced imaging

NASA Astrophysics Data System (ADS)

Palumbo, Daniel; Johnson, Michael; Doeleman, Sheperd; Chael, Andrew; Bouman, Katherine

2018-01-01

The Event Horizon Telescope (EHT) is a multinational Very Long Baseline Interferometry (VLBI) network of dishes joined to resolve general relativistic behavior near a supermassive black hole. The imaging quality of the EHT is largely dependent upon the sensitivity and spatial frequency coverage of the many baselines between its constituent telescopes. The EHT already contains many highly sensitive dishes, including the crucial Atacama Large Millimeter/Submillimeter Array (ALMA), making it viable to add smaller, cheaper telescopes to the array, greatly improving future capabilities of the EHT. We develop tools for optimizing the positions of new dishes in planned arrays. We also explore the feasibility of adding small orbiting dishes to the EHT, and develop orbital optimization tools for space-based VLBI imaging. Unlike the Millimetron mission planned to be at L2, we specifically treat near-earth orbiters, and find rapid filling of spatial frequency coverage across a large range of baseline lengths. Finally, we demonstrate significant improvement in image quality when adding small dishes to planned arrays in simulated observations.

Long-term monitoring of Sgr A* at 7 mm with VERA and KaVA

NASA Astrophysics Data System (ADS)

Akiyama, K.; Kino, M.; Sohn, B.; Lee, S.; Trippe, S.; Honma, M.

2014-05-01

We present the results of radio monitoring observations of Sgr A* at 7 mm (i.e. 43 GHz) with the VLBI Exploration of Radio Astrometry (VERA), which is a VLBI array in Japan. VERA provides angular resolution on millisecond scales, resolving structures within 100 Schwarzschild radii of Sgr A* , similar to the Very Large Baseline Array (VLBA). We performed multi-epoch observations of Sgr A* in 2005 - 2008, and started monitoring it again with VERA from 2013 January to trace the current G2 encounter event. Our preliminary results in 2013 show that Sgr A* on mas scales has been in an ordinary state as of August 2013, although some fraction of the G2 cloud already passed the pericenter of Sgr A* in April 2013. We will continue monitoring Sgr A* with VERA and the newly developed KaVA (KVN and VERA Array).

VLBI: A Fascinating Technique for Geodesy and Astrometry

NASA Technical Reports Server (NTRS)

Schuh, H.; Behrend, Dirk

2012-01-01

Since the 1970s Very Long Baseline Interferometry (VLBI) has proven to be a primary space-geodetic technique by determining precise coordinates on the Earth, by monitoring the variable Earth rotation and orientation with highest precision, and by deriving many other parameters of the Earth system. VLBI provides an important linkage to astronomy through, for instance, the determination of very precise coordinates of extragalactic radio sources. Additionally, it contributes to determining parameters of relativistic and cosmological models. After a short review of the history of geodetic VLBI and a summary of recent results, this paper describes future perspectives of this fascinating technique. The International VLBI Service for Geodesy and Astrometry (IVS), as a service of the International Association of Geodesy (IAG) and the International Astronomical Union (IAU), is well on its way to fully defining a next generation VLBI system, called VLBI2010. The goals of the new system are to achieve on scales up to the size of the Earth an accuracy of 1 mm in position and of 0.1 mm/year in velocity. Continuous observations shall be carried out 24 h per day 7 days per week in the future with initial results to be delivered within 24 h after taking the data. Special sessions, e.g. for monitoring the Earth rotation parameters, will provide the results in near real-time. These goals require a completely new technical and conceptual design of VLBI measurements. Based on extensive simulation studies, strategies have been developed by the IVS to significantly improve its product accuracy through the use of a network of small (approx 12 m) fast-slewing antennas. A new method for generating high precision delay measurements as well as improved methods for handling biases related to radio source structure, system electronics, and deformations of the antenna structures has been developed. Furthermore, as of January 2012, the construction of ten new VLBI2010 sites has been funded, with good prospects for one dozen more antennas, which will improve the geographical distribution of geodetic VLBI sites on Earth and provide an important step toward a global VLBI2010 network. Within this paper, the Global Geodetic Observing System (GGOS) of the IAG will also be introduced and the contribution of VLBI to GGOS will be described.

Analysis of Vlbi, Slr and GPS Site Position Time Series

NASA Astrophysics Data System (ADS)

Angermann, D.; Krügel, M.; Meisel, B.; Müller, H.; Tesmer, V.

Conventionally the IERS terrestrial reference frame (ITRF) is realized by the adoption of a set of epoch coordinates and linear velocities for a set of global tracking stations. Due to the remarkable progress of the space geodetic observation techniques (e.g. VLBI, SLR, GPS) the accuracy and consistency of the ITRF increased continuously. The accuracy achieved today is mainly limited by technique-related systematic errors, which are often poorly characterized or quantified. Therefore it is essential to analyze the individual techniques' solutions with respect to systematic differences, models, parameters, datum definition, etc. Main subject of this presentation is the analysis of GPS, SLR and VLBI time series of site positions. The investigations are based on SLR and VLBI solutions computed at DGFI with the software systems DOGS (SLR) and OCCAM (VLBI). The GPS time series are based on weekly IGS station coordinates solutions. We analyze the time series with respect to the issues mentioned above. In particular we characterize the noise in the time series, identify periodic signals, and investigate non-linear effects that complicate the assignment of linear velocities for global tracking sites. One important aspect is the comparison of results obtained by different techniques at colocation sites.

First Space VLBI Observations and Images Using the VLBA and VSOP

NASA Astrophysics Data System (ADS)

Romney, J. D.; Benson, J. M.; Claussen, M. J.; Desai, K. M.; Flatters, C.; Mioduszewski, A. J.; Ulvestad, J. S.

1997-12-01

The National Radio Astronomy Observatory (NRAO) is a participant in the VSOP Space VLBI mission, an international collaboration led by Japan's Institute of Space and Astronautical Science. NRAO has committed up to 30% of scheduled observing time on the Very Long Baseline Array (VLBA), and corresponding correlation resources, to Space VLBI observations. The NRAO Space VLBI Project, funded by NASA, has been working for several years to complete the necessary enhancements to the VLBA correlator and the AIPS image processing system. These developments were completed by the time of the successful launch of the VSOP mission's Halca spacecraft on 1997 February 12. As part of the in-orbit checkout phase, the first Space VLBI fringes from a VLBA observation were detected on 1997 June 12, and the VSOP mission's first images, in both the 1.6- and 5-GHz bands, were obtained shortly thereafter. In-orbit test observations continued through early September, with the first General Observing Time (GOT) scientific observations beginning in July. Through mid-October, a total of 20 Space VLBI observations, comprising 190 hours, had been completed at the VLBA correlator. This paper reviews the unique features of correlation and imaging of Space VLBI observations. These include, for correlation, the ephemeris for an orbiting VLBI ``station'' which is not fixed on the surface of the earth, and the requirement to close the loop on the phase-transfer process from a frequency standard on the ground to the spacecraft. Images from a number of early tests and scientific observations are presented. NRAO's user-support program, providing expert assistance in data analysis to Space VLBI observers, is also described.

EOP and scale from continuous VLBI observing: CONT campaigns to future VGOS networks

NASA Astrophysics Data System (ADS)

MacMillan, D. S.

2017-07-01

Continuous (CONT) VLBI campaigns have been carried out about every 3 years since 2002. The basic idea of these campaigns is to acquire state-of-the-art VLBI data over a continuous time period of about 2 weeks to demonstrate the highest accuracy of which the current VLBI system is capable. In addition, these campaigns support scientific studies such as investigations of high-resolution Earth rotation, reference frame stability, and daily to sub-daily site motions. The size of the CONT networks and the observing data rate have increased steadily since 1994. Performance of these networks based on reference frame scale precision and polar motion/LOD comparison with global navigation satellite system (GNSS) earth orientation parameters (EOP) has been substantially better than the weekly operational R1 and R4 series. The precisions of CONT EOP and scale have improved by more than a factor of two since 2002. Polar motion precision based on the WRMS difference between VLBI and GNSS for the most recent CONT campaigns is at the 30 μas level, which is comparable to that of GNSS. The CONT campaigns are a natural precursor to the planned future VLBI observing networks, which are expected to observe continuously. We compare the performance of the most recent CONT campaigns in 2011 and 2014 with the expected performance of the future VLBI global observing system network using simulations. These simulations indicate that the expected future precision of scale and EOP will be at least 3 times better than the current CONT precision.

Differences Between S/X and VLBI2010 Operation

NASA Technical Reports Server (NTRS)

Hase, Hayo; Himwich, Ed; Neidhardt, Alexander

2010-01-01

The intended VLBI2010 operation has some significant differences to the current S/X operation. The presentation focuses on the problem of extending the operation of a global VLBI network to continuous operation within the frame of the same given amount of human resources. Remote control operation is a suitable solution to minimize operational expenses. The implementation of remote control operation requires more site specific information. A concept of a distributed-centralized remote control of the operation and its implications is presented.

Multiband VLBI Observations of CTA102

NASA Technical Reports Server (NTRS)

Rantakyro, F. T.; Baath, L. B.; Dallacasa, D.; Jones, D. L.; Wehrle, A. E.

1995-01-01

The source CTA102, known to exhibit low frequency variability, has been observed at six epochs (three at lambda 32 cm, two at lambda 18 cm, and one at lambda l.3 cm) with intercontinental VLBI arrays. On the basis of the changes observed in the structure, we believe that the flux density variations at these wavelengths are due to intrinsic processes and not due to interstellar scintillation. This source exhibits behaviour suggestive of being expanding with a very high apparent transverse velocity.

Monte Carlo simulations of the impact of troposphere, clock and measurement errors on the repeatability of VLBI positions

NASA Astrophysics Data System (ADS)

Pany, A.; Böhm, J.; MacMillan, D.; Schuh, H.; Nilsson, T.; Wresnik, J.

2011-01-01

Within the International VLBI Service for Geodesy and Astrometry (IVS) Monte Carlo simulations have been carried out to design the next generation VLBI system ("VLBI2010"). Simulated VLBI observables were generated taking into account the three most important stochastic error sources in VLBI, i.e. wet troposphere delay, station clock, and measurement error. Based on realistic physical properties of the troposphere and clocks we ran simulations to investigate the influence of the troposphere on VLBI analyses, and to gain information about the role of clock performance and measurement errors of the receiving system in the process of reaching VLBI2010's goal of mm position accuracy on a global scale. Our simulations confirm that the wet troposphere delay is the most important of these three error sources. We did not observe significant improvement of geodetic parameters if the clocks were simulated with an Allan standard deviation better than 1 × 10-14 at 50 min and found the impact of measurement errors to be relatively small compared with the impact of the troposphere. Along with simulations to test different network sizes, scheduling strategies, and antenna slew rates these studies were used as a basis for the definition and specification of VLBI2010 antennas and recording system and might also be an example for other space geodetic techniques.

New VLBI2010 scheduling strategies and implications on the terrestrial reference frames.

PubMed

Sun, Jing; Böhm, Johannes; Nilsson, Tobias; Krásná, Hana; Böhm, Sigrid; Schuh, Harald

In connection with the work for the next generation VLBI2010 Global Observing System (VGOS) of the International VLBI Service for Geodesy and Astrometry, a new scheduling package (Vie_Sched) has been developed at the Vienna University of Technology as a part of the Vienna VLBI Software. In addition to the classical station-based approach it is equipped with a new scheduling strategy based on the radio sources to be observed. We introduce different configurations of source-based scheduling options and investigate the implications on present and future VLBI2010 geodetic schedules. By comparison to existing VLBI schedules of the continuous campaign CONT11, we find that the source-based approach with two sources has a performance similar to the station-based approach in terms of number of observations, sky coverage, and geodetic parameters. For an artificial 16 station VLBI2010 network, the source-based approach with four sources provides an improved distribution of source observations on the celestial sphere. Monte Carlo simulations yield slightly better repeatabilities of station coordinates with the source-based approach with two sources or four sources than the classical strategy. The new VLBI scheduling software with its alternative scheduling strategy offers a promising option with respect to applications of the VGOS.

New VLBI2010 scheduling strategies and implications on the terrestrial reference frames

NASA Astrophysics Data System (ADS)

Sun, Jing; Böhm, Johannes; Nilsson, Tobias; Krásná, Hana; Böhm, Sigrid; Schuh, Harald

2014-05-01

In connection with the work for the next generation VLBI2010 Global Observing System (VGOS) of the International VLBI Service for Geodesy and Astrometry, a new scheduling package (Vie_Sched) has been developed at the Vienna University of Technology as a part of the Vienna VLBI Software. In addition to the classical station-based approach it is equipped with a new scheduling strategy based on the radio sources to be observed. We introduce different configurations of source-based scheduling options and investigate the implications on present and future VLBI2010 geodetic schedules. By comparison to existing VLBI schedules of the continuous campaign CONT11, we find that the source-based approach with two sources has a performance similar to the station-based approach in terms of number of observations, sky coverage, and geodetic parameters. For an artificial 16 station VLBI2010 network, the source-based approach with four sources provides an improved distribution of source observations on the celestial sphere. Monte Carlo simulations yield slightly better repeatabilities of station coordinates with the source-based approach with two sources or four sources than the classical strategy. The new VLBI scheduling software with its alternative scheduling strategy offers a promising option with respect to applications of the VGOS.

Terrestrial reference frame solution with the Vienna VLBI Software VieVS and implication of tropospheric gradient estimation

NASA Astrophysics Data System (ADS)

Spicakova, H.; Plank, L.; Nilsson, T.; Böhm, J.; Schuh, H.

2011-07-01

The Vienna VLBI Software (VieVS) has been developed at the Institute of Geodesy and Geophysics at TU Vienna since 2008. In this presentation, we present the module Vie_glob which is the part of VieVS that allows the parameter estimation from multiple VLBI sessions in a so-called global solution. We focus on the determination of the terrestrial reference frame (TRF) using all suitable VLBI sessions since 1984. We compare different analysis options like the choice of loading corrections or of one of the models for the tropospheric delays. The effect of atmosphere loading corrections on station heights if neglected at observation level will be shown. Time series of station positions (using a previously determined TRF as a priori values) are presented and compared to other estimates of site positions from individual IVS (International VLBI Service for Geodesy and Astrometry) Analysis Centers.

MK3TOOLS & NetCDF - storing VLBI data in a machine independent array oriented data format

NASA Astrophysics Data System (ADS)

Hobiger, T.; Koyama, Y.; Kondo, T.

2007-07-01

In the beginning of 2002 the International VLBI Service (IVS) has agreed to introduce a Platform-independent VLBI exchange format (PIVEX) which permits the exchange of observational data and stimulates the research across different analysis groups. Unfortunately PIVEX has never been implemented and many analysis software packages are still depending on prior processing (e.g. ambiguity resolution and computation of ionosphere corrections) done by CALC/SOLVE. Thus MK3TOOLS which handles MK3 databases without CALC/SOLVE being installed has been developed. It uses the NetCDF format to store the data and since interfaces exist for a variety of programming languages (FORTRAN, C/C++, JAVA, Perl, Python) it can be easily incorporated in existing and upcoming analysis software packages.

THE FIRST VERY LONG BASELINE INTERFEROMETRIC SETI EXPERIMENT

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rampadarath, H.; Morgan, J. S.; Tingay, S. J.

2012-08-15

The first Search for Extra-Terrestrial Intelligence (SETI) conducted with very long baseline interferometry (VLBI) is presented. By consideration of the basic principles of interferometry, we show that VLBI is efficient at discriminating between SETI signals and human generated radio frequency interference (RFI). The target for this study was the star Gliese 581, thought to have two planets within its habitable zone. On 2007 June 19, Gliese 581 was observed for 8 hr at 1230-1544 MHz with the Australian Long Baseline Array. The data set was searched for signals appearing on all interferometer baselines above five times the noise limit. Amore » total of 222 potential SETI signals were detected and by using automated data analysis techniques were ruled out as originating from the Gliese 581 system. From our results we place an upper limit of 7 MW Hz{sup -1} on the power output of any isotropic emitter located in the Gliese 581 system within this frequency range. This study shows that VLBI is ideal for targeted SETI including follow-up observations. The techniques presented are equally applicable to next-generation interferometers, such as the long baselines of the Square Kilometre Array.« less

Theory and Realization of Global Terrestrial Reference Systems

NASA Technical Reports Server (NTRS)

Ma, C.; Bolotin, S.; Gipson, J.; Gordon, D.; Le Bail, K.; MacMillan, D.

2010-01-01

Comparison of realizations of the terrestrial reference frame. IGN and DGFI both generated realizations of the terrestrial reference frame under the auspices of the IERS from combination of the same space geodetic data. We examined both results for VLBI sites using the full geodetic VLBI data set with respect to site positions and velocities and time series of station positions, baselines and Earth orientation parameters. One of the difficulties encountered was matching episodic breaks and periods of non-linear motion of the two realizations with the VLBI models. Our analysis and conclusions will be discussed.

Studies of regional and global tectonics and the rotation of the earth using very-long baseline interferometry

NASA Technical Reports Server (NTRS)

Shapiro, Irwin I.

1989-01-01

Progress in the areas of data analysis, atmospheric delay calibration and software conversion is reported. Over 800 very long baseline interferometry (VLBI) experiments were analyzed in the last 6 months. Reprocessing of the Mark III VLBI data set is almost completed. Results of analysis of the water-vapor radiometer (WVR) data were submitted and a preprint of a related paper is attached. Work on conversion of the VLBI analysis software from HP1000 to Unix based workstations is continuing.

Crustal dynamics project data analysis, 1987. Volume 2: Mobile VLBI geodetic results, 1982-1986

NASA Technical Reports Server (NTRS)

Ma, C.; Ryan, J. W.

1987-01-01

The Goddard VLBI group reports the results of analyzing 101 Mark III data sets acquired from mobile observing sites through the end of 1986 and available to the Crustal Dynamics Project. The fixed VLBI observations at Hat Creek, Ft. Davis, Mojave, and OVRO are included as they participate heavily in the mobile schedules. One large solution GLB171 was used to obtain baseline length and transverse evolutions. Radio source positions were estimated globally, while nutation offsets were estimated from each data set. The results include 28 mobile sites.

Generation of Global Geodetic Networks for GGOS

NASA Astrophysics Data System (ADS)

MacMillan, Daniel; Pavlis, Erricos C.; Kuzmicz-Cieslak, Magda; Koenig, Daniel

2016-12-01

We simulated future networks of VLBI+SLR sites to assess their performance. The objective is to build a global network of geographically well distributed, co-located next-generation sites from each of the space geodetic techniques. The network is being designed to meet the GGOS terrestrial reference frame goals of 1 mm in accuracy and 0.1 mm/yr in stability. We simulated the next generation networks that should be available in five years and in ten years to assess the likelihood that these networks will meet the reference frame goals. Simulations were based on the expectation that 17 broadband VLBI stations will be available in five years and 27 stations in ten years. We also consider the improvement resulting from expanding the network by six additional VLBI sites to improve the global distribution of the network. In the simulations, the networks will operate continuously, but we account for station downtime for maintenance or because of bad weather. We ran SLR+VLBI combination TRF solutions, where site ties were used to connect the two networks in the same way as in combination solutions with observed data. The strengths of VLBI and SLR allows them to provide the necessary reference frame accuracy in scale, geocenter, and orientation. With the +10-year extended network operating for ten years, simulations indicate that scale, origin, and orientation accuracies will be at the level of 0.02 ppb, 0.2 mm, and 6 μas. Combining the +5-year and +10-year network realizations will provide better estimates of accuracy and estimates of stability.

The Global GNSS, SLR, VLBI, and DORIS Networks and their Support of GGOS: IGS+ILRS+IVS+IDS

NASA Technical Reports Server (NTRS)

Noll, Carey

2008-01-01

The global network of the International GNSS Service (IGS), the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), and the International DORIS Service (IDS) are part of the ground-based infrastructure for GGOS. The observations obtained from these global networks provide for the determination and maintenance of the International Terrestrial Reference Frame (ITRF), an accurate set of positions and velocities that provides a stable coordinate system allowing scientists ts to link measurements over space and time. Many of these sites offer co-location of two or more techniques. Co-location provides integration of technique-specific networks into the ITRF as well as an assessment/validation of the quality and accuracy of the resulting measurements. As of fall 2008, these networks consisted of 410 GNSS sites, 42 laser ranging sites, 45 VLBI sites, and 58 DORIS sites. This poster will illustrate the global coverage of these networks, highlighting inter-technique co-locations, and show the importance of these networks 60 the underlying goals of GGOS including providing the observational basis to maintain a stable, accurate, global reference frame.

VLBI Monitoring of the Nucleus of Centaurus A

NASA Technical Reports Server (NTRS)

Preston, Robert

1995-01-01

A series of 8.4 GHz VLBI images of the nucleus of Centaurus A have been made with a Southern Hemisphere array over more than a 3-year time span. The nuclear radio jet is approximately 50 mas in length, or about 1 pc at the 3.5 Mpc distance of Centaurus A. Sub-luminal motion is seen and structural changes observed on time-scales shorter than four months. Observations at both 4.8 and 8.4 GHz at one epoch allow identification of the core at the southwestern end of the jet.

Status and plans for the future of the Vienna VLBI Software

NASA Astrophysics Data System (ADS)

Madzak, Matthias; Böhm, Johannes; Böhm, Sigrid; Girdiuk, Anastasiia; Hellerschmied, Andreas; Hofmeister, Armin; Krasna, Hana; Kwak, Younghee; Landskron, Daniel; Mayer, David; McCallum, Jamie; Plank, Lucia; Schönberger, Caroline; Shabala, Stanislav; Sun, Jing; Teke, Kamil

2016-04-01

The Vienna VLBI Software (VieVS) is a VLBI analysis software developed and maintained at Technische Universität Wien (TU Wien) since 2008 with contributions from groups all over the world. It is used for both academic purposes in university courses as well as for providing VLBI analysis results to the geodetic community. Written in a modular structure in Matlab, VieVS offers easy access to the source code and the possibility to adapt the programs for particular purposes. The new version 2.3, released in December 2015, includes several new parameters to be estimated in the global solution, such as tidal ERP variation coefficients. The graphical user interface was slightly modified for an improved user functionality and, e.g., the possibility of deriving baseline length repeatabilities. The scheduling of satellite observations was refined, the simulator newly includes the effect of source structure which can also be corrected for in the analysis. This poster gives an overview of all VLBI-related activities in Vienna and provides an outlook to future plans concerning the Vienna VLBI Software.

The LBA Calibrator Survey of Southern Compact Extragalactic Radio Sources - LCS1

NASA Technical Reports Server (NTRS)

Petrov, Leonid; Phillips, Chris; Bertarini, Alessandra; Murphy, Tara; Sadler, Elaine M.

2011-01-01

We present a catalogue of accurate positions and correlated flux densities for 410 flat-spectrum, compact extragalactic radio sources previously detected in the Australia Telescope 20 GHz (AT20G) survey. The catalogue spans the declination range [-90deg, -40deg] and was constructed from four 24-h very long baseline interferometry (VLBI) observing sessions with the Australian Long Baseline Array at 8.3 GHz. The VLBI detection rate in these experiments is 97 per cent, the median uncertainty of the source positions is 2.6 mas and the median correlated flux density on projected baselines longer than 1000 km is 0.14 Jy. The goals of this work are (1) to provide a pool of southern sources with positions accurate to a few milliarcsec, which can be used for phase-referencing observations, geodetic VLBI and space navigation; (2) to extend the complete flux-limited sample of compact extragalactic sources to the Southern hemisphere; and (3) to investigate the parsec-scale properties of high-frequency selected sources from the AT20G survey. As a result of this VLBI campaign, the number of compact radio sources south of declination -40deg which have measured VLBI correlated flux densities and positions known to milliarcsec accuracy has increased by a factor of 3.5.

Detection of Intrinsic Source Structure at ∼3 Schwarzschild Radii with Millimeter-VLBI Observations of SAGITTARIUS A*

NASA Astrophysics Data System (ADS)

Lu, Ru-Sen; Krichbaum, Thomas P.; Roy, Alan L.; Fish, Vincent L.; Doeleman, Sheperd S.; Johnson, Michael D.; Akiyama, Kazunori; Psaltis, Dimitrios; Alef, Walter; Asada, Keiichi; Beaudoin, Christopher; Bertarini, Alessandra; Blackburn, Lindy; Blundell, Ray; Bower, Geoffrey C.; Brinkerink, Christiaan; Broderick, Avery E.; Cappallo, Roger; Crew, Geoffrey B.; Dexter, Jason; Dexter, Matt; Falcke, Heino; Freund, Robert; Friberg, Per; Greer, Christopher H.; Gurwell, Mark A.; Ho, Paul T. P.; Honma, Mareki; Inoue, Makoto; Kim, Junhan; Lamb, James; Lindqvist, Michael; Macmahon, David; Marrone, Daniel P.; Martí-Vidal, Ivan; Menten, Karl M.; Moran, James M.; Nagar, Neil M.; Plambeck, Richard L.; Primiani, Rurik A.; Rogers, Alan E. E.; Ros, Eduardo; Rottmann, Helge; SooHoo, Jason; Spilker, Justin; Stone, Jordan; Strittmatter, Peter; Tilanus, Remo P. J.; Titus, Michael; Vertatschitsch, Laura; Wagner, Jan; Weintroub, Jonathan; Wright, Melvyn; Young, Ken H.; Zensus, J. Anton; Ziurys, Lucy M.

2018-05-01

We report results from very long baseline interferometric (VLBI) observations of the supermassive black hole in the Galactic center, Sgr A*, at 1.3 mm (230 GHz). The observations were performed in 2013 March using six VLBI stations in Hawaii, California, Arizona, and Chile. Compared to earlier observations, the addition of the APEX telescope in Chile almost doubles the longest baseline length in the array, provides additional uv coverage in the N–S direction, and leads to a spatial resolution of ∼30 μas (∼3 Schwarzschild radii) for Sgr A*. The source is detected even at the longest baselines with visibility amplitudes of ∼4%–13% of the total flux density. We argue that such flux densities cannot result from interstellar refractive scattering alone, but indicate the presence of compact intrinsic source structure on scales of ∼3 Schwarzschild radii. The measured nonzero closure phases rule out point-symmetric emission. We discuss our results in the context of simple geometric models that capture the basic characteristics and brightness distributions of disk- and jet-dominated models and show that both can reproduce the observed data. Common to these models are the brightness asymmetry, the orientation, and characteristic sizes, which are comparable to the expected size of the black hole shadow. Future 1.3 mm VLBI observations with an expanded array and better sensitivity will allow more detailed imaging of the horizon-scale structure and bear the potential for a deep insight into the physical processes at the black hole boundary.

The Pearson-Readhead Survey of Compact Extragalactic Radio Sources from Space. I. The Images

NASA Astrophysics Data System (ADS)

Lister, M. L.; Tingay, S. J.; Murphy, D. W.; Piner, B. G.; Jones, D. L.; Preston, R. A.

2001-06-01

We present images from a space-VLBI survey using the facilities of the VLBI Space Observatory Programme (VSOP), drawing our sample from the well-studied Pearson-Readhead survey of extragalactic radio sources. Our survey has taken advantage of long space-VLBI baselines and large arrays of ground antennas, such as the Very Long Baseline Array and European VLBI Network, to obtain high-resolution images of 27 active galactic nuclei and to measure the core brightness temperatures of these sources more accurately than is possible from the ground. A detailed analysis of the source properties is given in accompanying papers. We have also performed an extensive series of simulations to investigate the errors in VSOP images caused by the relatively large holes in the (u,v)-plane when sources are observed near the orbit normal direction. We find that while the nominal dynamic range (defined as the ratio of map peak to off-source error) often exceeds 1000:1, the true dynamic range (map peak to on-source error) is only about 30:1 for relatively complex core-jet sources. For sources dominated by a strong point source, this value rises to approximately 100:1. We find the true dynamic range to be a relatively weak function of the difference in position angle (P.A.) between the jet P.A. and u-v coverage major axis P.A. For regions with low signal-to-noise ratios, typically located down the jet away from the core, large errors can occur, causing spurious features in VSOP images that should be interpreted with caution.

PMD IVS Analysis Center

NASA Technical Reports Server (NTRS)

Tornatore, Vincenza

2013-01-01

The main activities carried out at the PMD (Politecnico di Milano DIIAR) IVS Analysis Center during 2012 are briefly higlighted, and future plans for 2013 are sketched out. We principally continued to process European VLBI sessions using different approaches to evaluate possible differences due to various processing choices. Then VLBI solutions were also compared to the GPS ones as well as the ones calculated at co-located sites. Concerning the observational aspect, several tests were performed to identify the most suitable method to achieve the highest possible accuracy in the determination of GNSS (GLOBAL NAVIGATION SATELLITE SYSTEM) satellite positions using the VLBI technique.

HIGH-PRECISION ASTROMETRIC MILLIMETER VERY LONG BASELINE INTERFEROMETRY USING A NEW METHOD FOR ATMOSPHERIC CALIBRATION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rioja, M.; Dodson, R., E-mail: maria.rioja@icrar.org

2011-04-15

We describe a new method which achieves high-precision very long baseline interferometry (VLBI) astrometry in observations at millimeter (mm) wavelengths. It combines fast frequency-switching observations, to correct for the dominant non-dispersive tropospheric fluctuations, with slow source-switching observations, for the remaining ionospheric dispersive terms. We call this method source-frequency phase referencing. Provided that the switching cycles match the properties of the propagation media, one can recover the source astrometry. We present an analytic description of the two-step calibration strategy, along with an error analysis to characterize its performance. Also, we provide observational demonstrations of a successful application with observations using themore » Very Long Baseline Array at 86 GHz of the pairs of sources 3C274 and 3C273 and 1308+326 and 1308+328 under various conditions. We conclude that this method is widely applicable to mm-VLBI observations of many target sources, and unique in providing bona fide astrometrically registered images and high-precision relative astrometric measurements in mm-VLBI using existing and newly built instruments, including space VLBI.« less

Combination of Vlbi, GPS and Slr Observations At The Observation Level For The Realization of Terrestrial and Celestial Reference Frames

NASA Astrophysics Data System (ADS)

Andersen, P. H.

Forsvarets forskningsinstitutt (FFI, the Norwegian Defence Research Establishment) has during the last 17 years developed a software system called GEOSAT, for the analysis of any type of high precision space geodetic observations. A unique feature of GEOSAT is the possibility of combining any combination of different space geode- tic data at the observation level with one consistent model and one consistent strategy. This is a much better strategy than the strategy in use today where different types of observations are processed separately using analysis software developed specifically for each technique. The results from each technique are finally combined a posteriori. In practice the models implemented in the software packages differ at the 1-cm level which is almost one order of magnitude larger than the internal precision of the most precise techniques. Another advantage of the new proposed combination method is that for example VLBI and GPS can use the same tropospheric model with common parameterization. The same is the case for the Earth orientation parameters, the geo- center coordinates and other geodetic or geophysical parameters where VLBI, GPS and SLR can have a common estimate for each of the parameters. The analysis with GEOSAT is automated for the combination of VLBI, SLR and GPS observations. The data are analyzed in batches of one day where the result from each daily arc is a SRIF array (Square Root Information Filter). A large number of SRIF arrays can be combined into a multi-year solution using the CSRIFS program (Com- bination Square Root Information Filter and Smoother). Four parameter levels are available and any parameter can, at each level, either be represented as a constant or a stochastic parameter (white noise, colored noise, or random walk). The batch length (i.e. the time interval between the addition of noise to the SRIF array) can be made time- and parameter dependent. GEOSAT and CSRIFS have been applied in the analysis of selected VLBI and SLR data (LAGEOS I &II) from the period January 1993 to July 2001. A selected number of arcs also include GPS data. Earth orientation parameters, geocenter motion, sta- tion coordinates and velocities were estimated simultaneously with the coordinates of the radio sources and satellite orbital parameters. Recent software improvements and 1 results of analyses will be presented at the meeting. 2

Possible systematics in the VLBI catalogs as seen from Gaia

NASA Astrophysics Data System (ADS)

Liu, N.; Zhu, Z.; Liu, J.-C.

2018-01-01

Aims: In order to investigate the systematic errors in the very long baseline interferometry (VLBI) positions of extragalactic sources (quasars) and the global differences between Gaia and VLBI catalogs, we use the first data release of Gaia (Gaia DR1) quasar positions as the reference and study the positional offsets of the second realization of the International Celestial Reference Frame (ICRF2) and the Goddard VLBI solution 2016a (gsf2016a) catalogs. Methods: We select a sample of 1032 common sources among three catalogs and adopt two methods to represent the systematics: considering the differential orientation (offset) and declination bias; analyzing with the vector spherical harmonics (VSH) functions. Results: Between two VLBI catalogs and Gaia DR1, we find that: i) the estimated orientation is consistent with the alignment accuracy of Gaia DR1 to ICRF, of 0.1 mas, but the southern and northern hemispheres show opposite orientations; ii) the declination bias in the southern hemisphere between Gaia DR1 and ICRF2 is estimated to be +152 μas, much larger than that between Gaia DR1 and gsf2016a which is +34 μas. Between two VLBI catalogs, we find that: i) the rotation component shows that ICRF2 and gsf2016a are generally consistent within 30 μas; ii) the glide component and quadrupole component report two declination-dependent offsets: dipolar deformation of +50 μas along the Z-axis, and quadrupolar deformation of -50 μas that would induce a pattern of sin2δ. Conclusions: The significant declination bias between Gaia DR1 and ICRF2 catalogs reported in previous studies is possibly attributed to the systematic errors of ICRF2 in the southern hemisphere. The global differences between ICRF2 and gsf2016a catalogs imply that possible, mainly declination-dependent systematics exit in the VLBI positions and need further investigations in the future Gaia data release and the next generation of ICRF.

Cryogenic Integration of the 2-14 GHz Eleven Feed in a Wideband Receiver for VLBI2010

NASA Technical Reports Server (NTRS)

Pantaleev, Miroslaw; Jang, Jian; Karadikar, Yogesh; Helldner, Leif; Klein, Benjamin; Haas, Rudiger; Zaman, Ashraf; Zamani, Mojtaba; Kildal, Per-Simon

2010-01-01

The next generation VLBI systems require the design of a wideband receiver covering the 2-14 GHz range, necessitating a wideband feed. Presented here are the 2009 development of a cryogenic 2-14 GHz Eleven feed for reflector radio telescope antennas, including its integration into a cryogenic receiver. The Eleven feed is designed for dual linear polarization and consists of four log-periodic folded dipole arrays. Each pair of arrays is fed by a differential two-wire transmission line connected either to balun or a differential LNA. The present configuration has been measured in many configurations, at various independent labs - corresponding simulations have been done. The results show (across the band) a high polarization efficiency for the feed, with a nearly constant beam width, a reflection coefficient below -10dB, and a constant phase center. Electrical parameters under cryogenic conditions and measured receiver noise temperatures are presented.

The Parsec-Scale Morphology of Southern GPS Sources

NASA Astrophysics Data System (ADS)

Edwards, P. G.; Tingay, S. J.

2016-12-01

Multi-frequency, multi-epoch ATCA observations of a sample of AGN resulted in the identification of nine new candidate Giga-hertz Peaked Spectrum sources. Here, we present Long Baseline Array observations at 4.8 GHz of the four candidates with no previously published VLBI image, and consider these together with previously published VLBI images of the other five sources. We find core-jet or compact double morphologies dominate, with further observations required to distinguish between these two possibilities for some sources. One of the nine candidates, PKS 1831-711, displays appreciable variability, suggesting its GPS spectrum is more ephemeral in nature. We focus in particular on the apparent relationship between a narrow spectral width and `compact double' parsec-scale morphology, finding further examples, but also exceptions to this trend. An examination of the VLBI morphologies high-redshift (z > 3) sub-class of GPS sources suggests that core-jet morphologies predominate in this class.

Crustal dynamics project data analysis, 1988: VLBI geodetic results, 1979 - 1987

NASA Technical Reports Server (NTRS)

Ma, C.; Ryan, J. W.; Caprette, D.

1989-01-01

The results obtained by the Goddard VLBI (very long base interferometry) Data Analysis Team from the analysis of 712 Mark 3 VLBI geodetic data sets acquired from fixed and mobile observing sites through the end of 1987 are reported. A large solution, GLB401, was used to obtain earth rotation parameters and site velocities. A second large solution, GLB405, was used to obtain baseline evolutions. Radio source positions were estimated globally while nutation offsets were estimated from each data set. Site positions are tabulated on a yearly basis from 1979 through 1988. The results include 55 sites and 270 baselines.

First steps of processing VLBI data of space probes with VieVS

NASA Astrophysics Data System (ADS)

Plank, L.; Böhm, J.; Schuh, H.

2011-07-01

Since 2008 the VLBI group at the Institute of Geodesy and Geophysics (IGG) of the Vienna University of Technology has developed the Vienna VLBI Software VieVS which is capable to process geodetic VLBI data in NGS format. Constantly we are working on upgrading the new software, e.g. by developing a scheduling tool or extending the software from single session solution to a so-called global solution, allowing the joint analysis of many sessions covering several years. In this presentation we report on first steps to enable the processing of space VLBI data with the software. Driven by the recently increasing number of space VLBI applications, our goal is the geodetic usage of such data, primarily concerning frame ties between various reference frames, e. g. by connecting the dynamic reference frame of a space probe with the kinematically defined International Celestial Reference Frame (ICRF). Main parts of the software extension w.r.t. the existing VieVS are the treatment of fast moving targets, the implementation of a delay model for radio emitters at finite distances, and the adequate mathematical model and adjustment of the particular unknowns. Actual work has been done for two mission scenarios so far: On the one hand differential VLBI (D-VLBI) data from the two sub-satellites of the Japanese lunar mission Selene were processed, on the other hand VLBI observations of GNSS satellites were modelled in VieVS. Besides some general aspects, we give details on the calculation of the theoretical delay (delay model for moving sources at finite distances) and its realization in VieVS. First results with real data and comparisons with best fit mission orbit data are also presented.'

Very long baseline interferometer measurements of plasma turbulence in the solar wind

DOE Office of Scientific and Technical Information (OSTI.GOV)

Takayuki Sakurai; Spangler, S.R.; Armstrong, J.W.

Turbulence in the solar wind plasma was studied using angular broadening measurements of 10 extragalactic compact radio sources (quasars) with a very long baseline interferometer (VLBI) at 4.99 GHz. Unlike other angular broadening studies, the measured broadening size was corrected for intrinsic source structures which were obtained from a separate VLBI observation. The solar elongations of the sources ranged from 18 R{sub S} to 243 R{sub S}, and five sources with elongations {<=} 60 R{sub S} showed varying degrees of broadening. The measured angular sizes are considerably less than predicted by the well-known empirical relationship of Erickson, as well asmore » two other models for strength of scattering as a function of solar elongation. However, the data are in good agreement with a model for the spatial power spectrum of the turbulence proposed by Coles and Harmon. This model consists of a Kolmogorov spectrum at large scales, but with an enhancement of power near the wavenumber corresponding o the ion inertial length. Two of these sources, 1148-001 and 1253-053 (3C279), show substantial differences in the amount of scattering, even though they are at similar solar elongations (29 versus 35 R{sub S}). Data to which the authors have access indicate that the state of the corona along the lines of sight to these sources may have been quite different. Angular broadening measurements with VLBI interferometers currently under development (primarily the very long baseline array) will allow a global view of plasma turbulence out of the ecliptic plane and thus be complementary to the point in situ measurements with Ulysses. 37 refs., 4 figs., 1 tab.« less

The Parallax of the Red Hypergiant VX Sgr with Accurate Tropospheric Delay Calibration

NASA Astrophysics Data System (ADS)

Xu, Shuangjing; Zhang, Bo; Reid, Mark J.; Menten, Karl M.; Zheng, Xingwu; Wang, Guangli

2018-05-01

We report astrometric results of VLBI phase-referencing observations of 22 GHz H2O masers emission toward the red hypergiant VX Sgr, one of most massive and luminous red hypergiant stars in our Galaxy, using the Very Long Baseline Array. A background source, J1820‑2528, projected 4.°4 from the target VX Sgr, was used as the phase reference. For the low decl. of these sources, such a large separation normally would seriously degrade the relative astrometry. We use a two-step method of tropospheric delay calibration, which combines the VLBI geodetic-block (or Global Positioning System) calibration with an image-optimization calibration, to obtain a trigonometric parallax of 0.64 ± 0.04 mas, corresponding to a distance of {1.56}-0.10+0.11 kpc. The measured proper motion of VX Sgr is 0.36 ± 0.76 and ‑2.92 ± 0.78 mas yr‑1 in the eastward and northward directions. The parallax and proper motion confirms that VX Sgr belong to the Sgr OB1 association. Rescaling bolometric luminosities in the literature to our parallax distance, we find that the luminosity of VX Sgr is (1.95 ± 0.62) × 105 L ⊙, where the uncertainty is dominated by differing photometry measurements.

Ultra-High Resolution Observations Of Selected Blazars

NASA Astrophysics Data System (ADS)

Hodgson, Jeffrey A.

2015-01-01

Active Galactic Nuclei are the luminous centres of active galaxies that produce powerful relativistic jets from central super massive black holes (SMBH). When these jets are oriented towards the observer's line-of-sight, they become very bright, very variable and very energetic. These sources are known as blazars and Very Long Baseline Interferometry (VLBI) provides a direct means of observing into the heart of these objects. VLBI performed at 3 mm with the Global mm-VLBI Array (GMVA) and 7 mm VLBI performed with the Very Long Baseline Array (VLBA), allows some of the highest angular resolution images of blazars to be produced. In this thesis, we present the first results of an ongoing monitoring program of blazars known to emit at γ-ray energies. The physical processes that produce these jets and the γ-ray emission are still not well known. The jets are thought to be produced by converting gravitational energy around the black hole into relativistic particles that are accelerated away at near the speed of light. However, the exact mechanisms for this and the role that magnetic fields play is not fully clear. Similarly, γ-rays have been long known to have been emitted from blazars and that their production is often related to the up-scattering of synchrotron radiation from the jet. However, the origin of seed photons for the up-scattering (either from within the jet itself or from an external photon field) and the location of the γ-ray emission regions has remained inconclusive. In this thesis, we aim to describe the likely location of γ-ray emission in jets, the physical structure of blazar jets, the location of the VLBI features relative to the origin of the jet and the nature of the magnetic field, both of the VLBI scale jet and in the region where the jet is produced. We present five sources that have been monitored at 3 mm using the GMVA from 2008 until 2012. These sources have been analysed with near-in-time 7 mm maps from the Very Long Baseline Array (VLBA), γ-ray light curves from the Fermi/LAT space telescope and cm to mm-wave total-intensity light curves. In one source, OJ 287, the source has additionally been analysed with monthly imaging at 7 mm with the VLBA and near-in-time 2 cm VLBI maps. We use these resources to analyse high angular resolution structural and spectral changes and see if they correlate with flaring (both radio and γ-ray) activity and with VLBI component ejections. By spectrally decomposing sources, we can determine the spatially resolved magnetic field structure in the jets at the highest yet performed resolutions and at frequencies that are near or above the turnover frequency for synchrotron self-absorption (SSA). We compute the magnetic field estimates from SSA theory and by assuming equipartition between magnetic fields and relativistic particle energies. All sources analysed exhibit downstream quasi-stationary features which sometimes exhibit higher brightness temperatures and flux density variability than the VLBI "core", which we interpret as being recollimation or oblique shocks. We find that γ-ray flaring, mm-wave radio flaring and changes in opacity from optically thick to optically thin, is in many cases consistent with component ejections past both the VLBI "core" and these quasi-stationary downstream features. We find decreasing apparent brightness temperatures and Doppler factors as a function of increased "core" separation, which is interpreted as consistent with a slowly accelerating jet over the de-projected inner ˜10-20 pc. Assuming equipartition between magnetic energy and relativistic particle energy, the magnetic field strengths within the jets at these scales are, on average, between B ˜ 0.3 - 0.9 G, with the highest strengths found within the VLBI "core". From the observed gradient in magnetic field strengths, we can place the mmwave "core" ˜1-3 pc downstream of the base of the jet. Additionally, we estimate the the magnetic field is Bapex ˜ 3000 - 18000 G at the base of the jet. We computed theoretical estimates based on jet production under magnetically arrested disks (MAD) and find our estimates to be consistent. In the BL Lac source OJ 287, we included monthly 7 mm and near-in-time 2 cm VLBA maps to provide full kinematics and increased spectral coverage. Following a previously reported radical change in inner-jet PA of ˜100° we find unusually discrepant PAs compared with the previous jet direction, that follow very different trajectories. The source exhibits a downstream quasi-stationary feature that at times has higher brightness temperatures than the "core". The source also exhibited a large change in apparent component speeds as compared with previous epochs, which we propose could be due to changes in jet pressure causing changes in the location of downstream recollimation or oblique shocks and hence their line-of-sight viewing angle. The addition of 2 cm VLBA data allows for a comparison of magnetic fields derived from SSA and equipartition. The magnetic field estimates are consistent within 20%, with BSSA ≥ 1.6 G and Bequi ≥ 1.2 G in the "core" and BSSA ≤ 0.4 G and Bequi ≤ 0.3 G in the stationary feature. Gamma-ray emission appears to originate in the "core" and the stationary feature. The decrease in magnetic field strengths places the mmwave "core' downstream of the jet base by ≤6 pc and likely outside of the broad line region (BLR). This, combined with the results in other sources are consistent with γ-rays being produced in the vicinity of the VLBI "core" of in further downstream stationary features, which are likely over a parsec downstream of the central black hole, favouring the scenario of photons being up-scattered within the relativistic jet.

Empirical Corrections to Nutation Amplitudes and Precession Computed from a Global VLBI Solution

NASA Astrophysics Data System (ADS)

Schuh, H.; Ferrandiz, J. M.; Belda-Palazón, S.; Heinkelmann, R.; Karbon, M.; Nilsson, T.

2017-12-01

The IAU2000A nutation and IAU2006 precession models were adopted to provide accurate estimations and predictions of the Celestial Intermediate Pole (CIP). However, they are not fully accurate and VLBI (Very Long Baseline Interferometry) observations show that the CIP deviates from the position resulting from the application of the IAU2006/2000A model. Currently, those deviations or offsets of the CIP (Celestial Pole Offsets - CPO), can only be obtained by the VLBI technique. The accuracy of the order of 0.1 milliseconds of arc (mas) allows to compare the observed nutation with theoretical prediction model for a rigid Earth and constrain geophysical parameters describing the Earth's interior. In this study, we empirically evaluate the consistency, systematics and deviations of the IAU 2006/2000A precession-nutation model using several CPO time series derived from the global analysis of VLBI sessions. The final objective is the reassessment of the precession offset and rate, and the amplitudes of the principal terms of nutation, trying to empirically improve the conventional values derived from the precession/nutation theories. The statistical analysis of the residuals after re-fitting the main nutation terms demonstrates that our empirical corrections attain an error reduction by almost 15 micro arc seconds.

AuScope VLBI Project and Hobart 26-m Antenna

NASA Technical Reports Server (NTRS)

Lovell, Jim; Dickey, John; Reid, Brett; McCallum, Jamie; Shabala, Stas; Watson, Christopher; Ellingsen, Simon; Memin, Anthony

2013-01-01

This is a report on the activities carried out at the three AuScope VLBI observatories and the Hobart 26-m antenna. In 2012 the three AuScope 12-m antennas at Hobart (Hb), Katherine (Ke), and Yarragadee (Yg) completed their first full year of operations as an array. The Hobart 26-m antenna (Ho) continued to make a contribution to IVS, providing overlap with the Hb time series. In total the AuScope antennas and the Hobart 26 m observed for 146 antenna days in 2012. In this report we also briefly highlight our research activities during 2012 and our plans for 2013.

Crustal dynamics project data analysis, 1987. Volume 1: Fixed station VLBI geodetic results, 1979-1986

NASA Technical Reports Server (NTRS)

Ryan, J. W.; Ma, C.

1987-01-01

The Goddard VLBI group reports the results of analyzing Mark III data sets from fixed observatories through the end of 1986 and available to the Crustal Dynamics Project. All full-day data from POLARIS/IRIS are included. The mobile VLBI sites at Platteville (Colorado), Penticton (British Columbia), and Yellowknife (Northwest Territories) are also included since these occupations bear on the study of plate stability. Two large solutions, GLB121 and GLB122, were used to obtain Earth rotation parameters and baseline evolutions, respectively. Radio source positions were estimated globally while nutation offsets were estimated from each data set. The results include 25 sites and 108 baselines.

Continental hydrology loading observed by VLBI measurements

NASA Astrophysics Data System (ADS)

Eriksson, David; MacMillan, D. S.

2014-07-01

Variations in continental water storage lead to loading deformation of the crust with typical peak-to-peak variations at very long baseline interferometry (VLBI) sites of 3-15 mm in the vertical component and 1-2 mm in the horizontal component. The hydrology signal at VLBI sites has annual and semi-annual components and clear interannual variations. We have calculated the hydrology loading series using mass loading distributions derived from the global land data assimilation system (GLDAS) hydrology model and alternatively from a global grid of equal-area gravity recovery and climate experiment (GRACE) mascons. In the analysis of the two weekly VLBI 24-h R1 and R4 network sessions from 2003 to 2010 the baseline length repeatabilities are reduced in 79 % (80 %) of baselines when GLDAS (GRACE) loading corrections are applied. Site vertical coordinate repeatabilities are reduced in about 80 % of the sites when either GLDAS or GRACE loading is used. In the horizontal components, reduction occurs in 70-80 % of the sites. Estimates of the annual site vertical amplitudes were reduced for 16 out of 18 sites if either loading series was applied. We estimated loading admittance factors for each site and found that the average admittances were 1.01 0.05 for GRACE and 1.39 0.07 for GLDAS. The standard deviations of the GRACE admittances and GLDAS admittances were 0.31 and 0.68, respectively. For sites that have been observed in a set of sufficiently temporally dense daily sessions, the average correlation between VLBI vertical monthly averaged series and GLDAS or GRACE loading series was 0.47 and 0.43, respectively.

VLBI tracking of GNSS satellites: recent achievements

NASA Astrophysics Data System (ADS)

Liu, Li; Heinkelmann, Robert; Tornatore, Vincenza; Li, Jinling; Mora-Diaz, Julian; Nilsson, Tobias; Karbon, Maria; Raposo-Pulido, Virginia; Soja, Benedikt; Xu, Minghui; Lu, Cuixian; Schuh, Harald

2014-05-01

While the ITRF (International Terrestrial Reference Frame) is realized by the combination of the various space geodetic techniques, VLBI (Very Long Baseline Interferometry) is the only technique for determining the ICRF (International Celestial Reference Frame) through its observations of extragalactic radio sources. Therefore, small inconsistencies between the two important frames do exist. According to recent comparisons of parameters derived by GNSS (Global Navigation Satellite Systems) and VLBI (e.g. troposphere delays, gradients, UT1-UTC), evidences of discrepancies obtained by the vast amounts of data become obvious. Terrestrial local ties can provide a way to interlink the otherwise independent technique-specific reference frames but only to some degree. It is evident that errors in the determination of the terrestrial ties, e.g. due to the errors when transforming the locally surveyed coordinates into global Cartesian three dimensional coordinates, introduce significant errors in the combined analysis of space geodetic techniques. A new concept for linking the space geodetic techniques might be to introduce celestial ties, e.g. realized by technique co-location on board of satellites. A small satellite carrying a variety of space geodetic techniques is under investigation at GFZ. Such a satellite would provide a new observing platform with its own additional unknowns, such as the orbit or atmospheric drag parameters. A link of the two techniques VLBI and GNSS might be achieved in a more direct way as well: by VLBI tracking of GNSS satellites. Several tests of this type of observation were already successfully carried out. This new kind of hybrid VLBI-GNSS observation would comprise a new direct inter-technique tie without the involvement of surveying methods and would enable improving the consistency of the two space geodetic techniques VLBI and GNSS, in particular of their celestial frames. Recently the radio telescopes Wettzell and Onsala have successfully observed a GNSS satellite for the first time, using also new receiver developments, done at Wettzell. In this contribution we want to develop the motivation for this kind of innovative observation and we will show first results of the test observations.

THE FIRST VERY LONG BASELINE INTERFEROMETRY IMAGE OF A 44 GHz METHANOL MASER WITH THE KVN AND VERA ARRAY (KaVA)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Matsumoto, Naoko; Hirota, Tomoya; Honma, Mareki

2014-07-01

We have carried out the first very long baseline interferometry (VLBI) imaging of a 44 GHz class I methanol maser (7{sub 0}-6{sub 1} A {sup +}) associated with a millimeter core MM2 in a massive star-forming region IRAS 18151–1208 with KaVA (KVN and VERA Array), which is a newly combined array of KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry). We have succeeded in imaging compact maser features with a synthesized beam size of 2.7 milliarcseconds × 1.5 milliarcseconds (mas). These features are detected at a limited number of baselines within the length of shorter than ≈ 650 km corresponding to 100 Mλ in the uv-coverage.more » The central velocity and the velocity width of the 44 GHz methanol maser are consistent with those of the quiescent gas rather than the outflow traced by the SiO thermal line. The minimum component size among the maser features is ∼5 mas × 2 mas, which corresponds to the linear size of ∼15 AU × 6 AU assuming a distance of 3 kpc. The brightness temperatures of these features range from ∼3.5 × 10{sup 8} to 1.0 × 10{sup 10} K, which are higher than the estimated lower limit from a previous Very Large Array observation with the highest spatial resolution of ∼50 mas. The 44 GHz class I methanol maser in IRAS 18151–1208 is found to be associated with the MM2 core, which is thought to be less evolved than another millimeter core MM1 associated with the 6.7 GHz class II methanol maser.« less

Dual-Frequency VLBI Study of Centaurus A on Sub-Parsec Scales: The Highest-Resolution View of an Extragalactic Jet

NASA Technical Reports Server (NTRS)

Mueller, C.; Kadler, M.; Ojha, R.; Wilms, J.; Boeck, M.; Edwards, P.; Fromm, C. M.; Hase, H.; Horiuchi, S.; Katz, U.;

EARLY SCIENCE WITH THE KOREAN VLBI NETWORK: THE QCAL-1 43 GHz CALIBRATOR SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Petrov, Leonid; Lee, Sang-Sung; Kim, Jongsoo

2012-11-01

This paper presents the catalog of correlated flux densities in three ranges of baseline projection lengths of 637 sources from a 43 GHz (Q band) survey observed with the Korean VLBI Network. Of them, 14 objects used as calibrators were previously observed, but 623 sources have not been observed before in the Q band with very long baseline interferometry (VLBI). The goal of this work in the early science phase of the new VLBI array is twofold: to evaluate the performance of the new instrument that operates in a frequency range of 22-129 GHz and to build a list ofmore » objects that can be used as targets and as calibrators. We have observed the list of 799 target sources with declinations down to -40 Degree-Sign . Among them, 724 were observed before with VLBI at 22 GHz and had correlated flux densities greater than 200 mJy. The overall detection rate is 78%. The detection limit, defined as the minimum flux density for a source to be detected with 90% probability in a single observation, was in the range of 115-180 mJy depending on declination. However, some sources as weak as 70 mJy have been detected. Of 623 detected sources, 33 objects are detected for the first time in VLBI mode. We determined their coordinates with a median formal uncertainty of 20 mas. The results of this work set the basis for future efforts to build the complete flux-limited sample of extragalactic sources at frequencies of 22 GHz and higher at 3/4 of the celestial sphere.« less

Millimetron and Earth-Space VLBI

NASA Astrophysics Data System (ADS)

Likhachev, S.

2014-01-01

The main scientific goal of the Millimetron mission operating in Space VLBI (SVLBI) mode will be the exploration of compact radio sources with extremely high angular resolution (better than one microsecond of arc). The space-ground interferometer Millimetron has an orbit around L2 point of the Earth - Sun system and allows operating with baselines up to a hundred Earth diameters. SVLBI observations will be accomplished by space and ground-based radio telescopes simultaneously. At the space telescope the received baseband signal is digitized and then transferred to the onboard memory storage (up to 100TB). The scientific and service data transfer to the ground tracking station is performed by means of both synchronization and communication radio links (1 GBps). Then the array of the scientific data is processed at the correlation center. Due to the (u,v) - plane coverage requirements for SVLBI imaging, it is necessary to propose observations at two different frequencies and two circular polarizations simultaneously with frequency switching. The total recording bandwidth (2x2x4 GHz) defines of the on-board memory size. The ground based support of the Millimetron mission in the VLBI-mode could be Atacama Large Millimeter Array (ALMA), Pico Valletta (Spain), Plateau de Bure interferometer (France), SMT telescope in the US (Arizona), LMT antenna (Mexico), SMA array, (Mauna Kea, USA), as well as the Green Bank and Effelsberg 100 m telescopes (for 22 GHz observations). We will present simulation results for Millimetron-ALMA interferometer. The sensitivity estimate of the space-ground interferometer will be compared to the requirements of the scientific goals of the mission. The possibility of multi-frequency synthesis (MFS) to obtain high quality images will also be considered.

The first simultaneous mapping of four 7 mm SiO maser lines using the OCTAVE system

NASA Astrophysics Data System (ADS)

Oyama, Tomoaki; Kono, Yusuke; Suzuki, Syunsaku; Kanaguchi, Masahiro; Nishikawa, Takashi; Kawaguchi, Noriyuki; Hirota, Tomoya; Nagayama, Takumi; Kobayashi, Hideyuki; Imai, Hiroshi; Kuwahara, Sho; Kano, Amane; Oyadomari, Miyako; Chong, Sze Ning

2016-12-01

We report on simultaneous very long baseline interferometry (VLBI) mapping of 28SiO v = 1, 2, 3, and 29SiO v = 0 J = 1 → 0 maser lines at the 7 mm band toward the semi-regular variable star, W Hydrae (W Hya), using the new data acquisition system (OCTAVE-DAS), installed in the VLBI Exploration of Radio Astrometry (VERA) array and temporarily operated in the 45 m telescope of the Nobeyama Radio Observatory. Although these masers were spatially resolved, their compact maser spots were fortunately detected in the 1000 km baselines of VERA. We found the locations of the v = 3 maser emission which are unexpected from the currently proposed maser pumping models. Mapping of the 29SiO maser line in W Hya is the third result after those in WX Psc and R Leo. This paper shows the scientific implication of simultaneous VLBI observations of multiple SiO maser lines as realized by using the OCTAVE system.

Capabilities and prospects of the East Asia Very Long Baseline Interferometry Network

NASA Astrophysics Data System (ADS)

An, T.; Sohn, B. W.; Imai, H.

2018-02-01

The very long baseline interferometry (VLBI) technique offers angular resolutions superior to any other instruments at other wavelengths, enabling unique science applications of high-resolution imaging of radio sources and high-precision astrometry. The East Asia VLBI Network (EAVN) is a collaborative effort in the East Asian region. The EAVN currently consists of 21 telescopes with diverse equipment configurations and frequency setups, allowing flexible subarrays for specific science projects. The EAVN provides the highest resolution of 0.5 mas at 22 GHz, allowing the fine imaging of jets in active galactic nuclei, high-accuracy astrometry of masers and pulsars, and precise spacecraft positioning. The soon-to-be-operational Five-hundred-meter Aperture Spherical radio Telescope (FAST) will open a new era for the EAVN. This state-of-the-art VLBI array also provides easy access to and crucial training for the burgeoning Asian astronomical community. This Perspective summarizes the status, capabilities and prospects of the EAVN.

The recent progress of Chinese VLBI Network

NASA Astrophysics Data System (ADS)

Zheng, Weimin

2015-08-01

At present, Chinese VLBI Network (CVN) consists of 5 antennas (Seshan 25m, Urumqi 25m, Kunming 30m, Miyun 50m and Tianma 65m) and one data processing center in Shanghai Observatory, Chinese academy of sciences. It is a synthetic aperture radio telescope with the equivalent diameter up to 3000 Km. Through e-VLBI (electronic VLBI) technology, CVN is connected by the commuication network. It is a multi-purpose scientific research platform radio for geodesy, astronomy, as well as deep space exploration. In Geodesy, CVN is the component of the Crustal Movement Observation Network of China. Since the year of 2006, more than 20 geodetic domestic observations have been carried out. A set of phase-referencing observations of pulsars with CVN has carried out and got preliminary results. CVN also joined the Chinese lunar exploration Project from 2007 and supported 4 Chang’E series lunar probe missions. In Chang’E-3 mission, using the in-beam VLBI observations, the relative position accuracy of Rover and Lander is up to 1 meter.In recent years, we have updated the facilities of CVN from antenna, receivers, VLBI terminals to correlator. Participation of Tianma 65m antennas increases its performance. In 2012, Shanghai correlator was accepted as the IVS correlator. After upgrade, Shanghai correlator will try to provide the data process service for IVS community from 2015. To drive the construction of the planned VGOS (VLBI2010 Global Observing System) station, at least two VOGS 13m antenna will join CVN in the near future. Construction of the first VOGS antenna in Shanghai hopes to begin this year.The new VLBI correlator and digital terminal are under development. From participation in VGOS, we plan to study the earth rotation especially of high frequency and corresponding geophysical signals, to link China’s regional reference frame to ITRF, and etc. CVN is willing to join the research corporation with IVS, EVN, VLBA and AOV (Asia- Oceania VLBI Group for Geodesy and Astrometry) in the future.

What is the impact of different VLBI analysis setups of the tropospheric delay on precipitable water vapor trends?

NASA Astrophysics Data System (ADS)

Balidakis, Kyriakos; Nilsson, Tobias; Heinkelmann, Robert; Glaser, Susanne; Zus, Florian; Deng, Zhiguo; Schuh, Harald

2017-04-01

The quality of the parameters estimated by global navigation satellite systems (GNSS) and very long baseline interferometry (VLBI) are distorted by erroneous meteorological observations applied to model the propagation delay in the electrically neutral atmosphere. For early VLBI sessions with poor geometry, unsuitable constraints imposed on the a priori tropospheric gradients is a source of additional hassle of VLBI analysis. Therefore, climate change indicators deduced from the geodetic analysis, such as the long-term precipitable water vapor (PWV) trends, are strongly affected. In this contribution we investigate the impact of different modeling and parameterization of the propagation delay in the troposphere on the estimates of long-term PWV trends from geodetic VLBI analysis results. We address the influence of the meteorological data source, and of the a priori non-hydrostatic delays and gradients employed in the VLBI processing, on the estimated PWV trends. In particular, we assess the effect of employing temperature and pressure from (i) homogenized in situ observations, (ii) the model levels of the ERA Interim reanalysis numerical weather model and (iii) our own blind model in the style of GPT2w with enhanced parameterization, calculated using the latter data set. Furthermore, we utilize non-hydrostatic delays and gradients estimated from (i) a GNSS reprocessing at GeoForschungsZentrum Potsdam, rigorously considering tropospheric ties, and (ii)) direct ray-tracing through ERA Interim, as additional observations. To evaluate the above, the least-squares module of the VieVS@GFZ VLBI software was appropriately modified. Additionally, we study the noise characteristics of the non-hydrostatic delays and gradients estimated from our VLBI and GNSS analyses as well as from ray-tracing. We have modified the Theil-Sen estimator appropriately to robustly deduce PWV trends from VLBI, GNSS, ray-tracing and direct numerical integration in ERA Interim. We disseminate all our solutions in the latest Tropo-SINEX format.

Combining GPS and VLBI earth-rotation data for improved universal time

NASA Technical Reports Server (NTRS)

Freedman, A. P.

1991-01-01

The Deep Space Network (DSN) routinely measures Earth orientation in support of spacecraft tracking and navigation using very long-baseline interferometry (VLBI) with the deep-space tracking antennas. The variability of the most unpredictable Earth-orientation component, Universal Time 1 (UT1), is a major factor in determining the frequency with which the DSN measurements must be made. The installation of advanced Global Positioning System (GPS) receivers at the DSN sites and elsewhere may soon permit routine measurements of UT1 variation with significantly less dependence on the deep-space tracking antennas than is currently required. GPS and VLBI data from the DSN may be combined to generate a precise UT1 series, while simultaneously reducing the time and effort the DSN must spend on platform-parameter calibrations. This combination is not straightforward, however, and a strategy for the optimal combination of these data is presented and evaluated. It appears that, with the aid of GPS, the frequency of required VLBI measurements of Earth orientation could drop from twice weekly to once per month. More stringent real-time Earth orientation requirements possible in the future would demand significant improvements in both VLBI and GPS capabilities, however.

From Wagons to Race Cars, At Least Now We Have A Chassis

NASA Astrophysics Data System (ADS)

Morrow, A. L.

2003-12-01

For the past 30 years Very-Long-Baseline Interferometry (VLBI) has provided astronomers with the most accurate measurements to date of both distant radio sources as well as the tectonic plates. The resolutions attainable through VLBI are orders of magnitude better than other instruments. In order to transmit radio signals collected at different sites to a correlator for processing the VLBI data was stored on magnetic tapes, and then the magnetic tapes were shipped through the mail to the central processing site. This was not only arduous and inefficient, it was also costly. Now this means of shipment can be replaced by global high speed networks. This means of transmission is called e-VLBI. New protocols must be developed so e-VLBI can become a proficient high bandwidth background user. The protocol agreed upon uses a Real-time Transport Protocol (RTP) framework to preserve timing information and synchronization. The RTP is then transported using the Internet User Datagram Protocol (UDP) with RTP Control Protocol (RTCP) monitoring the networks performance. When this protocol is fully functional astronomers will be able to observe all over the world and receive results in real time.

Long-term millimeter VLBI monitoring of M 87 with KVN at milliarcsecond resolution: nuclear spectrum

NASA Astrophysics Data System (ADS)

Kim, Jae-Young; Lee, Sang-Sung; Hodgson, Jeffrey A.; Algaba, Juan-Carlos; Zhao, Guang-Yao; Kino, Motoki; Byun, Do-Young; Kang, Sincheol

2018-02-01

We study the centimeter- to millimeter-wavelength synchrotron spectrum of the core of the radio galaxy M 87 at ≲0.8 mas 110Rs spatial scales using four years of fully simultaneous, multi-frequency VLBI data obtained by the Korean VLBI Network (KVN). We find a core spectral index α of ≳‑0.37 (S ∝ ν+α) between 22 and 129 GHz. By combining resolution-matched flux measurements from the Very Long Baseline Array (VLBA) at 15 GHz and taking the Event Horizon Telescope (EHT) 230 GHz core flux measurements in epochs 2009 and 2012 as lower limits, we find evidence of a nearly flat core spectrum across 15 and 129 GHz, which could naturally connect the 230 GHz VLBI core flux. The extremely flat spectrum is a strong indication that the jet base does not consist of a simple homogeneous plasma, but of inhomogeneous multi-energy components, with at least one component with the turn-over frequency ≳ 100 GHz. The spectral shape can be qualitatively explained if both the strongly (compact, optically thick at >100 GHz) and the relatively weakly magnetized (more extended, optically thin at <100 GHz) plasma components are colocated in the footprint of the relativistic jet.

Love numbers for the long-period tides estimated by VLBI

NASA Astrophysics Data System (ADS)

Krásná, Hana; Böhm, Johannes; Haas, Rüdiger; Schuh, Harald

2013-04-01

Love and Shida numbers are proportionality factors characterizing the deformation of the anelastic Earth which arises as a response to external forces from the Moon and Sun. The increasing precision and quality of the Very Long Baseline Interferometry (VLBI) measurements allow determining those parameters. In particular, the long history of the VLBI data enables the estimation of Love and Shida numbers at the low frequencies of the tidal waves including the periods from 14 days to 18.6 years. In this study we analyse 27 years of VLBI measurements (1984.0 - 2011.0) following the recent IERS Conventions 2010. In several global solutions, we estimate the complex Love and Shida numbers of the solid Earth tides for the main long-period tidal waves. Furthermore, we determine the Love and Shida numbers of the rotational deformation due to polar motion, the so-called pole tide. We also focus on station displacement where still some deficiencies in the long-period signal modelling can be seen.

Planning of an Experiment for VLBI Tracking of GNSS Satellites

NASA Technical Reports Server (NTRS)

Tornatore, Vincenza; Hass, Ruediger; Molera, Guifre; Pogrebenko, Sergei

2010-01-01

As a preparation for future possible orbit determination of global navigation satellite system (GNSS) satellites by VLBI observations an initial three-station experiment was planned and performed in January 2009. The goal was to get first experience and to verify the feasibility of using the method for accurate satellite tracking. GNSS orbits related to a satellite constellation can be expressed in the Terrestrial Reference Frame. A comparison with orbit results that might be obtained by VLBI can give valuable information on how the GNSS reference frame and the VLBI reference frame are linked. We present GNSS transmitter specifications and experimental results of the observations of some GLONASS satellites together with evaluations for the expected signal strengths at telescopes. The satellite flux densities detected on the Earth s surface are very high. The narrow bandwidth of the GNSS signal partly compensates for potential problems at the receiving stations, and signal attenuation is necessary. Attempts to correlate recorded data have been performed with different software.

First Local Ties from Data of the Wettzell Triple Radio Telescope Array

NASA Astrophysics Data System (ADS)

Schüler, T.; Plötz, C.; Mähler, S.; Klügel, T.; Neidhardt, A.; Bertarini, A.; Halsig, S.; Nothnagel, A.; Lösler, M.; Eschelbach, C.; Anderson, J.

2016-12-01

The Geodetic Observatory Wettzell features three radio telescopes. Local ties between the reference points are available from terrestrial precision surveying with an expected accuracy below 0.7 mm. In addition, local VLBI data analysis is currently investigated to provide independent vectors and to provide quality feedback to the engineers. The preliminary results presented in this paper show a deviation from the local survey at the level of one millimeter with a clear systematic component. Sub-millimeter precision is reached after removal of this bias. This systematic effect is likely caused by omission of thermal expansion and gravity deformation, which is not yet implemented in our local VLBI analysis software.

STOCHASTIC OPTICS: A SCATTERING MITIGATION FRAMEWORK FOR RADIO INTERFEROMETRIC IMAGING

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Michael D., E-mail: mjohnson@cfa.harvard.edu

2016-12-10

Just as turbulence in the Earth’s atmosphere can severely limit the angular resolution of optical telescopes, turbulence in the ionized interstellar medium fundamentally limits the resolution of radio telescopes. We present a scattering mitigation framework for radio imaging with very long baseline interferometry (VLBI) that partially overcomes this limitation. Our framework, “stochastic optics,” derives from a simplification of strong interstellar scattering to separate small-scale (“diffractive”) effects from large-scale (“refractive”) effects, thereby separating deterministic and random contributions to the scattering. Stochastic optics extends traditional synthesis imaging by simultaneously reconstructing an unscattered image and its refractive perturbations. Its advantages over direct imagingmore » come from utilizing the many deterministic properties of the scattering—such as the time-averaged “blurring,” polarization independence, and the deterministic evolution in frequency and time—while still accounting for the stochastic image distortions on large scales. These distortions are identified in the image reconstructions through regularization by their time-averaged power spectrum. Using synthetic data, we show that this framework effectively removes the blurring from diffractive scattering while reducing the spurious image features from refractive scattering. Stochastic optics can provide significant improvements over existing scattering mitigation strategies and is especially promising for imaging the Galactic Center supermassive black hole, Sagittarius A*, with the Global mm-VLBI Array and with the Event Horizon Telescope.« less

The Determination of Earth Orientation by VLBI and GNSS: Principles and Results

NASA Astrophysics Data System (ADS)

Capitaine, Nicole

2017-10-01

The Earth Orientation Parameters (EOP) connect the International Terrestrial Reference System (ITRS) to the Geocentric Celestial Reference System (GCRS). These parameters, i.e., Universal Time, UT1, and pole coordinates in the ITRS and in the GCRS, describe the irregularities of the Earth's rotation. They are mainly determined by two modern astro-geodetic techniques, VLBI (Very Long Baseline Radio Interferometry) on extragalactic radio sources, which is used to realize and maintain the International Celestial Reference System (ICRS), and Global Navigation Satellite System (GNSS), especially GPS (Global Positioning System), which has an important contribution to the realization of the ITRS. The aim of this presentation is twofold: to present the modern bases for the consider- ation of Earth orientation and to discuss how the principles of VLBI and GPS give access to the measure of different components of the EOP variations, especially UT1. The accuracy that can be achieved is based on the improved concepts, definitions, and models that have been adopted by IAU/IUGG resolutions on reference systems and Earth's rotation, as well as on the refined strategy of the observations.

Mark 6: A Next-Generation VLBI Data System

NASA Astrophysics Data System (ADS)

Whitney, A. R.; Lapsley, D. E.; Taveniku, M.

2011-07-01

A new real-time high-data-rate disk-array system based on entirely commercial-off-the-shelf hardware components is being evaluated for possible use as a next-generation VLBI data system. The system, developed by XCube Communications of Nashua, NH, USA was originally developed for the automotive industry for testing/evaluation of autonomous driving systems that require continuous capture of an array of video cameras and automotive sensors at ~8Gbps from multiple 10GigE data links and other data sources. In order to sustain the required recording data rate, the system is designed to account for slow and/or failed disks by shifting the load to other disks as necessary in order to maintain the target data rate. The system is based on a Linux OS with some modifications to memory management and drivers in order to guarantee the timely movement of data, and the hardware/software combination is highly tuned to achieve the target data rate; data are stored in standard Linux files. A kit is also being designed that will allow existing Mark 5 disk modules to be modified to be used with the XCube system (though PATA disks will need to be replaced by SATA disks). Demonstrations of the system at Haystack Observatory and NRAO Socorro have proved very encouraging; some modest software upgrades/revisions are being made by XCube in order to meet VLBI-specific requirements. The system is easily expandable, with sustained 16 Gbps likely to be supported before end CY2011.

Crustal deformation at very long baseline interferometry sites due to seasonal air-mass and ground water variations

NASA Technical Reports Server (NTRS)

Stolz, A.; Larden, D. R.

1980-01-01

The seasonal deformation normal to the Earth's surface was calculated at stations involved or interested in very long baseline interferometry (VLBI) geodesy and at hypothetical sites in Australia and Brazil using global atmospheric pressure data, values for groundwater storage, and load Love numbers deduced from current Earth models. It was found that the annual range of deformation approached the centimeter level measuring potential of the VLBI technqiue at Greenbank, Haystack, and the Brazil site.

VLBI Digital-Backend Intercomparison Test Report

NASA Technical Reports Server (NTRS)

Whitney, Alan; Beaudoin, Christopher; Cappallo, Roger; Niell, Arthur; Petrachenko, Bill; Ruszczyk, Chester A.; Titus, Mike

2013-01-01

Issues related to digital-backend (DBE) systems can be difficult to evaluate in either local tests or actual VLBI experiments. The 2nd DBE intercomparison workshop at Haystack Observatory on 25-26 October 2012 provided a forum to explicitly address validation and interoperability issues among independent global developers of DBE equipment. This special report discusses the workshop. It identifies DBE systems that were tested at the workshop, describes the test objectives and procedures, and reports and discusses the results of the testing.

Collaboration and Development of Radio Astronomy in Australasia and South-Pacific Region: New Zealand Perspectives

NASA Astrophysics Data System (ADS)

Gulyaev, S.; Natusch, T.

2006-08-01

Radio telescopes in the Asia-Pacific region form a natural network for VLBI observations, similar to the very successful networks in North America (Network Users Group) and Europe (European VLBI Network). New Zealand's VLBI facility, which we are developing since 2005, has the potential to strengthen the Asian-Pacific VLBI network and its role in astronomy, geodesy and geoscience. It will positively influence regional and international activities in geoscience and geodesy that advance New Zealand's national interests. A self-contained radio astronomy system for VLBI, including a 1.658 GHz (centre frequency), 16 MHz bandwidth RF system (feed and downconversion system locked to a Rubidium maser and GPS clock), an 8-bit sampler/digitisation system, and a disk-based recording system built around a commodity PC was developed in New Zealand Centre for Radiophysics and Space Research. This was designed as a portable system for use on various radio telescopes. A number of Trans-Tasman tests has been conducted in 2005-2006 between the CRSR system installed on a 6 metre dish located in Auckland and the Australia Telescope Compact Array in Narrabri, Australia. This work has been successful, with fringes located from the recorded data and high resolution image of the quasar PKS1921-231 obtained. Experiments were recently conducted with Japan; new tests are planned with Korea and Fiji. Plans have been made to build a new 16.5 m antenna in New Zealand's North Island and to upgrade an 11 m dish in the South Island. A possible future of New Zealand's participation in the SKA is being discussed.

Active galactic nuclei cores in infrared-faint radio sources. Very long baseline interferometry observations using the Very Long Baseline Array

NASA Astrophysics Data System (ADS)

Herzog, A.; Middelberg, E.; Norris, R. P.; Spitler, L. R.; Deller, A. T.; Collier, J. D.; Parker, Q. A.

2015-06-01

Context. Infrared-faint radio sources (IFRS) form a new class of galaxies characterised by radio flux densities between tenths and tens of mJy and faint or absent infrared counterparts. It has been suggested that these objects are radio-loud active galactic nuclei (AGNs) at significant redshifts (z ≳ 2). Aims: Whereas the high redshifts of IFRS have been recently confirmed based on spectroscopic data, the evidence for the presence of AGNs in IFRS is mainly indirect. So far, only two AGNs have been unquestionably confirmed in IFRS based on very long baseline interferometry (VLBI) observations. In this work, we test the hypothesis that IFRS contain AGNs in a large sample of sources using VLBI. Methods: We observed 57 IFRS with the Very Long Baseline Array (VLBA) down to a detection sensitivity in the sub-mJy regime and detected compact cores in 35 sources. Results: Our VLBA detections increase the number of VLBI-detected IFRS from 2 to 37 and provide strong evidence that most - if not all - IFRS contain AGNs. We find that IFRS have a marginally higher VLBI detection fraction than randomly selected sources with mJy flux densities at arcsec-scales. Moreover, our data provide a positive correlation between compactness - defined as the ratio of milliarcsec- to arcsec-scale flux density - and redshift for IFRS, but suggest a decreasing mean compactness with increasing arcsec-scale radio flux density. Based on these findings, we suggest that IFRS tend to contain young AGNs whose jets have not formed yet or have not expanded, equivalent to very compact objects. We found two IFRS that are resolved into two components. The two components are spatially separated by a few hundred milliarcseconds in both cases. They might be components of one AGN, a binary black hole, or the result of gravitational lensing.

Refined discrete and empirical horizontal gradients in VLBI analysis

NASA Astrophysics Data System (ADS)

Landskron, Daniel; Böhm, Johannes

2018-02-01

Missing or incorrect consideration of azimuthal asymmetry of troposphere delays is a considerable error source in space geodetic techniques such as Global Navigation Satellite Systems (GNSS) or Very Long Baseline Interferometry (VLBI). So-called horizontal troposphere gradients are generally utilized for modeling such azimuthal variations and are particularly required for observations at low elevation angles. Apart from estimating the gradients within the data analysis, which has become common practice in space geodetic techniques, there is also the possibility to determine the gradients beforehand from different data sources than the actual observations. Using ray-tracing through Numerical Weather Models (NWMs), we determined discrete gradient values referred to as GRAD for VLBI observations, based on the standard gradient model by Chen and Herring (J Geophys Res 102(B9):20489-20502, 1997. https://doi.org/10.1029/97JB01739) and also for new, higher-order gradient models. These gradients are produced on the same data basis as the Vienna Mapping Functions 3 (VMF3) (Landskron and Böhm in J Geod, 2017.https://doi.org/10.1007/s00190-017-1066-2), so they can also be regarded as the VMF3 gradients as they are fully consistent with each other. From VLBI analyses of the Vienna VLBI and Satellite Software (VieVS), it becomes evident that baseline length repeatabilities (BLRs) are improved on average by 5% when using a priori gradients GRAD instead of estimating the gradients. The reason for this improvement is that the gradient estimation yields poor results for VLBI sessions with a small number of observations, while the GRAD a priori gradients are unaffected from this. We also developed a new empirical gradient model applicable for any time and location on Earth, which is included in the Global Pressure and Temperature 3 (GPT3) model. Although being able to describe only the systematic component of azimuthal asymmetry and no short-term variations at all, even these empirical a priori gradients slightly reduce (improve) the BLRs with respect to the estimation of gradients. In general, this paper addresses that a priori horizontal gradients are actually more important for VLBI analysis than previously assumed, as particularly the discrete model GRAD as well as the empirical model GPT3 are indeed able to refine and improve the results.

Joint Meteorological Statistics of Observing Sites for the Event Horizon Telescope

NASA Astrophysics Data System (ADS)

Lope Córdova Rosado, Rodrigo Eduardo; Doeleman, Sheperd; Paine, Scott; Johnson, Michael; Event Horizon Telescope (EHT)

2018-01-01

The Event Horizon Telescope (EHT) aims to resolve the general relativistic shadow of Sgr A*, the supermassive black hole at the center of our galaxy, via Very Long Baseline Interferometry (VLBI) measurements with a multinational array of radio observatories. In order to optimize the scheduling of future observations, we have developed tools to model the atmospheric opacity at each EHT site using the past 10 years of Global Forecast System (GFS) data describing the atmospheric state. These tools allow us to determine the ideal observing windows for EHT observations and to assess the suitability and impact of new EHT sites. We describe our modeling framework, compare our models to in-situ measurements at EHT sites, and discuss the implications of weather limitations for planned extensions of the EHT to higher frequencies, as well as additional sites and observation windows.

Impact of quasar proper motions on the alignment between the International Celestial Reference Frame and the Gaia reference frame

NASA Astrophysics Data System (ADS)

Liu, J.-C.; Malkin, Z.; Zhu, Z.

2018-03-01

The International Celestial Reference Frame (ICRF) is currently realized by the very long baseline interferometry (VLBI) observations of extragalactic sources with the zero proper motion assumption, while Gaia will observe proper motions of these distant and faint objects to an accuracy of tens of microarcseconds per year. This paper investigates the difference between VLBI and Gaia quasar proper motions and it aims to understand the impact of quasar proper motions on the alignment of the ICRF and Gaia reference frame. We use the latest time series data of source coordinates from the International VLBI Service analysis centres operated at Goddard Space Flight Center (GSF2017) and Paris observatory (OPA2017), as well as the Gaia auxiliary quasar solution containing 2191 high-probability optical counterparts of the ICRF2 sources. The linear proper motions in right ascension and declination of VLBI sources are derived by least-squares fits while the proper motions for Gaia sources are simulated taking into account the acceleration of the Solar system barycentre and realistic uncertainties depending on the source brightness. The individual and global features of source proper motions in GSF2017 and OPA2017 VLBI data are found to be inconsistent, which may result from differences in VLBI observations, data reduction and analysis. A comparison of the VLBI and Gaia proper motions shows that the accuracies of the components of rotation and glide between the two systems are 2-4 μas yr- 1 based on about 600 common sources. For the future alignment of the ICRF and Gaia reference frames at different wavelengths, the proper motions of quasars must necessarily be considered.

Impact of the galactic acceleration on the terrestrial reference frame and the scale factor in VLBI

NASA Astrophysics Data System (ADS)

Krásná, Hana; Titov, Oleg

2017-04-01

The relative motion of the solar system barycentre around the galactic centre can also be described as an acceleration of the solar system directed towards the centre of the Galaxy. So far, this effect has been omitted in the a priori modelling of the Very Long Baseline Interferometry (VLBI) observable. Therefore, it results in a systematic dipole proper motion (Secular Aberration Drift, SAD) of extragalactic radio sources building the celestial reference frame with a theoretical maximum magnitude of 5-7 microarcsec/year. In this work, we present our estimation of the SAD vector obtained within a global adjustment of the VLBI measurements (1979.0 - 2016.5) using the software VieVS. We focus on the influence of the observed radio sources with the maximum SAD effect on the terrestrial reference frame. We show that the scale factor from the VLBI measurements estimated for each source individually discloses a clear systematic aligned with the direction to the Galactic centre-anticentre. Therefore, the radio sources located near Galactic anticentre may cause a strong systematic effect, especially, in early VLBI years. For instance, radio source 0552+398 causes a difference up to 1 mm in the estimated baseline length. Furthermore, we discuss the scale factor estimated for each radio source after removal of the SAD systematic.

Feasibility study of an optically coherent telescope array in space

NASA Technical Reports Server (NTRS)

Traub, W. A.

1983-01-01

Numerical methods of image construction which can be used to produce very high angular resolution images at optical wavelengths of astronomical objects from an orbiting array of telescopes are discussed and a concept is presented for a phase-coherent optical telescope array which may be deployed by space shuttle in the 1990's. The system would start as a four-element linear array with a 12 m baseline. The initial module is a minimum redundant array with a photon-counting collecting area three times larger than space telescope and a one dimensional resolution of better than 0.01 arc seconds in the visible range. Later additions to the array would build up facility capability. The advantages of a VLBI observatory in space are considered as well as apertures for the telescopes.

Influence of ocean tides on the diurnal and semidiurnal earth rotation variations from VLBI observations

NASA Astrophysics Data System (ADS)

Gubanov, V. S.; Kurdubov, S. L.

2015-05-01

The International astrogeodetic standard IERS Conventions (2010) contains a model of the diurnal and semidiurnal variations in Earth rotation parameters (ERPs), the pole coordinates and the Universal Time, arising from lunisolar tides in the world ocean. This model was constructed in the mid-1990s through a global analysis of Topex/Poseidon altimetry. The goal of this study is to try to estimate the parameters of this model by processing all the available VLBI observations on a global network of stations over the last 35 years performed within the framework of IVS (International VLBI Service) geodetic programs. The complexity of the problemlies in the fact that the sought-for corrections to the parameters of this model lie within 1 mm and, thus, are at the limit of their detectability by all currently available methods of ground-based positional measurements. This requires applying universal software packages with a high accuracy of reduction calculations and a well-developed system of controlling the simultaneous adjustment of observational data to analyze long series of VLBI observations. This study has been performed with the QUASAR software package developed at the Institute of Applied Astronomy of the Russian Academy of Sciences. Although the results obtained, on the whole, confirm a high accuracy of the basic model in the IERS Conventions (2010), statistically significant corrections that allow this model to be refined have been detected for some harmonics of the ERP variations.

The first VLBI detection of a spiral DRAGN core

NASA Astrophysics Data System (ADS)

Mao, Minnie Y.; Blanchard, Jay M.; Owen, Frazer; Sjouwerman, Loránt O.; Singh, Vikram; Scaife, Anna; Paragi, Zsolt; Norris, Ray P.; Momjian, Emmanuel; Johnson, Gia; Browne, Ian

2018-07-01

We present the first observation of 0313-192, the archetypal spiral DRAGN, at very long baseline interferometry (VLBI) resolutions. Spiral DRAGNs are Double-lobed Radio sources Associated with Galactic Nuclei (DRAGNs) that are hosted by spiral galaxies. 0313-192 is an edge-on spiral galaxy that appears to host a 360 kpc double-lobed radio source. The core of this galaxy is clearly detected at L, S, and X bands using the Very Long Baseline Array, signifying an ongoing active nucleus in the galaxy. This rules out the possibility that the spiral DRAGN is merely a chance alignment. The radio core has L1.4 GHz ˜ 3.0 × 1023 W Hz-1. Radio components are detected to the south-west of the core, but there are no detections of a counterjet. Assuming a symmetric, relativistic jet, we estimate an upper limit to the inclination angle of θ ≲ 72 deg. The VLBI-detected radio jet components are extremely well aligned with the larger scale radio source suggesting little to no jet disruption or interaction with the interstellar medium of the host galaxy.

Location of γ -ray emission and magnetic field strengths in OJ 287

DOE PAGES

Hodgson, J. A.; Krichbaum, T. P.; Marscher, A. P.; ...

2017-01-06

We report the γ-ray BL Lac object OJ 287 is known to exhibit inner-parsec “jet-wobbling”, high degrees of variability at all wavelengths and quasi-stationary features, including an apparent (≈100°) position-angle change in projection on the sky plane. Sub-50 micro-arcsecond resolution 86 GHz observations with the global mm-VLBI array (GMVA) supplement ongoing multi-frequency VLBI blazar monitoring at lower frequencies. Using these maps, together with cm/mm total intensity and γ-ray observations from Fermi-LAT from 2008-2014, we aim to Observations with the GMVA offer approximately double the angular resolution compared with 43 GHz VLBA observations and enable us to observe above the synchrotronmore » self-absorption peak frequency. Fermi-LAT γ-ray data were reduced and analysed. The jet was spectrally decomposed at multiple locations along the jet. From this, we could derive estimates of the magnetic field using equipartition and synchrotron self-absorption arguments. How the field decreases down the jet provided an estimate of the distance to the jet apex and an estimate of the magnetic field strength at the jet apex and in the broad line region. Combined with accurate kinematics, we attempt to locate the site of γ-ray activity, radio flares, and spectral changes. Strong γ-ray flares appeared to originate from either the so-called core region, a downstream stationary feature, or both, with γ-ray activity significantly correlated with radio flaring in the downstream quasi-stationary feature. Magnetic field estimates were determined at multiple locations along the jet, with the magnetic field found to be ≥1.6 G in the core and ≤0.4 G in the downstream quasi-stationary feature. Finally, we therefore found upper limits on the location of the VLBI core as ≲6.0 pc from the jet apex and determined an upper limit on the magnetic field near the jet base of the order of thousands of Gauss.« less

Location of γ -ray emission and magnetic field strengths in OJ 287

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hodgson, J. A.; Krichbaum, T. P.; Marscher, A. P.

We report the γ-ray BL Lac object OJ 287 is known to exhibit inner-parsec “jet-wobbling”, high degrees of variability at all wavelengths and quasi-stationary features, including an apparent (≈100°) position-angle change in projection on the sky plane. Sub-50 micro-arcsecond resolution 86 GHz observations with the global mm-VLBI array (GMVA) supplement ongoing multi-frequency VLBI blazar monitoring at lower frequencies. Using these maps, together with cm/mm total intensity and γ-ray observations from Fermi-LAT from 2008-2014, we aim to Observations with the GMVA offer approximately double the angular resolution compared with 43 GHz VLBA observations and enable us to observe above the synchrotronmore » self-absorption peak frequency. Fermi-LAT γ-ray data were reduced and analysed. The jet was spectrally decomposed at multiple locations along the jet. From this, we could derive estimates of the magnetic field using equipartition and synchrotron self-absorption arguments. How the field decreases down the jet provided an estimate of the distance to the jet apex and an estimate of the magnetic field strength at the jet apex and in the broad line region. Combined with accurate kinematics, we attempt to locate the site of γ-ray activity, radio flares, and spectral changes. Strong γ-ray flares appeared to originate from either the so-called core region, a downstream stationary feature, or both, with γ-ray activity significantly correlated with radio flaring in the downstream quasi-stationary feature. Magnetic field estimates were determined at multiple locations along the jet, with the magnetic field found to be ≥1.6 G in the core and ≤0.4 G in the downstream quasi-stationary feature. Finally, we therefore found upper limits on the location of the VLBI core as ≲6.0 pc from the jet apex and determined an upper limit on the magnetic field near the jet base of the order of thousands of Gauss.« less

Global Velocities from VLBI

NASA Technical Reports Server (NTRS)

Ma, Chopo; Gordon, David; MacMillan, Daniel

1999-01-01

Precise geodetic Very Long Baseline Interferometry (VLBI) measurements have been made since 1979 at about 130 points on all major tectonic plates, including stable interiors and deformation zones. From the data set of about 2900 observing sessions and about 2.3 million observations, useful three-dimensional velocities can be derived for about 80 sites using an incremental least-squares adjustment of terrestrial, celestial, Earth rotation and site/session-specific parameters. The long history and high precision of the data yield formal errors for horizontal velocity as low as 0.1 mm/yr, but the limitation on the interpretation of individual site velocities is the tie to the terrestrial reference frame. Our studies indicate that the effect of converting precise relative VLBI velocities to individual site velocities is an error floor of about 0.4 mm/yr. Most VLBI horizontal velocities in stable plate interiors agree with the NUVEL-1A model, but there are significant departures in Africa and the Pacific. Vertical precision is worse by a factor of 2-3, and there are significant non-zero values that can be interpreted as post-glacial rebound, regional effects, and local disturbances.

Permanent Monitoring of the Reference Point of the 20m Radio Telescope Wettzell

NASA Technical Reports Server (NTRS)

Neidhardt, Alexander; Losler, Michael; Eschelbach, Cornelia; Schenk, Andreas

2010-01-01

To achieve the goals of the VLBI2010 project and the Global Geodetic Observing System (GGOS), an automated monitoring of the reference points of the various geodetic space techniques, including Very Long Baseline Interferometry (VLBI), is desirable. The resulting permanent monitoring of the local-tie vectors at co-location stations is essential to obtain the sub-millimeter level in the combinations. For this reason a monitoring system was installed at the Geodetic Observatory Wettzell by the Geodetic Institute of the University of Karlsruhe (GIK) to observe the 20m VLBI radio telescope from May to August 2009. A specially developed software from GIK collected data from automated total station measurements, meteorological sensors, and sensors in the telescope monument (e.g., Invar cable data). A real-time visualization directly offered a live view of the measurements during the regular observation operations. Additional scintillometer measurements allowed refraction corrections during the post-processing. This project is one of the first feasibility studies aimed at determining significant deformations of the VLBI antenna due to, for instance, changes in temperature.

IVS Working Group 4: VLBI Data Structures

NASA Astrophysics Data System (ADS)

Gipson, J.

2012-12-01

I present an overview of the "openDB format" for storing, archiving, and processing VLBI data. In this scheme, most VLBI data is stored in NetCDF files. NetCDF has the advantage that there are interfaces to most common computer languages including Fortran, Fortran-90, C, C++, Perl, etc, and the most common operating systems including Linux, Windows, and Mac. The data files for a particular session are organized by special ASCII "wrapper" files which contain pointers to the data files. This allows great flexibility in the processing and analysis of VLBI data. For example it allows you to easily change subsets of the data used in the analysis such as troposphere modeling, ionospheric calibration, editing, and ambiguity resolution. It also allows for extending the types of data used, e.g., source maps. I present a roadmap to transition to this new format. The new format can already be used by VieVS and by the global mode of solve. There are plans in work for other software packages to be able to use the new format.

Imaging black holes: past, present and future

NASA Astrophysics Data System (ADS)

Falcke, Heino

2017-12-01

This paper briefly reviews past, current, and future efforts to image black holes. Black holes seem like mystical objects, but they are an integral part of current astrophysics and are at the center of attempts to unify quantum physics and general relativity. Yet, nobody has ever seen a black hole. What do they look like? Initially, this question seemed more of an academic nature. However, this has changed over the past two decades. Observations and theoretical considerations suggest that the supermassive black hole, Sgr A*, in the center of our Milky Way is surrounded by a compact, foggy emission region radiating at and above 230 GHz. It has been predicted that the event horizon of Sgr A* should cast its shadow onto that emission region, which could be detectable with a global VLBI array of radio telescopes. In contrast to earlier pictures of black holes, that dark feature is not supposed to be due to a hole in the accretion flow, but would represent a true negative image of the event horizon. Currently, the global Event Horizon Telescope consortium is attempting to make such an image. In the future those images could be improved by adding more telescopes to the array, in particular at high sites in Africa. Ultimately, a space array at THz frequencies, the Event Horizon Imager, could produce much more detailed images of black holes. In combination with numerical simulations and precise measurements of the orbits of stars - ideally also of pulsars - these images will allow us to study black holes with unprecedented precision.

IVS Working Group 2 for Product Specification and Observing Programs

NASA Astrophysics Data System (ADS)

Schuh, H.; Charlot, P.; Hase, H.; Himwich, E.; Kingham, K.; Klatt, C.; Ma, C.; Malkin, Z.; Niell, A.; Nothnagel, A.; Schluter, W.; Takashima, K.; Vandenberg, N.

2002-03-01

After the scientific rationale is given in the introduction the Terms of Reference and the proceeding of IVS Working Group 2 are presented. Then the present status and future goals of all international activities within IVS are described. In particular the current products of IVS are described in terms of accuracy, reliability, frequency of observing sessions, temporal resolution of the parameters estimated by VLBI data analysis, time delay from observing to product, i.e. time which has passed after the end of the last session included in the VLBI solution till availability of the final products and frequency of solution (in the case of "global solutions";, when all existing or a high number of VLBI sessions are used to determine so-called global parameters). All IVS products and their potential users are covered in the report. This includes the Earth orientation parameters (EOP), the reference frames (TRF and CRF), geodynamical and geophysical parameters and physical parameters. Measures which should be taken within IVS to meet the goals defined in the first steps are presented. As most of the measures are related to the observing programs, these are the main focus for improving the current status of IVS products. The report shows that due to various requirements of the different users of IVS products the following aspects must be accomplished: - significant improvement of the accuracy of VLBI products, - shorter time delay from observation to availability of results, - almost continuous temporal coverage by VLBI sessions. A first scenario of the IVS observing program for 2002 and 2003 considers an increase of observing time by about 30%-40% and includes sessions carried out by S2 and K4 technology. The midterm observing program for the next 4-5 years seems to be rather ambitious. However, it appears feasible if all efforts are concentrated and the necessary resources are made available. From: Bidzina Kapanadze (Ilia State Iniversity) Address: bidzina_kapanadze@iliauni.edu.ge Database: ast

Troposphere gradients from the ECMWF in VLBI analysis

NASA Astrophysics Data System (ADS)

Boehm, Johannes; Schuh, Harald

2007-06-01

Modeling path delays in the neutral atmosphere for the analysis of Very Long Baseline Interferometry (VLBI) observations has been improved significantly in recent years by the use of elevation-dependent mapping functions based on data from numerical weather models. In this paper, we present a fast way of extracting both, hydrostatic and wet, linear horizontal gradients for the troposphere from data of the European Centre for Medium-range Weather Forecasts (ECMWF) model, as it is realized at the Vienna University of Technology on a routine basis for all stations of the International GNSS (Global Navigation Satellite Systems) Service (IGS) and International VLBI Service for Geodesy and Astrometry (IVS) stations. This approach only uses information about the refractivity gradients at the site vertical, but no information from the line-of-sight. VLBI analysis of the CONT02 and CONT05 campaigns, as well as all IVS-R1 and IVS-R4 sessions in the first half of 2006, shows that fixing these a priori gradients improves the repeatability for 74% (40 out of 54) of the VLBI baseline lengths compared to fixing zero or constant a priori gradients, and improves the repeatability for the majority of baselines compared to estimating 24-h offsets for the gradients. Only if 6-h offsets are estimated, the baseline length repeatabilities significantly improve, no matter which a priori gradients are used.

THE KCAL VERA 22 GHz CALIBRATOR SURVEY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Petrov, L.; Honma, M.; Shibata, S. M., E-mail: Leonid.Petrov@lpetrov.net

2012-02-15

We observed a sample of 1536 sources with correlated flux densities brighter than 200 mJy at 8 GHz with the very long baseline interferometry (VLBI) array VLBI Exploration of Radio Astrometry at 22 GHz. One half of the target sources has been detected. The detection limit was around 200 mJy. We derived the correlated flux densities of 877 detected sources in three ranges of projected baseline lengths. The objective of these observations was to determine the suitability of given sources as phase calibrators for dual-beam and phase-referencing observations at high frequencies. Preliminary results indicate that the number of compact extragalacticmore » sources at 22 GHz brighter than a given correlated flux density level is two times less than that at 8 GHz.« less

Plate Motions, Regional Deformation, and Time-Variation of Plate Motions

NASA Technical Reports Server (NTRS)

Gordon, R. G.

1998-01-01

The significant results obtained with support of this grant include the following: (1) Using VLBI data in combination with other geodetical, geophysical, and geological data to bound the present rotation of the Colorado Plateau, and to evaluate to its implications for the kinematics and seismogenic potential of the western half of the conterminous U.S. (2) Determining realistic estimates of uncertainties for VLBI data and then applying the data and uncertainties to obtain an upper bound on the integral of deformation within the "stable interior" of the North American and other plates and thus to place an upper bound on the seismogenic potential within these regions. (3) Combining VLBI data with other geodetic, geophysical, and geologic data to estimate the motion of coastal California in a frame of reference attached to the Sierra Nevada-Great Valley microplate. This analysis has provided new insights into the kinematic boundary conditions that may control or at least strongly influence the locations of asperities that rupture in great earthquakes along the San Andreas transform system. (4) Determining a global tectonic model from VLBI geodetic data that combines the estimation of plate angular velocities with individual site linear velocities where tectonically appropriate. and (5) Investigation of the some of the outstanding problems defined by the work leading to global plate motion model NUVEL-1. These problems, such as the motion between the Pacific and North American plates and between west Africa and east Africa, are focused on regions where the seismogenic potential may be greater than implied by published plate tectonic models.

Estimation of Geodetic and Geodynamical Parameters with VieVS

NASA Technical Reports Server (NTRS)

Spicakova, Hana; Bohm, Johannes; Bohm, Sigrid; Nilsson, tobias; Pany, Andrea; Plank, Lucia; Teke, Kamil; Schuh, Harald

2010-01-01

Since 2008 the VLBI group at the Institute of Geodesy and Geophysics at TU Vienna has focused on the development of a new VLBI data analysis software called VieVS (Vienna VLBI Software). One part of the program, currently under development, is a unit for parameter estimation in so-called global solutions, where the connection of the single sessions is done by stacking at the normal equation level. We can determine time independent geodynamical parameters such as Love and Shida numbers of the solid Earth tides. Apart from the estimation of the constant nominal values of Love and Shida numbers for the second degree of the tidal potential, it is possible to determine frequency dependent values in the diurnal band together with the resonance frequency of Free Core Nutation. In this paper we show first results obtained from the 24-hour IVS R1 and R4 sessions.

Seasonal station variations in the Vienna VLBI terrestrial reference frame VieTRF16a

NASA Astrophysics Data System (ADS)

Krásná, Hana; Böhm, Johannes; Madzak, Matthias

2017-04-01

The special analysis center of the International Very Long Baseline Interferometry (VLBI) Service for Geodesy and Astrometry (IVS) at TU Wien (VIE) routinely analyses the VLBI measurements and estimates its own Terrestrial Reference Frame (TRF) solutions. We present our latest solution VieTRF16a (1979.0 - 2016.5) computed with the software VieVS version 3.0. Several recent updates of the software have been applied, e.g., the estimation of annual and semi-annual station variations as global parameters. The VieTRF16a is determined in the form of the conventional model (station position and its linear velocity) simultaneously with the celestial reference frame and Earth orientation parameters. In this work, we concentrate on the seasonal station variations in the residual time series and compare our TRF with the three combined TRF solutions ITRF2014, DTRF2014 and JTRF2014.

Measurement of horizontal motions in Alaska using very long baseline interferometry

NASA Technical Reports Server (NTRS)

Ma, C.; Sauber, J. M.; Clark, T. A.; Ryan, J. W.; Bell, L. J.; Gordon, D.; Himwich, W. E.

1990-01-01

Results are presented on an analysis of VLBI measurements performed between 1984 and 1990 by means of a network of 53 sites in Alaska, the Yukon Territory, and the conterminous United States to determine the extent of horizontal motions in Alaska. Results are presented in two ways, one showing the evolution of individual baselines and the other yielding site velocities; both approaches use VLBI data from other permanent stations in order to define a global reference frame. It was found that VLBI sites within the Alaska-Aleutian subduction boundary zone (Yakataga, Kodiak, and Sand Point) had higher instantaneous velocities relative to eastern North America than the interior sites of Alaska. The results of Yakataga data modeling suggests that the observed motion is the result of elastic straining of the overriding plate due to a locked main thrust zone with a component of oblique slip.

R&D at JIVE: transforming the way VLBI is done

NASA Astrophysics Data System (ADS)

Szomoru, Arpad; van Langevelde, Huib

2015-08-01

Arpad Szomoru, Huib van Langevelde and the JIVE staffFor many years, the heart of operations at JIVE has been the MkIV hardware correlator, a custom-built high-performance data processor. At this time the MkIV has been replaced by the locally developed EVN software correlator (SFXC).This development has vastly improved the science capacity of the EVN, by providing higher spectral resolution and polarization accuracy, but most notably, by enabling completely new observing modes. Observing multiple simultaneous field centers has enabled wide-field imaging, while a phased-array mode has made it possible to do pulsar time series with the EVN. New algorithms have been developed for near-field VLBI, making it possible to focus on objects within our solar system. This has been used to track the RadioAstron satellite, and by applying the derived orbital parameters to improve subsequent space VLBI observations.New digital baseband convertors will allow higher observing bandwidths in the EVN. In anticipation of this, and of the even higher bandwidths of future mm-VLBI observations, added to the deployment of much larger arrays (including the AVN, the SKA precursors and the SKA itself), we are investigating more powerful and economical solutions. The JIVE UniBoard Correlator is the first FPGA-based EVN correlator; its scalability and flexibility are now under assessment. The new UniBoard2 project, also sponsored by the EC, will skip two generations of FPGA technology and deliver enormous processing power at lower power consumption.Maybe just as importantly, research is ongoing into software tools to enable the efficient handling of the vast data sets that the EVN and other current and future instruments will produce. New data processing pipelines are being designed that will be able to cache intermediate products, and upon changing parameters only re-calculate what is needed, as opposed to re-starting every time from scratch.Finally, we will discuss the development of time and frequency transfer via public networks, in the context of a new H2020 project aimed at the astronomy, astrophysics and astroparticle physics faciclities within the ESFRI roadmap .

Optimized scheduling of VLBI UT1 intensive sessions for twin telescopes employing impact factor analysis

NASA Astrophysics Data System (ADS)

Leek, Judith; Artz, Thomas; Nothnagel, Axel

2015-09-01

Daily Very Long Baseline Interferometry (VLBI) intensive measurements make an important contribution to the regular monitoring of Earth rotation variations. Since these variations are quite rapid, their knowledge is important for navigation with global navigation satellite system and for investigations in Earth sciences. Unfortunately, the precision of VLBI intensive observations is 2-3 times worse than the precision of regular 24h-VLBI measurements with networks of 5-10 radio telescopes. The major advancement of research in this paper is the improvement of VLBI intensive results by (a) using twin telescopes instead of single telescopes and (b) applying an entirely new scheduling concept for the individual observations. Preparatory investigations of standardintensive sessions suggest that the impact factors of the observations are well suited for the identification of the most influential observations which are needed for the determination of certain parameters within the entire design of a VLBI session. Based on this experience, the scheduling method is designed for optimizing the observations' geometry for a given network of radio telescopes and a predefined set of parameters to be estimated. The configuration of at least two twin telescopes, or one twin and two single telescopes, offers the possibility of building pairwise sub-nets that observe two different sources simultaneously. In addition to an optimized observing plan, a special parametrization for twin telescopes leads to an improved determination of Earth rotation variations, as it is shown by simulated observations. In general, an improvement of about 50 % in the formal errors can be realized using twin radio telescopes. This result is only due to geometric improvements as higher slew rates of the twin telescopes are not taken into account.

The African VLBI network project

NASA Astrophysics Data System (ADS)

Loots, Anita

2015-01-01

The AVN is one of the most significant vehicles through which capacity development in Africa for SKA participation will be realized. It is a forerunner to the long baseline Phase 2 component of the mid-frequency SKA. Besides the 26m HartRAO telescope in South Africa, Ghana is expected to be the first to establish a VLBI-capable telescope through conversion of a redundant 32m telecommunications system near Accra. The most widely used receivers in the EVN are L-band and C-band (5 GHz). L-band is divided into a low band around the hydrogen (HI) line frequency of 1420 MHz, and a high band covering the hydroxyl line frequencies of 1612-1720 MHz. The high band is much more commonly used for VLBI as it provides more bandwidth. For the AVN, the methanol maser line at 6668 MHz is a key target for the initial receiver and the related 12178MHz methanol maser line also seen in star-forming regions a potential future Ku-band receiver. In the potential future band around 22GHz(K-band), water masers in star-forming regions and meg-maser galaxies at 22.235 GHz are targets, as are other radio continuum sources such as AGNs. The AVN system will include 5GHz and 6.668GHz receiver systems with recommendation to partner countries that the first upgrade should be L-band receivers. The original satellite telecommunications feed horns cover 3.8 - 6.4 GHz and should work at 5 GHz and operation at 6.668 GHz for the methanol maser is yet to be verified. The first light science will be conducted in the 6.7 GHz methanol maser band. Telescopes developed for the AVN will initially join other global networks for VLBI. When at least four VLBI-capable telescopes are operational on the continent, it will be possible to initiate stand-alone AVN VLBI. Each country where an AVN telescope becomes operational will have its own single-dish observing program. Capacity building to run an observatory includes the establishment of competent core essential observatory staff in partner countries who can train larger teams in science, engineering and technology issues and collaborate with the broader global science community to develop new African radio astronomy science communities.

Operational VGOS Scheduling

NASA Astrophysics Data System (ADS)

Searle, Anthony; Petrachenko, Bill

2016-12-01

The VLBI Global Observing System (VGOS) has been designed to take advantage of advances in data recording speeds and storage capacity, allowing for smaller and faster antennas, wider bandwidths, and shorter observation durations. Here, schedules for a ``realistic" VGOS network, frequency sequences, and expanded source lists are presented using a new source-based scheduling algorithm. The VGOS aim for continuous observations presents new operational challenges. As the source-based strategy is independent of the observing network, there are operational advantages which allow for more flexible scheduling of continuous VLBI observations. Using VieVS, simulations of several schedules are presented and compared with previous VGOS studies.

A comparative study of amplitude calibrations for the East Asia VLBI Network: A priori and template spectrum methods

NASA Astrophysics Data System (ADS)

Cho, Ilje; Jung, Taehyun; Zhao, Guang-Yao; Akiyama, Kazunori; Sawada-Satoh, Satoko; Kino, Motoki; Byun, Do-Young; Sohn, Bong Won; Shibata, Katsunori M.; Hirota, Tomoya; Niinuma, Kotaro; Yonekura, Yoshinori; Fujisawa, Kenta; Oyama, Tomoaki

2017-12-01

We present the results of a comparative study of amplitude calibrations for the East Asia VLBI Network (EAVN) at 22 and 43 GHz using two different methods of an "a priori" and a "template spectrum", particularly on lower declination sources. Using observational data sets of early EAVN observations, we investigated the elevation-dependence of the gain values at seven stations of the KaVA (KVN and VERA Array) and three additional telescopes in Japan (Takahagi 32 m, Yamaguchi 32 m, and Nobeyama 45 m). By comparing the independently obtained gain values based on these two methods, we found that the gain values from each method were consistent within 10% at elevations higher than 10°. We also found that the total flux densities of two images produced from the different amplitude calibrations were in agreement within 10% at both 22 and 43 GHz. By using the template spectrum method, furthermore, the additional radio telescopes can participate in KaVA (i.e., EAVN), giving a notable sensitivity increase. Therefore, our results will constrain the detailed conditions in order to measure the VLBI amplitude reliably using EAVN, and discuss the potential of possible expansion to telescopes comprising EAVN.

Exploring the Variability of the Flat-spectrum Radio Source 1633+382. II. Physical Properties

NASA Astrophysics Data System (ADS)

Algaba, Juan-Carlos; Lee, Sang-Sung; Rani, Bindu; Kim, Dae-Won; Kino, Motoki; Hodgson, Jeffrey; Zhao, Guang-Yao; Byun, Do-Young; Gurwell, Mark; Kang, Sin-Cheol; Kim, Jae-Young; Kim, Jeong-Sook; Kim, Soon-Wook; Park, Jong-Ho; Trippe, Sascha; Wajima, Kiyoaki

2018-06-01

The flat-spectrum radio quasar 1633+382 (4C 38.41) showed a significant increase of its radio flux density during the period 2012 March–2015 August, which correlates with γ-ray flaring activity. Multi-frequency simultaneous very long baseline interferometry (VLBI) observations were conducted as part of the interferometric monitoring of gamma-ray bright active galactic nuclei (iMOGABA) program and supplemented with additional radio monitoring observations with the OVRO 40 m telescope, the Boston University VLBI program, and the Submillimeter Array. The epochs of the maxima for the two largest γ-ray flares coincide with the ejection of two respective new VLBI components. Analysis of the spectral energy distribution indicates a higher turnover frequency after the flaring events. The evolution of the flare in the turnover frequency-turnover flux density plane probes the adiabatic losses in agreement with the shock-in-jet model. The derived synchrotron self-absorption magnetic fields, of the order of 0.1 mG, do not seem to change dramatically during the flares, and are much weaker, by a factor 104, than the estimated equipartition magnetic fields, indicating that the source of the flare may be associated with a particle-dominated emitting region.

Plate tectonics from VLBI and SLR global data

NASA Technical Reports Server (NTRS)

Harrison, Christopher G. A.; Robaudo, Stefano

1992-01-01

This study is based on data derived from fifteen years of observations of the SLR (side-looking radar) network and six years of the VLBI (very long baseline interferometry) network. In order to use all available information VLBI and SLR global data sets were combined in a least squares fashion to calculate station horizontal velocities. All significant data pertaining to a single site contribute to the station horizontal motion. The only constraint on the solution is that no vertical motion is allowed. This restriction does not greatly affect the precision of the overall solution given the fact that the expected vertical motion for most stations, even those experiencing post glacial uplift, is well under 1 cm/yr. Since the average baseline is under 4,000 km, only a small fraction of the station vertical velocity is translated into baseline rates so that the error introduced in the solution by restricting up-down station movement is minimal. As a reference, station velocities were then compared to the ones predicted by the NUVEL-1 geological model of DeMets et al. (1990). The focus of the study is on analyzing these discrepancies for global plate tectonics as well as regional tectonic settings. The method used also allows us not only to derive horizontal motion for individual stations but also to calculate Euler vectors for those plates that have enough stations located on the stable interior like North America, Pacific, Eurasia, and Australia.

The Event Horizon Telescope: Future Polarimetric Capabilities

NASA Astrophysics Data System (ADS)

Fish, Vincent L.; Doeleman, S. S.; EHT Collaboration

2010-01-01

The Event Horizon Telescope (EHT) is a (sub)mm wavelength VLBI network that will achieve angular resolutions sufficient to resolve and image the Event Horizons of the nearest supermassive black holes. Recent millimeter observations with the initial three stations of the EHT (located in Hawaii, Arizona, and California) have constrained the size of the emitting region of Sgr A*, the Galactic Center radio/infrared/X-ray source associated with a supermassive black hole, to be no more than a few Schwarzschild radii. While EHT observations have heretofore focused on detecting Sgr A* in total intensity, theoretical models predict large polarization signatures well in excess of the few percent linear polarization detected by low resolution arrays (e.g., SMA). Here, we generalize our previous total intensity simulations of future EHT observations to include full polarimetric quantities. Ratios of polarimetric visibilities provide baseline-based observables that are robust against most calibration errors. We find that the shortest VLBI baselines track the integrated polarization fraction and position angle, as presently observed by connected-element arrays, while longer VLBI baselines are sensitive to highly-polarized substructures that are beam-diluted at lower angular resolution. Ratios of polarized visibilities may be even more sensitive to detecting periodic structural changes, as might be expected from a hot spot near the innermost stable circular orbit of the black hole, than closure quantities obtained from total intensity quantities. These results suggest that high priority should be given to upgrading telescopes in the EHT collaboration in order to allow full polarimetric observations of Sgr A* and other supermassive black hole sources. This research is made possible thanks to funding provided by the National Science Foundation.

Multi-disciplinary contributions of HartRAO to global geodesy and geodynamics

NASA Astrophysics Data System (ADS)

Combrinck, Ludwig

2015-04-01

The Hartebeesthoek Radio Astronomy Observatory (South Africa) supports global initiatives in both geodesy and geodynamics through an active programme of science platform provision in Africa, the Atlantic Ocean, Indian Ocean and Antarctica. Our involvement ranges from the installation of tide gauges, Global Navigation Satellite Systems stations, seismometers and accelerometers on remote islands to the installation of radar reflectors in Antarctica which enable accurate, geo-referenced maps of the Antarctic coast line to be made. Currently we also participate in the African VLBI Network (AVN), with the aim to densify not only astronomical observatories in Africa, but to improve the geometry and distribution of advanced geodetic and geophysical equipment to facilitate development of research platforms in Africa, which can be used for geodynamics and related sciences, supporting international projects such as the WEGENER initiative. We present our multi-disciplinary activities during the last decade and sketch the way forward. Participation of Africa in the global arena of astronomy, geodesy, geodynamics and related fields will receive a major boost during the next decade. This is partially due to the development of a component of the Square Kilometre Array (SKA) in Africa but also due to the Global Geodetic Observing System (GGOS) project and the international objectives of higher geodetic accuracies and more stable reference frames. Consequent spinoffs into many disciplines relying on global reference frames and sub-cm positional accuracies stand to benefit and Africa can play a major role in improving both science and network geometries.

A Strategic Independent Geodetic VLBI Network for Europe

NASA Astrophysics Data System (ADS)

Dale, Denise; Combrinck, Ludwig; de Witt, Alet

2014-12-01

Irregularities of the rotation of the Earth in space are described by the Earth Orientation Parameters (EOPs). An independent EOP network, applying the Very Long Baseline Interferometry (VLBI) technique and using the Vienna VLBI Software (VieVS), are strategically essential for Europe to minimize its reliance on foreign global support in terms of required infrastructure for the realization of such a network. The generation of independent EOPs is already achievable by countries such as the USA, the People's Republic of China, and the Russian Federation due to their large extent of land mass that allows for long baselines in both the North-South and East-West directions and thus allows for accurate determination of all EOPs. These three countries need not rely on foreign partnerships to generate EOPs, as they all have independent geodetic VLBI networks capable of determining EOPs for precise positioning, navigation, and satellite launch/orbital purposes. They also have or are developing independent Global Navigation Satellite Systems (GNSS) constellations; so does the European Union (EU). Accurate EOPs are essential for long-term orbital maintenance of GNSS constellations, leaving the EU GALILEO GNSS vulnerable and reliant on the three superpowers. Generation of accurate EOPs by Europe is not possible due to its much smaller land mass and thus smaller achievable baselines. Even though there are many radio telescopes spread across Europe, these are separated by relatively short distances. The proposed stations that will be used to investigate this independent EOP network for Europe are the WETTZELL radio telescope in Germany, two German owned radio telescopes, TIGOCONC in Concepción, Chile, and OHIGGINS in Antarctica, as well as the HartRAO radio telescope in South Africa.

Application of laser ranging and VLBI data to a study of plate tectonic driving forces. [finite element method

NASA Technical Reports Server (NTRS)

Solomon, S. C.

1980-01-01

The measurability of changes in plate driving or resistive forces associated with plate boundary earthquakes by laser rangefinding or VLBI is considered with emphasis on those aspects of plate forces that can be characterized by such measurements. Topics covered include: (1) analytic solutions for two dimensional stress diffusion in a plate following earthquake faulting on a finite fault; (2) two dimensional finite-element solutions for the global state of stress at the Earth's surface for possible plate driving forces; and (3) finite-element solutions for three dimensional stress diffusion in a viscoelastic Earth following earthquake faulting.

Crustal dynamics project data analysis, 1991: VLBI geodetic results, 1979 - 1990

NASA Technical Reports Server (NTRS)

Ma, C.; Ryan, J. W.; Caprette, D. S.

1992-01-01

The Goddard VLBI group reports the results of analyzing 1412 Mark II data sets acquired from fixed and mobile observing sites through the end of 1990 and available to the Crustal Dynamics Project. Three large solutions were used to obtain Earth rotation parameters, nutation offsets, global source positions, site velocities, and baseline evolution. Site positions are tabulated on a yearly basis from 1979 through 1992. Site velocities are presented in both geocentric Cartesian coordinates and topocentric coordinates. Baseline evolution is plotted for 175 baselines. Rates are computed for earth rotation and nutation parameters. Included are 104 sources, 88 fixed stations and mobile sites, and 688 baselines.

Potential Refinement of the ICRF

NASA Technical Reports Server (NTRS)

Ma, Chopo

2003-01-01

The analysis and data used for the ICRF represented the state of the art in global, extragalactic, X/S band microwave astrometry in 1995. The same general analysis method was used to extend the ICRF with subsequent VLBI data in a manner consistent with the original catalog. Since 1995 there have been considerable advances in the geodetic/astrometric VLBI data set and in the analysis that would significantly improve the systematic errors, stability, and density of the next realization of the ICRS when the decision is made to take this step. In particular, data acquired since 1990, including extensive use of the VLBA, are of higher quality and astrometric utility because of changes in instrumentation, schedule design, and networks as well as specifically astrometric intent. The IVS (International VLBI Service for Geodesy and Astrometry) continues to devote a portion of its observing capability to systematic extension of the astrometric data set. Sufficient data distribution exists to select a better set of defining sources. Improvements in troposphere modeling will minimize known systematic astrometric errors while accurate modeling and estimation of station effects from loading and nonlinear motions will permit the reintegration of the celestial reference frame, terrestrial reference frame and Earth orientation parameters though a single VLBI solution. The differences between the current ICRF and the potential next realization will be described.

Comparison of Kalman filter estimates of zenith atmospheric path delays using the global positioning system and very long baseline interferometry

NASA Technical Reports Server (NTRS)

Tralli, David M.; Lichten, Stephen M.; Herring, Thomas A.

1992-01-01

Kalman filter estimates of zenith nondispersive atmospheric path delays at Westford, Massachusetts, Fort Davis, Texas, and Mojave, California, were obtained from independent analyses of data collected during January and February 1988 using the GPS and VLBI. The apparent accuracy of the path delays is inferred by examining the estimates and covariances from both sets of data. The ability of the geodetic data to resolve zenith path delay fluctuations is determined by comparing further the GPS Kalman filter estimates with corresponding wet path delays derived from water vapor radiometric data available at Mojave over two 8-hour data spans within the comparison period. GPS and VLBI zenith path delay estimates agree well within one standard deviation formal uncertainties (from 10-20 mm for GPS and 3-15 mm for VLBI) in four out of the five possible comparisons, with maximum differences of 5 and 21 mm over 8- to 12-hour data spans.

Proper Motion of Components in 4C 39.25

NASA Technical Reports Server (NTRS)

Guirado, J. C.; Marcaide, J. M.; Alberdi, A.; Elosegui, P.; Ratner, M. I.; Shapiro, I. I.; Kilger, R.; Mantovani, F.; Venturi, T.; Rius, A.;

High-Resolution Millimeter-VLBI Imaging of the super-massive black hole candidate at the Galactic center - Sgr A*

NASA Astrophysics Data System (ADS)

Shen, Zqs

Sagittarius A* (Sgr A*), the extremely compact radio source at the Galactic center (GC), is the best candidate for the single super-massive black hole (SMBH). The accurate measurements of its mass (as a gravitational source) and size (as a radiative source) are of great importance in testing its SMBH hypothesis. Great progress has been made on determining its central dark mass of 3.7 million solar masses. Here, we will present the highest resolution VLBI imaging observations of Sgr A* made at both 7.0 and 3.5 millimeters with the Very Long Baseline Array (VLBA) plus the Green Bank Telescope (GBT) and the VLBA, respectively. Both the imaging and the model-fitting with the closure amplitudes show a consistent East-West elongated elliptical Gaussian emission. The inferred possible intrinsic emitting region is less than 1 AU at the distance of 8 kpc to GC.

SN 1986J VLBI. IV. The Nature of the Central Component

NASA Astrophysics Data System (ADS)

Bietenholz, Michael F.; Bartel, Norbert

2017-12-01

We report on Very Large Array measurements between 1 and 45 GHz of the evolving radio spectral energy distribution (SED) of SN 1986J, made in conjunction with very long baseline interferometry (VLBI) imaging. The SED of SN 1986J is unique among supernovae, and shows an inversion point and a high-frequency turnover. Both are due to the central component seen in the VLBI images, and both are progressing downward in frequency with time. The optically thin spectral index of the central component is almost the same as that of the shell. We fit a simple model to the evolving SED consisting of an optically thin shell and a partly absorbed central component. The evolution of the SED is consistent with that of a homologously expanding system. Both components are fading, but the shell is fading more rapidly. We conclude that the central component is physically inside the expanding shell, and not a surface hotspot central only in projection. Our observations are consistent with the central component being due to interaction of the shock with the dense and highly structured circumstellar medium that resulted from a period of common-envelope evolution of the progenitor. However, a young pulsar-wind nebula or emission from an accreting black hole can also not be ruled out at this point.

A demonstration of centimeter-level monitoring of polar motion with the Global Positioning System

NASA Technical Reports Server (NTRS)

Lindqwister, U. J.; Freedman, A. P.; Blewitt, G.

1992-01-01

Daily estimates of the Earth's pole position were obtained with the Global Positioning System (GPS) by using measurements obtained during the GPS IERS (International Earth Rotation Service) and Geodynamics (GIG'91) experiment from 22 Jan. to 13 Feb. 1991. Data from a globally distributed network consisting of 21 Rogue GPS receivers were chosen for the analysis. A comparison of the GPS polar motion series with nine 24-hour very long baseline interferometry (VLBI) estimates yielded agreement in the day-to-day pole position of about 1.5 cm for both X and Y polar motion. A similar comparison of GPS and satellite laser ranging (SLR) data showed agreement to about 1.0 cm. These preliminary results indicate that polar motion can be determined by GPS independent of, and at a level comparable to, that which is obtained from either VLBI or SLR. Furthermore, GPS can provide these data with a daily frequency that neither alternative technique can readily achieve. Thus, GPS promises to be a powerful tool for determining high-frequency platform parameter variation, essential for the ultraprecise spacecraft-tracking requirements of the coming years.

On principles, methods and recent advances in studies towards a GPS-based control system for geodesy and geodynamics

NASA Technical Reports Server (NTRS)

Delikaraoglou, Demitris

1989-01-01

Although Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) are becoming increasingly important tools for geodynamic studies, their future role may well be fulfilled by using alternative techniques such as those utilizing the signals from the Global Positioning System (GPS). GPS, without the full implementation of the system, already offers a favorable combination of cost and accuracy and has consistently demonstrated the capability to provide high precision densification control in the regional and local areas of the VLBI and SLR networks. This report reviews VLBI and SLR vis-a-vis GPS and outlines the capabilities and limitations of each technique and how their complementary application can be of benefit to geodetic and geodynamic operations. It demonstrates, albeit with a limited data set, that dual-frequency GPS observations and interferometric type analysis techniques make possible the modelling of the GPS orbits for several days with an accuracy of a few meters. The use of VLBI or SLR sites as fiducial stations together with refinements in the orbit determination procedures can greatly reduce the systematic errors in the GPS satellite orbits used to compute the positions of non-fiducial locations. In general, repeatability and comparison with VLBI of the GPS determined locations are of the order of between 2 parts in 10 to the 7th power and 5 parts in 10 to the 8th power for baseline lengths less than 2000 km. This report is mainly a synthesis of problems, assumptions, methods and recent advances in the studies towards the establishment of a GPS-based system for geodesy and geodynamics and is one phase in the continuing effort for the development of such a system. To some, including the author, it seems reasonable to expect within the next few years that more evidence will show GPS to be as a powerful and reliable a tool as mobile VLBI and SLR are today, but largely more economical.

The Acceleration of the Barycenter of Solar System Obtained from VLBI Observations and Its Impact on the ICRS

NASA Astrophysics Data System (ADS)

Xu, M. H.

2016-03-01

Since 1998 January 1, instead of the traditional stellar reference system, the International Celestial Reference System (ICRS) has been realized by an ensemble of extragalactic radio sources that are located at hundreds of millions of light years away (if we accept their cosmological distances), so that the reference frame realized by extragalactic radio sources is assumed to be space-fixed. The acceleration of the barycenter of solar system (SSB), which is the origin of the ICRS, gives rise to a systematical variation in the directions of the observed radio sources. This phenomenon is called the secular aberration drift. As a result, the extragalactic reference frame fixed to the space provides a reference standard for detecting the secular aberration drift, and the acceleration of the barycenter with respect to the space can be determined from the observations of extragalactic radio sources. In this thesis, we aim to determine the acceleration of the SSB from astrometric and geodetic observations obtained by Very Long Baseline Interferometry (VLBI), which is a technique using the telescopes globally distributed on the Earth to observe a radio source simultaneously, and with the capacity of angular positioning for compact radio sources at 10-milliarcsecond level. The method of the global solution, which allows the acceleration vector to be estimated as a global parameter in the data analysis, is developed. Through the formal error given by the solution, this method shows directly the VLBI observations' capability to constrain the acceleration of the SSB, and demonstrates the significance level of the result. In the next step, the impact of the acceleration on the ICRS is studied in order to obtain the correction of the celestial reference frame (CRF) orientation. This thesis begins with the basic background and the general frame of this work. A brief review of the realization of the CRF based on the kinematical and the dynamical methods is presented in Chapter 2, along with the definition of the CRF and its relationship with the inertial reference frame. Chapter 3 is divided into two parts. The first part describes various effects that modify the geometric direction of an object, especially the parallax, the aberration, and the proper motion. Then the derivative model and the principle of determination of the acceleration are introduced in the second part. The VLBI data analysis method, including VLBI data reduction (solving the ambiguity, identifying the clock break, and determining the ionospheric effect), theoretical delay model, parameterization, and datum definition, is discussed in detail in Chapter 4. The estimation of the acceleration by more than 30-year VLBI observations and the results are then described in Chapter 5. The evaluation and the robust check of our results by different solutions and the comparison to that from another research group are performed. The error sources for the estimation of the acceleration, such as the secular parallax caused by the velocity of the barycenter in space, are quantitatively studied by simulation and data analysis in Chapter 6. The two main impacts of the acceleration on the CRF, the apparent proper motion with the magnitude of the μ as\\cdot yr^{-1} level and the global rotation in the CRF due to the un-uniformed distribution of radio sources on the sky, are discussed in Chapter 7. The definition and the realization of the epoch CRF are presented as well. The future work concerning the explanation of the estimated acceleration and potential research on several main problems in modern astrometry are discussed in the last chapter.

MERI: an ultra-long-baseline Moon-Earth radio interferometer.

NASA Astrophysics Data System (ADS)

Burns, J. O.

Radiofrequency aperture synthesis, pioneered by Ryle and his colleagues at Cambridge in the 1960's, has evolved to ever longer baselines and larger arrays in recent years. The limiting resolution at a given frequency for modern ground-based very-long-baseline interferometry is simply determined by the physical diameter of the Earth. A second-generation, totally space-based VLB network was proposed recently by a group at the Naval Research Laboratory. The next logical extension of space-based VLBI would be a station or stations on the Moon. The Moon could serve as an outpost or even the primary correlator station for an extended array of space-based antennas.

Tropospheric delays derived from Kalman-filtered VLBI observations

NASA Astrophysics Data System (ADS)

Soja, Benedikt; Nilsson, Tobias; Karbon, Maria; Balidakis, Kyriakos; Lu, Cuixian; Anderson, James; Glaser, Susanne; Liu, Li; Mora-Diaz, Julian A.; Raposo-Pulido, Virginia; Xu, Minghui; Heinkelmann, Robert; Schuh, Harald

2015-04-01

One of the most important error sources in the products of space geodetic techniques is the troposphere. Currently, it is not possible to model the rapid variations in the path delay caused by water vapor with sufficient accuracy, thus it is necessary to estimate these delays in the data analysis. Very long baseline interferometry (VLBI) is well suited to determine wet delays with high accuracy and precision. Compared to GNSS, the analysis does not need to deal with effects related to code biases, multipath, satellite orbit mismodeling, or antenna phase center variations that are inherent in GNSS processing. VLBI data are usually analyzed by estimating geodetic parameters in a least squares adjustment. However, once the VLBI Global Observing System (VGOS) will have become operational, algorithms providing real-time capability, for instance a Kalman filter, should be preferable for data analysis. Even today, certain advantages of such a filter, for example, allowing stochastic modeling of geodetic parameters, warrant its application. The estimation of tropospheric wet delays, in particular, greatly benefits from the stochastic approach of the filter. In this work we have investigated the benefits of applying a Kalman filter in the VLBI data analysis for the determination of tropospheric parameters. The VLBI datasets considered are the CONT campaigns, which demonstrate state-of-the-art capabilities of the VLBI system. They are unique in following a continuous observation schedule over 15 days and in having data recorded at higher bandwidth than usual. The large amount of observations leads to a very high quality of geodetic products. CONT campaigns are held every three years; we have analyzed all CONT campaigns between 2002 and 2014 for this study. In our implementation of a Kalman filter in the VLBI software VieVS@GFZ, the zenith wet delays (ZWD) are modeled as random walk processes. We have compared the resulting time series to corresponding ones obtained from other sources (water vapor radiometers, GNSS, ray-traced delays from numerical weather models) and from a classical least squares solution of the VLBI data. Taking the radiometer time series as a reference, the Kalman filter solution showed the smallest root mean square. Due to the high correlation between the ZWD and station coordinates, investigations of the baseline lengths are of great interest in this context as well. Comparing baseline length repeatabilities from the classical least squares fit with those from the Kalman filter, the filter results present a better performance of up to 15%. To further improve the performance of the ZWD estimation, the noise parameters of the Kalman filter were modeled individually for each station. From ZWD time series at all involved VLBI sites, the power spectral densities of the white noise processes which are driving the random walk processes have been derived. Applying this station-based model results in an improvement of the baseline length repeatabilities of additional 2-3%.

The Most Compact Bright Radio-loud AGNs. II. VLBA Observations of 10 Sources at 43 and 86 GHz

NASA Astrophysics Data System (ADS)

Cheng, X.-P.; An, T.; Hong, X.-Y.; Yang, J.; Mohan, P.; Kellermann, K. I.; Lister, M. L.; Frey, S.; Zhao, W.; Zhang, Z.-L.; Wu, X.-C.; Li, X.-F.; Zhang, Y.-K.

2018-01-01

Radio-loud active galactic nuclei (AGNs), hosting powerful relativistic jet outflows, provide an excellent laboratory for studying jet physics. Very long baseline interferometry (VLBI) enables high-resolution imaging on milli-arcsecond (mas) and sub-mas scales, making it a powerful tool to explore the inner jet structure, shedding light on the formation, acceleration, and collimation of AGN jets. In this paper, we present Very Long Baseline Array observations of 10 radio-loud AGNs at 43 and 86 GHz that were selected from the Planck catalog of compact sources and are among the brightest in published VLBI images at and below 15 GHz. The image noise levels in our observations are typically 0.3 and 1.5 mJy beam‑1 at 43 and 86 GHz, respectively. Compared with the VLBI data observed at lower frequencies from the literature, our observations with higher resolutions (with the highest resolution being up to 0.07 mas at 86 GHz and 0.18 mas at 43 GHz) and at higher frequencies detected new jet components at sub-parsec scales, offering valuable data for studies of the physical properties of the innermost jets. These include the compactness factor of the radio structure (the ratio of core flux density to total flux density), and core brightness temperature ({T}{{b}}). In all these sources, the compact core accounts for a significant fraction (> 60 % ) of the total flux density. Their correlated flux density at the longest baselines is higher than 0.16 Jy. The compactness of these sources make them good phase calibrators of millimeter-wavelength ground-based and space VLBI.

Impact of different NWM-derived mapping functions on VLBI and GPS analysis

NASA Astrophysics Data System (ADS)

Nikolaidou, Thalia; Balidakis, Kyriakos; Nievinski, Felipe; Santos, Marcelo; Schuh, Harald

2018-06-01

In recent years, numerical weather models have shown the potential to provide a good representation of the electrically neutral atmosphere. This fact has been exploited for the modeling of space geodetic observations. The Vienna Mapping Functions 1 (VMF1) are the NWM-based model recommended by the latest IERS Conventions. The VMF1 are being produced 6 hourly based on the European Centre for Medium-Range Weather Forecasts operational model. UNB-VMF1 provide meteorological parameters aiding neutral atmosphere modeling for VLBI and GNSS, based on the same concept but utilizing the Canadian Meteorological Centre model. This study presents comparisons between the VMF1 and the UNB-VMF1 in both delay and position domains, using global networks of VLBI and GPS stations. It is shown that the zenith delays agree better than 3.5 mm (hydrostatic) and 20 mm (wet) which implies an equivalent predicted height error of less than 2 mm. In the position domain and VLBI analysis, comparison of the weighted root-mean-square error (wrms) of the height component showed a maximum difference of 1.7 mm. For 48% of the stations, the use of VMF1 reduced the height wrms of the stations by 2.6% on average compared to a respective reduction of 1.7% for 41% of the stations employing the UNB-VMF1. For the subset of VLBI stations participating in a large number of sessions, neither mapping function outranked the other. GPS analysis using Precise Point Positioning had a sub-mm respective difference, while the wrms of the individual solutions had a maximum value of 12 mm for the 1-year-long analysis. A clear advantage of one NWM over the other was not shown, and the statistics proved that the two mapping functions yield equal results in geodetic analysis.

Applying Kalman filtering to investigate tropospheric effects in VLBI

NASA Astrophysics Data System (ADS)

Soja, Benedikt; Nilsson, Tobias; Karbon, Maria; Heinkelmann, Robert; Liu, Li; Lu, Cuixian; Andres Mora-Diaz, Julian; Raposo-Pulido, Virginia; Xu, Minghui; Schuh, Harald

2014-05-01

Very Long Baseline Interferometry (VLBI) currently provides results, e.g., estimates of the tropospheric delays, with a delay of more than two weeks. In the future, with the coming VLBI2010 Global Observing System (VGOS) and increased usage of electronic data transfer, it is planned that the time between observations and results is decreased. This may, for instance, allow the integration of VLBI-derived tropospheric delays into numerical weather prediction models. Therefore, future VLBI analysis software packages need to be able to process the observational data autonomously in near real-time. For this purpose, we have extended the Vienna VLBI Software (VieVS) by a Kalman filter module. This presentation describes the filter and discusses its application for tropospheric studies. Instead of estimating zenith wet delays as piece-wise linear functions in a least-squares adjustment, the Kalman filter allows for more sophisticated stochastic modeling. We start with a random walk process to model the time-dependent behavior of the zenith wet delays. Other possible approaches include the stochastic model described by turbulence theory, e.g. the model by Treuhaft and Lanyi (1987). Different variance-covariance matrices of the prediction error, depending on the time of the year and the geographic latitude, have been tested. In winter and closer to the poles, lower variances and covariances are appropriate. The horizontal variations in tropospheric delays have been investigated by comparing three different strategies: assumption of a horizontally stratified troposphere, using north and south gradients modeled, e.g., as Gauss-Markov processes, and applying a turbulence model assuming correlations between observations in different azimuths. By conducting Monte-Carlo simulations of current standard VLBI networks and of future VGOS networks, the different tropospheric modeling strategies are investigated. For this purpose, we use the simulator module of VieVS which takes into account the errors due to the atomic clocks at the stations, the troposphere, and white noise processes. The simulated data as well as actual observational data from the two-week CONT11 campaign are analyzed using the Kalman filter, focusing on the tropospheric effects. The results of the different strategies are compared with solutions applying the classical least-squares method. An advantage of the Kalman filter is the possibility of easily integrating additional external information. It is expected that by including tropospheric delays from GNSS, water vapor radiometers, or ray-traced delays from numerical weather prediction models, the accuracy of the VLBI solution could be improved.

A two-level approach to VLBI terrestrial and celestial reference frames using both least-squares adjustment and Kalman filter algorithms

NASA Astrophysics Data System (ADS)

Soja, B.; Krasna, H.; Boehm, J.; Gross, R. S.; Abbondanza, C.; Chin, T. M.; Heflin, M. B.; Parker, J. W.; Wu, X.

2017-12-01

The most recent realizations of the ITRS include several innovations, two of which are especially relevant to this study. On the one hand, the IERS ITRS combination center at DGFI-TUM introduced a two-level approach with DTRF2014, consisting of a classical deterministic frame based on normal equations and an optional coordinate time series of non-tidal displacements calculated from geophysical loading models. On the other hand, the JTRF2014 by the combination center at JPL is a time series representation of the ITRF determined by Kalman filtering. Both the JTRF2014 and the second level of the DTRF2014 are thus able to take into account short-term variations in the station coordinates. In this study, based on VLBI data, we combine these two approaches, applying them to the determination of both terrestrial and celestial reference frames. Our product has two levels like DTRF2014, with the second level being a Kalman filter solution like JTRF2014. First, we compute a classical TRF and CRF in a global least-squares adjustment by stacking normal equations from 5446 VLBI sessions between 1979 and 2016 using the Vienna VLBI and Satellite Software VieVS (solution level 1). Next, we obtain coordinate residuals from the global adjustment by applying the level-1 TRF and CRF in the single-session analysis and estimating coordinate offsets. These residuals are fed into a Kalman filter and smoother, taking into account the stochastic properties of the individual stations and radio sources. The resulting coordinate time series (solution level 2) serve as an additional layer representing irregular variations not considered in the first level of our approach. Both levels of our solution are implemented in VieVS in order to test their individual and combined performance regarding the repeatabilities of estimated baseline lengths, EOP, and radio source coordinates.

GARS O'Higgins as a core station for geodesy in Antarctica

NASA Astrophysics Data System (ADS)

Klügel, Thomas; Diedrich, Erhard; Falk, Reinhard; Hessels, Uwe; Höppner, Kathrin; Kühmstedt, Elke; Metzig, Robert; Plötz, Christian; Reinhold, Andreas; Schüler, Torben; Wojdziak, Reiner

2014-05-01

The German Antarctic Receiving Station GARS O'Higgins at the northern tip of the Antarctic Peninsula is a dual purpose facility for Earth observation since more than 20 years. It serves as a satellite ground station for payload data downlink and telecommanding of remote sensing satellites as well as a geodetic observatory for global reference frames and global change. Both applications use the same 9m diameter radio telescope. For space geodesy and astrometry the radio telescope significantly improves the coverage on the southern hemisphere and plays an essential role within the global Very Long Baseline Interferometry (VLBI) network. In particular the determination of the Earth Orientation Parameters (EOP) and the sky coverage of the International Celectial Reference Frame (ICRF) benefit from the location at high southern latitude. Further geodetic instrumentation includes different permanent GNSS receivers (since 1995), two SAR corner reflectors (since 2013) and in the past a PRARE system (1996 - 2004). In addition absolute gravity measurements were performed in 1997 and 2011. All geodetic reference points are tied together by a local survey network. The various geodetic instrumentation and the long time series at O'Higgins allow a reliable determination of crustal motions. VLBI station velocities, continuous GNSS time series and absolute gravity measurements consistently document an uplift rate of about 5 mm/a. A pressure gauge and a radar tide gauge being refererenced to space by a GNSS antenna on top allow the measurement of sea level changes independently from crustal motions, and the determination of the ellipsoidal height of the sea surface, which is, the geoid height plus the mean dynamic topography. The outstanding location on the Antarctic continent makes GARS O'Higgins also in future attractive for polar orbiting satellite missions and an essential station for the global VLBI network. Future plans envisage a development towards an observatory for environmentally relevant research.

Local effects of redundant terrestrial and GPS-based tie vectors in ITRF-like combinations

NASA Astrophysics Data System (ADS)

Abbondanza, Claudio; Altamimi, Zuheir; Sarti, Pierguido; Negusini, Monia; Vittuari, Luca

2009-11-01

Tie vectors (TVs) between co-located space geodetic instruments are essential for combining terrestrial reference frames (TRFs) realised using different techniques. They provide relative positioning between instrumental reference points (RPs) which are part of a global geodetic network such as the international terrestrial reference frame (ITRF). This paper gathers the set of very long baseline interferometry (VLBI)-global positioning system (GPS) local ties performed at the observatory of Medicina (Northern Italy) during the years 2001-2006 and discusses some important aspects related to the usage of co-location ties in the combinations of TRFs. Two measurement approaches of local survey are considered here: a GPS-based approach and a classical approach based on terrestrial observations (i.e. angles, distances and height differences). The behaviour of terrestrial local ties, which routinely join combinations of space geodetic solutions, is compared to that of GPS-based local ties. In particular, we have performed and analysed different combinations of satellite laser ranging (SLR), VLBI and GPS long term solutions in order to (i) evaluate the local effects of the insertion of the series of TVs computed at Medicina, (ii) investigate the consistency of GPS-based TVs with respect to space geodetic solutions, (iii) discuss the effects of an imprecise alignment of TVs from a local to a global reference frame. Results of ITRF-like combinations show that terrestrial TVs originate the smallest residuals in all the three components. In most cases, GPS-based TVs fit space geodetic solutions very well, especially in the horizontal components (N, E). On the contrary, the estimation of the VLBI RP Up component through GPS technique appears to be awkward, since the corresponding post fit residuals are considerably larger. Besides, combination tests including multi-temporal TVs display local effects of residual redistribution, when compared to those solutions where Medicina TVs are added one at a time. Finally, the combination of TRFs turns out to be sensitive to the orientation of the local tie into the global frame.

The Gamma-Ray Emitting Radio-Loud Narrow-Line Seyfert 1 Galaxy PKS 2004-447 II. The Radio View

NASA Technical Reports Server (NTRS)

Schulz, R.; Kreikenbohm, A.; Kadler, M.; Ojha, R.; Ros, E.; Stevens, J.; Edwards, P. G.; Carpenter, B.; Elsaesser, D.; Gehrels, N.;

NASA's Contribution to Global Space Geodesy Networks

NASA Technical Reports Server (NTRS)

Bosworth, John M.

1999-01-01

The NASA Space Geodesy program continues to be a major provider of space geodetic data for the international earth science community. NASA operates high performance Satellite Laser Ranging (SLR), Very Long Baseline Interferometry (VLBI) and Global Positioning System (GPS) ground receivers at well over 30 locations around the world and works in close cooperation with space geodetic observatories around the world. NASA has also always been at the forefront in the quest for technical improvement and innovation in the space geodesy technologies to make them even more productive, accurate and economical. This presentation will highlight the current status of NASA's networks; the plans for partnerships with international groups in the southern hemisphere to improve the geographic distribution of space geodesy sites and the status of the technological improvements in SLR and VLBI that will support the new scientific thrusts proposed by interdisciplinary earth scientists. In addition, the expanding role of the NASA Space geodesy data archive, the CDDIS will be described.

Atmospheric gradients from GNSS, VLBI, and DORIS analyses and from Numerical Weather Models during CONT14

NASA Astrophysics Data System (ADS)

Heinkelmann, Robert; Dick, Galina; Nilsson, Tobias; Soja, Benedikt; Wickert, Jens; Zus, Florian; Schuh, Harald

2015-04-01

Observations from space-geodetic techniques are nowadays increasingly used to derive atmospheric information for various commercial and scientific applications. A prominent example is the operational use of GNSS data to improve global and regional weather forecasts, which was started in 2006. Atmosphere gradients describe the azimuthal asymmetry of zenith delays. Estimates of geodetic and other parameters significantly improve when atmosphere gradients are determined in addition. Here we assess the capability of several space geodetic techniques (GNSS, VLBI, DORIS) to determine atmosphere gradients of refractivity. For this purpose we implement and compare various strategies for gradient estimation, such as different values for the temporal resolution and the corresponding parameter constraints. Applying least squares estimation the gradients are usually deterministically modelled as constants or piece-wise linear functions. In our study we compare this approach with a stochastic approach modelling atmosphere gradients as random walk processes and applying a Kalman Filter for parameter estimation. The gradients, derived from space geodetic techniques are verified by comparison with those derived from Numerical Weather Models (NWM). These model data were generated using raytracing calculations based on European Centre for Medium-Range Weather Forecast (ECMWF) and National Centers for Environmental Prediction (NCEP) analyses with different spatial resolutions. The investigation of the differences between the ECMWF and NCEP gradients hereby in addition allow for an empirical assessment of the quality of model gradients and how suitable the NWM data are for verification. CONT14 (2014-05-06 until 2014-05-20) is the youngest two week long continuous VLBI campaign carried out by IVS (International VLBI Service for Geodesy and Astrometry). It presents the state-of-the-art VLBI performance in terms of number of stations and number of observations and presents thus an excellent test period for comparisons with other space geodetic techniques. During the VLBI campaign CONT14 the HOBART12 and HOBART26 (Hobart, Tasmania, Australia) VLBI antennas were involved that co-locate with each other. The investigation of the gradient estimate differences from these co-located antennas allows for a valuable empirical quality assessment. Another quality criterion for gradient estimates are the differences of parameters at the borders of adjacent 24h-sessions. Both are investigated in our study.

VLBI-SLR Combination Solution Using GEODYN

NASA Technical Reports Server (NTRS)

MacMillan, Dan; Pavlis, Despina; Lemoine, Frank; Chinn, Douglas; Rowlands, David

2010-01-01

We would like to generate a multi-technique solution combining all of the geodetic techniques (VLBI, SLR, GPS, and DORIS) using the same software and using the same a priori models. Here we use GEODYN software and consider only the VLBI-SLR combination. Here we report initial results of our work on the combination. We first performed solutions with GEODYN using only VLBI data and found that VLBI EOP solution results produced with GEODYN agree with results using CALC/SOLVE at the 1-sigma level. We then combined the VLBI normal equations in GEODYN with weekly SLR normal equations for the period 2007-2008. Agreement of estimated Earth orientation parameters with IERS C04 were not significantly different for the VLBI-only, SLR-only, and VLBI+SLR solutions

Interferometric Monitoring of Gamma-Ray Bright AGNs: S5 0716+714

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Jee Won; Lee, Sang-Sung; Hodgson, Jeffrey A.

We present the results of very long baseline interferometry (VLBI) observations of gamma-ray bright blazar S5 0716+714 using the Korean VLBI Network (KVN) at the 22, 43, 86, and 129 GHz bands, as part of the Interferometric Monitoring of Gamma-ray Bright active galactic nuclei (iMOGABA) KVN key science program. Observations were conducted in 29 sessions from 2013 January 16 to 2016 March 1, with the source being detected and imaged at all available frequencies. In all epochs, the source was compact on the milliarcsecond scale, yielding a compact VLBI core dominating the synchrotron emission on these scales. Based on themore » multiwavelength data between 15 GHz (Owens Valley Radio Observatory) and 230 GHz (Submillimeter Array), we found that the source shows multiple prominent enhancements of the flux density at the centimeter (cm) and millimeter (mm) wavelengths, with mm enhancements leading cm enhancements by −16 ± 8 days. The turnover frequency was found to vary between 21 and 69 GHz during our observations. By assuming a synchrotron self-absorption model for the relativistic jet emission in S5 0716+714, we found the magnetic field strength in the mas emission region to be ≤5 mG during the observing period, yielding a weighted mean of 1.0 ± 0.6 mG for higher turnover frequencies (e.g., >45 GHz).« less

The Space Geodesy Project and Radio Frequency Interference Characterization and Mitigation

NASA Technical Reports Server (NTRS)

Lawrence, Hilliard M.; Beaudoin, C.; Corey, B. E.; Tourain, C. L.; Petrachenko, B.; Dickey, John

2013-01-01

The Space Geodesy Project (SGP) development by NASA is an effort to co-locate the four international geodetic techniques Satellite Laser Ranging (SLR) and Lunar Laser Ranging (LLR), Very Long Baseline Interferometry (VLBI), Global Navigation Satellite System (GNSS), and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) into one tightly referenced campus and coordinated reference frame analysis. The SGP requirement locates these stations within a small area to maintain line-of-sight and frequent automated survey known as the vector tie system. This causes a direct conflict with the new broadband VLBI technique. Broadband means 2-14 GHz, and RFI susceptibility at -80 dBW or higher due to sensitive RF components in the front end of the radio receiver.

18 centimeter VLBI observations of the quasar NRAO 140 during and after a low-frequency outburst

NASA Astrophysics Data System (ADS)

Marscher, Alan P.; Broderick, John J.; Padrielli, Lucia; Bartel, Norbert; Romney, Jonathan D.

1987-08-01

The authors have observed the quasar NRAO 140 using an eight station very long baseline array at 18 cm in 1984 April and a seven station array at 6 cm in 1984 May. They compare both the map and the data at 18 cm with those obtained by Marscher and Broderick in 1981 October. The latter coincided with a ≡25% outburst in flux density at wavelengths greater than ≡30 cm. The analysis indicates that a component ≡5 milli-arc seconds southeast of the "core" dropped significantly in brightness between 1981 October and 1984 April. The authors identify this component as the likely site of the low-frequency variations.

VizieR Online Data Catalog: Radio obs. of NGC 6251 jet (Tseng+, 2016)

NASA Astrophysics Data System (ADS)

Tseng, C.-Y.; Asada, K.; Nakamura, M.; Pu, H.-Y.; Algaba, J.-C.; Lo, W.-P.

2017-05-01

We conducted European VLBI Network (EVN) observations of NGC 6251 on 2013 March 10 at 1.6GHz with the stations at Badary, Svetloe, Zelenchukskaya (Russia), Effelsberg (Germany), Jodrell Bank (UK), Medicina, Noto (Italy), Onsala (Sweden), Shanghai, Urumqi (China), Torun (Poland), and Westerbork (Netherlands). Archival Very Long Baseline Array (VLBA) data at 5GHz are used and calibrated in the same manner as the EVN data. Also, 12 epochs of the VLBA data at 15GHz are obtained from the MOJAVE database (Lister+ 2009, J/AJ/137/3718). Observations were conducted during 1998-2013. We also use a published VLA image of NGC 6251 at 1.4GHz to compare with the VLBI measurements in Section 4.1. The image, as well as the calibration processes, is shown in Sambruna+ (2004A&A...414..885S). Observations were conducted on 1995 August 15 using the full VLA in its A-configuration. The beam is restored to be circular with an FWHM of 2". (1 data file).

Tsukuba VLBI Correlator

NASA Technical Reports Server (NTRS)

Kurihara, Shinobu; Nozawa, Kentaro

2013-01-01

The K5/VSSP software correlator (Figure 1), located in Tsukuba, Japan, is operated by the Geospatial Information Authority of Japan (GSI). It is fully dedicated to processing the geodetic VLBI sessions of the International VLBI Service for Geodesy and Astrometry. All of the weekend IVS Intensives (INT2) and the Japanese domestic VLBI observations organized by GSI were processed at the Tsukuba VLBI Correlator.

DSN Network e-VLBI Calibration of Earth Orientation

NASA Technical Reports Server (NTRS)

Zhang, Liwei Dennis; Steppe, A.; Lanyi, G.; Jacobs, C.

2006-01-01

This viewgraph presentation reviews the calibration of the Earth's orientation by using the Deep Space Network (DSN) e Very Large Base Integration (VLBI). The topics include: 1) Background: TEMPO; 2) Background: UT1 Knowledge Error; 3) e-VLBI: WVSR TEMPO Overview; 4) e-VLBI: WVSR TEMPO Turnaround; 5) e-VLBI: WVSR TEMPO R&D Tests; and 6) WVSR TEMPO Test Conclusion.

Location of γ-ray emission and magnetic field strengths in OJ 287

NASA Astrophysics Data System (ADS)

Hodgson, J. A.; Krichbaum, T. P.; Marscher, A. P.; Jorstad, S. G.; Rani, B.; Marti-Vidal, I.; Bach, U.; Sanchez, S.; Bremer, M.; Lindqvist, M.; Uunila, M.; Kallunki, J.; Vicente, P.; Fuhrmann, L.; Angelakis, E.; Karamanavis, V.; Myserlis, I.; Nestoras, I.; Chidiac, C.; Sievers, A.; Gurwell, M.; Zensus, J. A.

2017-01-01

Context. The γ-ray BL Lac object OJ 287 is known to exhibit inner-parsec "jet-wobbling", high degrees of variability at all wavelengths and quasi-stationary features, including an apparent (≈100°) position-angle change in projection on the sky plane. Aims: Sub-50 micro-arcsecond resolution 86 GHz observations with the global mm-VLBI array (GMVA) supplement ongoing multi-frequency VLBI blazar monitoring at lower frequencies. Using these maps, together with cm/mm total intensity and γ-ray observations from Fermi-LAT from 2008-2014, we aim to determine the location of γ-ray emission and to explain the inner-mas structural changes. Methods: Observations with the GMVA offer approximately double the angular resolution compared with 43 GHz VLBA observations and enable us to observe above the synchrotron self-absorption peak frequency. Fermi-LAT γ-ray data were reduced and analysed. The jet was spectrally decomposed at multiple locations along the jet. From this, we could derive estimates of the magnetic field using equipartition and synchrotron self-absorption arguments. How the field decreases down the jet provided an estimate of the distance to the jet apex and an estimate of the magnetic field strength at the jet apex and in the broad line region. Combined with accurate kinematics, we attempt to locate the site of γ-ray activity, radio flares, and spectral changes. Results: Strong γ-ray flares appeared to originate from either the so-called core region, a downstream stationary feature, or both, with γ-ray activity significantly correlated with radio flaring in the downstream quasi-stationary feature. Magnetic field estimates were determined at multiple locations along the jet, with the magnetic field found to be ≥1.6 G in the core and ≤0.4 G in the downstream quasi-stationary feature. We therefore found upper limits on the location of the VLBI core as ≲6.0 pc from the jet apex and determined an upper limit on the magnetic field near the jet base of the order of thousands of Gauss. The 3 mm GMVA data are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/597/A80

Navigating highly elliptical earth orbiters with simultaneous VLBI from orthogonal baseline pairs

NASA Technical Reports Server (NTRS)

Frauenholz, Raymond B.

1986-01-01

Navigation strategies for determining highly elliptical orbits with VLBI are described. The predicted performance of wideband VLBI and Delta VLBI measurements obtained by orthogonal baseline pairs are compared for a 16-hr equatorial orbit. It is observed that the one-sigma apogee position accuracy improves two orders of magnitude to the meter level when Delta VLBI measurements are added to coherent Doppler and range, and the simpler VLBI strategy provides nearly the same orbit accuracy. The effects of differential measurement noise and acquisition geometry on orbit accuracy are investigated. The data reveal that quasar position uncertainty limits the accuracy of wideband Delta VLBI measurements, and that polar motion and baseline uncertainties and offsets between station clocks affect the wideband VLBI data. It is noted that differential one-way range (DOR) has performance nearly equal to that of the more complex Delta DOR and is recommended for use on spacecraft in high elliptical orbits.

Instrumentation for single-dish observations with The Greenland Telescope

NASA Astrophysics Data System (ADS)

Grimes, Paul K.; Asada, K.; Blundell, R.; Burgos, R.; Chang, H.-H.; Chen, M. T.; Goldie, D.; Groppi, C.; Han, C. C.; Ho, P. T. P.; Huang, Y. D.; Inoue, M.; Kubo, D.; Koch, P.; Leech, J.; de Lera Acedo, E.; Martin-Cocher, P.; Nishioka, H.; Nakamura, M.; Matsushita, S.; Paine, S. N.; Patel, N.; Raffin, P.; Snow, W.; Sridharan, T. K.; Srinivasan, R.; Thomas, C. N.; Tong, E.; Wang, M.-J.; Wheeler, C.; Withington, S.; Yassin, G.; Zeng, L.-Z.

2014-07-01

The Greenland Telescope project will deploy and operate a 12m sub-millimeter telescope at the highest point of the Greenland i e sheet. The Greenland Telescope project is a joint venture between the Smithsonian As- trophysical Observatory (SAO) and the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA). In this paper we discuss the concepts, specifications, and science goals of the instruments being developed for single-dish observations with the Greenland Telescope, and the coupling optics required to couple both them and the mm-VLBI receivers to antenna. The project will outfit the ALMA North America prototype antenna for Arctic operations and deploy it to Summit Station,1 a NSF operated Arctic station at 3,100m above MSL on the Greenland I e Sheet. This site is exceptionally dry, and promises to be an excellent site for sub-millimeter astronomical observations. The main science goal of the Greenland Telescope is to carry out millimeter VLBI observations alongside other telescopes in Europe and the Americas, with the aim of resolving the event horizon of the super-massive black hole at the enter of M87. The Greenland Telescope will also be outfitted for single-dish observations from the millimeter-wave to Tera-hertz bands. In this paper we will discuss the proposed instruments that are currently in development for the Greenland Telescope - 350 GHz and 650 GHz heterodyne array receivers; 1.4 THz HEB array receivers and a W-band bolometric spectrometer. SAO is leading the development of two heterodyne array instruments for the Greenland Telescope, a 48- pixel, 325-375 GHz SIS array receiver, and a 4 pixel, 1.4 THz HEB array receiver. A key science goal for these instruments is the mapping of ortho and para H2D+ in old protostellar ores, as well as general mapping of CO and other transitions in molecular louds. An 8-pixel prototype module for the 350 GHz array is currently being built for laboratory and operational testing on the Greenland Telescope. Arizona State University are developing a 650 GHz 256 pixel SIS array receiver based on the KAPPa SIS mixer array technology and ASIAA are developing 1.4 THz HEB single pixel and array receivers. The University of Cambridge and SAO are collaborating on the development of the CAMbridge Emission Line Surveyor (CAMELS), a W-band `on- hip' spectrometer instrument with a spectral resolution of R ~ 3000. CAMELS will consist of two pairs of horn antennas, feeding super conducting niobium nitride filter banks read by tantalum based Kinetic Inductance Detectors.

GSFC VLBI Analysis Center

NASA Technical Reports Server (NTRS)

Gordon, David; Ma, Chopo; MacMillan, Dan; Gipson, John; Bolotin, Sergei; Le Bail, Karine; Baver, Karen

2013-01-01

This report presents the activities of the GSFC VLBI Analysis Center during 2012. The GSFC VLBI Analysis Center analyzes all IVS sessions, makes regular IVS submissions of data and analysis products, and performs research and software development aimed at improving the VLBI technique.

VLBI Technology Development at SHAO

NASA Technical Reports Server (NTRS)

Zhang, Xiuzhong; Shu, Fengchun; Xiang, Ying; Zhu, Renjie; Xu, Zhijun; Chen, Zhong; Zheng, Weimin; Luo, Jintao; Wu, Yajun

2010-01-01

VLBI technology development made significant progress at SHAO in the last few years. The development status of the Chinese DBBC, the software and FPGA-based correlators, and the new VLBI antenna, as well as VLBI applications are summarized in this paper.

The "Quasar" Network Observations in e-VLBI Mode Within the Russian Domestic VLBI Programs

NASA Technical Reports Server (NTRS)

Finkelstein, Andrey; Ipatov, Alexander; Kaidanovsky, Michael; Bezrukov, Ilia; Mikhailov, Andrey; Salnikov, Alexander; Surkis, Igor; Skurikhina, Elena

2010-01-01

The purpose of the Russian VLBI "Quasar" Network is to carry out astrometrical and geodynamical investigations. Since 2006 purely domestic observational programs with data processing at the IAA correlator have been carried out. To maintain these geodynamical programs e-VLBI technology is being developed and tested. This paper describes the IAA activity of developing a real-time VLBI system using high-speed digital communication links.

Crustal Dynamics Project data analysis, 1990

NASA Technical Reports Server (NTRS)

Caprette, D. S.; Ma, C.; Ryan, J. W.

1990-01-01

The Goddard Very Long Baseline Interferometry (VLBI) group reports the results of analyzing 1073 Mark 3 data sets acquired from fixed and mobile observing sites through the end of 1989 and available to the Crustal Dynamics Project. Two large solutions, GLB656 and GLB657, were used to establish a VLBI reference frame with an origin coincident with the ITRF89. Another large solution, GLB658, was used to obtain Earth rotation parameters, nutation offsets, and global source positions. Site velocities were obtained from another large solution, GLB659. A fifth large solution, GLB660, was used to obtain baseline evolution. Site positions are tabulated on a yearly basis from 1979 through 1992. Site velocities are presented in both Cartesian and topocentric coordinates. The results include 76 sources, 80 sites, and 422 baselines.

International VLBI Service for Geodesy and Astrometry. Delivering high-quality products and embarking on observations of the next generation

NASA Astrophysics Data System (ADS)

Nothnagel, A.; Artz, T.; Behrend, D.; Malkin, Z.

2017-07-01

The International VLBI Service for Geodesy and Astrometry (IVS) regularly produces high-quality Earth orientation parameters from observing sessions employing extensive networks or individual baselines. The master schedule is designed according to the telescope days committed by the stations and by the need for dense sampling of the Earth orientation parameters (EOP). In the pre-2011 era, the network constellations with their number of telescopes participating were limited by the playback and baseline capabilities of the hardware (Mark4) correlators. This limitation was overcome by the advent of software correlators, which can now accommodate many more playback units in a flexible configuration. In this paper, we describe the current operations of the IVS with special emphasis on the quality of the polar motion results since these are the only EOP components which can be validated against independent benchmarks. The polar motion results provided by the IVS have improved continuously over the years, now providing an agreement with IGS results at the level of 20-25 μas in a WRMS sense. At the end of the paper, an outlook is given for the realization of the VLBI Global Observing System.

Discovering the invisible universe

NASA Astrophysics Data System (ADS)

Friedman, Herbert

1991-02-01

The history of astronomical observations outside the visible range is surveyed in a review for general readers. Consideration is given to Jansky's discovery of cosmic radio emission, the pioneering radio observers of the 1940s, the larger radio telescopes built since 1950, aperture synthesis and the Very Large Array, terrestrial and space VLBI networks, ground-based and satellite observations in the IR band, the discovery and early laboratory characterization of X-rays, and X-ray observations from sounding rockets and satellites. Extensive photographs, drawings, diagrams, and sample images are provided.

Prospect of Continuous VLBI Measurement of Earth Rotation in Monitoring Geophysical Fluids

NASA Technical Reports Server (NTRS)

Chao, Benjamin F.; Ma, Chopo; Clark, Thomas

1998-01-01

Large-scale mass transports in the geophysical fluids of the Earth system excite Earth's rotational variations in both length-of-day and polar motion. The excitation process is via the conservation of angular momentum. Therefore Earth rotation observations contain information about the integrated angular momentum (consisting of both the mass term and the motion term) of the geophysical fluids, which include atmosphere, hydrosphere, mantle, and the outer and inner cores. Such global information is often important and otherwise unattainable depending on the nature of the mass transport, its magnitude and time scale. The last few years have seen great advances in VLBI measurement of Earth rotation in precision and temporal resolution. These advances have opened new. areas in geophysical fluid studies, such as oceanic tidal angular momentum, atmospheric tides, Earth librations, and rapid atmospheric angular momentum fluctuations. Precision of 10 microseconds in UTI and 200 microarcseconds in polar motion can now be achieved on hourly basis. Building upon this heritage, the multi-network geodetic VLBI project, Continuous Observation of the Rotation of the Earth (CORE), promises to further these studies and to make possible studies on elusive but tell-tale geophysical processes such as oscillatory modes in the core and in the atmosphere. Currently the early phase of CORE is underway. Within a few years into the new mellinnium, the upcoming space gravity missions (such as GRACE) will measure the temporal variations in Earth's gravitational field, thus providing complementary information to that from Earth rotation study for a better understanding of global geophysical fluid processes.

Simulations of VLBI observations of a geodetic satellite providing co-location in space

NASA Astrophysics Data System (ADS)

Anderson, James M.; Beyerle, Georg; Glaser, Susanne; Liu, Li; Männel, Benjamin; Nilsson, Tobias; Heinkelmann, Robert; Schuh, Harald

2018-02-01

We performed Monte Carlo simulations of very-long-baseline interferometry (VLBI) observations of Earth-orbiting satellites incorporating co-located space-geodetic instruments in order to study how well the VLBI frame and the spacecraft frame can be tied using such measurements. We simulated observations of spacecraft by VLBI observations, time-of-flight (TOF) measurements using a time-encoded signal in the spacecraft transmission, similar in concept to precise point positioning, and differential VLBI (D-VLBI) observations using angularly nearby quasar calibrators to compare their relative performance. We used the proposed European Geodetic Reference Antenna in Space (E-GRASP) mission as an initial test case for our software. We found that the standard VLBI technique is limited, in part, by the present lack of knowledge of the absolute offset of VLBI time to Coordinated Universal Time at the level of microseconds. TOF measurements are better able to overcome this problem and provide frame ties with uncertainties in translation and scale nearly a factor of three smaller than those yielded from VLBI measurements. If the absolute time offset issue can be resolved by external means, the VLBI results can be significantly improved and can come close to providing 1 mm accuracy in the frame tie parameters. D-VLBI observations with optimum performance assumptions provide roughly a factor of two higher uncertainties for the E-GRASP orbit. We additionally simulated how station and spacecraft position offsets affect the frame tie performance.

On the long-term stability of terrestrial reference frame solutions based on Kalman filtering

NASA Astrophysics Data System (ADS)

Soja, Benedikt; Gross, Richard S.; Abbondanza, Claudio; Chin, Toshio M.; Heflin, Michael B.; Parker, Jay W.; Wu, Xiaoping; Nilsson, Tobias; Glaser, Susanne; Balidakis, Kyriakos; Heinkelmann, Robert; Schuh, Harald

2018-06-01

The Global Geodetic Observing System requirement for the long-term stability of the International Terrestrial Reference Frame is 0.1 mm/year, motivated by rigorous sea level studies. Furthermore, high-quality station velocities are of great importance for the prediction of future station coordinates, which are fundamental for several geodetic applications. In this study, we investigate the performance of predictions from very long baseline interferometry (VLBI) terrestrial reference frames (TRFs) based on Kalman filtering. The predictions are computed by extrapolating the deterministic part of the coordinate model. As observational data, we used over 4000 VLBI sessions between 1980 and the middle of 2016. In order to study the predictions, we computed VLBI TRF solutions only from the data until the end of 2013. The period of 2014 until 2016.5 was used to validate the predictions of the TRF solutions against the measured VLBI station coordinates. To assess the quality, we computed average WRMS values from the coordinate differences as well as from estimated Helmert transformation parameters, in particular, the scale. We found that the results significantly depend on the level of process noise used in the filter. While larger values of process noise allow the TRF station coordinates to more closely follow the input data (decrease in WRMS of about 45%), the TRF predictions exhibit larger deviations from the VLBI station coordinates after 2014 (WRMS increase of about 15%). On the other hand, lower levels of process noise improve the predictions, making them more similar to those of solutions without process noise. Furthermore, our investigations show that additionally estimating annual signals in the coordinates does not significantly impact the results. Finally, we computed TRF solutions mimicking a potential real-time TRF and found significant improvements over the other investigated solutions, all of which rely on extrapolating the coordinate model for their predictions, with WRMS reductions of almost 50%.

(abstract) A VLBI Test of Tropospheric Delay Calibration with WVRs

NASA Technical Reports Server (NTRS)

Linfield, R. P.; Teitelbaum, L. P.; Keihm, S. J.; Resch, G. M.; Mahoney, M. J.; Treuhaft, R. N.

1994-01-01

Dual frequency (S/X band) very long baseline interferometry (VLBI) observations were used to test troposphere calibration by water vapor radiometers (WVRs). Comparison of the VLBI and WVR measurements show a statistical agreement (specifically, their structure functions agree) on time scales less than 700 seconds. On longer time scales, VLBI instrumental errors become important. The improvement in VLBI residual delays from WVR calibration was consistent with the measured level of tropospheric fluctuations.

Shuttle VLBI experiment. Technical working group summary report

NASA Technical Reports Server (NTRS)

Morgan, S. H. (Editor); Roberts, D. H. (Editor)

1982-01-01

The gain in interferometric resolution of extragalactic sources at radio frequencies which can be achieved by placing a very long baseline interferometry (VLBI) antenna in space is quantitatively described and a VLBI demonstration experiment using a large deployable antenna, which if realized could be a very acceptable first venture for VLBI in space is discussed. A tutorial on VLBI, a summary of the technology available for the experiment, and a preliminary mission scenario are included.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Yun, Youngjoo; Cho, Se-Hyung; Kim, Jaeheon

We present the first images of the v = 1 and v = 2 J = 1 → 0 SiO maser lines taken with KaVA, i.e., the combined array of the Korean Very Long Baseline Interferometry (VLBI) Network and the VLBI Exploration of Radio Astrometry (VERA), toward the OH/IR star WX Psc. The combination of long and short antenna baselines enabled us to detect a large number of maser spots, which exhibit a typical ring-like structure in both the v = 1 and v = 2 J = 1 → 0 SiO masers as those that have been found inmore » previous VLBI observational results of WX Psc. The relative alignment of the v = 1 and v = 2 SiO maser spots are precisely derived from astrometric analysis, due to the absolute coordinates of the reference maser spot that were well determined in an independent astrometric observation with VERA. The superposition of the v = 1 and v = 2 maser spot maps shows a good spatial correlation between the v = 1 and v = 2 SiO maser features. Nevertheless, it is also shown that the v = 2 SiO maser spot is distributed in an inner region compared to the v = 1 SiO maser by about 0.5 mas on average. These results provide good support for the recent theoretical studies of the SiO maser pumping, in which both the collisional and the radiative pumping predict the strong spatial correlation and the small spatial discrepancy between the v = 1 and v = 2 SiO maser.« less

Towards Cloud Processing of GGOS Big Data

NASA Astrophysics Data System (ADS)

Weston, Stuart; Kim, Bumjun; Litchfield, Alan; Gulyaev, Sergei; Hall, Dylan; Chorao, Carlos; Ruthven, Andrew; Davies, Glyn; Lagos, Bruno; Christie, Don

2017-04-01

We report on our initial steps towards development of a cloud-like correlation infrastructure for geodetic Very Long Baseline Interferometry (VLBI), which in its raw format is of the order of 10-100 TB (big data). Data is generated by multiple VLBI radio telescopes, and is then used by for geodetic, geophysical, and astrometric research and operational activities through the International VLBI Service (IVS), as well as for corrections of GPS satellite orbits. Currently IVS data is correlated in several international Correlators (Correlation Centres), which receive data from individual radio telescope stations either in hard drives via regular mail service or via fibre using e-transfer mode. The latter is strongly limited by connectivity of existing correlation centres, which creates bottle necks and slows down the turnover of the data. This becomes critical in many applications - for example, it currently takes 1-2 weeks to generate the dUT1 parameter for corrections of GNSS orbits while less than 1-2 days delay is desirable. We started with a blade server at the AUT campus to emulate a cloud server using Virtual Machines (VMWare). The New Zealand Data Head node is connected to the high speed (100 Gbps) network ring circuit courtesy of the Research and Education Advanced Network New Zealand (REANNZ), with the additional nodes at remote physical sites connected via 10 Gbps fibre. We use real Australian Long Baseline Array (LBA) observational data from 6 radio telescopes in Australia, South Africa and New Zealand (15 baselines) of 1.5 hours in duration making 8 TB to emulate data transfer from remote locations and to provide a meaningful benchmark dataset for correlation. Data was successfully transferred using bespoke UDT network transfer tools and correlated with the speed-up factor of 0.8 using DiFX software correlator. In partnership with the New Zealand office of Catalyst IT Ltd we have moved this environment into Catalyst Cloud and report on the first correlation of a VLBI Dataset in a true cloud environment.

Estimability of geodetic parameters from space VLBI observables

NASA Technical Reports Server (NTRS)

Adam, Jozsef

1990-01-01

The feasibility of space very long base interferometry (VLBI) observables for geodesy and geodynamics is investigated. A brief review of space VLBI systems from the point of view of potential geodetic application is given. A selected notational convention is used to jointly treat the VLBI observables of different types of baselines within a combined ground/space VLBI network. The basic equations of the space VLBI observables appropriate for convariance analysis are derived and included. The corresponding equations for the ground-to-ground baseline VLBI observables are also given for a comparison. The simplified expression of the mathematical models for both space VLBI observables (time delay and delay rate) include the ground station coordinates, the satellite orbital elements, the earth rotation parameters, the radio source coordinates, and clock parameters. The observation equations with these parameters were examined in order to determine which of them are separable or nonseparable. Singularity problems arising from coordinate system definition and critical configuration are studied. Linear dependencies between partials are analytically derived. The mathematical models for ground-space baseline VLBI observables were tested with simulation data in the frame of some numerical experiments. Singularity due to datum defect is confirmed.

IVS Working Group 4: VLBI Data Structures

NASA Technical Reports Server (NTRS)

Gipson, John

2010-01-01

In 2007 the IVS Directing Board established IVS Working Group 4 on VLBI Data Structures. This note discusses the current VLBI data format, goals for a new format, the history and formation of the Working Group, and a timeline for the development of a new VLBI data format.

First Phase Development of Korea-Japan Joint VLBI Correlator and Its Current Progress

NASA Technical Reports Server (NTRS)

Oh, Se-Jin; Roh, Duk-Gyoo; Yeom, Jae-Hwan; Kobayashi, Hideyuki; Kawaguchi, Noriyuki

2010-01-01

The first phase of the Korea-Japan Joint VLBI Correlator (KJJVC) development has been completed and installed to correlate the observed data from KVN (Korean VLBI Network) and VERA (VLBI Exploration of Radio Astrometry) in October 2009. KJJVC is able to process 16 stations, a maximum of 8 Gbps/station, and 8,192 output channels for VLBI data. The system configuration, the experimental results, and future plans are introduced in this paper.

Impact of seasonal and postglacial surface displacement on global reference frames

NASA Astrophysics Data System (ADS)

Krásná, Hana; Böhm, Johannes; King, Matt; Memin, Anthony; Shabala, Stanislav; Watson, Christopher

2014-05-01

The calculation of actual station positions requires several corrections which are partly recommended by the International Earth Rotation and Reference Systems Service (IERS) Conventions (e.g., solid Earth tides and ocean tidal loading) as well as other corrections, e.g. accounting for hydrology and atmospheric loading. To investigate the pattern of omitted non-linear seasonal motion we estimated empirical harmonic models for selected stations within a global solution of suitable Very Long Baseline Interferometry (VLBI) sessions as well as mean annual models by stacking yearly time series of station positions. To validate these models we compare them to displacement series obtained from the Gravity Recovery and Climate Experiment (GRACE) data and to hydrology corrections determined from global models. Furthermore, we assess the impact of the seasonal station motions on the celestial reference frame as well as on Earth orientation parameters derived from real and also artificial VLBI observations. In the second part of the presentation we apply vertical rates of the ICE-5G_VM2_2012 vertical land movement grid on vertical station velocities. We assess the impact of postglacial uplift on the variability in the scale given different sampling of the postglacial signal in time and hence on the uncertainty in the scale rate of the estimated terrestrial reference frame.

Global Lunar Gravity Field Recovery from SELENE

NASA Technical Reports Server (NTRS)

Matsumoto, Koji; Heki, Kosuke; Hanada, Hideo

2002-01-01

Results of numerical simulation are presented to examine the global gravity field recovery capability of the Japanese lunar exploration project SELENE (Selenological and Engineering Explorer) which will be launched in 2005. New characteristics of the SELENE lunar gravimetry include four-way satellite-to-satellite Doppler tracking of main orbiter and differential VLBI tracking of two small free-flier satellites. It is shown that planned satellites configuration will improve lunar gravity field in wide range of wavelength as well as far-side selenoid.

VLBI height corrections due to gravitational deformation of antenna structures

NASA Astrophysics Data System (ADS)

Sarti, P.; Negusini, M.; Abbondanza, C.; Petrov, L.

2009-12-01

From an analysis of regional European VLBI data we evaluate the impact of a VLBI signal path correction model developed to account for gravitational deformations of the antenna structures. The model was derived from a combination of terrestrial surveying methods applied to telescopes at Medicina and Noto in Italy. We find that the model corrections shift the derived height components of these VLBI telescopes' reference points downward by 14.5 and 12.2 mm, respectively. No other parameter estimates nor other station positions are affected. Such systematic height errors are much larger than the formal VLBI random errors and imply the possibility of significant VLBI frame scale distortions, of major concern for the International Terrestrial Reference Frame (ITRF) and its applications. This demonstrates the urgent need to investigate gravitational deformations in other VLBI telescopes and eventually correct them in routine data analysis.

International VLBI Service for Geodesy and Astrometry 2004 Annual Report

NASA Technical Reports Server (NTRS)

Behrend, Dirk (Editor); Baver, Karen D. (Editor)

2005-01-01

Contents include the following: Combination Studies using the Cont02 Campaign. Coordinating Center report. Analysis coordinator report. Network coordinator report. IVS Technology coordinator report. Algonquin Radio observatory. Fortaleza Station report for 2004. Gilmore Creek Geophysical Observatory. Goddard Geophysical and Astronomical observatory. Hartebeesthoek Radio Astronomy Observatory (HartRAO). Hbart, Mt Pleasant, station report for 2004. Kashima 34m Radio Telescope. Kashima and Koganei 11-m VLBI Stations. Kokee Park Geophysical Observatory. Matera GGS VLBI Station. The Medicina Station status report. Report of the Mizusawa 10m Telescope. Noto Station Activity. NYAL Ny-Alesund 20 metre Antenna. German Antarctic receiving Station (GARS) O'higgins. The IVS network station Onsala space Observatory. Sheshan VLBI Station report for 2004. 10 Years of Geodetic Experiments at the Simeiz VLBI Station. Svetloe RAdio Astronomical Observatory. JARE Syowa Station 11-m Antenna, Antarctica. Geodetic Observatory TIGO in Concepcion. Tsukuba 32-m VLBI Station. Nanshan VLBI Station Report. Westford Antenna. Fundamental-station Wettzell 20m Radiotelescope. Observatorio Astroonomico Nacional Yebes. Yellowknife Observatory. The Bonn Geodetic VLBI Operation Center. CORE Operation Center Report. U.S. Naval Observatory Operation Center. The Bonn Astro/Geo Mark IV Correlator.

New International Agreements About Space Techniques Among Argentina, China and France

NASA Astrophysics Data System (ADS)

Pacheco, A. M.; Podestá, R.; Actis, E.; Adarvez, S.; Quinteros, J.; Li, J.; Saunier, J.; Podestá, F.; Ramos, F.; Aguilera, J.; Sosa, G.; Hauser, D.

2018-01-01

The International Earth Rotation and Reference Systems (IERS) is in charge of defining and materializing celestial reference systems (ICRS - ICRF) and terrestrial reference systems (ITRS - ITRF). In order to perform this task it uses data from the following techniques: Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite System (GNSS) and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS). Nowadays, the Observatorio Astronómico Félix Aguilar (OAFA) has two instruments with these advanced techniques: SLR and a permanent GNSS station. In the nearby future a 40 m diameter radio telescope will be available that will be operated in VLBI mode along with a DORIS buoy which will be co-localized with a SLR telescope and GNSS antennas. In this way OAFA will become a zero station, first class, of the ITRF 2014 frame.

JPL VLBI Analysis Center Report for 2012

NASA Technical Reports Server (NTRS)

Jacobs, Chris

2013-01-01

This report describes the activities of the JPL VLBI Analysis Center for the year 2012. The highlight of the year was the successful MSL rover Mars landing, which was supported by VLBI-based navigation using our combined spacecraft, celestial reference frame, terrestrial reference frame, earth orientation, and planetary ephemeris VLBI systems. We also supported several other missions with VLBI navigation measurements. A combined NASA-ESA network was demonstrated with first Ka-band fringes to ESA's Malargue, Argentina 35 m. We achieved first fringes with our new digital back end and Mark 5C recorders.

The Comparison of VLBI Data Analysis Using Software Globl and Globk

NASA Astrophysics Data System (ADS)

Guangli, W.; Xiaoya, W.; Jinling, L.; Wenyao, Z.

The comparison of different geodetic data analysis software is one of the quite of- ten mentioned topics. In this paper we try to find out the difference between software GLOBL and GLOBK when use them to process the same set of VLBI data. GLOBL is a software developed by VLBI team, geodesy branch, GSFC/NASA to process geode- tic VLBI data using algorithm of arc-parameter-elimination, while GLOBK using al- gorithm of kalman filtering is mainly used in GPS data analysis, and it is also used in VLBI data analysis. Our work focus on whether there are significant difference when use the two softwares to analyze the same VLBI data set and investigate the reasons caused the difference.

Mark-III VLBI astrometry of pulsars

NASA Technical Reports Server (NTRS)

Jones, Dayton L.; Dewey, Rachel J.; Gwinn, Carl R.; Davis, Michael M.

1991-01-01

Differential 18-cm VLBI measurements of the position of PSR 1937 + 214 with respect to the quasar 1923 + 210 are reported. The preliminary results for VLBI minus timing position are +21 +/- 2 milliarcsec in right ascension and +14 +/- 9 milliarcsec in declination. The results show the value of combining the large collecting area of the Arecibo telescope with the Mk-III VLBI recording system.

Radio astronomy interferometer network testing for a Malaysia-China real-time e-VLBI

NASA Astrophysics Data System (ADS)

Abidin, Zamri Zainal; Hashim, Shaiful Jahari; Wei, Lim Yang; Zhong, Chen; Rosli, Zulfazli

2018-01-01

The uv-coverage of the current VLBI network between Australia northern Asia will be significantly enhanced with an existence of a middle baseline VLBI station located in Malaysia. This paper investigated the connecting route of the first half of the Asia-Oceania VLBI network i.e. from Malaysia to China. The investigation of transmission network characteristics between Malaysia and China was carried out in order to perform a real-time and reliable data transfer within the e-VLBI network for future eVLBI observations. MyREN (Malaysia) and CSTNET (China) high-speed research networks were utilized for this proposed e-VLBI connection. Preliminary network test was performed by ping, traceroute, and iperf prior to data transfer tests, which were evaluated with three types of protocols namely FTP, Tsunami-UDT and UDT. The results showed that, on average, there were eighteen hops between Malaysia and China networks with 98 ms round trip time (RTT) delay. Overall UDP protocol has a better throughput compared to TCP protocol. UDP can reach a maximum rate of 90 Mbps with 0% packet loss. In this feasibility test, the VLBI test data was successfully transferred between Malaysia and China by utilizing the three types of data transfer protocols.

Design Aspects of the VLBI2010 System - Progress Report of the IVS VLBI2010 Committee

NASA Technical Reports Server (NTRS)

Petrachenko, Bill; Niell, Arthur; Behrend, Dirk; Corey, Brian; Boehm, Johannes; Chralot, Patrick; Collioud, Arnaud; Gipson, John; Haas, Ruediger; Hobiger, Thomas;

Development of an e-VLBI Data Transport Software Suite with VDIF

NASA Technical Reports Server (NTRS)

Sekido, Mamoru; Takefuji, Kazuhiro; Kimura, Moritaka; Hobiger, Thomas; Kokado, Kensuke; Nozawa, Kentarou; Kurihara, Shinobu; Shinno, Takuya; Takahashi, Fujinobu

2010-01-01

We have developed a software library (KVTP-lib) for VLBI data transmission over the network with the VDIF (VLBI Data Interchange Format), which is the newly proposed standard VLBI data format designed for electronic data transfer over the network. The software package keeps the application layer (VDIF frame) and the transmission layer separate, so that each layer can be developed efficiently. The real-time VLBI data transmission tool sudp-send is an application tool based on the KVTP-lib library. sudp-send captures the VLBI data stream from the VSI-H interface with the K5/VSI PC-board and writes the data to file in standard Linux file format or transmits it to the network using the simple- UDP (SUDP) protocol. Another tool, sudp-recv , receives the data stream from the network and writes the data to file in a specific VLBI format (K5/VSSP, VDIF, or Mark 5B). This software system has been implemented on the Wettzell Tsukuba baseline; evaluation before operational employment is under way.

Single baseline GLONASS observations with VLBI: data processing and first results

NASA Astrophysics Data System (ADS)

Tornatore, V.; Haas, R.; Duev, D.; Pogrebenko, S.; Casey, S.; Molera Calvés, G.; Keimpema, A.

2011-07-01

Several tests to observe signals transmitted by GLONASS (GLObal NAvigation Satellite System) satellites have been performed using the geodetic VLBI (Very Long Baseline Interferometry) technique. The radio telescopes involved in these experiments were Medicina (Italy) and Onsala (Sweden), both equipped with L-band receivers. Observations at the stations were performed using the standard Mark4 VLBI data acquisition rack and Mark5A disk-based recorders. The goals of the observations were to develop and test the scheduling, signal acquisition and processing routines to verify the full tracking pipeline, foreseeing the cross-correlation of the recorded data on the baseline Onsala-Medicina. The natural radio source 3c286 was used as a calibrator before the starting of the satellite observation sessions. Delay models, including the tropospheric and ionospheric corrections, which are consistent for both far- and near-field sources are under development. Correlation of the calibrator signal has been performed using the DiFX software, while the satellite signals have been processed using the narrow band approach with the Metsaehovi software and analysed with a near-field delay model. Delay models both for the calibrator signals and the satellites signals, using the same geometrical, tropospheric and ionospheric models, are under investigation to make a correlation of the satellite signals possible.

DSN Beowulf Cluster-Based VLBI Correlator

NASA Technical Reports Server (NTRS)

Rogstad, Stephen P.; Jongeling, Andre P.; Finley, Susan G.; White, Leslie A.; Lanyi, Gabor E.; Clark, John E.; Goodhart, Charles E.

2009-01-01

The NASA Deep Space Network (DSN) requires a broadband VLBI (very long baseline interferometry) correlator to process data routinely taken as part of the VLBI source Catalogue Maintenance and Enhancement task (CAT M&E) and the Time and Earth Motion Precision Observations task (TEMPO). The data provided by these measurements are a crucial ingredient in the formation of precision deep-space navigation models. In addition, a VLBI correlator is needed to provide support for other VLBI related activities for both internal and external customers. The JPL VLBI Correlator (JVC) was designed, developed, and delivered to the DSN as a successor to the legacy Block II Correlator. The JVC is a full-capability VLBI correlator that uses software processes running on multiple computers to cross-correlate two-antenna broadband noise data. Components of this new system (see Figure 1) consist of Linux PCs integrated into a Beowulf Cluster, an existing Mark5 data storage system, a RAID array, an existing software correlator package (SoftC) originally developed for Delta DOR Navigation processing, and various custom- developed software processes and scripts. Parallel processing on the JVC is achieved by assigning slave nodes of the Beowulf cluster to process separate scans in parallel until all scans have been processed. Due to the single stream sequential playback of the Mark5 data, some ramp-up time is required before all nodes can have access to required scan data. Core functions of each processing step are accomplished using optimized C programs. The coordination and execution of these programs across the cluster is accomplished using Pearl scripts, PostgreSQL commands, and a handful of miscellaneous system utilities. Mark5 data modules are loaded on Mark5 Data systems playback units, one per station. Data processing is started when the operator scans the Mark5 systems and runs a script that reads various configuration files and then creates an experiment-dependent status database used to delegate parallel tasks between nodes and storage areas (see Figure 2). This script forks into three processes: extract, translate, and correlate. Each of these processes iterates on available scan data and updates the status database as the work for each scan is completed. The extract process coordinates and monitors the transfer of data from each of the Mark5s to the Beowulf RAID storage systems. The translate process monitors and executes the data conversion processes on available scan files, and writes the translated files to the slave nodes. The correlate process monitors the execution of SoftC correlation processes on the slave nodes for scans that have completed translation. A comparison of the JVC and the legacy Block II correlator outputs reveals they are well within a formal error, and that the data are comparable with respect to their use in flight navigation. The processing speed of the JVC is improved over the Block II correlator by a factor of 4, largely due to the elimination of the reel-to-reel tape drives used in the Block II correlator.

New Uses for the VLBI Network

NASA Technical Reports Server (NTRS)

Clark, Tom

2000-01-01

This paper suggests some potential new uses for the existing VLBI network. It seems that every VLBI group in the world faces some common problems: We do not have enough money to operate! We do not have enough money to make improvements!! In this contribution I discuss several possibilities for new business that might help to support the network stations without causing serious impacts on the primary VLBI programs.

Current Limitations on VLBI Accuracy

NASA Technical Reports Server (NTRS)

Ma, Chopo; Gipson, John; MacMillan, Daniel

1998-01-01

The contribution of VLBI to geophysics and geodesy arises from its ability to measure distances between stations in a network and to determine the orientation of stations in the network as well as the orientation of the network with respect to the external reference frame of extragalactic radio objects. Integrating nearly two decades of observations provides useful information about station positions and velocities and the orientation of the Earth, but the complications of the real world and the limitations of observing, modeling and analysis prevent recovery of all effects. Of the factors that limit the accuracy of seemingly straightforward geodetic parameters, the neutral propagation medium has been subject to the greatest scrutiny, but the treatment of the mapping function, the wet component and spatial/temporal inhomogeneities is still improving. These affect both the terrestrial scale and consistency over time. The modeling of non-secular site motions (tides and loading) has increased in sophistication, but there are some differences between the models and the observations. VLBI antennas are massive objects, so their behavior is quite unlike GPS monuments, but antenna deformations add some (generally) unmodeled signal. Radio sources used in geodetic VLBI observations are selected for strength and (relative) absence of structure, but apparent changes in position can leak into geodetic parameters. A linear rate of change of baseline or site parameters is the simplest model and its error improves with time span. However, in most cases the VLBI data distribution is insufficient to look for real non-linear behavior that might affect the average rate. A few sites have multiple VLBI antennas, and some show small differences in rate. VLBI intrinsically measures relative positions and velocities, but individual site positions and velocities are generally more useful. The creation of the VLBI terrestrial reference frame, which transforms relative information into individual results, is an empirical process that has intrinsic errors. While UT1 is uniquely measured by VLBI, the geographical distribution and availability of VLBI stations, especially in the southern hemisphere, and the consistency of the VLBI terrestrial reference frame may limit the accuracy of Earth orientation measurements. The effects of particular error sources on geodetic and geophysical parameters derived from VLBI data will be illustrated.

Binary black holes in nuclei of extragalactic radio sources

NASA Astrophysics Data System (ADS)

Roland, J.; Britzen, S.

If we assume that nuclei of extragalactic radio sources contain a Binary Black Hole system, the 2 black holes can eject VLBI components and in that case 2 families of different VLBI trajectories will be observed. An important consequence of the presence of a Binary Black Hole system is the following: the VLBI core is associated with one black hole and if a VLBI component is ejected by the second black hole, one expects to be able to detect the offset of the origin of the VLBI component ejected by the black hole not associated with the VLBI core. The ejection of VLBI components is perturbed by the precession of the accretion disk and the motion of the black holes around the gravity center of the BBH system. We modeled the ejection of the component taking into account the 2 perturbations and we obtained a method to fit the coordinates of a VLBI component and to deduce the characteristics of the BBH system, i.e. the ratio Tp/Tb where Tp is the precession period of the accretion disk and Tb the orbital period of the BBH system, the mass ratio M1/M2, the radius of the BBH system Rbin. We applied the method to component S1 of 1823+568 and to component C5 of 3C 279 which presents a large offset of the space origin from the VLBI core. We found that 1823+568 contains a BBH system which size is Rbin ≈ 60 mu as and 3C 279 contains a BBH system which size is Rbin ≈ 378 mu as. We were able to deduce the separation of the 2 black holes and the coordinates of the second black hole from the VLBI core, this information will be important to make the link between the radio reference frame system deduced from VLBI observations and the optical reference frame system deduced from GAIA.

Current Status of the Development of a Transportable and Compact VLBI System by NICT and GSI

NASA Technical Reports Server (NTRS)

Ishii, Atsutoshi; Ichikawa, Ryuichi; Takiguchi, Hiroshi; Takefuji, Kazuhiro; Ujihara, Hideki; Koyama, Yasuhiro; Kondo, Tetsuro; Kurihara, Shinobu; Miura, Yuji; Matsuzaka, Shigeru;

The Most Remote Point Method for the Site Selection of the Future VGOS Network

NASA Astrophysics Data System (ADS)

Hase, Hayo; Pedreros, Felipe

2014-12-01

The VLBI Global Observing System (VGOS) will be part of the Global Geodetic Observing System (GGOS) and will consist of globally well distributed geodetic observatories. The most remote point (MRP) method is used to identify gaps in the network geometry. In each iteration step the identified most remote points are assumed to become new observatory sites improving the homogeneity of the global network. New locations for VGOS observatories have been found in La Plata, Tahiti, O'Higgins, Galapagos, Colombo, and Syowa. This contribution is an excerpt of a work published in Journal of Geodesy (DOI: 10.1007/s00190-014-0731-y) covering the site selection for the GGOS.%

The State and Development Direction of the Geodetic VLBI Station in Korea

NASA Technical Reports Server (NTRS)

Ju, Hyunhee; Kim, Myungho; Kim, Suchul; Park, Jinsik; Kondo, Tetsuro; Kim, Tuhwan; Oh, Hongjong; Yi, Sangoh

2010-01-01

A permanent geodetic VLBI station with a 22-m diameter antenna will be newly constructed in Korea by the National Geographic Information Institute (NGII) under the project Korea VLBI system for Geodesy (KVG) that aims at maintaining the Korean geodetic datum accurately on the International Terrestrial Reference Frame (ITRF). KVG can receive 2, 8, 22, and 43 GHz bands simultaneously in order to conduct geodetic and astronomical VLBI observations with Korea astronomical VLBI stations along with geodetic observations with IVS stations. This simultaneous four-band receiving capability is a unique feature of the KVG system. The KVG has started officially in October 2008. A new geodetic VLBI station will be constructed at Sejong city (about 120 km south of Seoul and about 20 km north-northwest of Daejeon) and construction of all systems will be completed in 2011.

Launch Will Create a Radio Telescope Larger than Earth

NASA Astrophysics Data System (ADS)

NASA and the National Radio Astronomy Observatory are joining with an international consortium of space agencies to support the launch of a Japanese satellite next week that will create the largest astronomical "instrument" ever built -- a radio telescope more than two-and-a-half times the diameter of the Earth that will give astronomers their sharpest view yet of the universe. The launch of the Very Long Baseline Interferometry (VLBI) Space Observatory Program (VSOP) satellite by Japan's Institute of Space and Astronautical Science (ISAS) is scheduled for Feb. 10 at 11:50 p.m. EST (1:50 p.m. Feb. 11, Japan time.) The satellite is part of an international collaboration led by ISAS and backed by Japan's National Astronomical Observatory; NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA; the National Science Foundation's National Radio Astronomy Observatory (NRAO), Socorro, NM; the Canadian Space Agency; the Australia Telescope National Facility; the European VLBI Network and the Joint Institute for Very Long Baseline Interferometry in Europe. Very long baseline interferometry is a technique used by radio astronomers to electronically link widely separated radio telescopes together so they work as if they were a single instrument with extraordinarily sharp "vision," or resolving power. The wider the distance between telescopes, the greater the resolving power. By taking this technique into space for the first time, astronomers will approximately triple the resolving power previously available with only ground-based telescopes. The satellite system will have resolving power almost 1,000 times greater than the Hubble Space Telescope at optical wavelengths. The satellite's resolving power is equivalent to being able to see a grain of rice in Tokyo from Los Angeles. "Using space VLBI, we can probe the cores of quasars and active galaxies, believed to be powered by super massive black holes," said Dr. Robert Preston, project scientist for the U.S. Space Very Long Baseline Interferometry project at JPL. "Observations of cosmic masers -- naturally-occurring microwave radio amplifiers -- will tell us new things about the process of star formation and activity in the heart of other galaxies." "By the 1980s, radio astronomers were observing the universe with assemblages of radio telescopes whose resolving power was limited only by the size of the Earth. Now, through a magnificent international effort, we will be able to break this barrier and see fine details of celestial objects that are beyond the reach of a purely ground-based telescope array. We anticipate a rich harvest of new scientific knowledge from VSOP," said Dr. Paul Vanden Bout, Director of NRAO. In the first weeks after launch, scientists and engineers will "test the deployment of the reflecting mesh telescope in orbit, the wide-band data link from the satellite to the ground, the performance of the low noise amplifiers in orbit, and the high-precision orbit determination and attitude control necessary for VLBI observations with an orbiting telescope," according to Dr. Joel Smith, manager of the U.S. Space VLBI project at JPL. Scientific observations are expected to begin in May. The 26-foot diameter orbiting radio telescope will observe celestial radio sources in concert with a number of the world's ground-based radio telescopes. The 1,830-pound satellite will be launched from ISAS' Kagoshima Space Center, at the southern tip of Kyushu, one of Japan's main islands, and will be the first launch with ISAS' new M-5 series rocket. The satellite will go into an elliptical orbit, varying between 620 to 12,400 miles above the Earth's surface. This orbit provides a wide range of distances between the satellite and ground-based telescopes, which is important for producing a high-quality image of the radio source being observed. One orbit of the Earth will take about six hours. The satellite's observations will concentrate on some of the most distant and intriguing objects in the universe, where the extremely sharp radio "vision" of the new system can provide much-needed information about a number of astronomical mysteries. For years, astronomers have known that powerful "engines" in the hearts of quasars and many galaxies are pouring out tremendous amounts of energy. They suspect that supermassive black holes, with gravitational fields so strong that not even light can escape them, lie in the centers of these "engines." The mechanism at work in the centers of quasars and active galaxies, however, remains a mystery. Ground-based radio telescopes, notably NRAO's Very Long Baseline Array (VLBA), have revealed fascinating new details in recent years, and VSOP is expected to add a wealth of new information on these objects, millions or billions of light-years distant from Earth. Many of these same objects act as super-powerful particle accelerators to eject "jets" of subatomic particles at nearly the speed of light. Scientists plan to use VSOP to monitor the changes and motions in these jets to learn more about how they originate and interact with their surroundings. The satellite also will aim at regions in the sky where giant collections of water and other molecules act as natural amplifiers of radio emission much as lasers amplify light. These regions, called cosmic masers, are found in areas where new stars are forming and near the centers of galaxies. Observations can provide the detail needed to measure motions of individual maser "spots" within these regions, and provide exciting new information about the star-forming regions and the galaxies where the masers reside. In addition, high-resolution studies of cosmic masers can allow astronomers to calculate distances to them with unprecedented accuracy, and thus help resolve continuing questions about the size and age of the universe. The project is a major international undertaking, with about 40 radio telescopes from more than 15 countries having committed time to co-observe with the satellite. This includes the National Science Foundation's Very Long Baseline Array (VLBA), an array of 10 telescopes spanning the United States from Hawaii to Saint Croix; NASA's Deep Space Network (DSN) sites in California, Spain, and Australia; the European VLBI Network, more than a dozen telescopes ranging from the United Kingdom to China; a Southern Hemisphere array of telescopes stretching from eastern Australia to South Africa; and Japan's network of domestic radio telescopes. In the United States, NASA is funding critical roles in the VSOP mission at both JPL and NRAO. JPL has built an array of three new tracking stations at its DSN sites in Goldstone, CA; Madrid, Spain; and near Canberra, Australia. A large existing tracking station at each of these sites has also been converted to an extremely sensitive radio telescope for simultaneous observations with the satellite. JPL also is providing precision orbit determination, scientific and operational planning support to the Japanese, and advice to U.S. astronomers who wish to observe with the satellite. NRAO is building a new tracking station at Green Bank, WV; contributing observing time on the VLBA array of telescopes; modifying existing data analysis hardware and software, and aiding astronomers with the analysis of the VSOP data. Much of the observational data will be processed at NRAO's facility in Socorro, NM, using the VLBA Correlator, a special purpose high-performance computer designed to process VLBI data. VSOP is the culmination of many years of planning and work by scientists and engineers around the world. Tests using NASA's Tracking and Data Relay Satellite System (TDRSS) proved the feasibility of space VLBI in 1986. Just last year, those old data were used again to test successfully the data-reduction facilities for VSOP. JPL manages the U.S. Space Very Long Baseline Interferometry project for NASA's Office of Space Science, Washington, DC. The VLBA, headquartered in Socorro, NM, is part of the National Radio Astronomy Observatory, a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Tsukuba 32-m VLBI Station

NASA Technical Reports Server (NTRS)

Kawabata, Ryoji; Kurihara, Shinobu; Fukuzaki, Yoshihiro; Kuroda, Jiro; Tanabe, Tadashi; Mukai, Yasuko; Nishikawa, Takashi

2013-01-01

The Tsukuba 32-m VLBI station is operated by the Geospatial Information Authority of Japan. This report summarizes activities of the Tsukuba 32-m VLBI station in 2012. More than 200 sessions were observed with the Tsukuba 32-m and other GSI antennas in accordance with the IVS Master Schedule of 2012. We have started installing the observing facilities that will be fully compliant with VLBI2010 for the first time in Japan.

Comparison of VLBI TRF solutions based on Kalman filtering and recent ITRS realizations

NASA Astrophysics Data System (ADS)

Soja, Benedikt; Nilsson, Tobias; Glaser, Susanne; Balidakis, Kyriakos; Karbon, Maria; Heinkelmann, Robert; Gross, Richard; Schuh, Harald

2016-04-01

Compared to previous prominent global terrestrial reference frames (TRF) solutions, such as the ITRF2008 or DTRF2008, the current accuracy requirements demand among other things extended parameterization to account for various non-linear signals present in the time series of station coordinates. The next generation of TRFs, built upon geodetic data until the end of 2014, employs different approaches to tackle in particular seasonal variations and post-seismic deformations. The ITRF2014, developed at the International Earth Rotation and Reference Systems Service (IERS) Combination Center (CC) at Institut Géographique National, introduces harmonic, exponential and logarithmic functions to take into account aforementioned effects. In contrast, the ITRS realization of the IERS CC at Jet Propulsion Laboratory is based on Kalman filtering, which allows coordinate variations to be modeled in a stochastic sense besides the parameterized linear and seasonal signals. In our study, we compare these multi-technique TRFs with solutions solely based on VLBI data, including 104 radio telescopes and 4239 VLBI sessions, covering a time span of 34 years. We calculated a VLBI TRF based on the traditional least-squares adjustment of session-wise normal equations, and an ensemble of Kalman filter and smoother solutions with different parameterizations and stochastic models. In particular, we investigate the impact of different process noise levels for station coordinates, the choice of stochastic processes, e.g. random walks, and the application of time- and station-dependent noise models. For instance, we find that the estimation of seasonal signals, while important for predictions, does not affect the filtered coordinate time series when observational data is available. Furthermore, post-seismic deformations after major earthquakes require the process noise to be scaled accordingly. For instance, we detected coordinate differences of up to 5 cm immediately after the Chile 2010 earthquake when changing the process noise by a factor of 10. Finally, we investigated velocity differences and found the RMS of the differences between the VLBI solutions reaching 0.3 mm/yr for stations with good observational history.

VLBI2010: An Overview

NASA Technical Reports Server (NTRS)

Petrachenko, Bill

2010-01-01

The first concrete actions toward a next generation system for geodetic VLBI began in 2003 when the IVS initiated Working Group 3 to investigate requirements for a new system. The working group set out ambitious performance goals and sketched out initial recommendations for the system. Starting in 2006, developments continued under the leadership of the VLBI2010 Committee (V2C) in two main areas: Monte Carlo simulators were developed to evaluate proposed system changes according to their impact on IVS final products, and a proof-of-concept effort sponsored by NASA was initiated to develop next generation systems and verify the concepts behind VLBI2010. In 2009, the V2C produced a progress report that summarized the conclusions of the Monte Carlo work and outlined recommendations for the next generation system in terms of systems, analysis, operations, and network configuration. At the time of writing: two complete VLBI2010 signal paths have been completed and data is being produced; a number of VLBI2010 antenna projects are under way; and a VLBI2010 Project Executive Group (V2PEG) has been initiated to provide strategic leadership.

Establishing a celestial VLBI reference frame. 1: Searching for VLBI sources

NASA Technical Reports Server (NTRS)

Preston, R. A.; Morabito, D. D.; Williams, J. G.; Slade, M. A.; Harris, A. W.; Finley, S. G.; Skjerve, L. J.; Tanida, L.; Spitzmesser, D. J.; Johnson, B.

1978-01-01

The Deep Space Network is currently engaged in establishing a new high-accuracy VLBI celestial reference frame. The present status of the task of finding suitable celestial radio sources for constructing this reference frame is discussed. To date, 564 VLBI sources were detected, with 166 of these lying within 10 deg of the ecliptic plane. The variation of the sky distribution of these sources with source strength is examined.

Comparison of VLBI, TV and traveling clock techniques for time transfer

NASA Technical Reports Server (NTRS)

Spencer, J. H.; Waltman, E. B.; Johnston, K. J.; Santini, N. J.; Klepczynski, W. J.; Matsakis, D. N.; Angerhofer, P. E.; Kaplan, G. M.

1982-01-01

A three part experiment was conducted to develop and compare time transfer techniques. The experiment consisted of (1) a very long baseline interferometer (VLBI), (2) a high precision portable clock time transfer system between the two sites, and (3) a television time transfer. A comparison of the VLBI and traveling clock shows each technique can perform satisfactorily at the five nsec level. There was a systematic offset of 59 nsec between the two methods, which we attributed to a difference in epochs between VLBI formatter and station clock. The VLBI method had an internal random error of one nsec at the three sigma level for a two day period. Thus, the Mark II system performed well, and VLBI shows promise of being an accurate method of time transfer. The TV system, which had technical problems during the experiment, transferred time with a random error of about 50 nsec.

GSFC VLBI Analysis center

NASA Technical Reports Server (NTRS)

Gordon, David; Ma, Chopo; MacMillan, Dan; Petrov, Leonid; Baver, Karen

2005-01-01

This report presents the activities of the GSFC VLBI Analysis Center during 2004. The GSFC Analysis Center analyzes all IVS sessions, makes regular IVS submissions of data and analysis products, and performs research and software development activities aimed at improving the VLBI technique.

Receiving And Data Acquisition Systems Of Rt-32 For Vlbi Observations / Rt-32 Uztveršanas Un Datu Reģistrācijas Sistēmas Vlbi Novērojumiem

NASA Astrophysics Data System (ADS)

Bezrukovs, Vl.; Shmeld, I.; Nechaeva, M.; Trokss, J.; Bezrukovs, D.; Klapers, M.; Berzins, A.; Lesins, A.; Dugin, N.

2012-12-01

Radiotelescope RT-32 is a fully steerable 32-m parabolic antenna located at Irbene and belonging to Ventspils International Radio Astronomy Centre (VIRAC). Currently, the work on upgrading and repair of its receiving hardware and data acquisition systems is of high priority for the VIRAC. One of the main scientific objectives for the VIRAC Radioastronomical observatory is VLBI (very long baseline interferometry) observations in centimetre wavelengths in collaboration with world VLBI networks, such as European VLBI network (EVN), Low Frequency VLBI network (LFVN), and others. During the last years the room in the secondary focus of telescope was reconstructed, and several new receivers were installed. Currently, RT-32 observations are carried out in four different bands: 92 cm, 18 cm, 6 cm, and 2.5 cm. First three of them are already successfully employed in diversified VLBI experiments. The receiver on 2.5 cm band has only one linear polarized chain and is used mainly for the methanol maser single dish observations. The apparatus system of RT-32 is equipped with two independent VLBI data acquisition systems: TN-16, and DBBC in combination with MK5b. Both systems are employed in interferometric observations depending on the purpose of experiment and the enabled radiotelescopes. The current status of RT-32, the availability of its receiving and data acquisition units for VLBI observations and the previous VLBI sessions are discussed. Radioteleskops RT-32 ir Ventspils Starptautiskajam Radioastronomijas Centram (VSRC) piederoša pilnas piedziņas 32 m diametra paraboliskā antena. Pašreiz visaktuālākie VSRC veicamie darbi ir saistīti ar RT-32 uztverošās aparatūras un datu reģistrēšanas sistēmas labošanu un modernizāciju. Viens no radioastronomiskās observatorijas galvenajiem zinātniskajiem uzdevumiem ir sevišķi lielas bāzes interferometriskie (VLBI) novērojumi centimetru viļņu garumu diapazonā sadarbībā ar pasaules VLBI tīkla partneriem, tādiem kā Eiropas VLBI tīkls, Zemo frekvenču VLBI tīkls (LFVN) un citiem. Pēdējos gados rekonstruēta teleskopa sekundārajā fokusā izvietotā uztvērēju telpa un tajā uzstādīti vairāki jauni uztvērēji. Pašreiz radioteleskops ļauj veikt novērojumus četros viļņu garumu diapazonos: 92 cm, 18 cm, 6 cm un 2.5 cm. No minētajiem pirmie 3 jau tiek veiksmīgi izmantoti dažādos VLBI eksperimentos. 2.5 cm uztvērējam ir tikai viens lineārās polarizācijas kanāls, kuru izmanto galvenokārt metanola māzeru novērojumiem viena teleskopa režīmā. RT-32 aparatūru veido divas neatkarīgas VLBI datu reģistrācijas sistēmas: TN-16 un DBBC kopā ar Mark5b. Abas sistēmas izmanto interferometriskajos novērojumos atkarībā no eksperimentu mērķa un radioteleskopa iespējām. Aplūkots Irbenes RT-32 radioteleskopa pašreizējais statuss, tā VLBI novērojumiem piemērotās uztveršanas un datu reģistrācijas iekārtas, kā arī notikušajās VLBI sesijās uzkrātā pieredze.

GSFC VLBI Analysis Center Annual Report

NASA Technical Reports Server (NTRS)

Gordon, David; Ma, Chopo; MacMillan, Dan

1999-01-01

The GSFC VLBI group, located at NASA's Goddard Space Flight Center in Greenbelt, MD, is a part of the NASA Space Geodesy Program. Since its inception in the mid 1970's, this group has been involved with and been a leader in most aspects of geodetic and astrometric VLBI. Current major activities include coordination of the international geodetic observing program; coordination and analysis of the CORE program; VLBI technique development; and all types of data processing, analysis, and research activities.

Use of Very Long Baseline Array Interferometric Data for Spacecraft Navigation

NASA Technical Reports Server (NTRS)

Martin-Mur, Tomas J.; Antreasian, P.; Border, J.; Benson, J.; Dhawan, V.; Fomalont, E.; Graat, E.; Jacobson, R.; Lanyi, G.; McElrath, T.;

Height bias and scale effect induced by antenna gravitational deformations in geodetic VLBI data analysis

NASA Astrophysics Data System (ADS)

Sarti, Pierguido; Abbondanza, Claudio; Petrov, Leonid; Negusini, Monia

2011-01-01

The impact of signal path variations (SPVs) caused by antenna gravitational deformations on geodetic very long baseline interferometry (VLBI) results is evaluated for the first time. Elevation-dependent models of SPV for Medicina and Noto (Italy) telescopes were derived from a combination of terrestrial surveying methods to account for gravitational deformations. After applying these models in geodetic VLBI data analysis, estimates of the antenna reference point positions are shifted upward by 8.9 and 6.7 mm, respectively. The impact on other parameters is negligible. To simulate the impact of antenna gravitational deformations on the entire VLBI network, lacking measurements for other telescopes, we rescaled the SPV models of Medicina and Noto for other antennas according to their size. The effects of the simulations are changes in VLBI heights in the range [-3, 73] mm and a net scale increase of 0.3-0.8 ppb. The height bias is larger than random errors of VLBI position estimates, implying the possibility of significant scale distortions related to antenna gravitational deformations. This demonstrates the need to precisely measure gravitational deformations of other VLBI telescopes, to derive their precise SPV models and to apply them in routine geodetic data analysis.

Improved UT1 Predictions through Low-Latency VLBI Observations

DTIC Science & Technology

2010-03-14

J Geod (2010) 84:399–402 DOI 10.1007/s00190-010-0372-8 SHORT NOTE Improved UT1 predictions through low-latency VLBI observations Brian Luzum · Axel...polar motion and nutation on UT1 determinations from VLBI Intensive obser- vations. J Geod 82(12):863. doi:10.1007/s00190-008-0212-2 Ray JR, Carter WE...Behrend D (2007) The International VLBI Service for Geodesy and Astrometry (IVS): current capabilities and future prospects. J Geod 81(6–8):479. doi

Haystack Observatory VLBI Correlator

NASA Technical Reports Server (NTRS)

Titus, Mike; Cappallo, Roger; Corey, Brian; Dudevoir, Kevin; Niell, Arthur; Whitney, Alan

2013-01-01

This report summarizes the activities of the Haystack Correlator during 2012. Highlights include finding a solution to the DiFX InfiniBand timeout problem and other DiFX software development, conducting a DBE comparison test following the First International VLBI Technology Workshop, conducting a Mark IV and DiFX correlator comparison, more broadband delay experiments, more u- VLBI Galactic Center observations, and conversion of RDV session processing to the Mark IV/HOPS path. Non-real-time e-VLBI transfers and engineering support of other correlators continued.

Astrometria diferencial de precision con VLBI el triangulo de Draco (y estudios de SN1993J)

NASA Astrophysics Data System (ADS)

Ros, E.

1997-11-01

The Very Long Baseline Interferometry (VLBI) technique provides unprecedented resolutions in astronomy. In this PhD we show progress in the study of high precision phase-delay differential astrometry through observations of the radio source triangle formed by the BL-Lac objects 1803+784 and 2007+777, and the QSO 1928+738, in the Northern constellation of Draco (the Dragon), from observations carried out on 20/21 November 1991 with an intercontinental interferometric array simultaneously at the frequencies of 2.3 and 8.4 GHz. We have determined the angular separations among the three radio sources with submilliarcsecond accuracy from a weighted least squares analysis of the differential phase delay from the three celestial bodies. Our present work introduces important advances with respect to previous astrometric studies, carried out over radio source pairs separated by smaller angular distances. We have consistently modeled the parameters involved in an astrometric VLBI observation, in order to reproduce the differential phase observed for radio sources separated by almost 7o on the sky. We have demonstrated the possibility of phase-connection over these angular distances at 8.4 GHz, even at an epoch of a maximum in the solar activity. After the phase-connection we have corrected the effects of the extended structure of the radio source and of the ionosphere. This last correction is one of the main technical achievements of this thesis: it is possible to remove the ionospheric contribution with independent measurements of the ionosphere total electron content obtained at Global Positioning Systems (GPS) sites the VLBI observing stations. The triangular geometry introduces constraints in parameter space that allow a better estimation of the angular separations among the radio sources. It is possible to test the consistency of the astrometric results through the Sky-Closure, defined as the circular sum of the angular separations of the three radio sources, determined pairwise and independently. In our case it is consistent with zero, and verifies satisfactorily the data process followed. The comparison of the measurements of the separations of the pair 1928+738/2007+777 (1991 data) with previous measurements (data from 1985 and 1988), carried out with the same technique, allows us to register adequately the absolute position of 1928+738 relative to 2007+777. We estimate the proper motion of components in 1928+738, and also identify the position of the radio source core. We confirm the superluminal motion of the components of 1928+738. The comparison of our results with those obtained by Eubanks (USNO) from group delay measurements (without structure correction) show the incorrectness of the latter. We also include succinctly in this PhD my colaboration in the work on the radio supernova SN 1993J, in galaxy M81. We have discovered a shell-like structure of the radio emission of SN 1993J which exploded on March 1993. We have also elaborated a movie of its evolution, by monitoring the shell structure for different epochs, and determined the deceleration of its expansion.

Second Epoch VLBA Calibrator Survey Observations: VCS-II

NASA Astrophysics Data System (ADS)

Gordon, David; Jacobs, Christopher; Beasley, Anthony; Peck, Alison; Gaume, Ralph; Charlot, Patrick; Fey, Alan; Ma, Chopo; Titov, Oleg; Boboltz, David

2016-06-01

Six very successful Very Long Baseline Array (VLBA) calibrator survey campaigns were run between 1994 and 2007 to build up a large list of compact radio sources with positions precise enough for use as very long baseline interferometry (VLBI) phase reference calibrators. We report on the results of a second epoch VLBA Calibrator Survey campaign (VCS-II) in which 2400 VCS sources were re-observed in the X and S bands in order to improve the upcoming third realization of the International Celestial Reference Frame (ICRF3) as well as to improve their usefulness as VLBI phase reference calibrators. In this survey, some 2062 previously detected sources and 324 previously undetected sources were detected and revised positions are presented. Average position uncertainties for the re-observed sources were reduced from 1.14 and 1.98 mas to 0.24 and 0.41 mas in R.A. and decl., respectively, or by nearly a factor of 5. Minimum detected flux values were approximately 15 and 28 mJy in the X and S bands, respectively, and median total fluxes are approximately 230 and 280 mJy. The vast majority of these sources are flat-spectrum sources, with ˜82% having spectral indices greater than -0.5.

Basic research for the geodynamics program

NASA Technical Reports Server (NTRS)

1991-01-01

The mathematical models of space very long base interferometry (VLBI) observables suitable for least squares covariance analysis were derived and estimatability problems inherent in the space VLBI system were explored, including a detailed rank defect analysis and sensitivity analysis. An important aim is to carry out a comparative analysis of the mathematical models of the ground-based VLBI and space VLBI observables in order to describe the background in detail. Computer programs were developed in order to check the relations, assess errors, and analyze sensitivity. In order to investigate the estimatability of different geodetic and geodynamic parameters from the space VLBI observables, the mathematical models for time delay and time delay rate observables of space VLBI were analytically derived along with the partial derivatives with respect to the parameters. Rank defect analysis was carried out both by analytical and numerical testing of linear dependencies between the columns of the normal matrix thus formed. Definite conclusions were formed about the rank defects in the system.

A experiment on radio location of objects in the near-Earth space with VLBI in 2012

NASA Astrophysics Data System (ADS)

Nechaeva, M.; Antipenko, A.; Bezrukovs, V.; Bezrukov, D.; Dementjev, A.; Dugin, N.; Konovalenko, A.; Kulishenko, V.; Liu, X.; Nabatov, A.; Nesteruk, V.; Pupillo, G.; Reznichenko, A.; Salerno, E.; Shmeld, I.; Shulga, O.; Sybiryakova, Y.; Tikhomirov, Yu.; Tkachenko, A.; Volvach, A.; Yang, W.-J.

An experiment on radar location of space debris objects using of the method of VLBI was carried out in April, 2012. The radar VLBI experiment consisted in irradiation of some space debris objects (4 rocket stages and 5 inactive satellites) with a signal of the transmitter with RT-70 in Evpatoria, Ukraine. Reflected signals were received by a complex of radio telescopes in the VLBI mode. The following VLBI stations took part in the observations: Ventspils (RT-32), Urumqi (RT-25), Medicina (RT-32) and Simeiz (RT-22). The experiment included measurements of the Doppler frequency shift and the delay for orbit refining, and measurements of the rotation period and sizes of objects by the amplitudes of output interferometer signals. The cross-correlation of VLBI-data is performed at a correlator NIRFI-4 of Radiophysical Research Institute (Nizhny Novgorod). Preliminary data processing resulted in the series of Doppler frequency shifts, which comprised the information on radial velocities of the objects. Some results of the experiment are presented.

Verification of the astrometric performance of the Korean VLBI network, using comparative SFPR studies with the VLBA AT 14/7 mm

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rioja, María J.; Dodson, Richard; Jung, TaeHyun

The Korean VLBI Network (KVN) is a new millimeter VLBI dedicated array with the capability to simultaneously observe at multiple frequencies, up to 129 GHz. The innovative multi-channel receivers present significant benefits for astrometric measurements in the frequency domain. The aim of this work is to verify the astrometric performance of the KVN using a comparative study with the VLBA, a well-established instrument. For that purpose, we carried out nearly contemporaneous observations with the KVN and the VLBA, at 14/7 mm, in 2013 April. The KVN observations consisted of simultaneous dual frequency observations, while the VLBA used fast frequency switchingmore » observations. We used the Source Frequency Phase Referencing technique for the observational and analysis strategy. We find that having simultaneous observations results in superior compensation for all atmospheric terms in the observables, in addition to offering other significant benefits for astrometric analysis. We have compared the KVN astrometry measurements to those from the VLBA. We find that the structure blending effects introduce dominant systematic astrometric shifts, and these need to be taken into account. We have tested multiple analytical routes to characterize the impact of the low-resolution effects for extended sources in the astrometric measurements. The results from the analysis of the KVN and full VLBA data sets agree within 2σ of the thermal error estimate. We interpret the discrepancy as arising from the different resolutions. We find that the KVN provides astrometric results with excellent agreement, within 1σ, when compared to a VLBA configuration that has a similar resolution. Therefore, this comparative study verifies the astrometric performance of the KVN using SFPR at 14/7 mm, and validates the KVN as an astrometric instrument.« less

VSOP Monitoring of the Quasar 1928+738

NASA Technical Reports Server (NTRS)

Murphy, D. W.

1999-01-01

One limitation of the VSOP (VLBI Space Observatory Program) mission is that several famous superluminal sources such as 3C273 cannot be monitored with good uv-coverage throughout the lifetime of the VSOP Mission at regular intervals that are spaced closely enough to follow the evolution in the fine-scale source-structure. The reason for this is that the HALCA spacecraft cannot#observe sources outside certain restricted ranges of sun angle, defined to be the time variable angle between the source and the sun. However sources that lie within 10 degrees of the ecliptic poles can be observed throughout the year and observations are not restricted to narrow temporal windows. Furthermore, the best ground-based uv-coverages are obtained for circumpolar sources and consequently these will be the sources for which the maximum amount of space VLBI data will be obtained with a given ground array. We have began a VSOP monitoring campaign at 5 GHz on the relatively low redshift (z=0.3) superluminal quasar 1928+738 which is both a circumpolar source and lies 10 degrees away from the ecliptic pole. 1928+738 is in the S5 polar cap sample and has been well studied both on the arsecond-scale and mas-scale. 22 GHz observations have shown that the motion of the VLBI components in 1928+738 is inconsistent with simple linear expansion along a fixed position angle (PA) for all components. Indeed, 1928+738 was one of the first sources for which helical jet motion was proposed and it has been further proposed that a massive binary black hole (MBBH) system is responsible for the sinusoidal jet ridge line observed at 22 GHz over a S year period. our VSOP observations are designed to check this proposal.

Is 4C+29.48 a γ-ray source?

NASA Astrophysics Data System (ADS)

Gabányi, K. É.; Frey, S.; An, T.

2018-05-01

Context. The Fermi Large Area Telescope revealed that the extragalactic γ-ray sky is dominated by blazars, active galactic nuclei (AGN) whose jet is seen at very small angle to the line of sight. To associate and then classify the γ-ray sources, data have been collected from lower frequency surveys and observations. Since those have superior angular resolution and positional accuracy compared to the γ-ray observations, some associations are not straightforward. Aims: The γ-ray source 3FGL J1323.0+2942 is associated with the radio source 4C+29.48 and classified as a blazar of unknown type, lacking optical spectrum and redshift. The higher-resolution radio data showed that 4C+29.48 comprises three bright radio-emitting features located within a 1'-diameter area. We aim to reveal their nature and pinpoint the origin of the γ-ray emission. Methods: We (re-)analyzed archival Very Large Array (VLA) and unpublished very long baseline interferometry (VLBI) observations conducted by the Very Long Baseline Array (VLBA) and the European VLBI Network of 4C+29.48. We also collected data form optical, infrared and X-ray surveys. Results: According to the VLBI data, the northernmost complex of 4C+29.48 contains a blazar with a high brightness temperature compact core and a steep-spectrum jet feature. The blazar is positionally coincident with an optical source at a redshift of 1.142. Its mid-infrared colors also support its association with a γ-ray emitting blazar. The two other radio complexes have steep radio spectra similar to AGN-related lobes and do not have optical or infrared counterparts in currently available surveys. Based on the radio morphology, they are unlikely to be related to the blazar. There is an optical source between the two radio features, also detected in infrared wavebands. We discuss the possibilities whether the two radio features are lobes of a radio galaxy, or gravitationally lensed images of a background source. Conclusions: We propose to associate the γ-ray source 3FGL J1323.0+2942 in subsequent versions of the Fermi catalog with the blazar residing in northernmost complex. We suggest naming this radio source J1323+2941A to avoid misinterpretation arising from the fact that the coordinates of the currently listed radio counterpart 4C+29.48 is closer to a most probably unrelated radio source.

Deformations in VLBI antennas

NASA Technical Reports Server (NTRS)

Clark, T. A.; Thomsen, P.

1988-01-01

A study is presented of deformations in antennas with the emphasis on their influence on VLBI measurements. The GIFTS structural analysis program has been used to model the VLBI antenna in Fairbanks (Alaska). The report identifies key deformations and studies the effect of gravity, wind, and temperature. Estimates of expected deformations are given.

VLBI2010 in NASA's Space Geodesy Project

NASA Technical Reports Server (NTRS)

Ma, Chopo

2012-01-01

In the summer of 20 11 NASA approved the proposal for the Space Geodesy Project (SGP). A major element is developing at the Goddard Geophysical and Astronomical Observatory a prototype of the next generation of integrated stations with co-located VLBI, SLR, GNSS and DORIS instruments as well as a system for monitoring the vector ties. VLBI2010 is a key component of the integrated station. The objectives ofSGP, the role of VLBI20 lOin the context of SGP, near term plans and possible future scenarios will be discussed.

Research in geodesy and geophysics based upon radio-interferometric observations of extragalactic radio sources. Final report, December 1984-December 1985

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clark, T.A.; Davis, J.L.; Gwinn, C.R.

1986-10-01

This report consists of a collection of reprints and preprints. Subjects included: description of Mk-III system for very-long-baseline interferometry (VLBI); geodetic results from the Mk-I and Mk-III systems for VLBI; effects of modeling atmospheric propagation on estimates of baseline length and station height; an improved model for the dry propagation delay; corrections to IAU 1980 nutation series based on VLBI data and geophysical interpretation of those corrections; and a review of the contributions of VLBI to geodynamic studies.

Ka Band Objects: Observation and Monitoring (KaBOOM)

NASA Astrophysics Data System (ADS)

Geldzahler, B.

2012-09-01

NASA has embarked on a path that will enable the implementation of a high power, high resolution X/Ka band radar system using widely spaced 12m antennas to better track and characterize near Earth objects and orbital debris. This radar system also has applications for cost effective space situational awareness. We shall demonstrate Ka band coherent uplink arraying with real-time atmospheric compensation using three 12m antennas at the Kennedy Space Center (KSC). Our proposed radar system can complement and supplement the activities of the Space Fence. The proposed radar array has the advantages of filling the gap between dusk and dawn and offers the possibility of high range resolution (4 cm) and high spatial resolution (?10 cm at GEO) when used in a VLBI mode. KSC was chosen because [a] of reduced implementation costs, [b] there is a lot of water vapor in the air (not Ka band friendly), and [c] the test satellites have a low elevation adding more attenuation and turbulence to the demonstration. If Ka band coherent uplink arraying can be made to work at KSC, it will work anywhere. We expect to rebaseline X-band in 2013, and demonstrate Ka band uplink arraying in 2014.

ALMA observations towards G023.01-00.41 .

NASA Astrophysics Data System (ADS)

Sanna, A.; Moscadelli, L.; Cesaroni, R.; Caratti o Garatti, A.; Menten, K. M.; Kölligan, A.; Kuiper, R.

We want to understand whether or not young stars, with masses of tens of Solar masses, can form in the disk accretion scenario. This challenge requires to resolve the spatial morphology of gas and dust within a few 1000 au of a massive young stellar object, and to measure the gas kinematics with respect to the star. Also, because the gas kinematics near the young star can be a mixture of rotating, expanding, and infalling motions all together, to separate each velocity component it is necessary to map the emission of various gas tracers, as well as to image the circumstellar gas at different distances from the star. With this in mind, we made use of the Atacama Large Millimeter Array (ALMA) at wavelengths near 1 mm, with the aim to image the dense molecular gas in the vicinity of a well-known O-type young star. We previously observed this source with the Submillimeter Array (SMA), at scales of about 0.1 pc, with the Karl G. Jansky Very Large Array (VLA), at scales of the order of 1000 au, and with the Very Long Baseline Array (VLBA) and European VLBI Network (EVN), at scales of a few au.

eVLBI at Wettzell

NASA Astrophysics Data System (ADS)

Dassing, Reiner; Kronschnabl, Gerhard

Wettzell's radiotelescope is connected to 34 Mbps INTERNET connection. Since April, 2005, Wettzell is performing eVLBI observations for INT2 on a regular basis. The data is transfered to Tsukuba, and one day after the observations, the results of the correlation is produced. A gain of about 7 days is possible due to eVLBI.

Real-time Data Streams from ``e-RemoteCtrl'' to Central VLBI Network Status Monitoring Services Like IVS Live

NASA Astrophysics Data System (ADS)

Neidhardt, Alexander; Collioud, Arnaud

2014-12-01

A central VLBI network status monitoring can be realized by using online status information about current VLBI sessions, real-time, and status data directly from each radio telescope. Such monitoring helps to organize sessions or to get immediate feedback from the active telescopes. Therefore the remote control software for VLBI radio telescopes ``e-RemoteCtrl'' (http://www.econtrol-software.de), which enables remote access as extension to the NASA Field System, realizes real-time data streams to dedicated data centers. The software has direct access to the status information about the current observation (e.g., schedule, scan, source) and the telescope (e.g., current state, temperature, pressure) in real-time. This information are directly sent to ``IVS Live''. ``IVS Live'' (http://ivslive.obs.u-bordeaux1.fr/) is a Web tool that can be used to follow the observing sessions, organized by the International VLBI Service for Geodesy and Astrometry (IVS), navigate through past or upcoming sessions, or search and display specific information about sessions, sources (like VLBI images), and stations, by using an Internet browser.

Correlated flux densities from VLBI observations with the DSN

NASA Technical Reports Server (NTRS)

Coker, R. F.

1992-01-01

Correlated flux densities of extragalactic radio sources in the very long baseline interferometry (VLBI) astrometric catalog are required for the VLBI tracking of Galileo, Mars Observer, and future missions. A system to produce correlated and total flux density catalogs was developed to meet these requirements. A correlated flux density catalog of 274 sources, accurate to about 20 percent, was derived from more than 5000 DSN VLBI observations at 2.3 GHz (S-band) and 8.4 GHz (X-band) using 43 VLBI radio reference frame experiments during the period 1989-1992. Various consistency checks were carried out to ensure the accuracy of the correlated flux densities. All observations were made on the California-Spain and California-Australia DSN baselines using the Mark 3 wideband data acquisition system. A total flux density catalog, accurate to about 20 percent, with data on 150 sources, was also created. Together, these catalogs can be used to predict source strengths to assist in the scheduling of VLBI tracking passes. In addition, for those sources with sufficient observations, a rough estimate of source structure parameters can be made.

VLBI observations to the APOD satellite

NASA Astrophysics Data System (ADS)

Sun, Jing; Tang, Geshi; Shu, Fengchun; Li, Xie; Liu, Shushi; Cao, Jianfeng; Hellerschmied, Andreas; Böhm, Johannes; McCallum, Lucia; McCallum, Jamie; Lovell, Jim; Haas, Rüdiger; Neidhardt, Alexander; Lu, Weitao; Han, Songtao; Ren, Tianpeng; Chen, Lue; Wang, Mei; Ping, Jinsong

2018-02-01

The APOD (Atmospheric density detection and Precise Orbit Determination) is the first LEO (Low Earth Orbit) satellite in orbit co-located with a dual-frequency GNSS (GPS/BD) receiver, an SLR reflector, and a VLBI X/S dual band beacon. From the overlap statistics between consecutive solution arcs and the independent validation by SLR measurements, the orbit position deviation was below 10 cm before the on-board GNSS receiver got partially operational. In this paper, the focus is on the VLBI observations to the LEO satellite from multiple geodetic VLBI radio telescopes, since this is the first implementation of a dedicated VLBI transmitter in low Earth orbit. The practical problems of tracking a fast moving spacecraft with current VLBI ground infrastructure were solved and strong interferometric fringes were obtained by cross-correlation of APOD carrier and DOR (Differential One-way Ranging) signals. The precision in X-band time delay derived from 0.1 s integration time of the correlator output is on the level of 0.1 ns. The APOD observations demonstrate encouraging prospects of co-location of multiple space geodetic techniques in space, as a first prototype.

Imaging the supermassive black hole shadow and jet base of M87 with the event horizon telescope

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lu, Ru-Sen; Fish, Vincent L.; Doeleman, Sheperd S.

2014-06-20

The Event Horizon Telescope (EHT) is a project to assemble a Very Long Baseline Interferometry (VLBI) network of millimeter wavelength dishes that can resolve strong field general relativistic signatures near a supermassive black hole. As planned, the EHT will include enough dishes to enable imaging of the predicted black hole 'shadow', a feature caused by severe light bending at the black hole boundary. The center of M87, a giant elliptical galaxy, presents one of the most interesting EHT targets as it exhibits a relativistic jet, offering the additional possibility of studying jet genesis on Schwarzschild radius scales. Fully relativistic modelsmore » of the M87 jet that fit all existing observational constraints now allow horizon-scale images to be generated. We perform realistic VLBI simulations of M87 model images to examine the detectability of the black shadow with the EHT, focusing on a sequence of model images with a changing jet mass load radius. When the jet is launched close to the black hole, the shadow is clearly visible both at 230 and 345 GHz. The EHT array with a resolution of 20-30 μas resolution (∼2-4 Schwarzschild radii) is able to image this feature independent of any theoretical models and we show that imaging methods used to process data from optical interferometers are applicable and effective for EHT data sets. We demonstrate that the EHT is also capable of tracing real-time structural changes on a few Schwarzschild radii scales, such as those implicated by very high-energy flaring activity of M87. While inclusion of ALMA in the EHT is critical for shadow imaging, the array is generally robust against loss of a station.« less

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cseh, David; Corbel, Stephane; Kaaret, Philip

We present new radio, optical, and X-ray observations of three ultraluminous X-ray sources (ULXs) that are associated with large-scale nebulae. We report the discovery of a radio nebula associated with the ULX IC 342 X-1 using the Very Large Array (VLA). Complementary VLA observations of the nebula around Holmberg II X-1, and high-frequency Australia Telescope Compact Array and Very Large Telescope spectroscopic observations of NGC 5408 X-1 are also presented. We study the morphology, ionization processes, and the energetics of the optical/radio nebulae of IC 342 X-1, Holmberg II X-1, and NGC 5408 X-1. The energetics of the optical nebulamore » of IC 342 X-1 is discussed in the framework of standard bubble theory. The total energy content of the optical nebula is 6 Multiplication-Sign 10{sup 52} erg. The minimum energy needed to supply the associated radio nebula is 9.2 Multiplication-Sign 10{sup 50} erg. In addition, we detected an unresolved radio source at the location of IC 342 X-1 at the VLA scales. However, our Very Long Baseline Interferometry (VLBI) observations using the European VLBI Network likely rule out the presence of any compact radio source at milliarcsecond (mas) scales. Using a simultaneous Swift X-ray Telescope measurement, we estimate an upper limit on the mass of the black hole in IC 342 X-1 using the 'fundamental plane' of accreting black holes and obtain M{sub BH} {<=} (1.0 {+-} 0.3) Multiplication-Sign 10{sup 3} M{sub Sun }. Arguing that the nebula of IC 342 X-1 is possibly inflated by a jet, we estimate accretion rates and efficiencies for the jet of IC 342 X-1 and compare with sources like S26, SS433, and IC 10 X-1.« less

Low frequency VLBI in space using GAS-Can satellites: Report on the May 1987 JPL Workshop

NASA Technical Reports Server (NTRS)

Mahoney, M. J.; Jones, D. L.; Kuiper, T. B. H.; Preston, R. A.

1987-01-01

Summarized are the results of a workshop held at JPL on May 28 and 29, 1987, to study the feasibility of using small, very inexpensive spacecraft for a low-frequency radio interferometer array. Many technical aspects of a mission to produce high angular resolution images of the entire sky at frequencies from 2 to 20 MHz were discussed. The workshop conclusion was that such a mission was scientifically valuable and technically practical. A useful array could be based on six or more satellites no larger than those launched from Get-Away-Special canisters. The cost of each satellite could be $1-2M, and the mass less than 90 kg. Many details require further study, but as this report shows, there is good reason to proceed. No fundamental problems have been discovered involving the use of untraditional, very inexpensive spacecraft for this type of mission.

Crustal dynamics project observations: 1982 results and plans for 1983

NASA Technical Reports Server (NTRS)

Frey, H.

1983-01-01

The 1982 Crustal Dynamics Project observations by fixed and mobile SLR and VLBI systems are reviewed. Plate motion measurements between North America and Europe were conducted by both techniques and SLR measurements were also made between North America, the Pacific, Australia and South American plates. Regional deformation measurements by VLBI and SLR systems were restricted to the western United States in 1982, including a number of important intercomparison baseline measured by both techniques. In 1983 the observing program grows significantly, with new SLR systems in Mexico, Easter Island, the Pacific and Italy. New VLBI systems will include a dedicated VLBI site at Weltzell, in Germany. Two highly mobile SLR and two highly mobile VLBI systems will greatly increase the regional deformation measurements in California and through the Basin and Range, where more than 25 sites will be occupied in 1983.

Status and Plans for the Vienna VLBI and Satellite Software (VieVS 3.0)

NASA Astrophysics Data System (ADS)

Gruber, Jakob; Böhm, Johannes; Böhm, Sigrid; Girdiuk, Anastasiia; Hellerschmied, Andreas; Hofmeister, Armin; Krásná, Hana; Kwak, Younghee; Landskron, Daniel; Madzak, Matthias; Mayer, David; McCallum, Jamie; Plank, Lucia; Schartner, Matthias; Shabala, Stas; Teke, Kamil; Sun, Jing

2017-04-01

The Vienna VLBI and Satellite Software (VieVS) is a geodetic analysis software developed and maintained at Technische Universität Wien (TU Wien) with contributions from groups all over the world. It is used for both academic purposes in university courses as well as for providing Very Long Baseline Interferometry (VLBI) analysis results to the geodetic community. Written in a modular structure in Matlab, VieVS offers easy access to the source code and the possibility to adapt the programs for particular purposes. The new version 3.0, released in early 2017, includes several new features, e.g., improved scheduling capabilities for observing quasars and satellites. This poster gives an overview of all VLBI-related activities in Vienna and provides an outlook to future plans concerning the Vienna VLBI and Satellite Software (VieVS).

JPL VLBI Analysis Center IVS Annual Report for 2004

NASA Technical Reports Server (NTRS)

Jacobs, Chris

2005-01-01

This report describes the activities of the JPL VLBI analysis center for the year 2004. We continue to be celestial reference frame, terrestrial reference frame, earth orientation, and spacecraft navigation work using the VLBI technique. There are several areas of our work that are undergoing active development. In 2004 we demonstrated 1 mm level troposphere calibration on an intercontinental baseline. We detected our first X/Ka (8.4/32 GHz) VLBI fringes. We began to deploy Mark 5 recorders and to interface the Mark 5 units to our software correlator. We also have actively participated in the international VLBI community through our involvement in six papers at the February IVS meeting and by collaborating on a number of projects such as densifying the S/X celestial frame creating celestial frames at K (24 GHz) and Q-bands ($# GHz)>

Comparison of observation level versus 24-hour average atmospheric loading corrections in VLBI analysis

NASA Astrophysics Data System (ADS)

MacMillan, D. S.; van Dam, T. M.

2009-04-01

Variations in the horizontal distribution of atmospheric mass induce displacements of the Earth's surface. Theoretical estimates of the amplitude of the surface displacement indicate that the predicted surface displacement is often large enough to be detected by current geodetic techniques. In fact, the effects of atmospheric pressure loading have been detected in Global Positioning System (GPS) coordinate time series [van Dam et al., 1994; Dong et al., 2002; Scherneck et al., 2003; Zerbini et al., 2004] and very long baseline interferometery (VLBI) coordinates [Rabble and Schuh, 1986; Manabe et al., 1991; van Dam and Herring, 1994; Schuh et al., 2003; MacMillan and Gipson, 1994; and Petrov and Boy, 2004]. Some of these studies applied the atmospheric displacement at the observation level and in other studies, the predicted atmospheric and observed geodetic surface displacements have been averaged over 24 hours. A direct comparison of observation level and 24 hour corrections has not been carried out for VLBI to determine if one or the other approach is superior. In this presentation, we address the following questions: 1) Is it better to correct geodetic data at the observation level rather than applying corrections averaged over 24 hours to estimated geodetic coordinates a posteriori? 2) At the sub-daily periods, the atmospheric mass signal is composed of two components: a tidal component and a non-tidal component. If observation level corrections reduce the scatter of VLBI data more than a posteriori correction, is it sufficient to only model the atmospheric tides or must the entire atmospheric load signal be incorporated into the corrections? 3) When solutions from different geodetic techniques (or analysis centers within a technique) are combined (e.g., for ITRF2008), not all solutions may have applied atmospheric loading corrections. Are any systematic effects on the estimated TRF introduced when atmospheric loading is applied?

Gravity-dependent signal path variation in a large VLBI telescope modelled with a combination of surveying methods

NASA Astrophysics Data System (ADS)

Sarti, Pierguido; Abbondanza, C.; Vittuari, L.

2009-11-01

The very long baseline interferometry (VLBI) antenna in Medicina (Italy) is a 32-m AZ-EL mount that was surveyed several times, adopting an indirect method, for the purpose of estimating the eccentricity vector between the co-located VLBI and Global Positioning System instruments. In order to fulfill this task, targets were located in different parts of the telescope’s structure. Triangulation and trilateration on the targets highlight a consistent amount of deformation that biases the estimate of the instrument’s reference point up to 1 cm, depending on the targets’ locations. Therefore, whenever the estimation of accurate local ties is needed, it is critical to take into consideration the action of gravity on the structure. Furthermore, deformations induced by gravity on VLBI telescopes may modify the length of the path travelled by the incoming radio signal to a non-negligible extent. As a consequence, differently from what it is usually assumed, the relative distance of the feed horn’s phase centre with respect to the elevation axis may vary, depending on the telescope’s pointing elevation. The Medicina telescope’s signal path variation Δ L increases by a magnitude of approximately 2 cm, as the pointing elevation changes from horizon to zenith; it is described by an elevation-dependent second-order polynomial function computed as, according to Clark and Thomsen (Techical report, 100696, NASA, Greenbelt, 1988), a linear combination of three terms: receiver displacement Δ R, primary reflector’s vertex displacement Δ V and focal length variations Δ F. Δ L was investigated with a combination of terrestrial triangulation and trilateration, laser scanning and a finite element model of the antenna. The antenna gain (or auto-focus curve) Δ G is routinely determined through astronomical observations. A surprisingly accurate reproduction of Δ G can be obtained with a combination of Δ V, Δ F and Δ R.

Ultra-rapid earth rotation determination with VLBI during CONT11 and CONT14

NASA Astrophysics Data System (ADS)

Haas, Rüdiger; Hobiger, Thomas; Kurihara, Shinobu; Hara, Tetsuya

2015-08-01

In 2007 the Geospatial Information Authority of Japan (GSI) and the Onsala Space Observatory (OSO) started a collaboration project aiming at determining the earth rotation angle, usually expressed as UT1-UTC, in near real-time. In the beginning of this project dedicated one hour long one-baseline experiments were observed periodically using the VLBI stations Onsala (Sweden) and Tsukuba (Japan). The strategy is that the observed VLBI data are sent in real-time via the international optical fibre backbone to the correlator at Tsukuba where the data are correlated with a software correlator and analyzed in near-real time with the c5++ VLBI data analysis software, thus producing UT1-UTC results with very low latency. The latency between the observation at the stations and the determination of UT1-UTC is on the order of a few minutes, thus we can talk about an ultra-rapid determination of UT1-UTC. An offline version of this strategy was adopted in 2009 for the regular VLBI intensive series INT-2, organized by the International VLBI Service for Geodesy and Astrometry (IVS), that involves Wettzell (Germany) and Tsukuba. Since March 2010 the INT-2 is using real-time e-transfer, too, and since June 2010 also automated analysis. Starting in 2009 the ultra-rapid approach was applied to regular 24 hour long IVS VLBI-sessions that involve Tsukuba and Onsala, so that ultra-rapid UT1-UTC results can be produced already during ongoing VLBI-sessions. This strategy was successfully operated during the 15 days long continuous VLBI campaigns CONT11 and CONT14. In this presentation we give an overview of the ultra-rapid concept, present the results derived during CONT11 and CONT14, and compare these ultra-rapid results to results derived from post-processing

Ultra-rapid earth rotation determination with VLBI during CONT11 and CONT14

NASA Astrophysics Data System (ADS)

Haas, Rüdiger; Hobiger, Thomas; Kurihara, Shinobu; Hara, Tetsuya

2016-04-01

In 2007 the Geospatial Information Authority of Japan (GSI) and the Onsala Space Observatory (OSO) started a collaboration project aiming at determining the earth rotation angle, usually expressed as UT1-UTC, in near real-time. In the beginning of this project dedicated one hour long one-baseline experiments were observed periodically using the VLBI stations Onsala (Sweden) and Tsukuba (Japan). The strategy is that the observed VLBI data are sent in real-time via the international optical fibre backbone to the correlator at Tsukuba where the data are correlated with a software correlator and analyzed in near-real time with the c5++ VLBI data analysis software, thus producing UT1-UTC results with very low latency. The latency between the observation at the stations and the determination of UT1-UTC is on the order of a few minutes, thus we can talk about an ultra-rapid determination of UT1-UTC. An offline version of this strategy was adopted in 2009 for the regular VLBI intensive series INT-2, organized by the International VLBI Service for Geodesy and Astrometry (IVS), that involves Wettzell (Germany) and Tsukuba. Since March 2010 the INT-2 is using real-time e-transfer, too, and since June 2010 also automated analysis. Starting in 2009 the ultra-rapid approach was applied to regular 24 hour long IVS VLBI-sessions that involve Tsukuba and Onsala, so that ultra-rapid UT1-UTC results can be produced already during ongoing VLBI-sessions. This strategy was successfully operated during the 15 days long continuous VLBI campaigns CONT11 and CONT14. In this presentation we give an overview of the ultra-rapid concept, present the results derived during CONT11 and CONT14, and compare these ultra-rapid results to results derived from post-processing.

Prospects for UT1 Measurements from VLBI Intensive Sessions

NASA Technical Reports Server (NTRS)

Boehm, Johannes; Nilsson, Tobias; Schuh, Harald

2010-01-01

Very Long Baseline Interferometry (VLBI) Intensives are one-hour single baseline sessions to provide Universal Time (UT1) in near real-time up to a delay of three days if a site is not e-transferring the observational data. Due to the importance of UT1 estimates for the prediction of Earth orientation parameters, as well as any kind of navigation on Earth or in space, there is not only the need to improve the timeliness of the results but also their accuracy. We identify the asymmetry of the tropospheric delays as the major error source, and we provide two strategies to improve the results, in particular of those Intensives which include the station Tsukuba in Japan with its large tropospheric variation. We find an improvement when (1) using ray-traced delays from a numerical weather model, and (2) when estimating tropospheric gradients within the analysis of Intensive sessions. The improvement is shown in terms of reduction of rms of length-of-day estimates w.r.t. those derived from Global Positioning System observations

Global reference frame: Intercomparison of results (SLR, VLBI and GPS)

NASA Technical Reports Server (NTRS)

Ma, Chopo; Watkins, Michael M.; Heflin, M.

1994-01-01

The terrestrial reference frame (TRF) is realized by a set of positions and velocities derived from a combination of the three space geodetic techniques, SLR, VLBI and GPS. The standard International TRF is constructed by the International Earth Rotation Service in such a way that it is stable with time and the addition of new data. An adopted model for overall plate motion, NUVEL-1 NNR, defines the conceptual reference frame in which all the plates are moving. In addition to the measurements made between reference points within the space geodetic instruments, it is essential to have accurate, documented eccentricity measurements from the instrument reference points to ground monuments. Proper local surveys between the set of ground monuments at a site are also critical for the use of the space geodetic results. Eccentricities and local surveys are, in fact, the most common and vexing sources of error in the use of the TRF for such activities as collocation and intercomparison.

GPS inferred geocentric reference frame for satellite positioning and navigation

NASA Technical Reports Server (NTRS)

Malla, Rajendra P.; Wu, Sien-Chong

1989-01-01

Accurate geocentric three-dimensional positioning is of great importance for various geodetic and oceanographic applications. While relative positioning accuracy of a few centimeters has become a reality using Very Long Baseline Interferometry (VLBI), the uncertainty in the offset of the adopted coordinate system origin from the geocenter is still believed to be of the order of one meter. Satellite Laser Ranging (SLR) is capable of determining this offset to better than 10 cm, though, because of the limited number of satellites, this requires a long arc of data. The Global Positioning System (GPS) measurements provide a powerful alternative for an accurate determination of this origin offset in relatively short period of time. Two strategies are discussed, the first utilizes the precise relative positions predetermined by VLBI, whereas the second establishes a reference frame by holding only one of the tracking sites longitude fixed. Covariance analysis studies indicate that geocentric positioning to an accuracy of a few centimeters can be achieved with just one day of precise GPS pseudorange and carrier phase data.

Application of new radio tracking data types to critical spacecraft navigation problems

NASA Technical Reports Server (NTRS)

Ondrasik, V. J.; Rourke, K. H.

1972-01-01

Earth-based radio tracking data types are considered, which involve simultaneous or nearly simultaneous spacecraft tracking from widely separated tracking stations. These data types are conventional tracking instrumentation analogs of the very long baseline interferometry (VLBI) of radio astronomy-hence the name quasi-VLBI. A preliminary analysis of quasi-VLBI is presented using simplified tracking data models. The results of accuracy analyses are presented for a representative mission, Viking 1975. The results indicate that, contingent on projected tracking system accuracy, quasi-VLBI can be expected to significantly improve navigation performance over that expected from conventional tracking data types.

Nontidal Loading Applied in VLBI Geodetic Analysis

NASA Astrophysics Data System (ADS)

MacMillan, D. S.

2015-12-01

We investigate the application of nontidal atmosphere pressure, hydrology, and ocean loading series in the analysis of VLBI data. The annual amplitude of VLBI scale variation is reduced to less than 0.1 ppb, a result of the annual components of the vertical loading series. VLBI site vertical scatter and baseline length scatter is reduced when these loading models are applied. We operate nontidal loading services for hydrology loading (GLDAS model), atmospheric pressure loading (NCEP), and nontidal ocean loading (JPL ECCO model). As an alternative validation, we compare these loading series with corresponding series generated by other analysis centers.

International VLBI Service for Geodesy and Astrometry 2004 General Meeting Proceedings

NASA Technical Reports Server (NTRS)

Vandenberg, Nancy R. (Editor); Baver, Karen D. (Editor)

2004-01-01

This volume is the proceedings of the third General Meeting of the International VLBI Service for Geodesy and Astromctry IVS), held in Otlawa, Canada, February 9-11,2004. The keynote of the third GM was visions for the next decade following the main theme of "Today's Results and Tomorrow's Vision". with a recognition that the outstanding VLBI results available today are the foundation and motivation for the next generation VLBI system requirements. The goal of the meeting was to provide an interesting and informative program for a wide cross section of IVS members, including station operators, program managers, and analysts.

Establishing a Modern Ground Network for Space Geodesy Applications

NASA Technical Reports Server (NTRS)

Pearlman, M.; Pavlis, E.; Altamimi, Z.; Noll, C.

2010-01-01

Ground-based networks of co-located space-geodesy techniques (VLBI, SLR, GLASS, DORIS) are the basis for the development and maintenance of the :International Terrestrial deference Frame (ITRE), which is the basis for our metric measurements of global change. The Global Geodetic Observing System (GGOS) within the International Association of Geodesy has established a task to develop a strategy to design, integrate and maintain the fundamental geodetic network and supporting infrastructure in a sustainable way to satisfy the long-term requirements for the reference frame. The GGOS goal is an origin definition at I mm or better and a temporal stability on the order of 0.1 mm/y, with similar numbers for the scale and orientation components. These goals are based on scientific requirements to address sea level rise with confidence. As a first step, simulations focused on establishing the optimal global SLR and VLBI network, since these two techniques alone are sufficient to define the reference frame. The GLASS constellations will then distribute the reference frame to users anywhere on the Earth. Using simulated data to be collected by the future networks, we investigated various designs and the resulting accuracy in the origin, scale and orientation of the resulting ITRF. We present here the results of extensive simulation studies aimed at designing optimal global geodetic networks to support GGOS science products. Current estimates are the network will require 24 - 32 globally distributed co-location sites. Stations in the near global network will require geologically stable sites witla good weather, established infrastructure, and local support and personnel. EGOS will seek groups that are interested in participation. GGOS intends to issues a Call for Participation of groups that would like to take part in the network implementation and operation_ Some examples of integrated stations currently in operation or under development will be presented. We will examine necessary conditions and challenges in designing a co-location station.

Analysis of Polar Motion Series Differences Between VLBI, GNSS, and SLR

NASA Astrophysics Data System (ADS)

MacMillan, Daniel; Pavlis, Erricos

2017-04-01

We have compared polar motion series from VLBI, GNSS, and SLR generated with a reference frame aligned to ITRF2008. Three objectives of the comparisons are 1) to determine biases between the techniques, 2) to determine the precision of each technique via a 3-corner hat analysis after removing the relative biases, and 3) to evaluate the long-term stability of polar motion series. Between VLBI, GNSS,and SLR, there are clear variations ranging from 20 to 60 µas in peak-to-peak amplitude. We investigate the possible causes of these variations. In addition, there are other apparent systematic biases and rate differences. There are VLBI network dependent effects that appear in the VLBI-GNSS and VLBI-SLR differences, specifically between the operational R1 and R4 weekly 24-hour sessions. We investigate the origins of these differences including network station changes in these networks over the period from 2002-present. The polar motion biases and precisions of the five IVS VLBI continuous observing CONT campaigns (since 2002) are also analyzed since these 2-week campaigns were each designed to provide the highest quality results that could be produced at the time. A possible source of bias between the three techniques is the underlying chosen sub-network used by each technique to realize the adopted reference frame. We also consider the technique differences when ITRF2014 is used instead of ITRF2008

Uses of the ICRF and implications for future VLBI

NASA Technical Reports Server (NTRS)

Ma, Chopo

2006-01-01

Since its inception on 1 Jan 1998, the fundamental ICRF has been set by the VLBI positions of 212 "defining" extragalactic radio sources. In all there are approx.3000 sources with usefully accurate (< few mas) positions consistent with the ICRF. The uses of the ICRF include fundamental astrometry, monitoring of Earth orientation, and spacecraft navigation. For fundamental astrometry, stability and accuracy are most important, and realizations at different frequencies must be in proper registration. However, there is no preferred frequency, and the GAIA mission has the potential for an optical ICRF with 500,000 objects at the 50 microarcsec level some time after the planned 2011 launch. The radio ICRF should be properly prepared for a transition to assure long term stability and consistency. Earth orientation monitoring requires objects attached to the solid Earth, and VLBI will continue to be the fundamental technique. For this purpose it is essential that the new VLBI stations contemplated in the VLBI20l0 report be capable of observing a sufficiently large and well-distributed set of stable sources, and identifying these sources is an on-going effort. Spacecraft navigation by differential VLBI is planned using the Ka-band telemetry signal, and work has begun towards an ICRF realization suitable for this purpose. The balancing of different needs related to the VLBI ICRF will be discussed.

VLBI-based Products - Naval Oceanography Portal

Science.gov Websites

section Advanced Search... Sections Home Time Earth Orientation Astronomy Meteorology Oceanography Ice You are here: Home › USNO › Earth Orientation › VLBI-based Products USNO Logo USNO Navigation Earth determine Earth Orientation Parameters (EOP) is Very Long Baseline Interferometry (VLBI). USNO provides both

Advanced relativistic VLBI model for geodesy

NASA Astrophysics Data System (ADS)

Soffel, Michael; Kopeikin, Sergei; Han, Wen-Biao

2017-07-01

Our present relativistic part of the geodetic VLBI model for Earthbound antennas is a consensus model which is considered as a standard for processing high-precision VLBI observations. It was created as a compromise between a variety of relativistic VLBI models proposed by different authors as documented in the IERS Conventions 2010. The accuracy of the consensus model is in the picosecond range for the group delay but this is not sufficient for current geodetic purposes. This paper provides a fully documented derivation of a new relativistic model having an accuracy substantially higher than one picosecond and based upon a well accepted formalism of relativistic celestial mechanics, astrometry and geodesy. Our new model fully confirms the consensus model at the picosecond level and in several respects goes to a great extent beyond it. More specifically, terms related to the acceleration of the geocenter are considered and kept in the model, the gravitational time-delay due to a massive body (planet, Sun, etc.) with arbitrary mass and spin-multipole moments is derived taking into account the motion of the body, and a new formalism for the time-delay problem of radio sources located at finite distance from VLBI stations is presented. Thus, the paper presents a substantially elaborated theoretical justification of the consensus model and its significant extension that allows researchers to make concrete estimates of the magnitude of residual terms of this model for any conceivable configuration of the source of light, massive bodies, and VLBI stations. The largest terms in the relativistic time delay which can affect the current VLBI observations are from the quadrupole and the angular momentum of the gravitating bodies that are known from the literature. These terms should be included in the new geodetic VLBI model for improving its consistency.

E-GRASP/Eratosthenes: GRGS numerical simulations and millimetric TRF realization

NASA Astrophysics Data System (ADS)

Pollet, A.; Coulot, D.; Biancale, R.; Mandea, M.

2017-12-01

To accurately measuring and understanding changes in sea level, ice sheets and other elements of the dynamic Earth system, a stable Terrestrial Reference Frame (TRF) is needed. To reach the goals for the TRF realization of 1 mm accuracy and 0.1 mm/year stability (GGOS, Meeting the Requirements of a Global Society on a Changing Planet in 2020, Plag and Pearlman, 2009), The European - Geodetic Reference Antenna in Space (E-GRASP) has been recently proposed to the ESA EE9 call. This space mission is designed to build an enduring and stable TRF, by carrying very precise sensor systems for all the key geodetic techniques used to define and monitor the TRF (DORIS, GNSS, SLR and VLBI).In this study, we present the numerical simulations carried out by the French Groupe de Recherche en Géodésie Spatiale (GRGS). We simulated the measurements of the four geodetic techniques (DORIS and SLR measurements to E-GRASP, VLBI interferometric measurements on E-GRASP and GPS measurements from ground stations and from E-GRASP) over five years. Next, we have evaluated the expected exactitude and stability of the TRF provided by the processing of these measurements. In addition, we show the expected impact of the on-board instrument calibration on the TRF. Finally, we simulated the measurements of the two LAGEOS and four DORIS satellites, quasars for VLBI and we computed two multi-technique combinations, one with E-GRASP measurements and one without, to evaluate the contribution of this satellite to a combination.

Deriving a geocentric reference frame for satellite positioning and navigation

NASA Technical Reports Server (NTRS)

Malla, R. P.; Wu, S.-C.

1988-01-01

With the advent of Earth-orbiting geodetic satellites, nongeocentric datums or reference frames have become things of the past. Accurate geocentric three-dimensional positioning is now possible and is of great importance for various geodetic and oceanographic applications. While relative positioning accuracy of a few centimeters has become a reality using very long baseline interferometry (VLBI), the uncertainty in the offset of the adopted coordinate system origin from the geocenter is still believed to be on the order of 1 meter. Satellite laser ranging (SLR), however, is capable of determining this offset to better than 10 cm, but this is possible only after years of measurements. Global Positioning System (GPS) measurements provide a powerful tool for an accurate determination of this origin offset. Two strategies are discussed. The first strategy utilizes the precise relative positions that were predetermined by VLBI to fix the frame orientation and the absolute scaling, while the offset from the geocenter is determined from GPS measurements. Three different cases are presented under this strategy. The reference frame thus adopted will be consistent with the VLBI coordinate system. The second strategy establishes a reference frame by holding only the longitude of one of the tracking sites fixed. The absolute scaling is determined by the adopted gravitational constant (GM) of the Earth; and the latitude is inferred from the time signature of the Earth rotation in the GPS measurements. The coordinate system thus defined will be a geocentric Earth-fixed coordinate system.

New VLBI Observing System 'OCTAVE-Family' to Support VDIF Specifications with 10 GigE for VERA, JVN, and Japanese e-VLBI (OCTAVE)

NASA Astrophysics Data System (ADS)

Oyama, T.; Kono, Y.; Suzuki, S.; Mizuno, S.; Bushimata, T.; Jike, T.; Kawaguchi, N.; Kobayashi, H.; Kimura, M.

2012-12-01

The new VLBI observing system (OCTAVE-Family) has been designed and developed based on the VSI-H and VDIF specifications at NAOJ (National Astronomical Observatory of Japan). It consists of 1) a high speed 8-Gsps 3-bit ADC (OCTAD) enabling us to acquire not only wide intermediate frequencies but also radio frequencies up to 50 GHz, 2) a converter (OCTAVIA) between one 10 GigE port and four 2 Gbps input and output ports conformable to VSI-H, 3) new recorders (OCTADISK and OCTADISK2) at rates of 4.5 Gbps and above 8 Gbps, and 4) a high speed software correlator system (OCTACOR) using GICO3 which was developed by NICT. These OCTAVE systems are connected via 10 GigE network with VDIF and VSI specifications. These components are used for VERA, JVN (Japanese VLBI network), and KJJVC (Korea-Japan Joint VLBI Correlator).

Navigation of space VLBI missions: Radioastron and VSOP

NASA Technical Reports Server (NTRS)

Ellis, Jordan

1993-01-01

In the mid-1990s, Russian and Japanese space agencies will each place into highly elliptic earth orbit a radio telescope consisting of a large antenna and radio astronomy receivers. Very long baseline interferometry (VLBI) techniques will be used to obtain high resolution images of radio sources observed by the space and ground based antennas. Stringent navigation accuracy requirements are imposed on the space VLBI missions by the need to transfer an ultra-stable ground reference frequency standard to the spacecraft and by the demands of the VLBI correlation process. Orbit determination for the mission will be the joint responsibility of navigation centers in the U.S., Russia, and Japan with orbit estimates based on combining tracking data from NASA, Russian, and Japanese sites. This paper describes the operational plans, the inter-agency coordination, and data exchange between the navigation centers required for space VLBI navigation.

Height biases and scale variations in VLBI networks due to antenna gravitational deformations

NASA Astrophysics Data System (ADS)

Abbondanza, Claudio; Sarti, Pierguido; Petrov, Leonid; Negusini, Monia

2010-05-01

The impact of signal path variations (SPVs) caused by antenna gravity deformations on geodetic VLBI results is evaluated for the first time. Elevation-dependent models of SPV for Medicina and Noto (Italy) telescopes were derived from a combination of terrestrial surveying methods to account for gravitational deformations. After applying these models, estimates of the antenna reference point (ARP) positions are shifted upward by 8.9 mm and 6.7 mm, respectively. The impact on other parameters is negligible. To infer the impact of antenna gravity deformations on the entire VLBI network, lacking measurements for other telescopes, we rescaled the SPV models of Medicina and Noto for other antennas according to their size. The effects are changes in VLBI heights in the range [-3,73] mm and a significant net scale increase of 0.3 - 0.8 ppb. This demonstrates the need to include SPV models in routine VLBI data analysis.

Viscoelastic deformation near active plate boundaries

NASA Technical Reports Server (NTRS)

Ward, Steven N.

1991-01-01

Very Long Baseline Interferometry (VLBI) now has the capacity to monitor geodetic positions with precisions of a few 1 mm over continental baselines. For tectonic applications, one of the major products of the VLBI program is the determination of the rate of change of station locations. Vector site velocities are now routinely produced. One of the novel techniques, VLBI Euler poles, is discussed.

VLBI astrometry and the Hipparcos link to the extragalactic reference frame

NASA Technical Reports Server (NTRS)

Lestrade, J.-F.; Preston, R. A.; Gabuzda, D. C.; Phillips, R. B.

1991-01-01

Intermediate results are reported from a program of VLBI radio observations designed to establish a link between the rotating reference frame of the ESA Hipparcos astrometric satellite and the extragalactic VLBI frame being developed by the International Earth Rotation Service. A group of 12 link stars have been observed at various epochs since 1982, and more observations are being undertaken during the 3-yr Hipparcos mission (1989-1992). Analysis of data on Algol indicates that phase-reference VLBI can determine an expected sky displacement of 4 marcsec with an uncertainty of 0.5 marcsec, even when the activity is only a few mJy.

Crustal dynamics project data analysis, 1986. Volume 1: Fixed station VLBI geodetic results

NASA Technical Reports Server (NTRS)

Ma, C.; Ryan, J. W.

1987-01-01

The Goddard VLBI group reports the results of analyzing 361 Mark III VLBI data sets from fixed observatories through the end of 1985 which are available to the Crustal Dynamics Project. All POLARIS/IRIS full-day data sets are included. The mobile VLBI sites at Platteville, Colorado; Penticton, British Columbia; and Yellowknife, Northwest Territories are also included since these occupations bear on the study of plate stability. Two large solutions, GLB027 and GLB028, were used to obtain site/baseline evolutions and earth rotation parameters, respectively. Source positions and nutation offsets were also adjusted in each solution. The results include 23 sites and 101 baselines.

Application of Geodetic VLBI Data to Obtaining Long-Term Light Curves for Astrophysics

NASA Technical Reports Server (NTRS)

Kijima, Masachika

2010-01-01

The long-term light curve is important to research on binary black holes and disk instability in AGNs. The light curves have been drawn mainly using single dish data provided by the University of Michigan Radio Observatory and the Metsahovi Radio Observatory. Hence, thus far, we have to research on limited sources. I attempt to draw light curves using VLBI data for those sources that have not been monitored by any observatories with single dish. I developed software, analyzed all geodetic VLBI data available at the IVS Data Centers, and drew the light curves at 8 GHz. In this report, I show the tentative results for two AGNs. I compared two light curves of 4C39.25, which were drawn based on single dish data and on VLBI data. I confirmed that the two light curves were consistent. Furthermore, I succeeded in drawing the light curve of 0454-234 with VLBI data, which has not been monitored by any observatory with single dish. In this report, I suggest that the geodetic VLBI archive data is useful to obtain the long-term light curves at radio bands for astrophysics.

Southern California Permanent GPS Geodetic Array: Continuous measurements of regional crustal deformation between the 1992 Landers and 1994 Northridge earthquakes

USGS Publications Warehouse

Bock, Y.; Wdowinski, S.; Fang, P.; Zhang, Jiahua; Williams, S.; Johnson, H.; Behr, J.; Genrich, J.; Dean, J.; Van Domselaar, M.; Agnew, D.; Wyatt, F.; Stark, K.; Oral, B.; Hudnut, K.; King, R.; Herring, T.; Dinardo, S.; Young, W.; Jackson, D.; Gurtner, W.

1997-01-01

The southern California Permanent GPS Geodetic Array (PGGA) was established in 1990 across the Pacific-North America plate boundary to continuously monitor crustal deformation. We describe the development of the array and the time series of daily positions estimated for its first 10 sites in the 19-month period between the June 28, 1992 (Mw=7.3), Landers and January 17, 1994 (Mw=6.7), Northridge earthquakes. We compare displacement rates at four site locations with those reported by Feigl et al. [1993], which were derived from an independent set of Global Positioning System (GPS) and very long baseline interferometry (VLBI) measurements collected over nearly a decade prior to the Landers earthquake. The velocity differences for three sites 65-100 km from the earthquake's epicenter are of order of 3-5 mm/yr and are systematically coupled with the corresponding directions of coseismic displacement. The fourth site, 300 km from the epicenter, shows no significant velocity difference. These observations suggest large-scale postseismic deformation with a relaxation time of at least 800 days. The statistical significance of our observations is complicated by our incomplete knowledge of the noise properties of the two data sets; two possible noise models fit the PGGA data equally well as described in the companion paper by Zhang et al. [this issue]; the pre-Landers data are too sparse and heterogeneous to derive a reliable noise model. Under a fractal white noise model for the PGGA data we find that the velocity differences for all three sites are statistically different at the 99% significance level. A white noise plus flicker noise model results in significance levels of only 94%, 43%, and 88%. Additional investigations of the pre-Landers data, and analysis of longer spans of PGGA data, could have an important effect on the significance of these results and will be addressed in future work. Copyright 1997 by the American Geophysical Union.

The effect of the dynamic wet troposphere on VLBI measurements

NASA Technical Reports Server (NTRS)

Treuhaft, R. N.; Lanyi, G. E.

1986-01-01

Calculations using a statistical model of water vapor fluctuations yield the effect of the dynamic wet troposphere on Very Long Baseline Interferometry (VLBI) measurements. The statistical model arises from two primary assumptions: (1) the spatial structure of refractivity fluctuations can be closely approximated by elementary (Kolmogorov) turbulence theory, and (2) temporal fluctuations are caused by spatial patterns which are moved over a site by the wind. The consequences of these assumptions are outlined for the VLBI delay and delay rate observables. For example, wet troposphere induced rms delays for Deep Space Network (DSN) VLBI at 20-deg elevation are about 3 cm of delay per observation, which is smaller, on the average, than other known error sources in the current DSN VLBI data set. At 20-deg elevation for 200-s time intervals, water vapor induces approximately 1.5 x 10 to the minus 13th power s/s in the Allan standard deviation of interferometric delay, which is a measure of the delay rate observable error. In contrast to the delay error, the delay rate measurement error is dominated by water vapor fluctuations. Water vapor induced VLBI parameter errors and correlations are calculated. For the DSN, baseline length parameter errors due to water vapor fluctuations are in the range of 3 to 5 cm. The above physical assumptions also lead to a method for including the water vapor fluctuations in the parameter estimation procedure, which is used to extract baseline and source information from the VLBI observables.

Near real-time monitoring of UT1 with geodetic VLBI

NASA Astrophysics Data System (ADS)

Haas, R.; Hobiger, T.; Sekido, M.; Koyama, Y.; Kondo, T.; Takiguchi, H.; Kurihara, S.; Kokado, K.; Tanimoto, D.; Nozawa, K.; Wagner, J.; Ritakari, J.; Mujunen, A.; Uunila, M.

2011-07-01

Geodetic VLBI is unique among the geodetic space techniques since it provides a direct connection between the international terrestrial reference frame and the international celestial reference frame. The Earth rotation angle, usually expressed as UT1, can be determined directly from geodetic VLBI observations. Accurate information about the Earth rotation angle is necessary and important for navigation purposes, in particular for satellite missions and space navigation. A near real-time knowledge of UT1 with high accuracy is therefore highly desirable. During the last few years the advances in data transfer over high-speed optical fibre lines have made it possible to electronically send the observational data from a VLBI radio telescope on one side of the globe in real-time to a VLBI correlator on the other side of the globe. Thus, data of two telescopes on opposite sides of the Earth, forming a long east-west oriented baseline, can be correlated in near real-time. Furthermore, advances in automated processing of the correlation results have made it possible to derive the Earth rotation angle UT1 in near real-time. Since 2007, the VLBI research groups in Sweden, Finland and Japan collaborate to derive UT1 in near real-time. Several dedicated so-called ultra-rapid UT1-sessions with 1-2 hours duration were performed. It was shown that final UT1-results can be derived within a few minutes after the end of an observing session (Sekido et al., 2008; Matsuzaka et al., 2008). The quality of the UT1-results is on the same level as the so-called IERS rapid solutions, but with a much lower latency (Haas et al., 2010). Recently, the ultra-rapid approach has been applied to standard 24 hour long VLBI observing sessions that are organized by the International VLBI Service for Geodesy and Astrometry (IVS). The long east-west baseline between Onsala (Sweden) and Tsukuba (Japan) is used to derive UT1 with a sliding window approach already during the ongoing IVS-session. The data processing and analysis is performed with a fully automated analysis software (Hobiger et al., 2010). We present results from the ultra-rapid UT1-sessions, both, from dedicated one-baseline sessions, as well from 24-hour ultra-rapid sessions during standard IVS-experiments. The near real-time UT1 results are compared to corresponding post-processing results, and results from independent analyses and techniques. Refrences: Sekido et al. (2008) Ultra-rapid UT1 measurements by e-VLBI, Earth Planets and Space, Vol. 60, 865-870. Matsuzaka et al. (2008) Ultra Rapid UT1 Experiment with e-VLBI, In: Proc. 5th IVS General Meeting, 68-71. Haas R et al. (2010) Ultra-Rapid DUT1-Observations with E-VLBI. Artificial Satellites, 45, 75-79. Hobiger et al. (2010) Fully automated VLBI analysis with c5++ for ultra-rapid determination of UT1, Earth Planets Space.

Global Vertical Rates from VLBl

NASA Technical Reports Server (NTRS)

Ma, Chopo; MacMillan, D.; Petrov, L.

2003-01-01

The analysis of global VLBI observations provides vertical rates for 50 sites with formal errors less than 2 mm/yr and median formal error of 0.4 mm/yr. These sites are largely in Europe and North America with a few others in east Asia, Australia, South America and South Africa. The time interval of observations is up to 20 years. The error of the velocity reference frame is less than 0.5 mm/yr, but results from several sites with observations from more than one antenna suggest that the estimated vertical rates may have temporal variations or non-geophysical components. Comparisons with GPS rates and corresponding site position time series will be discussed.

High-resolution VLBA Observations of Three 7 mm SiO Masers toward VX Sgr at Five Epochs

NASA Astrophysics Data System (ADS)

Su, J. B.; Shen, Z.-Q.; Chen, X.; Yi, Jiyune; Jiang, D. R.; Yun, Y. J.

2012-07-01

VX Sgr is a red supergiant at an adopted distance of 1.6 kpc with intense 43 GHz SiO maser emission. In this paper, we present the high-resolution very long baseline interferometry (VLBI) observations of SiO masers toward VX Sgr at five epochs. We used the Very Long Baseline Array to map the J = 1→0 (v = 1, 2) 28SiO masers and confirmed a ring-like structure. In the first two epochs, the v = 1 masers form a ring, but v = 2 maser spots residing only in the southern and northern regions do not form a complete ring. In the third epoch, the two masers are distributed in a ring structure and the v = 2 masers are a bit closer to the central star. In the last two epochs, many new maser spots appear and overlap each other. These overlapping maser spots can be related to the shock waves and reflect the collisional pumping. We compare the observations with the pumping models and speculate that the real pumping mechanism may be complex in VX Sgr and vary with time. The J = 1→0 (v = 0) 29SiO line emission is also detected, but is too weak to produce any VLBI map.

Determining dynamical parameters of the Milky Way Galaxy based on high-accuracy radio astrometry

NASA Astrophysics Data System (ADS)

Honma, Mareki; Nagayama, Takumi; Sakai, Nobuyuki

2015-08-01

In this paper we evaluate how the dynamical structure of the Galaxy can be constrained by high-accuracy VLBI (Very Long Baseline Interferometry) astrometry such as VERA (VLBI Exploration of Radio Astrometry). We generate simulated samples of maser sources which follow the gas motion caused by a spiral or bar potential, with their distribution similar to those currently observed with VERA and VLBA (Very Long Baseline Array). We apply the Markov chain Monte Carlo analyses to the simulated sample sources to determine the dynamical parameter of the models. We show that one can successfully determine the initial model parameters if astrometric results are obtained for a few hundred sources with currently achieved astrometric accuracy. If astrometric data are available from 500 sources, the expected accuracy of R0 and Θ0 is ˜ 1% or higher, and parameters related to the spiral structure can be constrained by an error of 10% or with higher accuracy. We also show that the parameter determination accuracy is basically independent of the locations of resonances such as corotation and/or inner/outer Lindblad resonances. We also discuss the possibility of model selection based on the Bayesian information criterion (BIC), and demonstrate that BIC can be used to discriminate different dynamical models of the Galaxy.

The high-resolution structure of the Centaurus A nucleus at 2.3 and 8.4 GHz

NASA Technical Reports Server (NTRS)

Meier, David L.; Preston, Robert A.; Morabito, David D.; Skjerve, Lyle; Slade, Martin A.; Wehrle, Ann E.; Niell, Arthur E.; Jauncey, David L.; Batchelor, Robert; Tzioumis, Anastasios K.

1989-01-01

VLBI observations of the nucleus of Centaurus A have been made at two frequencies with an array of five Australian radio telescopes as part of the Southern Hemisphere VLBI Experiment. Observations were made at 2.3 GHz with all five antennas, while only two were employed at 8.4 GHz. At 2.3 GHz seven tracks in the (u,v) plane with coverage of 6-8 hr each were obtained, yielding significant information on the structure of the nuclear jet. At 8.4 GHz a compact unresolved core was detected as well. It is found that the source consists of the compact self-absorbed core, a jet containing a set of three knots extending from 100 to 160 mas from the core, and a very long, narrow component elongated along the same position angle as the knots. The allowable range for the position angle of the jet is 51 + or - 3 deg, in agreement with that of the radio and X-ray structure on arcsecond and arcminute scales. The jet has brightened at 2.3 GHz by about 4 Jy, a factor of nearly 3, since the early 1970s, 1.8 Jy of which has occurred in the last 2 yr with no discernable changes in structure.

VizieR Online Data Catalog: Optically Bright extragalactic Radio Sources II (Petrov, 2013)

NASA Astrophysics Data System (ADS)

Petrov, L.

2014-06-01

The first VLBI (Very Long Baseline Interferometry) observing campaign in 2007 resulted in the detection of 398 targets with the European VLBI Network (EVN; Bourda et al., 2010, cat. J/A+A/520/A113). During the second observing campaign, a subset of 105 sources detected in the previous campaign was observed (Bourda et al., 2011, cat. J/A+A/526/A102). Their positions were derived by Petrov (2011, cat. J/AJ/142/105) and formed the OBRS-1 (Optically Bright extragalactic Radio Sources) catalog. The remaining sources were observed in the third campaign, called OBRS-2. During the OBRS-2 campaign, there were three observing sessions with 10 VLBA (Very Long Baseline Array) stations and 5-6 EVN stations from this list: EFLSBERG, MEDICINA, ONSALA60, YEBES40M, DSS63, HARTRAO, and NOTO. Observations were made on 2010 Mar 23 (session ID gc034a), on 2011 Nov 8 (gc034bcd), and on 2011 Mar 15 (gc034ef). The OBRS-2 catalog presents precise positions of the 295 extragalactic radio sources as well as median correlated flux densities at 8.4 and 2.2GHz at baseline lengths shorter than 900km and at baseline lengths longer than 5000km. (1 data file).

The next steps in Seti-Italia science and technology

NASA Astrophysics Data System (ADS)

Montebugnoli, Stelio; Cosmovici, Cristiano; Monari, Jader; Pluchino, Salvatore; Zoni, Luca; Bartolini, Marco; Orlati, Andrea; Salerno, Emma; Schillirò, Francesco; Pupillo, Giuseppe; Perini, Federico; Bianchi, Germano; Tani, Mattia; Amico, Leonardo

2010-02-01

The Italian Medicina Radioastronomy Station (nearby Bologna) is equipped with two antennas: the 32 mt (VLBI) dish and the Northern Cross, a large T-shaped parabolic/cylindrical antenna (30.000 sqm). So far Seti observations have been performed using a SERENDIP IV high resolution spectrometer connected to the VLBI dish in "piggy back" mode configuration. In order to facilitate data interpretation and to introduce innovative methods to search for possible extraterrestrial signals, we are planning to make use of the large UHF Northern Cross transit telescope. Sky observations performed at least within two months, could provide for each day a number of matrices labeled according to the observing sidereal time. The entire set of matrices will be characterized by an averaged spectrum on each row per day. Keeping constant the transit antenna declination, a coherent signal coming from a definite position of the sky, would produce a "flag on" in the same submatrix at the same sidereal time. Detections collected in this way could be considered "confirmed" since they always come from the same region of the sky and are observed regularly. An extremely powerful processing board based on a multi-FPGAs (Field Programmable Gate Array) core was developed and is now under programming. This is conceived to be the processing core for this new kind of investigations.

The Australian Geodetic Observing Program. Current Status and Future Plans

NASA Astrophysics Data System (ADS)

Johnston, G.; Dawson, J. H.

2015-12-01

Over the last decade, the Australian government has through programs like AuScope, the Asia Pacific Reference Frame (APREF), and the Pacific Sea Level Monitoring (PSLM) Project made a significant contribution to the Global Geodetic Observing Program. In addition to supporting the national research priorities, this contribution is justified by Australia's growing economic dependence on precise positioning to underpin efficient transportation, geospatial data management, and industrial automation (e.g., robotic mining and precision agriculture) and the consequent need for the government to guarantee provision of precise positioning products to the Australian community. It is also well recognised within Australia that there is an opportunity to exploit our near unique position as being one of the few regions in the world to see all new and emerging satellite navigation systems including Galileo (Europe), GPS III (USA), GLONASS (Russia), Beidou (China), QZSS (Japan) and IRNSS (India). It is in this context that the Australian geodetic program will build on earlier efforts and further develop its key geodetic capabilities. This will include the creation of an independent GNSS analysis capability that will enable Australia to contribute to the International GNSS Service (IGS) and an upgrade of key geodetic infrastructure including the national VLBI and GNSS arrays. This presentation will overview the significant geodetic activities undertaken by the Australian government and highlight its future plans.

Frequency standards requirements of the NASA deep space network to support outer planet missions

NASA Technical Reports Server (NTRS)

Fliegel, H. F.; Chao, C. C.

1974-01-01

Navigation of Mariner spacecraft to Jupiter and beyond will require greater accuracy of positional determination than heretofore obtained if the full experimental capabilities of this type of spacecraft are to be utilized. Advanced navigational techniques which will be available by 1977 include Very Long Baseline Interferometry (VLBI), three-way Doppler tracking (sometimes called quasi-VLBI), and two-way Doppler tracking. It is shown that VLBI and quasi-VLBI methods depend on the same basic concept, and that they impose nearly the same requirements on the stability of frequency standards at the tracking stations. It is also shown how a realistic modelling of spacecraft navigational errors prevents overspecifying the requirements to frequency stability.

Korea Geodetic VLBI Station, Sejong

NASA Technical Reports Server (NTRS)

Donghyun, Baek; Sangoh, Yi; Hongjong, Oh; Sangchul, Han

2013-01-01

The Sejong VLBI station officially joined the IVS as a new Network Station in 2012. This report summarizes the activities of the Sejong station during 2012. The following are the activities at the station: 1) VLBI test observations were carried out with the Tsukuba 34-m antenna of the GSI in Japan. As a result, the Sejong antenna needs to improve its efficiency, which is currently in progress, 2) A survey to connect the VLBI reference point to GNSS and ground marks was conducted, and 3) To see the indirect effects of RFI (Radio Frequency Interference) at this place, we checked the omni-direction (AZ 0? to 360?, EL fixed at 7?) for RFI influence.

Archiving Space Geodesy Data for 20+ Years at the CDDIS

NASA Technical Reports Server (NTRS)

Noll, Carey E.; Dube, M. P.

2004-01-01

Since 1982, the Crustal Dynamics Data Information System (CDDIS) has supported the archive and distribution of geodetic data products acquired by NASA programs. These data include GPS (Global Positioning System), GLONASS (GLObal NAvigation Satellite System), SLR (Satellite Laser Ranging), VLBI (Very Long Baseline Interferometry), and DORIS (Doppler Orbitography and Radiolocation Integrated by Satellite). The data archive supports NASA's space geodesy activities through the Solid Earth and Natural Hazards (SENH) program. The CDDIS data system and its archive have become increasingly important to many national and international programs, particularly several of the operational services within the International Association of Geodesy (IAG), including the International GPS Service (IGS), the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS), the International DORIS Service (IDS), and the International Earth Rotation Service (IERS). The CDDIS provides easy and ready access to a variety of data sets, products, and information about these data. The specialized nature of the CDDIS lends itself well to enhancement and thus can accommodate diverse data sets and user requirements. All data sets and metadata extracted from these data sets are accessible to scientists through ftp and the web; general information about each data set is accessible via the web. The CDDIS, including background information about the system and its user communities, the computer architecture, archive contents, available metadata, and future plans will be discussed.

The Crustal Dynamics Data Information System: A Resource to Support Scientific Analysis Using Space Geodesy

NASA Technical Reports Server (NTRS)

Noll. Carey E.

2010-01-01

Since 1982. the Crustal Dynamics Data Information System (CDDIS) has supported the archive and distribution of geodetic data products acquired by the National Aeronautics and Space Administration (NASA) as well as national and international programs. The CDDIS provides easy, timely, and reliable access to a variety of data sets, products, and information about these data. These measurements. obtained from a global network of nearly 650 instruments at more than 400 distinct sites, include DORIS (Doppler Orbitography and Radiopositioning Integrated by Satellite), GNSS (Global Navigation Satellite System), SLR and LLR (Satellite and Lunar Laser Ranging), and VLBI (Very Long Baseline Interferometry). The CDDIS data system and its archive have become increasingly important to many national and international science communities, particularly several of the operational services within the International Association of Geodesy (IAG) and its observing system the Global Geodetic Observing System (GGOS), including the International DORIS Service (IDS), the International GNSS Service (IGS). the International Laser Ranging Service (ILRS), the International VLBI Service for Geodesy and Astrometry (IVS). and the International Earth rotation and Reference frame Service (IERS), Investigations resulting from the data and products available through the CDDIS support research in many aspects of Earth system science and global change. Each month, the CDDIS archives more than one million data and derived product files totaling over 90 Gbytes in volume. In turn. the global user community downloads nearly 1.2 TBytes (over 10.5 million files) of data and products from the CDDIS each month. The requirements of analysts have evolved since the start of the CDDIS; the specialized nature of the system accommodates the enhancements required to support diverse data sets and user needs. This paper discusses the CDDIS. including background information about the system and its. user communities. archive contents. available metadata, and future plans.

Calibración del retardo ionosférico en observaciones astrométricas y geodésicas a partir de observaciones GPS

NASA Astrophysics Data System (ADS)

Brunini, C.; Kleusberg, A.; Arias, E. F.; de Biasi, M. S.

Los parámetros astrométricos y geodésicos de precisión se determinan hoy mediante la observación con técnicas espaciales (VLBI, GPS y LSR). Las técnicas VLBI y GPS operan en la banda de microondas y en ella la ionósfera terrestre es dispersiva. Las señales que provienen de las radiofuentes y de los satélites atraviesan la ionósfera, donde el índice de refracción difiere de la unidad en una cantidad que es proporcional a la densidad de electrones libres e inversamente proporcional al cuadrado de la frecuencia de la onda portadora. Actualmente el International GPS Service for Geodynamics (IGS) mantiene operacional una red global integrada por más de 50 estaciones equipadas con receptores GPS de alta performance; las observaciones diarias son accesibles a los usuarios mediante ftp. La posibilidad de utilizar estas observaciones en un monitoreo continuo de la ionósfera fue señalada por diversos autores, razón por la cual en los últimos años se ha invertido un significativo esfuerzo en la producción de mapas ionosféricos regionales y globales. En el presente trabajo se utilizan 28 estaciones cuyas observaciones mapean la mayor parte de la ionósfera global. Los resultados obtenidos demuestran la posibilidad de obtener mapas ionosféricos globales con una resolución de medio día y con una precisión de (1.5 nseg (rms)). Dichos mapas proveen valores medios globales para el intervalo ajustado. Los residuos del ajuste por mínimos cuadrados constituyen una señal a partir de la cual pueden estudiarse mejor las variaciones geográficas de la ionósfera y las componentes estadísticas de su variación temporal.

The search for reference sources for delta VLBI navigation of the Galileo spacecraft

NASA Technical Reports Server (NTRS)

Ulvestad, J. S.; Linfield, R. P.

1986-01-01

A comprehensive search was made in order to identify celestial radio sources that can be used as references for navigation of the Galileo spacecraft by means of VLBI observations. The astronomical literature was seached for potential navigation sources, and several VLBI experiments were performed to determine the suitability of those sources for navigation. The results of such work performed since mid-1983 is reported. A summary is presented of the source properties required, the procedures used to identify candidate sources, and the results of the observations of these sources. The lists of souces presented are not meant to be taken directly and used for VLBI navigation, but they do provide a means of identifying the radio sources that could be used at various positions along the Galileo trajectory. Since the reference sources nearest the critical points of Jupiter encounter and probe release are rather weak, it would be extremely beneficial to use a pair of 70-m antennas for the VLBI measurements.

Determination of Galactic Aberration from VLBI Measurements and Its Effect on VLBI Reference Frames and Earth Orientation Parameters.

NASA Astrophysics Data System (ADS)

MacMillan, D. S.

2014-12-01

Galactic aberration is due to the motion of the solar system barycenter around the galactic center. It results in a systematic pattern of apparent proper motion of radio sources observed by VLBI. This effect is not currently included in VLBI analysis. Estimates of the size of this effect indicate that it is important that this secular aberration drift be accounted for in order to maintain an accurate celestial reference frame and allow astrometry at the several microarcsecond level. Future geodetic observing systems are being designed to be capable of producing a future terrestrial reference frame with an accuracy of 1 mm and stability of 0.1 mm/year. We evaluate the effect galactic aberration on attaining these reference frame goals. This presentation will discuss 1) the estimation of galactic aberration from VLBI data and 2) the effect of aberration on the Terrestrial and Celestial Reference Frames and the Earth Orientation Parameters that connect these frames.

RAEGE: An Atlantic Network of Geodynamical Fundamental Stations

NASA Technical Reports Server (NTRS)

Gonzalez, Jesus Gomez; Colomer, Francisco; LopezFernandez, Jose Antonio; Assis, Marlene C. S.

2010-01-01

Project RAEGE (Red Atlantica de Estaciones Geodinamicas y Espaciales) intends to set up a Spanish-Portuguese network of four Geodetic Fundamental Stations in Yebes (1), Canary Islands (1), and A cores Islands (2), as part of the developments needed for the IVS VLBI2010 scenario. It is envisaged that each Geodetic Fundamental Station will be equipped with one radio telescope of VLBI2010 specifications (at least 12-m diameter, fast slewing speed, but also able to operate up to 40 GHz), one gravimeter, one permanent GNSS station, and, at least at the Yebes site, one SLR facility. The National Geographical Institute of Spain (IGN) has experience in VLBI, having been a member of the European VLBI Network since 1993 and being one of the founding institutions of the Joint Institute for VLBI in Europe (JIVE), and it has been participating in geodetic VLBI campaigns with the 14-m radio telescope in Yebes since 1995. A new 40-m radio telescope has been built and was recently put into operation. It regularly participates in IVS sessions. There is infrastructure available for the new stations at Yebes and the Canary Islands. An agreement between IGN, the Portuguese Geographical Institute (IGP), and the Regional Government of the A cores ensures that the RAEGE project can become a reality by 2013.

Tidal atmospheric and ocean loading in VLBI analysis

NASA Astrophysics Data System (ADS)

Girdiuk, Anastasiia; Schindelegger, Michael; Böhm, Johannes

2016-04-01

In VLBI (Very Long Baseline Interferometry) analysis, reductions for tidal atmospheric and ocean loading are commonly used according to the IERS Conventions. In this presentation we examine such loading corrections from contemporary geophysical models within routine VLBI processing and discuss the internal consistency of the applied corrections for various effects. In detail, two gravitational ocean tide models, FES2004 and the recent FES2012 atlas with a much finer horizontal resolution and an improved description of hydrodynamic processes, are employed. Moreover, the contribution of atmospheric tidal loading is also re-considered based on data taken from two providers of station displacements, Goddard Space Flight Center and the TU Wien group. Those two models differ in terms of the underlying meteorological data, which can be a reason for inconsistency of VLBI reductions and may lead to systematics in the VLBI products at tidal frequencies. We validate this assumption in terms of Earth rotation parameters, by a tidal analysis of diurnal and semi-diurnal universal time and semi-diurnal polar motion variations as determined with the Vienna VLBI Software. Applying the loading models in a consistent way still leads to unexplained residuals at about 4-5 μas in the diurnal polar motion band, thus limiting the possibility of assessing geophysical models at this particular frequency.

Connecting kinematic and dynamic reference frames by D-VLBI

NASA Astrophysics Data System (ADS)

Schuh, Harald; Plank, Lucia; Madzak, Matthias; Böhm, Johannes

2012-08-01

In geodetic and astrometric practice, terrestrial station coordinates are usually provided in the kinematic International Terrestrial Reference Frame (ITRF) and radio source coordinates in the International Celestial Reference Frame (ICRF), whereas measurements of space probes such as satellites and spacecrafts, or planetary ephemerides rest upon dynamical theories. To avoid inconsistencies and errors during measurement and calculation procedures, exact frame ties between quasi - inertial, kinematic and dynamic reference frames have to be secured. While the Earth Orientation Parameters (EOP), e.g. measured by VLBI, link the ITRF to the ICRF, the ties with the dynamic frames can be established with the differential Very Long Baseline Interferometry (D - VLBI) method. By observing space probes alternately t o radio sources, the relative position of the targets to each other on the sky can be determined with high accuracy. While D - VLBI is a common technique in astrophysics (source imaging) and deep space navigation, just recently there have been several effort s to use it for geodetic purposes. We present investigations concerning possible VLBI observations to satellites. This includes the potential usage of available GNNS satellites as well as specifically designed missions, as e.g. the GRASP mission proposed b y JPL/NASA and an international consortium, where the aspect of co - location in space of various techniques (VLBI, SLR, GNSS, DORIS) is the main focus.

Development of Wideband Feed

NASA Astrophysics Data System (ADS)

Ujihara, Hideki; Takefuji, Kazuhiro; Sekido, Mamoru; Kondo, Tetsuro

2015-08-01

Wideband feeds have developed for Kashima 34m antenna and new 2.4m portable VLBI antennas. Prototypes of the wideband feeds are multimode horns, first one was set on 34m in the end of 2013, and then replaced next one with 6.5-15.0GHz receiving frequency. Now, a new feed for 3.2GHz-14.4GHz will be installed in 2.4m and 34m antennas in this spring, which are named NINJA feed, because of its design flexibility in beam shpae. Next, IGUANA feed is now under design and fabrication, which is aimed for 2.2-22GHz and covers VGOS(VLBI2010) specification. This has coaxial structure, the smaller "daughter feed" for 6.4-22GHz is placed in the center of the larger "Mother feed" for 2.2-6.4GHz.They are used for our project of time and frequency transfer between remote atomic clocks by wideband VLBI, named Gala-V(Garapagos VLBI), and will also be used wideband VLBI observation for astronmy and geodesy.Prototype feeds were tested in measurement of aperture efficiency, SEFD and Tsys of 34m "Super Kashima Antenna" and both 6.7/12.2GHz methanol maser detection in one reciever system, and then better one is used for wideband VLBI observations.

Mapping Sub-pc Structure in AGNs with Water Vapor Megamasers

NASA Astrophysics Data System (ADS)

Braatz, James; Reid, Mark; Condon, James; Lo, K. Y.; Henkel, Christian; Kuo, Cheng-Yu; Impellizzeri, Violette; Gao, Feng; Zhao, Wei; Greene, Jenny; Constantin, Anca

2012-12-01

VLBI observations of water vapor megamasers provide the only direct means of mapping gas in AGNs on sub-parsec scales. Altogether, megamasers have been detected in about 150 galaxies, most of them nearby (z < 0.05), type 2 AGNs. Twenty one of these systems have now been mapped with VLBI. In about 25% of all maser-detected galaxies, the single-dish spectrum of the emission reveals a triple-peaked profile characteristic of masers in a thin, circumnuclear, edge-on accretion disk, and VLBI observations of these megamasers confirm the disk geometry. The maps reveal the size, shape (i.e. warp), and orientation of the disks, they provide gold-standard black hole masses and, in several cases, they enable the measurement of geometric distances to the host galaxies. In a few megamasers with more ambiguous single-dish profiles, VLBI maps show that some of the emission originates not in a disk, but rather from clouds that appear to be associated with an outflow. In these megamasers, and possibly many of the systems not yet mapped with VLBI, the masers may be directly associated with optically opaque, dusty gas believed to obscure some AGNs. Here we summarize results from VLBI imaging of megamasers and discuss the prospects for using observations of masers to understand the nature of obscuring gas in AGNs.

Tropospheric Parameters Determined by VLBI Within the IVS

NASA Astrophysics Data System (ADS)

Schuh, H.; Boehm, J.

2003-12-01

In April 2002 the IVS (International VLBI Service for Geodesy and Astrometry) set up the Pilot Project - Tropospheric Parameters, and the Institute of Geodesy and Geophysics (IGG), Vienna, was put in charge of coordinating the project. Seven IVS Analysis Centers have joined the project and regularly submitted their estimates of tropospheric parameters (wet and total zenith delays, horizontal gradients) for all IVS-R1 and IVS-R4 sessions since January 1st, 2002. The individual submissions are combined by a two-step procedure to obtain stable, robust and highly accurate tropospheric parameter time series with one hour resolution. The internal accuracy of the combined wet zenith delays is between 2 and 4 mm. The zenith delays derived by VLBI are compared with those provided by the International GPS Service (IGS). At sites with co-located VLBI and GPS antennas the short-term variabilities of the GPS and VLBI derived zenith delays generally show a good agreement but biases are found between the results of the two techniques. Possible reasons for these biases are discussed. Since July 1st, 2003, within the IVS the tropospheric parameters are determined as operational products. The presentation also includes the VLBI CONT02 campaign of 15 days of continuous observing in the second half of October 2002.

The Quasar Network Observations in e-VLBI Mode

NASA Astrophysics Data System (ADS)

Bezrukov, I.; Finkelstein, A.; Ipatov, A.; Kaidanovsky, A.; Mikhailov, A.; Salnikov, A.; Yakovlev, V.

2011-07-01

This paper describes activity of the Institute of Applied Astronomy in developing real-time VLBI-system using high speed digital communication links. Real-time VLBI-technology has been developing at IAA since 2007 when the very first experiment was successfully done with Haystack observatory. All observatories of VLBI-Network Quasar were connected by "last mile" communication channels and via the Internet at 100 Mbps rate. Additional UNIX servers were installed for data buffering. Now e-VLBI sessions are carried out routinely within domestic VLBI-programs for UT1-determination. Observational data of 1-hour sessions are transmitted simultaneously from Svetloe, Zelenchukskaya and Badary observatories to the IAA Data Processing Center in Saint-Petersburg through fiber lines at 50-70 Mbps via Tsunami-UDP protocol. In September 2010 few scans were successfully transmitted from Quasar-Network observatories to Correlator Center at Shanghai observatory and vice-versa from Shanghai observatory to Correlator of RAS. Within these experiments observation data recorded by Mark 5B recorder are transmitted to the buffer server during time interval when an antenna pointed from one source to another. This procedure allows us to reduce total time of obtaining final result by 30%. Now an advanced algorithm for automation of the data transmitting process from the recorder to correlator is developing.

Testing impact of the strategy of VLBI data analysis on the estimation of Earth Orientation Parameters and station coordinates

NASA Astrophysics Data System (ADS)

Wielgosz, Agata; Tercjak, Monika; Brzeziński, Aleksander

2016-06-01

Very Long Baseline Interferometry (VLBI) is the only space geodetic technique capable to realise the Celestial Reference Frame and tie it with the Terrestrial Reference Frame. It is also the only technique, which measures all the Earth Orientation Parameters (EOP) on a regular basis, thus the role of VLBI in determination of the universal time, nutation and polar motion and station coordinates is invaluable. Although geodetic VLBI has been providing observations for more than 30 years, there are no clear guidelines how to deal with the stations or baselines having significantly bigger post-fit residuals than the other ones. In our work we compare the common weighting strategy, using squared formal errors, with strategies involving exclusion or down-weighting of stations or baselines. For that purpose we apply the Vienna VLBI Software VieVS with necessary additional procedures. In our analysis we focus on statistical indicators that might be the criterion of excluding or down-weighting the inferior stations or baselines, as well as on the influence of adopted strategy on the EOP and station coordinates estimation. Our analysis shows that in about 99% of 24-hour VLBI sessions there is no need to exclude any data as the down-weighting procedure is sufficiently efficient. Although results presented here do not clearly indicate the best algorithm, they show strengths and weaknesses of the applied methods and point some limitations of automatic analysis of VLBI data. Moreover, it is also shown that the influence of the adopted weighting strategy is not always clearly reflected in the results of analysis.

12th European VLBI Network Symposium and Users Meeting

NASA Astrophysics Data System (ADS)

Tarchi, Andrea; Giroletti, Marcello; Feretti, Luigina

The Istituto di Radioastronomia (IRA) di Bologna and the Osservatorio Astronomico di Cagliari (OAC), on behalf of the European VLBI Consortium, hosted the 12th European VLBI Network (EVN) Symposium and Users Meeting. The Conference was held from 7th to 10th of October at the Hotel Regina Margherita, in the center of Cagliari. The latest scientific results and technical developments from VLBI, and, in particular, e-VLBI and space-VLBI (RadioAstron) outcomes were reported. The timing of this meeting coincided with the first successful observational tests of the Sardinia Radio Telescopes within the EVN, and with a number of results from new and upgraded radio facilities around the globe, such as e-MERLIN, ALMA, and the SKA pathfinders. The symposium was attended by 133 participants from all over the world, with the Asian community represented by more than 20 colleagues. The program of the meeting consisted of 70 oral contributions (including 8 invited speakers) and 50 poster that covered a very wide range of VLBI topics both in galactic and extragalactic astrophysics (e.g., AGN, stellar evolution from birth to death, astrometry, and planetary science) as well as technological developments and future international collaborations. The scientific program also included a visit to the 64-m Sardinia Radio Telescope (SRT) and the EVN Users Meeting, where astronomers have provided useful feedback on various matters regarding EVN operations. The research leading to these results has received funding from the European Commission Seventh Framework Programme (FP/2007-2013) under grant agreement No 283393 (RadioNet3). EDITORIAL BOARD: Andrea Tarchi, Marcello Giroletti, Luigina Feretti

Ultra-rapid EOP determination with VLBI

NASA Astrophysics Data System (ADS)

Haas, Rüdiger; Kurihara, Shinobu; Nozawa, Kentaro; Hobiger, Thomas; Lovell, Jim; McCallum, Jamie; Quick, Jonathan

2013-04-01

In 2007 the Geospatial information Authority of Japan (GSI) and the Onsala Space Observatory (OSO) started a project aiming at determining the earth rotation angle, usually expressed as dUT1, in near real-time. In the beginning of this project dedicated one hour long one-baseline experiments were observed periodically using the VLBI stations Onsala (Sweden) and Tsukuba (Japan). The strategy is that the observed VLBI-data are sent in real-time via the international optical fibre backbone to the VLBI-correlator at Tsukuba where the data are correlated and analyzed in near-real time, producing ultra-rapid dUT1 results. An offline version of this strategy has been adopted in 2009 for the regular VLBI intensive series INT-2 involving Wettzell (Germany) and Tsukuba. Since March 2010 the INT-2 is using real-time e-transfer, too, and since June 2010 also automated analysis. Starting in 2009 the ultra-rapid approach was applied to regular 24 hour long VLBI-sessions that involve Tsukuba and Onsala, so that ultra-rapid dUT1 results can be produced already during ongoing VLBI-sessions. This strategy was successfully operated during the 15 days long CONT11 campaign. In 2011 the ultra-rapid strategy was extended to involve a network of VLBI-stations, so that not only dUT1 but also the polar motion components can be determined in near real-time. Initially, in November 2011 a dedicated three-station session was observed involving Onsala, Tsukuba and Hobart (Tasmania, Australia). In 2012 several regular 24 hour long IVS-sessions that involved Onsala, Tsukuba and HartRAO (South Africa) were operated with the ultra-rapid strategy, and in several cases also Hobart was added as a fourth station. For this project we use the new analysis software c5++ developed by the National Institute of Information and Communications Technology (NICT). In this presentation we give an overview of the UREOP-project, describe the recent developments, and discuss the obtained results.

The simulation of lunar gravity field recovery from D-VLBI of Chang’E-1 and SELENE lunar orbiters

NASA Astrophysics Data System (ADS)

Yan, Jianguo; Ping, Jingsong; Matsumoto, K.; Li, Fei

2008-07-01

The lunar gravity field is a foundation to study the lunar interior structure, and to recover the evolution history of the Moon. It is still an open and key topic for lunar science. For above mentioned reasons, it becomes one of the important scientific objectives of recent lunar missions, such as KAGUYA (SELENE) the Japanese lunar mission and Chang’E-1, the Chinese lunar mission. The Chang’E-1 and the SELENE were successfully launched in 2007. It is estimated that these two missions can fly around the Moon longer than 6 months simultaneously. In these two missions, the Chinese new VLBI (Very Long Baseline Interferometry) network will be applied for precise orbit determination (POD) by using a differential VLBI (D-VLBI) method during the mission period. The same-beam D-VLBI technique will contribute to recover the lunar gravity field together with other conventional observables, i.e. R&RR (Range and Range Rate) and multi-way Doppler. Taking VLBI tracking conditions into consideration and using the GEODYNII/SOVLE software of GSFC/NASA/USA [Rowlands, D.D., Marshall, J.A., Mccarthy, J., et al. GEODYN II System Description, vols. 1 5. Contractor Report, Hughes STX Corp. Greenbelt, MD, 1997; Ullman, R.E. SOLVE program: mathematical formulation and guide to user input, Hughes/STX Contractor Report, Contract NAS5-31760. NASA Goddard Space Flight Center, Greenbelt, Maryland, 1994], we simulated the lunar gravity field recovering ability with and without D-VLBI between the Chang’E-1 and SELENE main satellite. The cases of overlapped flying and tracking period of 30 days, 60 days and 90 days have been analyzed, respectively. The results show that D-VLBI tracking between two lunar satellites can improve the gravity field recovery remarkably. The results and methods introduced in this paper will benefit the actual missions.

US Space VLBI Proposed Outreach Web Site

NASA Technical Reports Server (NTRS)

2000-01-01

The study of how VLBI might be pursued in space began in the late 1970's, when it was realized that the size of the earth was a serious limitation to the study of compact radio sources. By going to space, achieving angular resolution at radio wavelengths that could not be obtained with VLBI systems that were limited by the size of the earth, important tests could not be made of quasar models. The technology appeared to be within reach, and an early space VLBI concept, QUASAT, emerged as a joint project, involving both US and European scientists. In 1984, a workshop was held in Gross Enzerdorf, Austria, under joint sponsorship of NASA and the European Space Agency (ESA). The principal conclusion of the workshop was that a VLBI station in space, telemetering its data to ground data stations, working in connection with ground-based radio telescopes, would give the opportunity to achieve angular resolution of a few tens of micro-arc-seconds, and could develop high-quality radio maps of many classes of radio sources. The ground telemetry stations would also function as the source of a stable local oscillator for the spacecraft, which needs a highly stable frequency reference. The Deep Space Network of NASA could play a vital role in both the frequency-locking system and data acquisition. One outcome of the Gross Enzerdorf workshop was the convening, by COSPAR, of an ad hoc Committee on Space VLBI, to review and recommend procedures by which international collaboration on VLBI in space might be coordinated and promoted. In October 1985, the committee met in Budapest and recommended that the Inter-Agency Consultative Group (IACG) would be an appropriate body to coordinate VLBI activities in space. At the same time ESA convened a committee to explore the technical aspects of coordinating ground and space VLBI activities. At this stage both NASA and ESA were supporting preliminary studies of the QUASAT mission, with effective coordination between the two groups. The Soviet Union had also begun planning a mission with the clear intent to fly it as soon as the could; the mission was designated RadioAstron. In December 1985, the Soviets formed an international study team for the RadioAstron mission, holding the first meeting in Moscow. Subsequent meetings of this international committee were held in Budapest in May 1986 and in Moscow in December 1986. Thus, in 1986 there were three VLBI mission concepts under study: the NASA QUASAT mission, an ESA counterpart, and the Soviet RadioAstron mission.

A New Astronomical Facility for Peru: Converting a Telecommunication's 32 Meter Parabolic Antenna into a Radio Telescope

NASA Astrophysics Data System (ADS)

Ishitsuka, J. K.; Ishitsuka, M.; Inoue, M.; Kaifu, N.; Miyama, S.; Tsuboi, M.; Ohishi, M.; Fujisawa, K.; Kasuga, T.; Kondo, T.; Horiuchi, S.; Umemoto, T.; Miyoshi, M.; Miyazawa, K.; Bushimata, T.; Vidal, E. D.

2006-08-01

In 1984 Nippon Electric Company constructed an INTELSAT antenna at 3,370 meters above the sea level on the Peruvian Andes. Entel Peru, the Peruvian telecommunications company, managed the antenna station until 1993. This year the government transferred the station to a private telecommunications company, Telefónica del Peru. Since the satellite communications were rapidly replaced by transoceanic fiber optics, the beautiful 32 meters parabolic antenna has been unused since 2002.. In cooperation with the National Astronomical Observatory of Japan we began to convert the antenna into a radio telescope. Because researches on interstellar medium around Young Stellar Objects (YSO) will be able to observe the methanol masers that emit at 6.7 GHz, initially we will monitor the 6.7 GHz methanol masers and survey the southern sky. An ambient temperature receiver with Trx= 60 K was developed at Nobeyama Radio Observatory and is ready to be installed. The antenna control system is the Field System FS9 software installed in a Linux PC. An interface between the antenna and the PC was developed at Kashima Space Research Center in Japan. In the near future we plan to install the 2 GHz, 8 GHz, 12 GHz and 22 GHz receivers. The unique location and altitude of the Peruvian Radio Observatory will be useful for VLBI observations in collaboration with global arrays such as the VLBA array for astronomical observation and geodetic measurements. For Peru where few or almost no astronomical observational instruments are available for research, the implementation of the first radio observatory is a big and challenging step, and foster sciences at graduate and postgraduate levels of universities. Worldwide telecommunications antennas possibly are unused and with relative few investment could be transformed into a useful observational instrument.

Italy INAF Analysis Center Report

NASA Technical Reports Server (NTRS)

Negusini, M.; Sarti, P.

2013-01-01

This report summarizes the activity of the Italian INAF VLBI Analysis Center. Our Analysis Center is located in Bologna, Italy and belongs to the Institute of Radioastronomy, which is part of the National Institute of Astrophysics. IRA runs the observatories of Medicina and Noto, where two 32-m VLBI AZ-EL telescopes are situated. This report contains the AC's VLBI data analysis activities and shortly outlines the investigations into the co-locations of space geodetic instruments.

Application of Raytracing Through the High Resolution Numerical Weather Model HIRLAM for the Analysis of European VLBI

NASA Technical Reports Server (NTRS)

Garcia-Espada, Susana; Haas, Rudiger; Colomer, Francisco

2010-01-01

An important limitation for the precision in the results obtained by space geodetic techniques like VLBI and GPS are tropospheric delays caused by the neutral atmosphere, see e.g. [1]. In recent years numerical weather models (NWM) have been applied to improve mapping functions which are used for tropospheric delay modeling in VLBI and GPS data analyses. In this manuscript we use raytracing to calculate slant delays and apply these to the analysis of Europe VLBI data. The raytracing is performed through the limited area numerical weather prediction (NWP) model HIRLAM. The advantages of this model are high spatial (0.2 deg. x 0.2 deg.) and high temporal resolution (in prediction mode three hours).

Report for 2012 from the Bordeaux IVS Analysis Center

NASA Technical Reports Server (NTRS)

Charlot, Patrick; Bellanger, Antoine; Bouffet, Romuald; Bourda, Geraldine; Collioud, Arnaud; Baudry, Alain

2013-01-01

This report summarizes the activities of the Bordeaux IVS Analysis Center during the year 2012. The work focused on (i) regular analysis of the IVS-R1 and IVS-R4 sessions with the GINS software package; (ii) systematic VLBI imaging of the RDV sessions and calculation of the corresponding source structure index and compactness values; (iii) investigation of the correlation between astrometric position instabilities and source structure variations; and (iv) continuation of our VLBI observational program to identify optically-bright radio sources suitable for the link with the future Gaia frame. Also of importance is the 11th European VLBI Network Symposium, which we organized last October in Bordeaux and which drew much attention from the European and International VLBI communities.

The Impact of Radio Frequency Interference (RFI) on VLBI2010

NASA Technical Reports Server (NTRS)

Petrachenko, William

2010-01-01

A significant motivation for the development of a next generation system for geodetic VLBI was to address growing problems related to RFI. In this regard, the broadband 2-14 GHz frequency range proposed for VLBI2010 has advantages and disadvantages. It has the advantage of flexible allocation of band frequencies and hence the ability to avoid areas of the spectrum where RFI is worst. However, the receiver is at the same time vulnerable to saturation from RFI anywhere in the full 2-14 GHz range. The impacts of RFI on the VLBI2010 analog signal path, the sampler, and the digital signal processing are discussed. In addition, a number of specific RFI examples in the 2-14 GHz range are presented.

A complete VLBI delay model for deforming radio telescopes: the Effelsberg case

NASA Astrophysics Data System (ADS)

Artz, T.; Springer, A.; Nothnagel, A.

2014-12-01

Deformations of radio telescopes used in geodetic and astrometric very long baseline interferometry (VLBI) observations belong to the class of systematic error sources which require correction in data analysis. In this paper we present a model for all path length variations in the geometrical optics of radio telescopes which are due to gravitational deformation. The Effelsberg 100 m radio telescope of the Max Planck Institute for Radio Astronomy, Bonn, Germany, has been surveyed by various terrestrial methods. Thus, all necessary information that is needed to model the path length variations is available. Additionally, a ray tracing program has been developed which uses as input the parameters of the measured deformations to produce an independent check of the theoretical model. In this program as well as in the theoretical model, the illumination function plays an important role because it serves as the weighting function for the individual path lengths depending on the distance from the optical axis. For the Effelsberg telescope, the biggest contribution to the total path length variations is the bending of the main beam located along the elevation axis which partly carries the weight of the paraboloid at its vertex. The difference in total path length is almost 100 mm when comparing observations at 90 and at 0 elevation angle. The impact of the path length corrections is validated in a global VLBI analysis. The application of the correction model leads to a change in the vertical position of mm. This is more than the maximum path length, but the effect can be explained by the shape of the correction function.

Sgr A* and general relativity

NASA Astrophysics Data System (ADS)

Johannsen, Tim

2016-06-01

General relativity has been widely tested in weak gravitational fields but still stands largely untested in the strong-field regime. According to the no-hair theorem, black holes in general relativity depend only on their masses and spins and are described by the Kerr metric. Mass and spin are the first two multipole moments of the Kerr spacetime and completely determine all higher-order moments. The no-hair theorem and, hence, general relativity can be tested by measuring potential deviations from the Kerr metric affecting such higher-order moments. Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, is a prime target for precision tests of general relativity with several experiments across the electromagnetic spectrum. First, near-infrared (NIR) monitoring of stars orbiting around Sgr A* with current and new instruments is expected to resolve their orbital precessions. Second, timing observations of radio pulsars near the Galactic center may detect characteristic residuals induced by the spin and quadrupole moment of Sgr A*. Third, the event horizon telescope, a global network of mm and sub-mm telescopes, aims to study Sgr A* on horizon scales and to image the silhouette of its shadow cast against the surrounding accretion flow using very-long baseline interferometric (VLBI) techniques. Both NIR and VLBI observations may also detect quasiperiodic variability of the emission from the accretion flow of Sgr A*. In this review, I discuss our current understanding of the spacetime of Sgr A* and the prospects of NIR, timing, and VLBI observations to test its Kerr nature in the near future.

Evaluation of current tropospheric mapping functions by Deep Space Network very long baseline interferometry

NASA Technical Reports Server (NTRS)

Sovers, O. J.; Lanyi, G. E.

1994-01-01

To compare the validity of current algorithms that map zenith tropospheric delay to arbitrary elevation angles, 10 different tropospheric mapping functions are used to analyze the current data base of Deep Space Network Mark 3 intercontinental very long baseline interferometric (VLBI) data. This analysis serves as a stringent test because of the high proportion of low-elevation observations necessitated by the extremely long baselines. Postfit delay and delay-rate residuals are examined, as well as the scatter of baseline lengths about the time-linear model that characterizes tectonic motion. Among the functions that utilize surface meteorological data as input parameters, the Lanyi 1984 mapping shows the best performance both for residuals and baselines, through the 1985 Davis function is statistically nearly identical. The next best performance is shown by the recent function of Niell, which is based on an examination of global atmospheric characteristics as a function of season and uses no weather data at the time of the measurements. The Niell function shows a slight improvement in residuals relative to Lanyi, but also an increase in baseline scatter that is significant for the California-Spain baseline. Two variants of the Chao mapping function, as well as the Chao tables used with the interpolation algorithm employed in the Orbit Determination Program software, show substandard behavior for both VLBI residuals and baseline scatter. The length of the California-Australia baseline (10,600 km) in the VLBI solution can vary by as much as 5 to 10 cm for the 10 mapping functions.

TIGO: a geodetic observatory for the improvement of the global reference frame

NASA Astrophysics Data System (ADS)

Schlueter, Wolfgang; Hase, Hayo; Boeer, Armin

1999-12-01

The Bundesamt fuer Kartographie und Geodaesie (BKG) will provide a major contribution to the improvement and maintenance of the global reference frames: ICRF (International Celestial Reference Frame), ITRF (International Terrestrial Reference Frame) with the operation of TIGO (Transportable Integrated Geodetic Observatory). TIGO is designed as a transportable geodetic observatory which consists of all relevant geodetic space techniques for a fundamental station (including VLBI, SLR, GPS). The transportability of the observatory enables to fill up gaps in the International Space Geodetic Network and to optimize the contribution to the global reference frames. TIGO should operate for a period of 2 to 3 years (at minimum) at one location. BKG is looking for a cooperation with countries willing to contribute to the ITRF and to support the operation of TIGO.

Multi-technique comparison of troposphere zenith delays and gradients during CONT08

NASA Astrophysics Data System (ADS)

Teke, Kamil; Böhm, Johannes; Nilsson, Tobias; Schuh, Harald; Steigenberger, Peter; Dach, Rolf; Heinkelmann, Robert; Willis, Pascal; Haas, Rüdiger; García-Espada, Susana; Hobiger, Thomas; Ichikawa, Ryuichi; Shimizu, Shingo

2011-07-01

CONT08 was a 15 days campaign of continuous Very Long Baseline Interferometry (VLBI) sessions during the second half of August 2008 carried out by the International VLBI Service for Geodesy and Astrometry (IVS). In this study, VLBI estimates of troposphere zenith total delays (ZTD) and gradients during CONT08 were compared with those derived from observations with the Global Positioning System (GPS), Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS), and water vapor radiometers (WVR) co-located with the VLBI radio telescopes. Similar geophysical models were used for the analysis of the space geodetic data, whereas the parameterization for the least-squares adjustment of the space geodetic techniques was optimized for each technique. In addition to space geodetic techniques and WVR, ZTD and gradients from numerical weather models (NWM) were used from the European Centre for Medium-Range Weather Forecasts (ECMWF) (all sites), the Japan Meteorological Agency (JMA) and Cloud Resolving Storm Simulator (CReSS) (Tsukuba), and the High Resolution Limited Area Model (HIRLAM) (European sites). Biases, standard deviations, and correlation coefficients were computed between the troposphere estimates of the various techniques for all eleven CONT08 co-located sites. ZTD from space geodetic techniques generally agree at the sub-centimetre level during CONT08, and—as expected—the best agreement is found for intra-technique comparisons: between the Vienna VLBI Software and the combined IVS solutions as well as between the Center for Orbit Determination (CODE) solution and an IGS PPP time series; both intra-technique comparisons are with standard deviations of about 3-6 mm. The best inter space geodetic technique agreement of ZTD during CONT08 is found between the combined IVS and the IGS solutions with a mean standard deviation of about 6 mm over all sites, whereas the agreement with numerical weather models is between 6 and 20 mm. The standard deviations are generally larger at low latitude sites because of higher humidity, and the latter is also the reason why the standard deviations are larger at northern hemisphere stations during CONT08 in comparison to CONT02 which was observed in October 2002. The assessment of the troposphere gradients from the different techniques is not as clear because of different time intervals, different estimation properties, or different observables. However, the best inter-technique agreement is found between the IVS combined gradients and the GPS solutions with standard deviations between 0.2 and 0.7 mm.

Tidal Love and Shida numbers estimated by geodetic VLBI.

PubMed

Krásná, Hana; Böhm, Johannes; Schuh, Harald

2013-10-01

Frequency-dependent Love and Shida numbers, which characterize the Earth response to the tidal forces, were estimated in a global adjustment of all suitable geodetic Very Long Baseline Interferometry (VLBI) sessions from 1984.0 to 2011.0. Several solutions were carried out to determine the Love and Shida numbers for the tidal constituents at periods in the diurnal band and in the long-period band in addition to values of the Love and Shida numbers common for all tides of degree two. Adding up all twelve diurnal tidal waves that were estimated, the total differences in displacement with respect to the theoretical conventional values of the Love and Shida numbers calculated from an Earth model reach 1.73 ± 0.29 mm in radial direction and 1.15 ± 0.15 mm in the transverse plane. The difference in the radial deformation following from the estimates of the zonal Love numbers is largest for the semi-annual tide S sa with 1.07 ± 0.19 mm.

A new estimator for VLBI baseline length repeatability

NASA Astrophysics Data System (ADS)

Titov, O.

2009-11-01

The goal of this paper is to introduce a more effective technique to approximate for the “repeatability-baseline length” relationship that is used to evaluate the quality of geodetic VLBI results. Traditionally, this relationship is approximated by a quadratic function of baseline length over all baselines. The new model incorporates the mean number of observed group delays of the reference radio sources (i.e. estimated as global parameters) used in the estimation of each baseline. It is shown that the new method provides a better approximation of the “repeatability-baseline length” relationship than the traditional model. Further development of the new approach comes down to modeling the repeatability as a function of two parameters: baseline length and baseline slewing rate. Within the framework of this new approach the station vertical and horizontal uncertainties can be treated as a function of baseline length. While the previous relationship indicated that the station vertical uncertainties are generally 4-5 times larger than the horizontal uncertainties, the vertical uncertainties as determined by the new method are only larger by a factor of 1.44 over all baseline lengths.

Effects of Tropospheric Spatio-Temporal Correlated Noise on the Analysis of Space Geodetic Data

NASA Technical Reports Server (NTRS)

Romero-Wolf, A.; Jacobs, C. S.; Ratcliff, J. T.

2012-01-01

The standard VLBI analysis models the distribution of measurement noise as Gaussian. Because the price of recording bits is steadily decreasing, thermal errors will soon no longer dominate. As a result, it is expected that troposphere and instrumentation/clock errors will increasingly become more dominant. Given that both of these errors have correlated spectra, properly modeling the error distributions will become increasingly relevant for optimal analysis. We discuss the advantages of modeling the correlations between tropospheric delays using a Kolmogorov spectrum and the frozen flow assumption pioneered by Treuhaft and Lanyi. We then apply these correlated noise spectra to the weighting of VLBI data analysis for two case studies: X/Ka-band global astrometry and Earth orientation. In both cases we see improved results when the analyses are weighted with correlated noise models vs. the standard uncorrelated models. The X/Ka astrometric scatter improved by approx.10% and the systematic Delta delta vs. delta slope decreased by approx. 50%. The TEMPO Earth orientation results improved by 17% in baseline transverse and 27% in baseline vertical.

VLBI2010 and the Westford Station - The Path Forward

NASA Astrophysics Data System (ADS)

Beaudoin, C.; Wilson, K.; Whittier, B.; Whitney, A.; McWhirter, R.; Smythe, J. SooHoo, D.; Ruszczyk, C.; Rogers, A.; Poirier, M.; Niell, A.; Corey, B.; Cappallo, R.; Byford, J.; Bolis, P.

2012-12-01

For the past three years the role of the Westford antenna in geodetic VLBI has been two-fold. Over this time its primary purpose has been to participate in standard S/X-band geodetic VLBI observations. In its secondary role the Westford antenna has been converted into a research instrument, facilitating the development of the broadband geodetic VLBI observing technique. As a research instrument, the Westford antenna incorporates a commercially-available ETS-Lindgren 3164 quadridge antenna as a radio telescope feed. The system also uses the VLBI2010 data acquisition system that incorporates digital backends (DBEs) implementing a polyphase filter bank processor. The process of converting the station from its mode of operations to a research instrument often introduces subtle anomalies that must be diagnosed prior to broadband observing. Furthermore, this bifurcation of the station's role is not in line with the goals of the VLBI2010 specifications. Until recently it has not been possible for the Westford station to serve as both an operational and research instrument without conversion for two reasons: poor sensitivity and incompatibility of backend baseband filter bandwidths. The poor sensitivity of the Westford antenna as a broadband radio telescope is in large part due to the commercial broadband feed which was readily available when the proof-of-concept VLBI2010 observations were initiated. However, with the materialization of the quadridge flared horn (QRFH) by the California Institute of Technology and with the improvements in the DiFX software correlator, the necessary components are now available to upgrade the Westford station to full-broadband capability while adhering to the mandate to maintain backwards compatibility with the legacy S/X systems. In this paper we will present the path forward for upgrading the Westford site to full-broadband capability while maintaining S/X compatibility.

Astrometry VLBI in Space (AVS)

NASA Technical Reports Server (NTRS)

Cheng, Li-Jen; Reyes, George

1995-01-01

This paper describes a proposal for a new space radio astronomy mission for astrometry using Very Long Baseline Interferometry (VLBI) called Astrometry VLBI in Space (AVS). The ultimate goals of AVS are improving the accuracy of radio astrometry measurements to the microarcsecond level in one epoch of measurements and improving the accuracy of the transformation between the inertial radio and optical coordinate reference frames. This study will also assess the impact of this mission on astrophysics astrometry and geophysics.

Radio observations of active galactic nuclei with mm-VLBI

NASA Astrophysics Data System (ADS)

Boccardi, B.; Krichbaum, T. P.; Ros, E.; Zensus, J. A.

2017-11-01

Over the past few decades, our knowledge of jets produced by active galactic nuclei (AGN) has greatly progressed thanks to the development of very-long-baseline interferometry (VLBI). Nevertheless, the crucial mechanisms involved in the formation of the plasma flow, as well as those driving its exceptional radiative output up to TeV energies, remain to be clarified. Most likely, these physical processes take place at short separations from the supermassive black hole, on scales which are inaccessible to VLBI observations at centimeter wavelengths. Due to their high synchrotron opacity, the dense and highly magnetized regions in the vicinity of the central engine can only be penetrated when observing at shorter wavelengths, in the millimeter and sub-millimeter regimes. While this was recognized already in the early days of VLBI, it was not until the very recent years that sensitive VLBI imaging at high frequencies has become possible. Ongoing technical development and wide band observing now provide adequate imaging fidelity to carry out more detailed analyses. In this article, we overview some open questions concerning the physics of AGN jets, and we discuss the impact of mm-VLBI studies. Among the rich set of results produced so far in this frequency regime, we particularly focus on studies performed at 43 GHz (7 mm) and at 86 GHz (3 mm). Some of the first findings at 230 GHz (1 mm) obtained with the Event Horizon Telescope are also presented.

Analysis of Differences Between VLBI, GNSS and SLR Earth Orientation Series

NASA Astrophysics Data System (ADS)

MacMillan, D. S.; Pavlis, E. C.; Griffiths, J.

2016-12-01

We have compared polar motion series from VLBI, GNSS, and SLR where the reference frames were aligned to ITRF2008. Three objectives of the comparisons are 1) to determine biases between the techniques and 2) to determine the precisions of each technique via a 3-corner hat analysis after removing the relative biases, and 3) to evaluate the long-term stability of EOP series. Between VLBI and GPS or SLR, there are clear annual variations ranging from 25 to 100 µas in peak-to-peak amplitude. We investigate the possible causes of these variations. In addition, there are other apparent systematic bias and rate differences. From the point of view of VLBI, it is evident that there are VLBI network dependent effects, specifically between the operational R1 and R4 weekly 24-hour sessions. We investigate the origins of these differences including network station changes in these networks over the period from 2002-present. The EOP biases and precisions of the five IVS VLBI CONT campaigns (since 2002) are also analyzed since these sessions were each designed to provide the highest quality results that could be produced at the time. A possible source of biases between the geodetic techniques is the underlying reference frame used by each technique. We also consider the technique differences when ITRF2014 was applied instead of ITRF2008.

North America-Pacific plate boundary, an elastic-plastic megashear - Evidence from very long baseline interferometry

NASA Technical Reports Server (NTRS)

Ward, Steven N.

1988-01-01

Data obtained by Mark III VLBI measurements of radio signals from permanent and mobile VLBI sites for 5.5 years of observations, starting in October 1982, were used to derive a picture of the earth crust deformation near the North America-Pacific plate boundary. The data, which included the vector positions of the VLBI sites and their rate of change, were used for comparison with a number of lithospheric deformation models based upon the concept that the motions of points near the North America-Pacific plate boundary are a linear combination of North America and Pacific velocities. The best of these models were found to fit 95 percent of the variance in 139 VLBI length and transverse velocity observations. Instantaneous shear deformation associated with plate tectonics is apparently developing in a zone 450 km wide paralleling the San Andreas Fault; some of this deformation will be recovered through elastic rebound, while the rest will be permanently set through plastic processes. Because the VLBI data have not been collected for a significant fraction of the earthquake cycle, they cannot discriminate between elastic and plastic behaviors.

Vienna VLBI and Satellite Software (VieVS) for Geodesy and Astrometry

NASA Astrophysics Data System (ADS)

Böhm, Johannes; Böhm, Sigrid; Boisits, Janina; Girdiuk, Anastasiia; Gruber, Jakob; Hellerschmied, Andreas; Krásná, Hana; Landskron, Daniel; Madzak, Matthias; Mayer, David; McCallum, Jamie; McCallum, Lucia; Schartner, Matthias; Teke, Kamil

2018-04-01

The Vienna VLBI and Satellite Software (VieVS) is state-of-the-art Very Long Baseline Interferometry (VLBI) analysis software for geodesy and astrometry. VieVS has been developed at Technische Universität Wien (TU Wien) since 2008, where it is used for research purposes and for teaching space geodetic techniques. In the past decade, it has been successfully applied on Very Long Baseline Interferometry (VLBI) observations for the determination of celestial and terrestrial reference frames as well as for the estimation of celestial pole offsets, universal Time (UT1-UTC), and polar motion based on least-squares adjustment. Furthermore, VieVS is equipped with tools for scheduling and simulating VLBI observations to extragalactic radio sources as well as to satellites and spacecraft, features which proved to be very useful for a variety of applications. VieVS is now available as version 3.0 and we do provide the software to all interested persons and institutions. A wiki with more information about VieVS is available at http://vievswiki.geo.tuwien.ac.at/.

Multi-technique approach for deriving a VLBI signal extra-path variation model induced by gravity: the example of Medicina

NASA Astrophysics Data System (ADS)

Sarti, P.; Abbondanza, C.; Negusini, M.; Vittuari, L.

2009-09-01

During the measurement sessions gravity might induce significant deformations in large VLBI telescopes. If neglected or mismodelled, these deformations might bias the phase of the incoming signal thus corrupting the estimate of some crucial geodetic parameters (e.g. the height component of VLBI Reference Point). This paper describes a multi-technique approach implemented for measuring and quantifying the gravity-dependent deformations experienced by the 32-m diameter VLBI antenna of Medicina (Northern Italy). Such an approach integrates three different methods: Terrestrial Triangulations and Trilaterations (TTT), Laser Scanning (LS) and a Finite Element Model (FEM) of the antenna. The combination of the observations performed with these methods allows to accurately define an elevation-dependent model of the signal path variation which appears to be, for the Medicina telescope, non negligible. In the range [0,90] deg the signal path increases monotonically by almost 2 cm. The effect of such a variation has not been introduced in actual VLBI analysis yet; nevertheless this is the task we are going to pursue in the very next future.

THE APPLICATION OF MULTIVIEW METHODS FOR HIGH-PRECISION ASTROMETRIC SPACE VLBI AT LOW FREQUENCIES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Dodson, R.; Rioja, M.; Imai, H.

2013-06-15

High-precision astrometric space very long baseline interferometry (S-VLBI) at the low end of the conventional frequency range, i.e., 20 cm, is a requirement for a number of high-priority science goals. These are headlined by obtaining trigonometric parallax distances to pulsars in pulsar-black hole pairs and OH masers anywhere in the Milky Way and the Magellanic Clouds. We propose a solution for the most difficult technical problems in S-VLBI by the MultiView approach where multiple sources, separated by several degrees on the sky, are observed simultaneously. We simulated a number of challenging S-VLBI configurations, with orbit errors up to 8 mmore » in size and with ionospheric atmospheres consistent with poor conditions. In these simulations we performed MultiView analysis to achieve the required science goals. This approach removes the need for beam switching requiring a Control Moment Gyro, and the space and ground infrastructure required for high-quality orbit reconstruction of a space-based radio telescope. This will dramatically reduce the complexity of S-VLBI missions which implement the phase-referencing technique.« less

Comparison of the calibration of ionospheric delay in VLBI data by the methods of dual frequency and Faraday rotation

NASA Technical Reports Server (NTRS)

Scheid, J. A.

1985-01-01

When both S-band and X-band data are recorded for a signal which has passed through the ionosphere, it is possible to calculate the ionospheric contribution to signal delay. In Very Long Baseline Interferometry (VLBI) this method is used to calibrate the ionosphere. In the absence of dual frequency data, the ionospheric content measured by Faraday rotation, using a signal from a geostationary satellite, is mapped to the VLBI observing direction. The purpose here is to compare the ionospheric delay obtained by these two methods. The principal conclusions are: (1) the correlation between delays obtained by these two methods is weak; (2) in mapping Faraday rotation measurements to the VLBI observing direction, a simple mapping algorithm which accounts only for changes in hour angle and elevation angle is better than a more elaborate algorithm which includes solar and geomagnetic effects; (3) fluctuations in the difference in total electron content as seen by two antennas defining a baseline limit the application of Faraday rotation data to VLBI.

Geocenter Motion Derived from the JTRF2014 Combination

NASA Astrophysics Data System (ADS)

Abbondanza, C.; Chin, T. M.; Gross, R. S.; Heflin, M. B.; Parker, J. W.; van Dam, T. M.; Wu, X.

2016-12-01

JTRF2014 represents the JPL Terrestrial Reference Frame (TRF) recently obtained as a result of the combination of the space-geodetic reprocessed inputs to the ITRF2014. Based upon a Kalman filter and smoother approach, JTRF2014 assimilates station positions and Earth-Orientation Parameters (EOPs) from GNSS, VLBI, SLR and DORIS and combine them through local tie measurements. JTRF is in its essence a time-series based TRF. In the JTRF2014 the dynamical evolution of the station positions is formulated by introducing linear and seasonal terms (annual and semi-annual periodic modes). Non-secular and non-seasonal motions of the geodetic sites are included in the smoothed time series by properly defining the station position process noise whose variance is characterized by analyzing station displacements induced by temporal changes of planetary fluid masses (atmosphere, oceans and continental surface water). With its station position time series output at a weekly resolution, JTRF2014 materializes a sub-secular frame whose origin is at the quasi-instantaneous Center of Mass (CM) as sensed by SLR. Both SLR and VLBI contribute to the scale of the combined frame. The sub-secular nature of the frame allows the users to directly access the quasi-instantaneous geocenter and scale information. Unlike standard combined TRF products which only give access to the secular component of the CM-CN motions, JTRF2014 is able to preserve -in addition to the long-term- the seasonal, non-seasonal and non-secular components of the geocenter motion. In the JTRF2014 assimilation scheme, local tie measurements are used to transfer the geocenter information from SLR to the space-geodetic techniques which are either insensitive to CM (VLBI) or whose geocenter motion is poorly determined (GNSS and DORIS). Properly tied to the CM frame through local ties and co-motion constraints, GNSS, VLBI and DORIS contribute to improve the SLR network geometry. In this paper, the determination of the weekly (CM-CN) time series as inferred from the JTRF2014 combination will be presented. Comparisons with geocenter time series derived from global inversions of GPS, GRACE and ocean bottom pressure models show the JTRF2014-derived geocenter favourably compares to the results of the inversion.

The first VLBI image of an infrared-faint radio source

NASA Astrophysics Data System (ADS)

Middelberg, E.; Norris, R. P.; Tingay, S.; Mao, M. Y.; Phillips, C. J.; Hotan, A. W.

2008-11-01

Context: We investigate the joint evolution of active galactic nuclei and star formation in the Universe. Aims: In the 1.4 GHz survey with the Australia Telescope Compact Array of the Chandra Deep Field South and the European Large Area ISO Survey - S1 we have identified a class of objects which are strong in the radio but have no detectable infrared and optical counterparts. This class has been called Infrared-Faint Radio Sources, or IFRS. 53 sources out of 2002 have been classified as IFRS. It is not known what these objects are. Methods: To address the many possible explanations as to what the nature of these objects is we have observed four sources with the Australian Long Baseline Array. Results: We have detected and imaged one of the four sources observed. Assuming that the source is at a high redshift, we find its properties in agreement with properties of Compact Steep Spectrum sources. However, due to the lack of optical and infrared data the constraints are not particularly strong.

Proceedings of the Sixth General Meeting of the International VLBI Service for Geodesy and Astrometry

NASA Technical Reports Server (NTRS)

Behrend, Dirk (Editor); Baver, Karen D. (Editor)

2010-01-01

This volume is the proceedings of the sixth General Meeting of the International VLBI Service for Geodesy and Astrometry (IVS), held in Hobart, Tasmania, Australia, February 7-13, 2010. The contents of this volume also appear on the IVS Web site at http://ivscc.gsfc.nasa.gov/publications/gm2010. The keynote of the sixth GM was the new perspectives of the next generation VLBI system under the theme "VLBI2010: From Vision to Reality". The goal of the meeting was to provide an interesting and informative program for a wide cross-section of IVS members, including station operators, program managers, and analysts. This volume contains 88 papers. All papers were edited by the editors for usage of the English language, form, and minor content-related issues.

Earth Orientation Effects on Mobile VLBI Baselines

NASA Technical Reports Server (NTRS)

Allen, S. L.

1984-01-01

Improvements in data quality for the mobile VLBI systems have placed higher accuracy requirements on Earth orientation calibrations. Errors in these calibrations may give rise to systematic effects in the nonlength components of the baselines. Various sources of Earth orientation data were investigated for calibration of Mobile VLBI baselines. Significant differences in quality between the several available sources of UT1-UTC were found. It was shown that the JPL Kalman filtered space technology data were at least as good as any other and adequate to the needs of current Mobile VLBI systems and observing plans. For polar motion, the values from all service suffice. The effect of Earth orientation errors on the accuracy of differenced baselines was also investigated. It is shown that the effect is negligible for the current mobile systems and observing plan.

Report on IVS-WG4

NASA Astrophysics Data System (ADS)

Gipson, John

2011-07-01

I describe the proposed data structure for storing, archiving and processing VLBI data. In this scheme, most VLBI data is stored in NetCDF files. NetCDF has the advantage that there are interfaces to most common computer languages including Fortran, Fortran-90, C, C++, Perl, etc, and the most common operating systems including linux, Windows and Mac. The data files for a particular session are organized by special ASCII "wrapper" files which contain pointers to the data files. This allows great flexibility in the processing and analysis of VLBI data, and also allows for extending the types of data used, e.g., source maps. I discuss the use of the new format in calc/solve and other VLBI analysis packages. I also discuss plans for transitioning to the new structure.

Supernova research with VLBI

NASA Astrophysics Data System (ADS)

Bartel, Norbert; Bietenholz, Michael F.

2016-06-01

Core-collapse supernovae have been monitored with VLBI from shortly after the explosion to many years thereafter. Radio emission is produced as the ejecta hit the stellar wind left over from the dyingstar. Images show the details of the interaction as the shock front expands into the circumstellar medium. Measurements of the velocity and deceleration of the expansion provide information on both the ejecta and the circumstellar medium. VLBI observations can also search for the stellar remnant of the explosion, a neutron star or a black hole. Combining the transverse expansion rate with the radial expansion rate from optical spectra allows a geometric determination of the distance to the host galaxy. We will present results from recent VLBI observations, focus on their interpretations, and show updated movies of supernovae from soon after their explosion to the present.

Comparison Campaign of VLBI Data Analysis Software - First Results

NASA Technical Reports Server (NTRS)

Plank, Lucia; Bohm, Johannes; Schuh, Harald

2010-01-01

During the development of the Vienna VLBI Software VieVS at the Institute of Geodesy and Geophysics at Vienna University of Technology, a special comparison setup was developed with the goal of easily finding links between deviations of results achieved with different software packages and certain parameters of the observation. The object of comparison is the computed time delay, a value calculated for each observation including all relevant models and corrections that need to be applied in geodetic VLBI analysis. Besides investigating the effects of the various models on the total delay, results of comparisons between VieVS and Occam 6.1 are shown. Using the same methods, a Comparison Campaign of VLBI data analysis software called DeDeCC is about to be launched within the IVS soon.

Correlator data analysis for the array feed compensation system

NASA Technical Reports Server (NTRS)

Iijima, B.; Fort, D.; Vilnrotter, V.

1994-01-01

The real-time array feed compensation system is currently being evaluated at DSS 13. This system recovers signal-to-noise ratio (SNR) loss due to mechanical antenna deformations by using an array of seven Ka-band (33.7-GHz) horns to collect the defocused signal fields. The received signals are downconverted and digitized, in-phase and quadrature samples are generated, and combining weights are applied before the samples are recombined. It is shown that when optimum combining weights are employed, the SNR of the combined signal approaches the sum of the channel SNR's. The optimum combining weights are estimated directly from the signals in each channel by the Real-Time Block 2 (RTB2) correlator; since it was designed for very-long-baseline interferometer (VLBI) applications, it can process broadband signals as well as tones to extract the required weight estimates. The estimation algorithms for the optimum combining weights are described for tones and broadband sources. Data recorded in correlator output files can also be used off-line to estimate combiner performance by estimating the SNR in each channel, which was done for data taken during a Jupiter track at DSS 13.

Global and regional kinematics with GPS

NASA Technical Reports Server (NTRS)

King, Robert W.

1994-01-01

The inherent precision of the doubly differenced phase measurement and the low cost of instrumentation made GPS the space geodetic technique of choice for regional surveys as soon as the constellation reached acceptable geometry in the area of interest: 1985 in western North America, the early 1990's in most of the world. Instrument and site-related errors for horizontal positioning are usually less than 3 mm, so that the dominant source of error is uncertainty in the reference frame defined by the satellites orbits and the tracking stations used to determine them. Prior to about 1992, when the tracking network for most experiments was globally sparse, the number of fiducial sites or the level at which they could be tied to an SLR or VLBI reference frame usually, set the accuracy limit. Recently, with a global network of over 30 stations, the limit is set more often by deficiencies in models for non-gravitational forces acting on the satellites. For regional networks in the northern hemisphere, reference frame errors are currently about 3 parts per billion (ppb) in horizontal position, allowing centimeter-level accuracies over intercontinental distances and less than 1 mm for a 100 km baseline. The accuracy of GPS measurements for monitoring height variations is generally 2-3 times worse than for horizontal motions. As for VLBI, the primary source of error is unmodeled fluctuations in atmospheric water vapor, but both reference frame uncertainties and some instrument errors are more serious for vertical than horizontal measurements. Under good conditions, daily repeatabilities at the level of 10 mm rms were achieved. This paper will summarize the current accuracy of GPS measurements and their implication for the use of SLR to study regional kinematics.

Vienna Special Analysis Center Annual Report 2012

NASA Technical Reports Server (NTRS)

Boehm, Johannes; Boehm, Sigrid; Krasna, Hana; Madzak, Matthias; Nilsson, Tobias; Plank, Lucia; Raposo, Virginia; Schuh, Harald; Soja, Benedikt; Sun Jing;

Mobile VLBI deployment plans of the Crustal Dynamics Project for the western United States and Alaska

NASA Technical Reports Server (NTRS)

Trask, D. W.; Vegos, C. J.

1983-01-01

Current plans for the Mobile VLBI program are addressed. Present mobile stations and their past activities are summarized, and past and future modes of obtaining data are compared, including the 'burst' and 'leap frog' modes. The observational campaign for Mobile VLBI is described, emphasizing the portions in Canada and Alaska. The extent to which the mobile stations are utilized and the ways in which the site visit yield may be increased are discussed.

The VLBI Data Analysis Software νSolve: Development Progress and Plans for the Future

NASA Astrophysics Data System (ADS)

Bolotin, S.; Baver, K.; Gipson, J.; Gordon, D.; MacMillan, D.

2014-12-01

The program νSolve is a part of the CALC/SOLVE VLBI data analysis system. It is a replacement for interactive SOLVE, the part of CALC/SOLVE that is used for preliminary data analysis of new VLBI sessions. νSolve is completely new software. It is written in C++ and has a modern graphical user interface. In this article we present the capabilities of the software, its current status, and our plans for future development.

On an Allan variance approach to classify VLBI radio-sources on the basis of their astrometric stability

NASA Astrophysics Data System (ADS)

Gattano, C.; Lambert, S.; Bizouard, C.

2017-12-01

In the context of selecting sources defining the celestial reference frame, we compute astrometric time series of all VLBI radio-sources from observations in the International VLBI Service database. The time series are then analyzed with Allan variance in order to estimate the astrometric stability. From results, we establish a new classification that takes into account the whole multi-time scales information. The algorithm is flexible on the definition of ``stable source" through an adjustable threshold.

VLBI phase-referencing for observations of weak radio sources

NASA Technical Reports Server (NTRS)

Lestrade, J.-F.

1991-01-01

Phase-referencing is a technique used in VLBI to extend the signal coherence time from a few minutes to a few hours in order to enhance significantly its sensitivity. With this technique, VLBI observations of milliJansky radio sources can be conducted for high-accuracy differential astrometry as well as imaging. We describe the technique in some details and present, as an example, a submilliarcsecond differential astrometric experiment design to identify the star responsible for the weak radio emission in the binary system Algol.

22 GHz VLBI Survey: Status Report and Preliminary Results

NASA Technical Reports Server (NTRS)

Moellenbrock, G.; Fujisawa, K.; Preston, R.; Gurvits, L.; Dewey, R.; Hirabayashi, H.; Inoue, M.; Jauncey, D.; Migenes, V.; Roberts, D.;

Haystack Observatory Technology Development Center

NASA Technical Reports Server (NTRS)

Beaudoin, Chris; Corey, Brian; Niell, Arthur; Cappallo, Roger; Whitney, Alan

2013-01-01

Technology development at MIT Haystack Observatory were focused on four areas in 2012: VGOS developments at GGAO; Digital backend developments and workshop; RFI compatibility at VLBI stations; Mark 6 VLBI data system development.

HIGH-RESOLUTION VLBA OBSERVATIONS OF THREE 7 mm SiO MASERS TOWARD VX Sgr AT FIVE EPOCHS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Su, J. B.; Shen, Z.-Q.; Chen, X.

2012-07-20

VX Sgr is a red supergiant at an adopted distance of 1.6 kpc with intense 43 GHz SiO maser emission. In this paper, we present the high-resolution very long baseline interferometry (VLBI) observations of SiO masers toward VX Sgr at five epochs. We used the Very Long Baseline Array to map the J = 1{yields}0 (v = 1, 2) {sup 28}SiO masers and confirmed a ring-like structure. In the first two epochs, the v = 1 masers form a ring, but v = 2 maser spots residing only in the southern and northern regions do not form a complete ring.more » In the third epoch, the two masers are distributed in a ring structure and the v = 2 masers are a bit closer to the central star. In the last two epochs, many new maser spots appear and overlap each other. These overlapping maser spots can be related to the shock waves and reflect the collisional pumping. We compare the observations with the pumping models and speculate that the real pumping mechanism may be complex in VX Sgr and vary with time. The J = 1{yields}0 (v = 0) {sup 29}SiO line emission is also detected, but is too weak to produce any VLBI map.« less

VLBI survey of compact broad absorption line quasars with balnicity index BI = 0

NASA Astrophysics Data System (ADS)

Cegłowski, M.; Kunert-Bajraszewska, M.; Roskowiński, C.

2015-06-01

We present high-resolution observations, using both the European VLBI Network (EVN) at 1.7 GHz and the Very Long Baseline Array (VLBA) at 5 and 8.4 GHz, to image radio structures of 14 compact sources classified as broad absorption line (BAL) quasars based on the absorption index (AI). All sources but one were resolved, with the majority showing core-jet morphology typical for radio-loud quasars. We discuss in detail the most interesting cases. The high radio luminosities and small linear sizes of the observed objects indicate they are strong young active galactic nuclei. Nevertheless, the distribution of the radio-loudness parameter, log RI, of a larger sample of AI quasars shows that the objects observed by us constitute the most luminous, small subgroup of the AI population. Additionally, we report that for the radio-loudness parameter, the distribution of AI quasars and that for those selected using the traditional balnicity index differ significantly. Strong absorption is connected with lower log RI and thus probably larger viewing angles. Since the AI quasars have on average larger log RI, the orientation can mean that we see them less absorbed. However, we suggest that the orientation is not the only parameter that affects the detected absorption. That the strong absorption is associated with the weak radio emission is equally important and worth exploring.

Digital Front End for Wide-Band VLBI Science Receiver

NASA Technical Reports Server (NTRS)

Jongeling, Andre; Sigman, Elliott; Navarro, Robert; Goodhart, Charles; Rogstad, Steve; Chandra, Kumar; Finley, Sue; Trinh, Joseph; Soriano, Melissa; White, Les;

Constraining the radio jet proper motion of the high-redshift quasar J2134-0419 at z = 4.3

NASA Astrophysics Data System (ADS)

Perger, Krisztina; Frey, Sándor; Gabányi, Krisztina É.; An, Tao; Britzen, Silke; Cao, Hong-Min; Cseh, Dávid; Dennett-Thorpe, Jane; Gurvits, Leonid I.; Hong, Xiao-Yu; Hook, Isobel M.; Paragi, Zsolt; Schilizzi, Richard T.; Yang, Jun; Zhang, Yingkang

2018-06-01

To date, PMN J2134-0419 (at a redshift z = 4.33) is the second most distant quasar known with a milliarcsecond-scale morphology permitting direct estimates of the jet proper motion. Based on two-epoch observations, we constrained its radio jet proper motion using the very long baseline interferometry (VLBI) technique. The observations were conducted with the European VLBI Network (EVN) at 5 GHz on 1999 November 26 and 2015 October 6. We imaged the central 10-pc scale radio jet emission and modelled its brightness distribution. By identifying a jet component at both epochs separated by 15.86 yr, a proper motion of μ = 0.035 ± 0.023 mas yr-1 is found. It corresponds to an apparent superluminal speed of βa = 4.1 ± 2.7 c. Relativistic beaming at both epochs suggests that the jet viewing angle with respect to the line of sight is smaller than 20°, with a minimum bulk Lorentz factor Γ = 4.3. The small value of the proper motion is in good agreement with the expectations from the cosmological interpretation of the redshift and the current cosmological model. Additionally we analysed archival Very Large Array observations of J2143-0419 and found indication of a bent jet extending to ˜30 kpc.

Detection of atmospheric pressure loading using very long baseline interferometry measurements

NASA Technical Reports Server (NTRS)

Vandam, T. M.; Herring, T. A.

1994-01-01

Loading of the Earth by the temporal redistribution of global atmospheric mass is likely to displace the positions of geodetic monuments by tens of millimeters both vertically and horizontally. Estimates of these displacements are determined by convolving National Meteorological Center (NMC) global values of atmospheric surface pressure with Farrell's elastic Green's functions. An analysis of the distances between radio telescopes determined by very long baseline interferometry (VLBI) between 1984 and 1992 reveals that in many of the cases studied there is a significant contribution to baseline length change due to atmospheric pressure loading. Our analysis covers intersite distances of between 1000 and 10,000 km and is restricted to those baselines measured more than 100 times. Accounting for the load effects (after first removing a best fit slope) reduces the weighted root-mean-square (WRMS) scatter of the baseline length residuals on 11 of the 22 baselines investigated. The slight degradation observed in the WRMS scatter on the remaining baselines is largely consistent with the expected statistical fluctuations when a small correction is applied to a data set having a much larger random noise. The results from all baselines are consistent with approximately 60% of the computed pressure contribution being present in the VLBI length determinations. Site dependent coefficients determined by fitting local pressure to the theoretical radial displacement are found to reproduce the deformation caused by the regional pressure to within 25% for most inland sites. The coefficients are less reliable at near coastal and island stations.

Goddard Geophysical and Astronomical Observatory

NASA Technical Reports Server (NTRS)

Covey, Rawland; Kodak, Charles

1999-01-01

This report summarizes the technical parameters and the technical staff of the VLBI system at GGAO. It also gives an overview of VLBI activities during the previous year. The outlook lists the tasks planned for 1999.

Development of a New VLBI Data Analysis Software

NASA Technical Reports Server (NTRS)

Bolotin, Sergei; Gipson, John M.; MacMillan, Daniel S.

2010-01-01

We present an overview of a new VLBI analysis software under development at NASA GSFC. The new software will replace CALC/SOLVE and many related utility programs. It will have the capabilities of the current system as well as incorporate new models and data analysis techniques. In this paper we give a conceptual overview of the new software. We formulate the main goals of the software. The software should be flexible and modular to implement models and estimation techniques that currently exist or will appear in future. On the other hand it should be reliable and possess production quality for processing standard VLBI sessions. Also, it needs to be capable of processing observations from a fully deployed network of VLBI2010 stations in a reasonable time. We describe the software development process and outline the software architecture.

Centimeter repeatability of the VLBI estimates of European baselines

NASA Technical Reports Server (NTRS)

Rius, Antonio; Zarraoa, Nestor; Sardon, Esther; Ma, Chopo

1992-01-01

In the last three years, the European Geodetic Very Long Baseline Interferometry (VLBI) Network has grown to a total of six fixed antennas placed in Germany, Italy, Spain and Sweden, all equipped with the standard geodetic VLBI instrumentation and data recording systems. During this period of time, several experiments have been carried out using this interferometer providing data of very high quality due to the excellent sensitivity and performance of the European stations. The purpose of this paper is to study the consistency of the VLBI geodetic results on the European baselines with respect to the different degrees of freedom in the analysis procedure. Used to complete this study were both real and simulated data sets, two different software packages (OCCAM 3.0 and CALC 7.4/SOLVE), and a variety of data analysis strategies.

Parallel algorithm of VLBI software correlator under multiprocessor environment

NASA Astrophysics Data System (ADS)

Zheng, Weimin; Zhang, Dong

2007-11-01

The correlator is the key signal processing equipment of a Very Lone Baseline Interferometry (VLBI) synthetic aperture telescope. It receives the mass data collected by the VLBI observatories and produces the visibility function of the target, which can be used to spacecraft position, baseline length measurement, synthesis imaging, and other scientific applications. VLBI data correlation is a task of data intensive and computation intensive. This paper presents the algorithms of two parallel software correlators under multiprocessor environments. A near real-time correlator for spacecraft tracking adopts the pipelining and thread-parallel technology, and runs on the SMP (Symmetric Multiple Processor) servers. Another high speed prototype correlator using the mixed Pthreads and MPI (Massage Passing Interface) parallel algorithm is realized on a small Beowulf cluster platform. Both correlators have the characteristic of flexible structure, scalability, and with 10-station data correlating abilities.

Goddard Geophysical and Astronomical Observatory

NASA Technical Reports Server (NTRS)

Figueroa, Ricardo

2013-01-01

This report summarizes the technical parameters and the technical staff of the VLBI system at the fundamental station GGAO. It also gives an overview about the VLBI activities during the report year. The Goddard Geophysical and Astronomical Observatory (GGAO) consists of a 5-meter radio telescope for VLBI, a new 12-meter radio telescope for VLBI2010 development, a 1-meter reference antenna for microwave holography development, an SLR site that includes MOBLAS-7, the NGSLR development system, and a 48" telescope for developmental two-color Satellite Laser Ranging, a GPS timing and development lab, a DORIS system, meteorological sensors, and a hydrogen maser. In addition, we are a fiducial IGS site with several IGS/IGSX receivers. GGAO is located on the east coast of the United States in Maryland. It is approximately 15 miles NNE of Washington, D.C. in Greenbelt, Maryland.

Geodetic measurement of deformation in the Loma Prieta, California earthquake with Very Long Baseline Interferometry (VLBI)

DOE Office of Scientific and Technical Information (OSTI.GOV)

Clark, T.A.; Ma, C.; Sauber, J.M.

Following the Loma Prieta earthquake, two mobile Very Long Baseline Interferometry (VLBI) systems operated by the NASA Crustal Dynamics Project and the NOAA National Geodetic Survey were deployed at three previously established VLBI sites in the earthquake area: Fort Ord (near Monterey), the Presidio (in San Francisco) and Point Reyes. From repeated VLBI occupations of these sites since 1983, the pre-earthquake rates of deformation have been determined with respect to a North American reference frame with 1{sigma} formal standard errors of {approximately}1 mm/yr. The VLBI measurements immediately following the earthquake showed that the Fort Ord site was displaced 49 {plusmore » minus} 4 mm at an azimuth of 11 {plus minus} 4{degree} and that the Presidio site was displaced 12 {plus minus} 5 mm at an azimuth of 148 {plus minus} 13{degree}. No anomalous change was detected at Point Reyes with 1{sigma} uncertainty of 4 mm. The estimated displacements at Fort Ord and the Presidio are consistent with the static displacements predicted on the basis of a coseismic slip model in which slip on the southern segment is shallower than slip on the more northern segment is shallower than slip on the more northern segment of the fault rupture. The authors also give the Cartesian positions at epoch 1990.0 of a set of VLBI fiducial stations and the three mobile sites in the vicinity of the earthquake.« less

Twin-Telescope Wettzell (TTW)

NASA Astrophysics Data System (ADS)

Hase, H.; Dassing, R.; Kronschnabl, G.; Schlüter, W.; Schwarz, W.; Lauber, P.; Kilger, R.

2007-07-01

Following the recommendations made by the VLBI2010 vision report of the IVS, a proposal has been made to construct a Twin Telescope for the Fundamental Station Wettzell in order to meet the future requirements of the next VLBI generation. The Twin Telescope consists of two identical radiotelescopes. It is a project of the Federal Agency for Cartography and Geodesy (BKG). This article summarizes the project and some design ideas for the Twin-Telescope. %ZALMA (2005). Technical Specification for Design, Manufacturing, Transport and Integration on Site of the ALMA ANTENNAS, Doc. ALMA-34.00.00.00.006-BSPE. Behrend, D. (2006). VLBI2010 Antenna Specs, Data sheet. DeBoer, D. (2001). The ATA Offset Gregorian Antenna, ATA Memo #16, February 10. Imbriale, W.A. (2006). Design of a Wideband Radio Telescope, Jet Propulsion Laboratory and S. Weinreb and H. Mandi, California Institute of Technology. Kilger, R. (2007). TWIN-Design studies, Presentation for the IVS board members (internal document),Wettzell. Kronschnabl, G. (2006). Subject: Memo from Bill Petrachenko, E-mail to the Twin-Working Group (in German), July. Lindgren, ETS-Lindgren (2005). The Model 3164-05 Open Boundary Quadridge Horn, Data Sheet. Niell, A., A. Whitney, W. Petrachenko, W. Schlüter, N. Vandenberg, H.Hase, Y. Koyama, C. Ma, H. Schuh, G. Tucari (2006). in: IVS Annual Report 2005, pg. 13-40, NASA/TP-2006-214136, April. Olsson, R., Kildal, P.-S., and Weinreb, S. (2006). IEEE Transactions on Antennas and Propagation, Vol. 54, No. 2, February. Petrachenko, B. (2006). The Case For and Against Multiple Antennas at a Site, IVS Memorandum, 2006-019v01. Petrachenko, B. (2006). IVS Memorandum, 2006-016v01. RFSpin (2004). Double Ridged Waveguide Horn-Model DRH20, Antenna Specifications, Data Sheet. Rohde&Schwarz (2004). SHF Antennas Crossed Log- Periodic Antennas HL024A1/S1, Data Sheet. Rohde&Schwarz (2004). SHF Antennas Log-Periodic Antennas HL050/HL050S1, Data Sheet. Rogers, A.E.E. (2006). Simulations of broadband delay measurements, Mark 5 Memo #043, MIT Haystack Observatory. Rogers, A.E.E. (2006). Some thoughts on the calibration of broadband geodetic VLBI, Mark 5 Memo #044, MIT Haystack Observatory. Rothacher M. (2006). GGOS: the IAG contribution to Earth observation, IGS Workshop 2006 "Perspectives and Visions for 2010 and beyond", May 8-12, Darmstadt, Germany Weinreb, S., Mandi, H. (2006). Pattern and Noise Tests of ETS-Lindgren 3164-05 Quadridge/Vivaldi Antenna, California Institute of Technology. Weinreb, S. (2007). Broadband feeds, E-mail, January. Welch, Wm. J. (2005). The Allen Telescope Array, URSI, UC Berkeley, January.

Tracking of Mars Express and Venus Express spacecraft with VLBI radio telescopes

NASA Astrophysics Data System (ADS)

Molera Calvés, G.; Pogrebenko, S. V.; Wagner, J.; Cimò, G.; Gurvits, L.; Duev, D.

2010-12-01

The ESA Mars Express and Venus Express spacecraft (S/C) have been observed for the last two years with the European VLBI radio telescopes of Metsähovi (FI), Wettzell (GE), Yebes (SP), Medicina, Matera, Noto (IT), Puschino (RU) and Onsala (SW). The campaign is in the framework of the assessment study and preparation of the European VLBI Network to the upcoming ESA and other deep space missions. It also offers new opportunities for applications of radio astronomy techniques to planetary science, geophysics and geodesy. Observations are carried out either in single- or multi-dish modes when S/C is locked to the ESA’s ESTRACK ground stations (Cebreros or New Nortia) observing the two way link. Data are recorded locally at the stations using standard VLBI equipment and transferred to the Metsähovi for processing. Further on, the data are transferred from Metsähovi to Joint Institute for VLBI in Europe for further post-analysis. High dynamic range of the S/C signal detections allowed us to determine the apparent topocentric frequency of the S/C carrier line and accompanying ranging tones down to milli-Hz spectral accuracy and to extract the phase of the S/C signal carrier line. With multi-station observations, the respective phases can be calibrated on the per-baseline basis using VLBI phase referencing technique and observations of background quasars close to S/C in their celestial position using far-field VLBI delay model for quasars and near-field model for S/C. The post-analysis of the S/C tracking data enables us to study several parameters of the S/C signals. Of these, the phase fluctuations of the signal can be used for characterization of the interplanetary plasma density fluctuations along the signal propagation line at different spatial and temporal scales and different solar elongations. These fluctuations are well represented by a near-Kolmogorov spectrum. Multi-station observations can distinguish the contributions of propagation effects in the plasma along the up- and down-link paths as seen from different VLBI stations through different Fresnel channels. We also demonstrated high accuracy S/C Doppler tracking with 3 EVN stations (Metsähovi, Wettzell and Yebes) during the MEX-Phobos flyby, which occurred on 2010 march 03. These multi-station observing sessions could help to better determine the Phobos gravity field and together with phase referencing provide additional geometrical constrains on the orbiter/Phobos trajectories. VEX S/C signal detection with four VLBI antennae (23/08/2010).

Comparison of pulsar positions from timing and very long baseline astrometry

NASA Astrophysics Data System (ADS)

Wang, J. B.; Coles, W. A.; Hobbs, G.; Shannon, R. M.; Manchester, R. N.; Kerr, M.; Yuan, J. P.; Wang, N.; Bailes, M.; Bhat, N. D. R.; Dai, S.; Dempsey, J.; Keith, M. J.; Lasky, P. D.; Levin, Y.; Osłowski, S.; Ravi, V.; Reardon, D. J.; Rosado, P. A.; Russell, C. J.; Spiewak, R.; van Straten, W.; Toomey, L.; Wen, L.; You, X.-P.; Zhu, X.-J.

2017-07-01

Pulsar positions can be measured with high precision using both pulsar timing methods and very long baseline interferometry (VLBI). Pulsar timing positions are referenced to a solar-system ephemeris, whereas VLBI positions are referenced to distant quasars. Here, we compare pulsar positions from published VLBI measurements with those obtained from pulsar timing data from the Nanshan and Parkes radio telescopes in order to relate the two reference frames. We find that the timing positions differ significantly from the VLBI positions (and also differ between different ephemerides). A statistically significant change in the obliquity of the ecliptic of 2.16 ± 0.33 mas is found for the JPL ephemeris DE405, but no significant rotation is found in subsequent JPL ephemerides. The accuracy with which we can relate the two frames is limited by the current uncertainties in the VLBI reference source positions and in matching the pulsars to their reference source. Not only do the timing positions depend on the ephemeris used in computing them, but also different segments of the timing data lead to varying position estimates. These variations are mostly common to all ephemerides, but slight changes are seen at the 10 μas level between ephemerides.

Supernova VLBI

NASA Astrophysics Data System (ADS)

Bartel, N.

2009-08-01

We review VLBI observations of supernovae over the last quarter century and discuss the prospect of imaging future supernovae with space VLBI in the context of VSOP-2. From thousands of discovered supernovae, most of them at cosmological distances, ˜50 have been detected at radio wavelengths, most of them in relatively nearby galaxies. All of the radio supernovae are Type II or Ib/c, which originate from the explosion of massive progenitor stars. Of these, 12 were observed with VLBI and four of them, SN 1979C, SN 1986J, SN 1993J, and SN 1987A, could be imaged in detail, the former three with VLBI. In addition, supernovae or young supernova remnants were discovered at radio wavelengths in highly dust-obscured galaxies, such as M82, Arp 299, and Arp 220, and some of them could also be imaged in detail. Four of the supernovae so far observed were sufficiently bright to be detectable with VSOP-2. With VSOP-2 the expansion of supernovae can be monitored and investigated with unsurpassed angular resolution, starting as early as the time of the supernova's transition from its opaque to transparent stage. Such studies can reveal, in a movie, the aftermath of a supernova explosion shortly after shock break out.

Concepts for VLBI Station Control as Part of NEXPReS

NASA Astrophysics Data System (ADS)

Ettl, M.; Neidhardt, A.; Schönberger, M.; Alef, W.; Himwich, E.; Beaudoin, C.; Plötz, C.; Lovell, J.; Hase, H.

2012-12-01

In the Novel EXploration Pushing Robust e-VLBI Services-project (NEXPReS) the Technische Universität München (TUM) realizes concepts for continuous quality monitoring and station remote control in cooperation with the Max-Planck-Institute for Radio Astronomy, Bonn. NEXPReS is a three-year project, funded within the European Seventh Framework program. It is aimed to develop e-VLBI services for the European VLBI Network (EVN), which can also support the IVS observations (VLBI2010). Within this project, the TUM focuses on developments of an operational remote control system (e-RemoteCtrl) with authentication and authorization. It includes an appropriate role management with different remote access states for future observation strategies. To allow a flexible control of different systems in parallel, sophisticated graphical user interfaces are designed and realized. The software is currently under test in the new AuScope network, Australia/New Zealand. Additional system parameters and information are collected with a new system monitoring (SysMon) for a higher degree of automation, which is currently under preparation for standardization within the IVS Monitoring and Control Infrastructure (MCI) Collaboration Group. The whole system for monitoring and control is fully compatible with the NASA Field System and extends it.

Plans for the New Korean VLBI Network (kvn)

NASA Astrophysics Data System (ADS)

Fletcher, Andre B.

The new Korean VLBI Network construction project started in 2001. Currently funds have been allocated until 2007. Three new 20m diameter fast-slewing high-precision radio antennae will be built in South Korea at Seoul Tamna and Ulsan. These will be exclusively for VLBI astronomy astrometry and geodesy with a focus on developing millimeter-wave VLBI. First 2/8 22 43 and 86 GHz HEMT receivers will be used; 100/150 GHz SIS receivers will be installed later. The KVN Data Acquisition System (DAS) is being designed to realize a multi-band dual-circular polarization wide-bandwidth system. Very-high-speed digital samplers will generate data streams at rates of at least 1 Gigabit/s and a digital filter system will select the recorded frequency channels according to several observation modes one or more of which would be compatible with the VLBA and VERA DASs. The data will be transmitted by optical fibers to the new MIT Mark 5B hard-disk recorders. The KVN correlator will be designed with new technological concepts in mind. The observational capabilities and science goals of KVN are outlined. International scientific and technical collaborations with VLBI institutes will be essential for the ultimate success of the KVN project

ostglacial rebound from VLBI Geodesy: On Establishing Vertical Reference

NASA Technical Reports Server (NTRS)

Argus, Donald .

1996-01-01

I propose that a useful reference frame for vertical motions is that found by minimizing differences between vertical motions observed with VLBI [Ma and Ryan, 1995] and predictions from postglacial rebound predictions [Peltier, 1995].

Subliminal expansion in NGC1275

NASA Astrophysics Data System (ADS)

Backer, Donald C.

The neraby, active galaxy NGC 1275 (3C 84, z = 0.018) has been the subject of many investigations in the past 20 years. The author reviews the evolution of the microwave flux density and the structure seen in 10.7 GHz VLBI images. Then he summarizes recent VLBI observations at 22 GHz and 90 GHz. Rapid changes in the core and the presence of a "hot spot" in the outflowing material are prominent features of these short-wavelength VLBI data. The case for subluminal proper velocities of the outflow is strong.

Preliminary design work on a DSN VLBI correlator. [Deep Space Network

NASA Technical Reports Server (NTRS)

Lushbaugh, W. A.; Layland, J. W.

1978-01-01

The Deep Space Network is in the process of fielding high-density digital instrumentation recorders for support of the Pioneer Venus 1978 entry experiment and other related tasks. It has long been obvious that these recorders would also serve well as the recording medium for very long base interferometry (VLBI) experiments with relatively weak radio sources, provided that a suitable correlation processor for these tape recordings could be established. The overall design and current status of a VLBI correlator designed to mate with these tape recorders are described.

Arcsecond positions for milliarcsecond VLBI nuclei of extragalactic radio sources. IV - Seventeen sources

NASA Technical Reports Server (NTRS)

Morabito, D. D.; Preston, R. A.; Linfield, R. P.; Slade, M. A.; Jauncey, D. L.

1986-01-01

VLBI measurements of time delay and delay rate at 2.29 and 8.42 GHz on baselines of 10,000 km have been used to determine the positions of the milliarcsecond nuclei in 17 extragalactic radio sources with estimated accuracies of 0.1 to 0.3 arcsec. The observed sources are part of an all-sky VLBI catalog of milliarcsecond radio sources. In addition, slightly improved positions are presented for 101 sources originally reported by Morabito et al. (1983). Arcsecond positions have now been determined for 836 sources.

Postseismic Transient after the 2002 Denali Fault Earthquake from VLBI Measurements at Fairbanks

NASA Technical Reports Server (NTRS)

MacMillan, Daniel; Cohen, Steven

2004-01-01

The VLBI antenna (GILCREEK) at Fairbanks, Alaska observes in networks routinely twice a week with operational networks and on additional days with other networks on a more uneven basis. The Fairbanks antenna position is about 150 km north of the Denali fault and from the earthquake epicenter. We examine the transient behavior of the estimated VLBI position during the year following the earthquake to determine how the rate of change of postseismic deformation has changed. This is compared with what is seen in the GPS site position series.

Crustal dynamics project session 4 validation and intercomparison experiments 1979-1980 report

NASA Technical Reports Server (NTRS)

Liebrecht, P.; Kolenkiewicz, R.; Ryan, J.; Hothem, L.

1983-01-01

As part of the Crustal Dynamics Project, an experiment was performed to verify the ability of Satellite Laser Ranging (SLR), Very Long Baseline interferometry (VLBI) and Doppler Satellite Positioning System (Doppler) techniques to estimate the baseline distances between several locations. The Goddard Space Flight Center (GSFC) lasers were in operation at all five sites available to them. The ten baselines involved were analyzed using monthly orbits and various methods of selecting data. The standard deviation of the monthly SLR baseline lengths was at the 7 cm level. The GSFC VLBI (Mark III) data was obtained during three separate experiments. November 1979 at Haystack and Owens Valley, and April and July 1980 at Haystack, Owens Valley, and Fort Davis. Repeatability of the VLBI in determining baseline lengths was calculated to be at the 2 cm level. Jet Propulsion Laboratory (JPL) VLBI (Mark II) data was acquired on the Owens Valley to Goldstone baseline on ten occasions between August 1979 and November 1980. The repeatability of these baseline length determinations was calculated to be at the 5 cm level. National Geodetic Survey (NGS) Doppler data was acquired at all five sites in January 1980. Repeatability of the Doppler determined baseline lengths results were calculated at approximately 30 cm. An intercomparison between baseline distances and associated parameters was made utilizing SLR, VLBI, and Doppler results on all available baselines. The VLBI and SLR length determinations were compared on four baselines with a resultant mean difference of -1 cm and a maximum difference of 12 cm. The SLR and Doppler length determinations were compared on ten baselines with a resultant mean difference of about 30 cm and a maximum difference of about 60 cm. The VLBI and Doppler lengths from seven baselines showed a resultant mean difference of about 30 cm and maximum difference of about 1 meter. The intercomparison of baseline orientation parameters were consistent with past analysis.

Complex demodulation in VLBI estimation of high frequency Earth rotation components

NASA Astrophysics Data System (ADS)

Böhm, S.; Brzeziński, A.; Schuh, H.

2012-12-01

The spectrum of high frequency Earth rotation variations contains strong harmonic signal components mainly excited by ocean tides along with much weaker non-harmonic fluctuations driven by irregular processes like the diurnal thermal tides in the atmosphere and oceans. In order to properly investigate non-harmonic phenomena a representation in time domain is inevitable. We present a method, operating in time domain, which is easily applicable within Earth rotation estimation from Very Long Baseline Interferometry (VLBI). It enables the determination of diurnal and subdiurnal variations, and is still effective with merely diurnal parameter sampling. The features of complex demodulation are used in an extended parameterization of polar motion and universal time which was implemented into a dedicated version of the Vienna VLBI Software VieVS. The functionality of the approach was evaluated by comparing amplitudes and phases of harmonic variations at tidal periods (diurnal/semidiurnal), derived from demodulated Earth rotation parameters (ERP), estimated from hourly resolved VLBI ERP time series and taken from a recently published VLBI ERP model to the terms of the conventional model for ocean tidal effects in Earth rotation recommended by the International Earth Rotation and Reference System Service (IERS). The three sets of tidal terms derived from VLBI observations extensively agree among each other within the three-sigma level of the demodulation approach, which is below 6 μas for polar motion and universal time. They also coincide in terms of differences to the IERS model, where significant deviations primarily for several major tidal terms are apparent. An additional spectral analysis of the as well estimated demodulated ERP series of the ter- and quarterdiurnal frequency bands did not reveal any significant signal structure. The complex demodulation applied in VLBI parameter estimation could be demonstrated a suitable procedure for the reliable reproduction of high frequency Earth rotation components and thus represents a qualified tool for future studies of irregular geophysical signals in ERP measured by space geodetic techniques.

VLBI Radar of the 2012 DA14 Asteroid

NASA Astrophysics Data System (ADS)

Nechaeva, M. B.; Dugin, N. A.; Antipenko, A. A.; Bezrukov, D. A.; Bezrukov, V. V.; Voytyuk, V. V.; Dement'ev, A. F.; Jekabsons, N.; Klapers, M.; Konovalenko, A. A.; Kulishenko, V. F.; Nabatov, A. S.; Nesteruk, V. N.; Putillo, D.; Reznichenko, A. M.; Salerno, E.; Snegirev, S. D.; Tikhomirov, Yu. V.; Khutornoy, R. V.; Skirmante, K.; Shmeld, I.; Chagunin, A. K.

2015-03-01

An experiment on VLBI radar of the 2012 DA14 asteroid was carried out on February 15-16, 2011 at the time of its closest approach to the Earth. The research teams of Kharkov (Institute of Radio Astronomy of the National Academy of Sciences of Ukraine), Evpatoria (National Space Facilities Control and Test Center), Nizhny Novgorod (Radiophysical Research Institute), Bologna (Istituto di Radioastronomia (INAF)), and Ventspils (Ventspils International Radioastronomy Center) took part in the experiment. The asteroid was irradiated by the RT-70 planetary radar (Evpatoria) at a frequency of 5 GHz. The reflected signal was received using two 32-m radio telescopes in Medicina (Italy) and Irbene (Latvia) in radiointerferometric mode. The Doppler frequency shifts in bi-static radar mode and interference frequency in VLBI mode were measured. Accuracy of the VLBI radar method for determining the radial and angular velocities of the asteroid were estimated.

Integration of Reference Frames Using VLBI

NASA Technical Reports Server (NTRS)

Ma, Chopo; Smith, David E. (Technical Monitor)

2001-01-01

Very Long Baseline Interferometry (VLBI) has the unique potential to integrate the terrestrial and celestial reference frames through simultaneous estimation of positions and velocities of approx. 40 active VLBI stations and a similar number of stations/sites with sufficient historical data, the position and position stability of approx. 150 well-observed extragalactic radio sources and another approx. 500 sources distributed fairly uniformly on the sky, and the time series of the five parameters that specify the relative orientation of the two frames. The full realization of this potential is limited by a number of factors including the temporal and spatial distribution of the stations, uneven distribution of observations over the sources and the sky, variations in source structure, modeling of the solid/fluid Earth and troposphere, logistical restrictions on the daily observing network size, and differing strategies for optimizing analysis for TRF, for CRF and for EOP. The current status of separately optimized and integrated VLBI analysis will be discussed.

Tropospheric Delay Raytracing Applied in VLBI Analysis

NASA Astrophysics Data System (ADS)

MacMillan, D. S.; Eriksson, D.; Gipson, J. M.

2013-12-01

Tropospheric delay modeling error continues to be one of the largest sources of error in VLBI analysis. For standard operational solutions, we use the VMF1 elevation-dependent mapping functions derived from ECMWF data. These mapping functions assume that tropospheric delay at a site is azimuthally symmetric. As this assumption does not reflect reality, we have determined the raytrace delay along the signal path through the troposphere for each VLBI quasar observation. We determined the troposphere refractivity fields from the pressure, temperature, specific humidity and geopotential height fields of the NASA GSFC GEOS-5 numerical weather model. We discuss results from analysis of the CONT11 R&D and the weekly operational R1+R4 experiment sessions. When applied in VLBI analysis, baseline length repeatabilities were better for 66-72% of baselines with raytraced delays than with VMF1 mapping functions. Vertical repeatabilities were better for 65% of sites.

SAND: an automated VLBI imaging and analysing pipeline - I. Stripping component trajectories

NASA Astrophysics Data System (ADS)

Zhang, M.; Collioud, A.; Charlot, P.

2018-02-01

We present our implementation of an automated very long baseline interferometry (VLBI) data-reduction pipeline that is dedicated to interferometric data imaging and analysis. The pipeline can handle massive VLBI data efficiently, which makes it an appropriate tool to investigate multi-epoch multiband VLBI data. Compared to traditional manual data reduction, our pipeline provides more objective results as less human interference is involved. The source extraction is carried out in the image plane, while deconvolution and model fitting are performed in both the image plane and the uv plane for parallel comparison. The output from the pipeline includes catalogues of CLEANed images and reconstructed models, polarization maps, proper motion estimates, core light curves and multiband spectra. We have developed a regression STRIP algorithm to automatically detect linear or non-linear patterns in the jet component trajectories. This algorithm offers an objective method to match jet components at different epochs and to determine their proper motions.

Six-hourly time series of horizontal troposphere gradients in VLBI analyis

NASA Astrophysics Data System (ADS)

Landskron, Daniel; Hofmeister, Armin; Mayer, David; Böhm, Johannes

2016-04-01

Consideration of horizontal gradients is indispensable for high-precision VLBI and GNSS analysis. As a rule of thumb, all observations below 15 degrees elevation need to be corrected for the influence of azimuthal asymmetry on the delay times, which is mainly a product of the non-spherical shape of the atmosphere and ever-changing weather conditions. Based on the well-known gradient estimation model by Chen and Herring (1997), we developed an augmented gradient model with additional parameters which are determined from ray-traced delays for the complete history of VLBI observations. As input to the ray-tracer, we used operational and re-analysis data from the European Centre for Medium-Range Weather Forecasts. Finally, we applied those a priori gradient parameters to VLBI analysis along with other empirical gradient models and assessed their impact on baseline length repeatabilities as well as on celestial and terrestrial reference frames.

IVS Pilot Project - Tropospheric Parameters

NASA Astrophysics Data System (ADS)

Boehm, J.; Schuh, H.; Engelhardt, G.; MacMillan, D.; Lanotte, R.; Tomasi, P.; Vereshchagina, I.; Haas, R.; Negusini, M.; Gubanov, V.

2003-04-01

In April 2002 the IVS (International VLBI Service for Geodesy and Astrometry) set up the IVS Pilot Project - Tropospheric Parameters and the Institute of Geodesy and Geophysics (IGG), Vienna, was asked to coordinate the project. After a call for participation six IVS Analysis Centers have joined the project and submitted their estimates of tropospheric parameters (wet and total zenith delays, horizontal gradients) for all IVS-R1 and IVS-R4 sessions since January 1st, 2002, on a regular basis. Using a two-step procedure the individual submissions are combined to stable and robust tropospheric parameters with 1h resolution and high accuracy. The zenith delays derived by VLBI are also compared with those provided by IGS (International GPS Service). At collocated sites (VLBI and GPS antennas at the same station) rather constant biases are found between the GPS and VLBI derived zenith delays, although both techniques are subject to the same tropospheric delays. Possible reasons for these biases are discussed.

Automatic system of collection of parameters and control of receiving equipment of the radiotelescope of VLBI complex "Quasar "

NASA Astrophysics Data System (ADS)

Syrovoy, Sergey

At present the radiointerferometry with Very Long Bases (VLBI) is more and more globalized, turning into the world network of observation posts. So the inclusion of the developing Russian system "Quasar" into the world VLBI community has a great importance to us. The important role in the work of radiotelescope as a part of VLBI network belongs to a question of ensuring the optimal interaction of the its sub-systems, which can only be done by means of automation of the whole process of observation. The possibility of participation of RTF-32 in the international VLBI sessions observation is taken into account in the system development. These observations have the stable technology of experiments on the base Mark-IV Field System. In this paper the description, the structured and the functional schemes of the system of automatic collection of parameters and control of receiving complex of radiotelescope RTF-32 are given. This system is to solve the given problem. The most important tasks of the system being developed are the ensuring of distant checking and control of the following systems of the radiotelescope: 1. the receivers system, which consists of the five dual-channel radiometers 21-18 sm, 13 sm, 6 sm, 3.5 sm, 1.35 sm brands; 2. the radiotelescope pointing system; 3. the frequency-time synchronizing system, which consists of the hydrogen standard of frequency, the system of ultrahigh frequency oscillators and the generators of picosecond impulses; 4. the signal transformation system; 5. the signal registration system; 6. the system of measurement of electrical features of atmosphere; 7. the power supply system. The part of the automatic system, ensuring the distant checking and control of the radiotelescope pointing system both in the local mode and in the state of working under control the Field System computer, was put into operation and is functioning at this moment. Now the part of the automatic system ensuring the checking and control of receiving system of radiotelescope is being developed. The functional scheme has been designed. The experimental model of the device of connection of control PC with the terminal has been produced. The algorithms of receiver control in the different modes of observation have been developed. The questions of interaction with the computer Field System have been solved. The radiotelescope RTF-32 is capable of functioning in two modes such as radio-astronomical and radio-interferometrical. The control of the transformation signal system and the registration signal system in these modes is different and is entrusted with the Field System computer. The automation of collection of the meteorological data and parameters of the power supply system of the radiotelescope is last stage of the development of the presented system.

Earth Rotation Parameters from DSN VLBI: 1994

NASA Technical Reports Server (NTRS)

Steppe, J. A.; Oliveau, S. H.; Sovers, O. J.

1994-01-01

In this report, Earth Rotation Parameter (ERP) estimates ahve been obtained from an analysis of Deep Space Network (DSN) VLBI data that directly aligns its celestial and terrestrial reference frames with those of the International Earth Rotation Service (IERS).

Kashima and Koganei 11-m VLBI Stations

NASA Technical Reports Server (NTRS)

Sekido, Mamoru; Ichikawa, Ryuichi

2013-01-01

Two 11-m VLBI antennas at Kashima and Koganei are continuously operated and maintained by the National Institute of Information and Communications Technology (NICT). This report summarizes the status of these antennas, the staff, and the activities in 2012.

Space Geodesy, VLBI, and the Fourth Pillar of Geodesy - Spacetime Curvature

NASA Astrophysics Data System (ADS)

Combrinck, Ludwig

2014-12-01

Typically geodesy is described as having ``three pillars'': the variations in Earth's shape, gravity field, and rotation. These pillars form the conceptual and observational basis for the celestial and terrestrial reference frames required for Earth and space observations. However, it is no longer adequate to base the conceptual and observational basis on only three pillars. Spacetime curvature as described by the General Theory of Relativity (GTR) is an integral component of all space geodesy techniques and influences all measurements, techniques, and data reduction. Spacetime curvature is therefore the fourth pillar. It is the measurement of the shape of spacetime and its variations. Due to accuracies of Very Long Baseline Interferometry (VLBI) and optical celestial reference frame measurements reaching the tens of micro-arcsecond level in the near future, it is essential to recognize the impact of spacetime seeing on the accuracy objectives of the Global Geodetic Observing System. Spacetime seeing (resulting from spacetime curvature) is analogous to astronomical seeing (resulting from atmospheric conditions), as all of spacetime is affected by microlensing/weak lensing to some extent as a result of mass (normal baryonic and darkmatter) distribution, placing a limit on the realization of the celestial reference frame.

Atmospheric Delay Reduction Using KARAT for GPS Analysis and Implications for VLBI

NASA Technical Reports Server (NTRS)

Ichikawa, Ryuichi; Hobiger, Thomas; Koyama, Yasuhiro; Kondo, Tetsuro

2010-01-01

We have been developing a state-of-the-art tool to estimate the atmospheric path delays by raytracing through mesoscale analysis (MANAL) data, which is operationally used for numerical weather prediction by the Japan Meteorological Agency (JMA). The tools, which we have named KAshima RAytracing Tools (KARAT)', are capable of calculating total slant delays and ray-bending angles considering real atmospheric phenomena. The KARAT can estimate atmospheric slant delays by an analytical 2-D ray-propagation model by Thayer and a 3-D Eikonal solver. We compared PPP solutions using KARAT with that using the Global Mapping Function (GMF) and Vienna Mapping Function 1 (VMF1) for GPS sites of the GEONET (GPS Earth Observation Network System) operated by Geographical Survey Institute (GSI). In our comparison 57 stations of GEONET during the year of 2008 were processed. The KARAT solutions are slightly better than the solutions using VMF1 and GMF with linear gradient model for horizontal and height positions. Our results imply that KARAT is a useful tool for an efficient reduction of atmospheric path delays in radio-based space geodetic techniques such as GNSS and VLBI.

Differences Between VLBI2010 and S/X Hardware

NASA Technical Reports Server (NTRS)

Corey, Brian

2010-01-01

While the overall architecture is similar for the station hardware in current S/X systems and in the VLBI2010 systems under development, various functions are implemented differently. Some of these differences, and the reasons behind them, are described here.

Goddard Geophysical and Astronomical Observatory

NASA Technical Reports Server (NTRS)

Redmond, Jay; Kodak, Charles

2004-01-01

This report summarizes the technical parameters and the technical staff of the VLBI system at the fundamental station GGAO. It also gives an overview about the VLBI activities during the previous year. The outlook lists the outstanding tasks to improve the performance of GGAO.

International VLBI Service for Geodesy and Astrometry 2005 Annual Report

NASA Technical Reports Server (NTRS)

Behrend, Dirk (Editor); Baver, Karen D. (Editor)

2006-01-01

This volume of reports is the 2005 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the components of IVS. The 2005 Annual Report documents the work of these IVS components over the period January 1, 2005 through December 31, 2005. The reports document changes, activities, and progress of the IVS. The entire contents of this Annual Report also appear on the IVS Web site at http://ivscc.gsfc.nasa.gov/publications/ar2005.

Radio stars - A possible link between the Hipparcos optical reference frame and an extra-galactic very long baseline interferometry reference frame

NASA Technical Reports Server (NTRS)

Lestrade, J.-F.; Preston, R. A.; Slade, M. A.

1983-01-01

The concept of typing the Hipparcos optical and the JPL VLBI frames of reference by means of VLBI measurements of the positions and proper motions of the radio components of some bright stars is considered. The properties of the thermal and non-thermal radio-stars are discussed and 22 candidate stars are selected to achieve this tie. A description is given of the first VLBI attempt to detect these stars on the intercontinental baselines of the Deep Space Network with the Mark II recording system.

International VLBI Service for Geodesy and Astrometry

NASA Technical Reports Server (NTRS)

Vandenberg, Nancy R. (Editor); Baver, Karen D. (Editor)

2001-01-01

This volume of reports is the 2000 Annual Report of the International Very Long Base Interferometry (VLBI) Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the components of IVS. The 2000 Annual Report documents the work of these IVS components over the period March 1, 1999, through December 31, 2000. The reports document changes, activities, and progress of the IVS. The entire contents of this Annual Report also appear on the IVS web site at http://ivscc.gsfc.nasa.gov/publications/ar2000.

International VLBI Service for Geodesy and Astrometry 2007 Annual Report

NASA Technical Reports Server (NTRS)

Behrend, D. (Editor); Baver, K. D. (Editor)

2008-01-01

This volume of reports is the 2007 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the components of IVS. The 2007 Annual Report documents the work of these IVS components over the period January 1, 2007 through December 31, 2007. The reports document changes, activities, and progress of the IVS. The entire contents of this Annual Report also appear on the IVS Web site at http://ivscc.gsfc.nasa.gov/publications/ar2007.

International VLBI Service for Geodesy and Astrometry 2008 Annual Report

NASA Technical Reports Server (NTRS)

Behrend, Dirk; Baver, Karen D.

2009-01-01

This volume of reports is the 2008 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the components of IVS. The 2008 Annual Report documents the work of these IVS components over the period January 1, 2008 through December 31, 2008. The reports document changes, activities, and progress of the IVS. The entire contents of this Annual Report also appear on the IVS Web site at http://ivscc.gsfc.nasa.gov/publications/ar2008.

International VLBI Service for Geodesy and Astrometry 2011 Annual Report

NASA Technical Reports Server (NTRS)

Baver, Karen D. (Editor); Behrend, Dirk

2012-01-01

This volume of reports is the 2011 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the components of IVS. The 2011 Annual Report documents the work of these IVS components over the period January 1, 2011 through December 31, 2011. The reports document changes, activities, and progress of the IVS. The entire contents of this Annual Report also appear on the IVS Web site at http://ivscc.gsfc.nasa.gov/publications/ar2011.

EVN e-VLBI detections of MAXI J1659-152

NASA Astrophysics Data System (ADS)

Paragi, Z.; van der Horst, A. J.; Granot, J.; Taylor, G. B.; Kouveliotou, C.; Garrett, M. A.; Wijers, R. A. M. J.; Ramirez-Ruiz, E.; Kuulkers, E.; Gehrels, N.; Woods, P. M.

2010-10-01

We observed MAXI J1659-152 (Negoro et al. 2010, ATel #2873; Mangano et al. 2010, GCN #11296) following its sub-millimeter and centimeter radio detections (de Ugarte Postigo et al. 2010, GCN #11304; van der Horst et al. 2010, ATel #2874) with the European VLBI Network (EVN) in real-time e-VLBI mode on 30 September 2010, from 13:30 to 18:30 UT at 4.9 GHz. The participating telescopes were Cambridge, Effelsberg, Jodrell Bank (MkII), Hartebeesthoek, Medicina, Onsala, Torun and Westerbork sending data at a rate of ~1024 Mbps to the EVN Data Processor at JIVE.

VLBI2010 Feed Comparison

NASA Technical Reports Server (NTRS)

Petrachenko, Bill

2013-01-01

VLBI2010 requires a feed that simultaneously has high efficiency over the full 2.2-14 GHz frequency range. The simultaneity requirement implies that the feed must operate at high efficiency over the full frequency range without the need to adjust its focal position to account for frequency dependent phase centre variations. Two feeds meet this specification: The Eleven Feed developed at Chalmers University. (For more information, contact Miroslav Pantaleev, miroslav.pantaleev@chalmers.se. The Eleven Feed, integrated with LNA's in a cryogenic receiver, is available as a product from Omnisys Instruments, info@omnisys.se). The Quadruple Ridged Flared Horn (QRFH) developed at the California Institute of Technology. (For more information please contact Ahmed Akgiray, aakgiray@ieee.org or Sander Weinreb, sweinreb@caltech.edu) Although not VLBI2010 compliant, two triband S/X/Ka feeds are also being developed for the commissioning of VLBI2010 antennas, for S/X observations during the VLBI2010 transition period, and to support X/Ka CRF observations. The two feeds are: The Twin Telescopes Wettzell (TTW) triband feed developed by Mirad Microwave. (For more information please contact Gerhard Kronschnabl, Gerhard.Kronschnabl@bkg.bund.de) The RAEGE (Spain) triband feed developed at Yebes Observatory. (For more information please contact Jose Antonio Lopez Perez, ja.lopezperez@oan.es)

South African Student Constructed Indlebe Radio Telescope

NASA Astrophysics Data System (ADS)

McGruder, Charles H.; MacPherson, Stuart; Janse Van Vuuren, Gary Peter

2017-01-01

The Indlebe Radio Telescope (IRT) is a small transit telescope with a 5 m diameter parabolic reflector working at 21 cm. It was completely constructed by South African (SA) students from the Durban University of Technology (DUT), where it is located. First light occurred on 28 July 2008, when the galactic center, Sagittarius A, was detected. As a contribution to the International Year of Astronomy in 2009, staff members in the Department of Electronic Engineering at DUT in 2006 decided to have their students create a fully functional radio telescope by 2009. The specific project aims are to provide a visible project that could generate interest in science and technology in high school students and to provide a real world system for research in radio astronomy in general and an optimization of low noise radio frequency receiver systems in particular. These aims must be understood in terms of the SA’s government interests in radio astronomy. SA is a partner in the Square Kilometer Array (SKA) project, has constructed the Karoo Array Telescope (KAT) and MeerKat, which is the largest and most sensitive radio telescope in the southern hemisphere. SA and its partners in Africa are investing in the construction of the African Very Long Baseline Interferometry Network (AVN), an array of radio telescopes throughout Africa as an extension of the existing global Very Long Baseline Interferometry Network (VLBI). These projects will allow SA to make significant contributions to astronomy and enable astronomy to contribute to the scientific education and development goals of the country. The IRT sees on a daily basis the transit of Sag A. The transit time is influenced by precession, nutation, polar motion, aberration, celestial pole offset, proper motion, length of the terrestrial day and variable ionospheric refraction. Of these eight factors six are either predictable or measureable. To date neither celestial pole offset nor variable ionospheric refraction are predicable. Currently, we are comparing the observed transit times of Sag A with the calculable predications in order to obtain information over these two factors, with a view to better understanding them.

Project PARAS: Phased array radio astronomy from space

NASA Technical Reports Server (NTRS)

Nuss, Kenneth; Hoffmann, Christopher; Dungan, Michael; Madden, Michael; Bendakhlia, Monia

1992-01-01

An orbiting radio telescope is proposed which, when operated in a very long baseline interferometry (VLBI) scheme, would allow higher than currently available angular resolution and dynamic range in the maps and the ability to observe rapidly changing astronomical sources. Using passive phased array technology, the proposed design consists of 656 hexagonal modules forming a 150-m diameter antenna dish. Each observatory module is largely autonomous, having its own photovoltaic power supply and low-noise receiver and processor for phase shifting. The signals received by the modules are channeled via fiber optics to the central control computer in the central bus module. After processing and multiplexing, the data are transmitted to telemetry stations on the ground. The truss frame supporting each observatory panel is a novel hybrid structure consisting of a bottom graphite/epoxy tubular triangle and rigidized inflatable Kevlar tubes connecting the top observatory panel and the bottom triangle. Attitude control and station keeping functions will be performed by a system of momentum wheels in the bus and four propulsion modules located at the compass points on the periphery of the observatory dish. Each propulsion module has four monopropellant thrusters and four hydrazine arcjets, the latter supported by either a photovoltaic array or a radioisotope thermoelectric generator. The total mass of the spacecraft is about 20,500 kg.

The European VLBI network

NASA Technical Reports Server (NTRS)

Schilizzi, R. T.

1980-01-01

The capabilities of the European very long baseline interferometry (VLBI) network are summarized. The range of baseline parameters, sensitivities, and recording and other equipment available are included. Plans for upgrading the recording facilities and the use of geostationary satellites for signal transfer and clock synchronization are discussed.

Evidence for Low Black Hole Spin and Physically Motivated Accretion Models from Millimeter-VLBI Observations of Sagittarius A*

NASA Astrophysics Data System (ADS)

Broderick, Avery E.; Fish, Vincent L.; Doeleman, Sheperd S.; Loeb, Abraham

2011-07-01

Millimeter very long baseline interferometry (mm-VLBI) provides the novel capacity to probe the emission region of a handful of supermassive black holes on sub-horizon scales. For Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, this provides access to the region in the immediate vicinity of the horizon. Broderick et al. have already shown that by leveraging spectral and polarization information as well as accretion theory, it is possible to extract accretion-model parameters (including black hole spin) from mm-VLBI experiments containing only a handful of telescopes. Here we repeat this analysis with the most recent mm-VLBI data, considering a class of aligned, radiatively inefficient accretion flow (RIAF) models. We find that the combined data set rules out symmetric models for Sgr A*'s flux distribution at the 3.9σ level, strongly favoring length-to-width ratios of roughly 2.4:1. More importantly, we find that physically motivated accretion flow models provide a significantly better fit to the mm-VLBI observations than phenomenological models, at the 2.9σ level. This implies that not only is mm-VLBI presently capable of distinguishing between potential physical models for Sgr A*'s emission, but further that it is sensitive to the strong gravitational lensing associated with the propagation of photons near the black hole. Based upon this analysis we find that the most probable magnitude, viewing angle, and position angle for the black hole spin are a = 0.0+0.64 + 0.86, \\theta ={68^\\circ }^{+5^\\circ +9^\\circ }_{-20^\\circ -28^\\circ }, and \\xi ={-52^\\circ }^{+17^\\circ +33^\\circ }_{-15^\\circ -24^\\circ } east of north, where the errors quoted are the 1σ and 2σ uncertainties.

Monte Carlo Simulations for VLBI2010

NASA Astrophysics Data System (ADS)

Wresnik, J.; Böhm, J.; Schuh, H.

2007-07-01

Monte Carlo simulations are carried out at the Institute of Geodesy and Geophysics (IGG), Vienna, and at Goddard Space Flight Center (GSFC), Greenbelt (USA), with the goal to design a new geodetic Very Long Baseline Interferometry (VLBI) system. Influences of the schedule, the network geometry and the main stochastic processes on the geodetic results are investigated. Therefore schedules are prepared with the software package SKED (Vandenberg 1999), and different strategies are applied to produce temporally very dense schedules which are compared in terms of baseline length repeatabilities. For the simulation of VLBI observations a Monte Carlo Simulator was set up which creates artificial observations by randomly simulating wet zenith delay and clock values as well as additive white noise representing the antenna errors. For the simulation at IGG the VLBI analysis software OCCAM (Titov et al. 2004) was adapted. Random walk processes with power spectrum densities of 0.7 and 0.1 psec2/sec are used for the simulation of wet zenith delays. The clocks are simulated with Allan Standard Deviations of 1*10^-14 @ 50 min and 2*10^-15 @ 15 min and three levels of white noise, 4 psec, 8 psec and, 16 psec, are added to the artificial observations. The variations of the power spectrum densities of the clocks and wet zenith delays, and the application of different white noise levels show clearly that the wet delay is the critical factor for the improvement of the geodetic VLBI system. At GSFC the software CalcSolve is used for the VLBI analysis, therefore a comparison between the software packages OCCAM and CalcSolve was done with simulated data. For further simulations the wet zenith delay was modeled by a turbulence model. This data was provided by Nilsson T. and was added to the simulation work. Different schedules have been run.

Evolution of the Global Space Geodesy Network

NASA Astrophysics Data System (ADS)

Pearlman, Michael R.; Bianco, Giuseppe; Ipatov, Alexander; Ma, Chopo; Neilan, Ruth; Noll, Carey; Park, Jong Uk; Pavlis, Erricos; Wetzel, Scott

2013-04-01

The improvements in the reference frame and other space geodesy data products spelled out in the GGOS 2020 plan will evolve over time as new space geodesy sites enhance the global distribution of the network and new technologies are implemented at the sites thus enabling improved data processing and analysis. The goal of 30 globally distributed core sites with VLBI, SLR, GNSS and DORIS (where available) will take time to materialize. Co-location sites with less than the full core complement will continue to play a very important role in filling out the network while it is evolving and even after full implementation. GGOS through its Call for Participation, bi-lateral and multi-lateral discussions and work through the scientific Services has been encouraging current groups to upgrade and new groups to join the activity. This talk will give an update on the current expansion of the global network and the projection for the network configuration that we forecast over the next 10 years.

MODEST - JPL GEODETIC AND ASTROMETRIC VLBI MODELING AND PARAMETER ESTIMATION PROGRAM

NASA Technical Reports Server (NTRS)

Sovers, O. J.

1994-01-01

Observations of extragalactic radio sources in the gigahertz region of the radio frequency spectrum by two or more antennas, separated by a baseline as long as the diameter of the Earth, can be reduced, by radio interferometry techniques, to yield time delays and their rates of change. The Very Long Baseline Interferometric (VLBI) observables can be processed by the MODEST software to yield geodetic and astrometric parameters of interest in areas such as geophysical satellite and spacecraft tracking applications and geodynamics. As the accuracy of radio interferometry has improved, increasingly complete models of the delay and delay rate observables have been developed. MODEST is a delay model (MOD) and parameter estimation (EST) program that takes into account delay effects such as geometry, clock, troposphere, and the ionosphere. MODEST includes all known effects at the centimeter level in modeling. As the field evolves and new effects are discovered, these can be included in the model. In general, the model includes contributions to the observables from Earth orientation, antenna motion, clock behavior, atmospheric effects, and radio source structure. Within each of these categories, a number of unknown parameters may be estimated from the observations. Since all parts of the time delay model contain nearly linear parameter terms, a square-root-information filter (SRIF) linear least-squares algorithm is employed in parameter estimation. Flexibility (via dynamic memory allocation) in the MODEST code ensures that the same executable can process a wide array of problems. These range from a few hundred observations on a single baseline, yielding estimates of tens of parameters, to global solutions estimating tens of thousands of parameters from hundreds of thousands of observations at antennas widely distributed over the Earth's surface. Depending on memory and disk storage availability, large problems may be subdivided into more tractable pieces that are processed sequentially. MODEST is written in FORTRAN 77, C-language, and VAX ASSEMBLER for DEC VAX series computers running VMS. It requires 6Mb of RAM for execution. The standard distribution medium for this package is a 1600 BPI 9-track magnetic tape in DEC VAX BACKUP format. It is also available on a TK50 tape cartridge in DEC VAX BACKUP format. Instructions for use and sample input and output data are available on the distribution media. This program was released in 1993 and is a copyrighted work with all copyright vested in NASA.

VLBI Correlators in Kashima

NASA Technical Reports Server (NTRS)

Sekido, Mamoru; Takefuji, Kazuhiro

2013-01-01

Kashima Space Technology Center (KSTC) is making use of two kinds of software correlators, the multi-channel K5/VSSP software correlator and the fast wide-band correlator 'GICO3,' for geodetic and R&D VLBI experiments. Overview of the activity and future plans are described in this paper.

Expected Improvements in VLBI Measurements of the Earth's Orientation

NASA Technical Reports Server (NTRS)

Ma, Chopo

2003-01-01

Measurements of the Earth s orientation since the 1970s using space geodetic techniques have provided a continually expanding and improving data set for studies of the Earth s structure and the distribution of mass and angular momentum. The accuracy of current one-day measurements is better than 100 microarcsec for the motion of the pole with respect to the celestial and terrestrial reference frames and better than 3 microsec for the rotation around the pole. VLBI uniquely provides the three Earth orientation parameters (nutation and UTI) that relate the Earth to the extragalactic celestial reference frame. The accuracy and resolution of the VLBI Earth orientation time series can be expected to improve substantially in the near future because of refinements in the realization of the celestial reference frame, improved modeling of the troposphere and non-linear station motions, larger observing networks, optimized scheduling, deployment of disk-based Mark V recorders, full use of Mark IV capabilities, and e-VLBI. More radical future technical developments will be discussed.

General Relativistic Theory of the VLBI Time Delay in the Gravitational Field of Moving Bodies

NASA Technical Reports Server (NTRS)

Kopeikin, Sergei

2003-01-01

The general relativistic theory of the gravitational VLBI experiment conducted on September 8, 2002 by Fomalont and Kopeikin is explained. Equations of radio waves (light) propagating from the quasar to the observer are integrated in the time-dependent gravitational field of the solar system by making use of either retarded or advanced solutions of the Einstein field equations. This mathematical technique separates explicitly the effects associated with the propagation of gravity from those associated with light in the integral expression for the relativistic VLBI time delay of light. We prove that the relativistic correction to the Shapiro time delay, discovered by Kopeikin (ApJ, 556, L1, 2001), changes sign if one retains direction of the light propagation but replaces the retarded for the advanced solution of the Einstein equations. Hence, this correction is associated with the propagation of gravity. The VLBI observation measured its speed, and that the retarded solution is the correct one.

The Potential for a Ka-band (32 GHz) Worldwide VLBI Network

NASA Astrophysics Data System (ADS)

Jacobs, C. S.; Bach, U.; Colomer, F.; Garcá-Miró, C.; Gómez-González, J.; Gulyaev, S.; Horiuchi, S.; Ichikawa, R.; Kraus, A.; Kronschnabl, G.; López-Fernández, J. A.; Lovell, J.; Majid, W.; T; Natusch; Neidhardt, A.; Phillips, C.; Porcas, R.; Romero-Wolf, A.; Saldana, L.; Schreiber, U.; Sotuela, I.; Takeuchi, H.; Trinh, J.; Tzioumis, A.; de Vincente, P.; Zharov, V.

2012-12-01

Ka-band (32 GHz, 9 mm) Very Long Baseline Interferometric (VLBI) networking has now begun and has tremendous potential for expansion over the next few years. Ka-band VLBI astrometry from NASA's Deep Space Network has already developed a catalog of 470 observable sources with highly accurate positions. Now, several antennas worldwide are planning or are considering adding Ka-band VLBI capability. Thus, there is now an opportunity to create a worldwide Ka-band network with potential for high resolution imaging and astrometry. With baselines approaching a Giga-lambda, a Ka-band network would be able to probe source structure at the nano-radian (200 as) level (100X better than Hubble) and thus gain insight into the astrophysics of the most compact regions of emission in active galactic nuclei. We discuss the advantages of Ka-band, show the known sources and candidates, simulate projected baseline (uv) coverage, and discuss potential radio frequency feeds. The combination of these elements demonstrates the feasibility of a worldwide Ka network within the next few years.

Navigation of the space VLBI mission-HALCA

NASA Technical Reports Server (NTRS)

You, Tung Han; Ellis, Jordan; Mottinger, Neil

1998-01-01

In February 1997, the Japanese Space Agency ISAS launched the first space VLBI satellite, HALCA, with an 8 meter diameter wire mesh antenna and radio astronomy receivers capable of observing at 1.6, 4.8, and 22 Ghz. In a 560 by 21000 km orbit with a 6 hour period and 31 degree inclination, it observes celestial radio sources in conjunction with a world wide network of ground radio telescopes as part of an international collaborative effort which includes facilities in Japan, the U.S., Canada, Australia, and Europe. JPL is providing tracking and navigation support using a dedicated subnet of 11 meter antennas as well as co-observations using the DSN 70 meter antennas. This paper describes the spacecraft dynamics model and orbit determination strategies developed to meet the stringent trajectory accuracy requirements for generating predictions for the transfer of a stable uplink frequency to the spacecraft and for determining reconstructed orbits for delivery to the NRAO VLBI correlator and the international VLBI science community.

The Potential for a Ka-band (32 GHz) Worldwide VLBI Network

NASA Technical Reports Server (NTRS)

Jacobs, C. S.; Bach, U.; Colomer, F.; Garcia-Miro, C.; Gomez-Gonzalez, J.; Gulyaev, S.; Horiuchi, S.; Ichikawa, R.; Kraus, A.; Kronschnabl, G.;

Going Digital - The Transition from Mark IV to DBBC at Onsala

NASA Astrophysics Data System (ADS)

Kareinen, Niko; Haas, Rüdiger; La Porta, Laura; Bertarini, Alessandra

2014-12-01

The Onsala Space Observatory is currently equipped with both a VLBI Mark IV rack and a digital BBC (DBBC). The Mark IV rack at Onsala has been used operationally for both astronomical and geodetic VLBI for more than 40 years. In 2011, Onsala purchased a DBBC and we started to test it and to gain experience with the new device, both for astronomical and geodetic VLBI. The DBBC was upgraded several times and the Field System (FS) interface was implemented. We did parallel recordings, with both the old Mark IV/Mark 5A system and the new DBBC/Mark 5B+ system, during numerous geodetic VLBI sessions. Several R1, T2, and Euro sessions were correlated during the last two years by the Bonn correlator with Onsala being included both as an analog station (two-letter code On) and as a digital station (two-letter code Od). We present results from these parallel sessions, both results from the original correlation and results from the analysis of the corresponding databases.

A Fast Radio Burst Search Method for VLBI Observation

NASA Astrophysics Data System (ADS)

Liu, Lei; Tong, Fengxian; Zheng, Weimin; Zhang, Juan; Tong, Li

2018-02-01

We introduce the cross-spectrum-based fast radio burst (FRB) search method for Very Long Baseline Interferometer (VLBI) observation. This method optimizes the fringe fitting scheme in geodetic VLBI data post-processing, which fully utilizes the cross-spectrum fringe phase information and therefore maximizes the power of single-pulse signals. Working with cross-spectrum greatly reduces the effect of radio frequency interference compared with using auto-power spectrum. Single-pulse detection confidence increases by cross-identifying detections from multiple baselines. By combining the power of multiple baselines, we may improve the detection sensitivity. Our method is similar to that of coherent beam forming, but without the computational expense to form a great number of beams to cover the whole field of view of our telescopes. The data processing pipeline designed for this method is easy to implement and parallelize, which can be deployed in various kinds of VLBI observations. In particular, we point out that VGOS observations are very suitable for FRB search.

UniBoard: generic hardware for radio astronomy signal processing

NASA Astrophysics Data System (ADS)

Hargreaves, J. E.

2012-09-01

UniBoard is a generic high-performance computing platform for radio astronomy, developed as a Joint Research Activity in the RadioNet FP7 Programme. The hardware comprises eight Altera Stratix IV Field Programmable Gate Arrays (FPGAs) interconnected by a high speed transceiver mesh. Each FPGA is connected to two DDR3 memory modules and three external 10Gbps ports. In addition, a total of 128 low voltage differential input lines permit connection to external ADC cards. The DSP capability of the board exceeds 644E9 complex multiply-accumulate operations per second. The first production run of eight boards was distributed to partners in The Netherlands, France, Italy, UK, China and Korea in May 2011, with a further production runs completed in December 2011 and early 2012. The function of the board is determined by the firmware loaded into its FPGAs. Current applications include beamformers, correlators, digital receivers, RFI mitigation for pulsar astronomy, and pulsar gating and search machines The new UniBoard based correlator for the European VLBI network (EVN) uses an FX architecture with half the resources of the board devoted to station based processing: delay and phase correction and channelization, and half to the correlation function. A single UniBoard can process a 64MHz band from 32 stations, 2 polarizations, sampled at 8 bit. Adding more UniBoards can expand the total bandwidth of the correlator. The design is able to process both prerecorded and real time (eVLBI) data.

Tectonic motion site survey of the National Radio Astronomy Observatory, Green Bank, West Virginia

NASA Technical Reports Server (NTRS)

Webster, W. J., Jr.; Allenby, R. J.; Hutton, L. K.; Lowman, P. D., Jr.; Tiedemann, H. A.

1979-01-01

A geological and geophysical site survey was made of the area around the National Radio Astronomy Observatory (NRAO) to determine whether there are at present local tectonic movements that could introduce significant errors to Very Long Baseline Interferometry (VLBI) geodetic measurements. The site survey consisted of a literature search, photogeologic mapping with Landsat and Skylab photographs, a field reconnaissance, and installation of a seismometer at the NRAO. It is concluded that local tectonic movement will not contribute significantly to VLBI errors. It is recommended that similar site surveys be made of all locations used for VLBI or laser ranging.

MODEST: A Tool for Geodesy and Astronomy

NASA Technical Reports Server (NTRS)

Sovers, Ojars J.; Jacobs, Christopher S.; Lanyi, Gabor E.

2004-01-01

Features of the JPL VLBI modeling and estimation software "MODEST" are reviewed. Its main advantages include thoroughly documented model physics, portability, and detailed error modeling. Two unique models are included: modeling of source structure and modeling of both spatial and temporal correlations in tropospheric delay noise. History of the code parallels the development of the astrometric and geodetic VLBI technique and the software retains many of the models implemented during its advancement. The code has been traceably maintained since the early 1980s, and will continue to be updated with recent IERS standards. Scripts are being developed to facilitate user-friendly data processing in the era of e-VLBI.

International VLBI Service for Geodesy and Astronomy

NASA Technical Reports Server (NTRS)

Vandenberg, Nancy R. (Editor); Baver, Karen D. (Editor)

2004-01-01

This volume of reports is the 2003 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the permanent components of IVS. The IVS 2003 Annual Report documents the work of the IVS components for the calendar year 2003, our fifih year of existence. The reports describe changes, activities, and progress of the IVS. Many thanks to all IVS components who contributed to this Annual Report. The entire contents of this Annual Report also appear on the IVS web site at http://ivscc.gsfc.nasa.gov/publications/ar2OO3

A complete sample of double-lobed radio quasars for VLBI tests of source models - Definition and statistics

NASA Technical Reports Server (NTRS)

Hough, D. H.; Readhead, A. C. S.

1989-01-01

A complete, flux-density-limited sample of double-lobed radio quasars is defined, with nuclei bright enough to be mapped with the Mark III VLBI system. It is shown that the statistics of linear size, nuclear strength, and curvature are consistent with the assumption of random source orientations and simple relativistic beaming in the nuclei. However, these statistics are also consistent with the effects of interaction between the beams and the surrounding medium. The distribution of jet velocities in the nuclei, as measured with VLBI, will provide a powerful test of physical theories of extragalactic radio sources.

Geodetic measurement of deformation in the Loma Prieta, California earthquake with very long baseline interferometry

NASA Technical Reports Server (NTRS)

Clark, T. A.; Ma, C.; Sauber, J. M.; Ryan, J. W.; Gordon, D.; Shaffer, D. B.; Carprette, D. S.; Vandenberg, N. R.

1990-01-01

VLBI measurements were conducted immediately after the Loma Prieta earthquake and compared with VLBI gathered at Monterey, San Francisco, and Point Reyes since 1983 to obtain preearthquake deformation rates with respect to a North American reference frame. The estimated displacements at Monterey and San Francisco are consistent with the static displacements predicted on the basis of a coseismic slip model in which slip on the southern segment is shallower than slip on the northern segment of the fault rupture. Cartesian positions are presented at epoch 1990.0 of a set of VLBI fiducial stations and the three mobile sites in the earthquake's vicinity.

Mission feasibility study of a very long baseline interferometer utilizing the space shuttle

NASA Technical Reports Server (NTRS)

Burke, B. F.

1978-01-01

An introductory overview of very long baseline interferometry (VLBI) as it exists and is used today is given and the scientific advances that have been achieved with this technique in the past decade are described. The report briefly reviews developments now in progress that will improve ground station VLBI in the next few years, and the limitations that still will exist. The advantages and the scientific return on investment that may be expected from a VLBI terminal in space are described. Practical problems that have to be faced range from system design through hardware implementation, to data recovery and analysis.

VLBI Phase-Referenced Observations on Southern Hemisphere HIPPARCOS Radio Start

NASA Technical Reports Server (NTRS)

Guirado, J. C.; Preston, R. A.; Jones, D. L.; Lestrade, J. F.; Reynolds, J. E.; Jauncey, D. L.; Tzioumis, A. K.; Ferris, R. H.; King, E. A.; Lovell, J. E. J.;

Warkworth 12-m VLBI Station: WARK12M

NASA Technical Reports Server (NTRS)

Weston, Stuart; Takiguchi, Hiroshi; Natusch, Tim; Woodburn, Lewis; Gulyaev, Sergei

2013-01-01

The Warkworth 12-m radio telescope is operated by the Institute for Radio Astronomy and Space Research (IRASR) at AUT University, Auckland, New Zealand. Here we review the characteristics of the 12-m VLBI station and report on a number of activities and technical developments in 2012.

Status and Prospects for Combined GPS LOD and VLBI UT1 Measurements

NASA Astrophysics Data System (ADS)

Senior, K.; Kouba, J.; Ray, J.

2010-01-01

A Kalman filter was developed to combine VLBI estimates of UT1-TAI with biased length of day (LOD) estimates from GPS. The VLBI results are the analyses of the NASA Goddard Space Flight Center group from 24-hr multi-station observing sessions several times per week and the nearly daily 1-hr single-baseline sessions. Daily GPS LOD estimates from the International GNSS Service (IGS) are combined with the VLBI UT1-TAI by modeling the natural excitation of LOD as the integral of a white noise process (i.e., as a random walk) and the UT1 variations as the integration of LOD, similar to the method described by Morabito et al. (1988). To account for GPS technique errors, which express themselves mostly as temporally correlated biases in the LOD measurements, a Gauss-Markov model has been added to assimilate the IGS data, together with a fortnightly sinusoidal term to capture errors in the IGS treatments of tidal effects. Evaluated against independent atmospheric and oceanic axial angular momentum (AAM + OAM) excitations and compared to other UT1/LOD combinations, ours performs best overall in terms of lowest RMS residual and highest correlation with (AAM + OAM) over sliding intervals down to 3 d. The IERS 05C04 and Bulletin A combinations show strong high-frequency smoothing and other problems. Until modified, the JPL SPACE series suffered in the high frequencies from not including any GPS-based LODs. We find, surprisingly, that further improvements are possible in the Kalman filter combination by selective rejection of some VLBI data. The best combined results are obtained by excluding all the 1-hr single-baseline UT1 data as well as those 24-hr UT1 measurements with formal errors greater than 5 μs (about 18% of the multi-baseline sessions). A rescaling of the VLBI formal errors, rather than rejection, was not an effective strategy. These results suggest that the UT1 errors of the 1-hr and weaker 24-hr VLBI sessions are non-Gaussian and more heterogeneous than expected, possibly due to the diversity of observing geometries used, other neglected systematic effects, or to the much shorter observational averaging interval of the single-baseline sessions. UT1 prediction services could benefit from better handling of VLBI inputs together with proper assimilation of IGS LOD products, including using the Ultra-rapid series that is updated four times daily with 15 hr delay.

Geodesy by radio interferometry - Water vapor radiometry for estimation of the wet delay

NASA Technical Reports Server (NTRS)

Elgered, G.; Davis, J. L.; Herring, T. A.; Shapiro, I. I.

1991-01-01

An important source of error in VLBI estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. This paper presents and discusses the method of using data from a water vapor radiomete (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data or Kalman filtering to correct for atmospheric propagation delay at the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. For the most frequently measured baseline in this study, the use of WVR data yielded a 13 percent smaller weighted-root-mean-square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the 'best' minimum elevationi angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass.

ON THE CONNECTION OF THE APPARENT PROPER MOTION AND THE VLBI STRUCTURE OF COMPACT RADIO SOURCES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moor, A.; Frey, S.; Lambert, S. B.

2011-06-15

Many of the compact extragalactic radio sources that are used as fiducial points to define the celestial reference frame are known to have proper motions detectable with long-term geodetic/astrometric very long baseline interferometry (VLBI) measurements. These changes can be as high as several hundred microarcseconds per year for certain objects. When imaged with VLBI at milliarcsecond (mas) angular resolution, these sources (radio-loud active galactic nuclei) typically show structures dominated by a compact, often unresolved 'core' and a one-sided 'jet'. The positional instability of compact radio sources is believed to be connected with changes in their brightness distribution structure. For themore » first time, we test this assumption in a statistical sense on a large sample rather than on only individual objects. We investigate a sample of 62 radio sources for which reliable long-term time series of astrometric positions as well as detailed 8 GHz VLBI brightness distribution models are available. We compare the characteristic direction of their extended jet structure and the direction of their apparent proper motion. We present our data and analysis method, and conclude that there is indeed a correlation between the two characteristic directions. However, there are cases where the {approx}1-10 mas scale VLBI jet directions are significantly misaligned with respect to the apparent proper motion direction.« less

Star Scheduling Mode—A New Observing Strategy for Monitoring Weak Southern Radio Sources with the AuScope VLBI Array

NASA Astrophysics Data System (ADS)

McCallum, Lucia; Mayer, David; Le Bail, Karine; Schartner, Matthias; McCallum, Jamie; Lovell, Jim; Titov, Oleg; Shu, Fengchun; Gulyaev, Sergei

2017-11-01

The International Celestial Reference Frame suffers from significantly less observations in the southern hemisphere compared to the northern one. One reason for this is the historically low number of very long baseline interferometry radio telescopes in the south. The AuScope very long baseline interferometry array with three new telescopes on the Australian continent and an identical antenna in New Zealand were built to address this issue. While the overall number of observations in the south has greatly improved since then, a closer look reveals that this improvement is only true for strong radio sources (source flux densities >0.6 Jy). The new array of small very long baseline interferometry antennas has a relatively low baseline sensitivity so that only strong sources can be observed within a short integration time. A new observing strategy, the star scheduling mode, was developed to enable efficient observations of weak sources during geodetic sessions, through the addition of a single more sensitive antenna to the network. This scheduling mode was implemented in the Vienna very long baseline interferometry Software and applied in four 24-h sessions in 2016. These observations provide updated positions and source flux densities for 42 weak southern radio sources and significantly reduce the formal uncertainties for these sources. The star scheduling mode now allows the AuScope very long baseline interferometry array to undertake greater responsibility in monitoring sources in the southern sky, without significantly weakening the session for geodetic purposes.

Digital Signal Processing for the Event Horizon Telescope

NASA Astrophysics Data System (ADS)

Weintroub, Jonathan

2015-08-01

A broad international collaboration is building the Event Horizon Telescope (EHT). The aim is to test Einstein’s theory of General Relativity in one of the very few places it could break down: the strong gravity regime right at the edge of a black hole. The EHT is an earth-size VLBI array operating at the shortest radio wavelengths, that has achieved unprecedented angular resolution of a few tens of μarcseconds. For nearby super massive black holes (SMBH) this size scale is comparable to the Schwarzschild Radius, and emission in the immediate neighborhood of the event horizon can be directly observed. We give an introduction to the science behind the CASPER-enabled EHT, and outline technical developments, with emphasis on the secret sauce of high speed signal processing.

Bubbles, voids, and bumps in time: The new cosmology

NASA Astrophysics Data System (ADS)

Cornell, James

The history and current status of theoretical and observational cosmology are examined in chapters based on the Lowell Lectures, given in Boston and Washington DC in spring 1987. Topics addressed include the Aristotelian, Copernican, Newtonian, and Einsteinian universes; the measurement of the universe (redshifts and standard candles); mapping the universe (slices and bubbles); dark matter and missing mass; and the big bang and cosmic inflation. Six basic outstanding problems are identified, and the potential contributions of planned ground-based and space observatories to their solution are discussed. Particular attention is given to CCD detectors for large ground-based telescopes, the VLA, VLBI arrays, the ESO Very Large Telescope, the 10-m Keck telescope on Mauna Kea, the Hubble Space Telescope, the Gamma-Ray Observatory, and the Advanced X-ray Astrophysics Facility.

GSFC VLBI Analysis Center

NASA Technical Reports Server (NTRS)

Gordon, David; Ma, Chopo; Petrov, Leonid; MacMillan, Dan

2001-01-01

This report presents the activities of the Goddard Space Flight Center's Very Long Base Interferometry (VLBI) Analysis Center during the period from March 1, 1999 through December 31, 2000. The center's primary software development, analysis, and research activities axe reported, and the responsible staff members are described. Plans for 2001 are also presented.

VLBI-based Products - Naval Oceanography Portal

Science.gov Websites

section Advanced Search... Sections Home Time Earth Orientation Astronomy Meteorology Oceanography Ice You terrestrial reference frames and to predict the variable orientation of the Earth in three-dimensional space antennas that define a VLBI-based Terrestrial Reference Frame (TRF) and the Earth Orientation Parameters

Goddard Geophysical and Astronomical Observatory

NASA Technical Reports Server (NTRS)

Redmond, Jay; Kodak, Charles

2001-01-01

This report summarizes the technical parameters and the technical staff of the Very Long Base Interferometry (VLBI) system at the fundamental station Goddard Geophysical and Astronomical Observatory (GGAO). It also gives an overview about the VLBI activities during the previous year. The outlook lists the outstanding tasks to improve the performance of GGAO.

The Medicina Station Status Report

NASA Technical Reports Server (NTRS)

Orfei, Alessandro; Orlati, Andrea; Maccaferri, Giuseppe

2013-01-01

General information about the Medicina Radio Astronomy Station, the 32-m antenna status, and the staff in charge of the VLBI observations is provided. In 2012, the data from geodetic VLBI observations were acquired using the Mark 5A recording system with good results. Updates of the hardware were performed and are briefly described.

Measuring Crustal Deformation in the American West.

ERIC Educational Resources Information Center

Jordan, Thomas H.; Minster, J. Bernard

1988-01-01

Suggests that there is a close relationship between deformation in the western United States and the large-scale motions of tectonic plates. Introduces very-long-baseline interferometry (VLBI) as one of the space-geodetic techniques, vector addition of the VLBI data and geological data, and a new geodetic network. (YP)

Tectonic deformation in southern California

NASA Technical Reports Server (NTRS)

Jackson, David D.

1993-01-01

Our objectives were to use modem geodetic data, especially those derived from space techniques like Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), and the Global Positioning System (GPS) to infer crustal deformation in southern California and relate it to plate tectonics and earthquake hazard. To do this, we needed to collect some original data, write computer programs to determine positions of survey markers from geodetic observables, interpret time dependent positions in terms of velocity and earthquake caused episodic displacements, and construct a model to explain these velocities and displacements in terms of fault slip and plate movements.

Clock synchronization by accelerated observers - Metric construction for arbitrary congruences of world lines

NASA Technical Reports Server (NTRS)

Henriksen, R. N.; Nelson, L. A.

1985-01-01

Clock synchronization in an arbitrarily accelerated observer congruence is considered. A general solution is obtained that maintains the isotropy and coordinate independence of the one-way speed of light. Attention is also given to various particular cases including, rotating disk congruence or ring congruence. An explicit, congruence-based spacetime metric is constructed according to Einstein's clock synchronization procedure and the equation for the geodesics of the space-time was derived using Hamilton-Jacobi method. The application of interferometric techniques (absolute phase radio interferometry, VLBI) to the detection of the 'global Sagnac effect' is also discussed.

Monitoring of Earth Rotation by VLBI

NASA Technical Reports Server (NTRS)

Ma., Chopo; Macmillan, D. S.

2000-01-01

Monitoring Earth rotation with Very Long Baseline Interferometry (VLBI) has unique potential because of direct access to the Celestial Reference System (CRF and Terrestrial Reference System (TRF) and the feasibility of re-analyzing the entire data set. While formal precision of better than 0.045 mas for pole and 0.002 ms for UT 1 has been seen in the best 24-hr data, the accuracy of the Earth Orientation Parameter (EOP) time series as a whole is subject to logistical, operational, analytical and conceptual constraints. The current issues related to the VLBI data set and the CORE program for greater time resolution such as analysis consistency, network jitter and reference frame stability will be discussed.

VLBI geodesy - 2 parts-per-billion precision in length determinations for transcontinental baselines

NASA Technical Reports Server (NTRS)

Davis, J. L.; Herring, T. A.; Shapiro, I. I.

1988-01-01

VLBI was to make twenty-two independent measurements, between September 1984 and December 1986, of the length of the 3900-km baseline between the Mojave site in California and the Haystack/Westford site in Massachusetts. These experiments differ from the typical geodetic VLBI experiments in that a large fraction of observations is obtained at elevation angles between 4 and 10 deg. Data from these low elevation angles allow the vertical coordinate of site position, and hence the baseline length, to be estimated with greater precision. For the sixteen experiments processed thus far, the weighted root-mean-square scatter of the estimates of the baseline length is 8 mm.

Estimating Accurate Relative Spacecraft Angular Position from DSN VLBI Phases Using X-Band Telemetry or DOR Tones

NASA Technical Reports Server (NTRS)

Bagri, Durgadas S.; Majid, Walid

2009-01-01

At present spacecraft angular position with Deep Space Network (DSN) is determined using group delay estimates from very long baseline interferometer (VLBI) phase measurements employing differential one way ranging (DOR) tones. As an alternative to this approach, we propose estimating position of a spacecraft to half a fringe cycle accuracy using time variations between measured and calculated phases as the Earth rotates using DSN VLBI baseline(s). Combining fringe location of the target with the phase allows high accuracy for spacecraft angular position estimate. This can be achieved using telemetry signals of at least 4-8 MSamples/sec data rate or DOR tones.

Crustal dynamics project data analysis fixed station VLBI geodetic results

NASA Technical Reports Server (NTRS)

Ryan, J. W.; Ma, C.

1985-01-01

The Goddard VLBI group reports the results of analyzing the fixed observatory VLBI data available to the Crustal Dynamics Project through the end of 1984. All POLARIS/IRIS full-day data are included. The mobile site at Platteville, Colorado is also included since its occupation bears on the study of plate stability. Data from 1980 through 1984 were used to obtain the catalog of site and radio source positions labeled S284C. Using this catalog two types of one-day solutions were made: (1) to estimate site and baseline motions; and (2) to estimate Earth rotation parameters. A priori Earth rotation parameters were interpolated to the epoch of each observation from BIH Circular D.

A comparison of VLBI with the ICE-3G glacial rebound model

NASA Technical Reports Server (NTRS)

James, Thomas S.; Lambert, Anthony

1993-01-01

Crustal motion predicted by the ICE-3G glacial rebound model exhibits a pattern of tangential (horizontal) divergence away from the centers of uplift, which in North America and Europe are located around Hudson Bay and the Gulf of Bothnia. Tangential velocities reach peak magnitudes of 1-2 mm/yr, and must be included when predicting VLBI baseline length change rates due to postglacial rebound. Out of 18 observed VLBI baselines examined three are situated such that their predicted length rates are around their 2 sigma uncertainties or greater. It is encouraging that two of these baselines exhibit predicted length rates within 2 sigma of the observed rates.

Haystack Analysis Center

NASA Technical Reports Server (NTRS)

Niell, Arthur; Cappallo, Roger; Corey, Brian; Titus, Mike

2013-01-01

Analysis activities at Haystack Observatory are directed towards improving the accuracy of geodetic measurements, whether these are from VLBI, GNSS, SLR, or any other technique. Those analysis activities that are related to technology development are reported elsewhere in this volume. In this report, a preliminary analysis of the first geodetic sessions with the new broadband geodetic VLBI system is reported.

IAA Correlator Center

NASA Technical Reports Server (NTRS)

Surkis, Igor; Ken, Voitsekh; Melnikov, Alexey; Mishin, Vladimir; Sokolova, Nadezda; Shantyr, Violet; Zimovsky, Vladimir

2013-01-01

The activities of the six-station IAA RAS correlator include regular processing of national geodetic VLBI programs Ru-E, Ru-U, and Ru-F. The Ru-U sessions have been transferred in e-VLBI mode and correlated in the IAA Correlator Center automatically since 2011. The DiFX software correlator is used at the IAA in some astrophysical experiments.

On the Impact of Inhomogeneities in Meteorological Data on VLBI Data Analysis

NASA Astrophysics Data System (ADS)

Balidakis, Kyriakos; Heinkelmann, Robert; Phogat, Apurva; Soja, Benedikt; Glaser, Susanne; Nilsson, Tobias; Karbon, Maria; Schuh, Harald

2016-12-01

In this study, we address the issue of the quality of meteorological data employed for VLBI data analysis. We use data from six numerical weather models (NWMs) to form references on which the homogenization process is based. We explore the impact of the choice of NWM as well as the way to extract data from it. Among our findings is that data from the surface fields of NWMs are not suitable for either geodetic analysis or homogenization efforts, whether they are in their original form or after they have been compensated for the height difference between the orography of the NWM and the actual elevation. The reason lies in the fact that for 77% of the VLBI stations a height bias larger than 2.5 mm appears, as well as an average bias in the zenith wet delay estimates of 12.2 mm. Should the proposed extraction approach be followed, the difference between operational and reanalysis NWMs is not significant for such an application. Our conclusions are based on the analysis of VLBI data over 13 years.

PARSEC-SCALE SHOCKS IN THE KILOPARSEC-SCALE JET OF CENTAURUS A

DOE Office of Scientific and Technical Information (OSTI.GOV)

Tingay, S. J.; Lenc, E.

2009-09-15

High angular resolution very long baseline interferometry (VLBI) observations of Centaurus A have been undertaken that allow access to a wide field of view, encompassing both the well-studied parsec-scale jet and the inner part of the kiloparsec-scale jet. The VLBI observations have detected compact regions of synchrotron emission in the kiloparsec-scale jet that coincide with three stationary features identified from previous VLA monitoring observations. Each of these stationary features is associated with strong localized X-ray emission. The VLBI results strengthen arguments made by previous authors suggesting that the stationary features may be the result of stellar objects or gas cloudsmore » traversing the jet flow, intercepting the jet and causing strong shocks. The VLBI data show that the most strongly shocked regions in these features are resolved but have extents no larger than a few pc, reducing the required mass of the typical intercepting object by a factor of {approx}10 relative to previous estimates, making explanations based on high mass-loss stars or low-density gas clouds more plausible.« less

First mm-VLBI Observations between the TRAO 14-m and the NRO 45-m Telescopes: Observations of 86 GHz SiO Masers in VY Canis Majoris

NASA Astrophysics Data System (ADS)

Shibata, Katsunori M.; Chung, Hyung-Soo; Kameno, Seiji; Roh, Duk-Gyoo; Umemoto, Tomofumi; Kim, Kwang-Dong; Asada, Keiichi; Han, Seog-Tae; Mochizuki, Nanako; Cho, Se-Hyung; Sawada-Satoh, Satoko; Kim, Hyun-Goo; Bushimata, Takeshi; Minh, Young Chol; Miyaji, Takeshi; Kuno, Nario; Mikoshiba, Hiroshi; Sunada, Kazuyoshi; Inoue, Makoto; Kobayashi, Hideyuki

2004-06-01

We have made VLBI observations at 86GHz using a 1000-km baseline between Korea and Japan with successful detections of SiO v = 1, J = 2 - 1 maser emissions from VY CMa and Orion KL in 2001 June. This was the first VLBI result for this baseline and the first astronomical VLBI observation for the Korean telescope. Since then, we observed SiO v = 1, J = 2 - 1 maser emission in VY CMa in 2002 January and 2003 February and derived the distributions of the maser emissions. Our results show that the maser emissions extend over 2-4 stellar radii, and were within the inner radius of the dust shell. We observed other SiO maser sources and continuum sources, and 86-GHz continuum emissions were detected from three continuum sources. It was verified that this baseline has a performance comparable to the most sensitive baseline in the VLBA and the CMVA, and is capable of investigating the proper motions of maser features in circumstellar envelopes using monitoring observations.

Jet outflow and gamma-ray emission correlations in S5 0716+714

DOE PAGES

Rani, B.; Krichbaum, T. P.; Marscher, A. P.; ...

2014-11-06

Here, using millimeter very long baseline interferometry (VLBI) observations of the BL Lac object S5 0716+714 from August 2008 to September 2013, we investigate variations in the core flux density and orientation of the sub-parsec scale jet, i.e. position angle. The γ-ray data obtained by the Fermi Large Area Telescope are used to investigate the high-energy flux variations over the same time period. For the first time in any blazar, we report a significant correlation between the γ-ray flux variations and the position angle variations in the VLBI jet. The cross-correlation analysis also indicates a positive correlation such that themore » mm-VLBI core flux density variations are delayed with respect to the γ-ray flux by 82±32 days. This suggests that the high-energy emission is coming from a region located ≥(3.8±1.9) parsecs upstream of the mm-VLBI core (closer to the central black hole). Lastly, these results imply that the observed inner jet morphology has a strong connection with the observed γ-ray flares.« less

Direct estimation of tidally induced Earth rotation variations observed by VLBI

NASA Astrophysics Data System (ADS)

Englich, S.; Heinkelmann, R.; BOHM, J.; Schuh, H.

2009-09-01

The subject of our study is the investigation of periodical variations induced by solid Earth tides and ocean tides in Earth rotation parameters (ERP: polar motion, UT1)observed by VLBI. There are two strategies to determine the amplitudes and phases of Earth rotation variations from observations of space geodetic techniques. The common way is to derive time series of Earth rotation parameters first and to estimate amplitudes and phases in a second step. Results obtained by this means were shown in previous studies for zonal tidal variations (Englich et al.; 2008a) and variations caused by ocean tides (Englich et al.; 2008b). The alternative method is to estimate the tidal parameters directly within the VLBI data analysis procedure together with other parameters such as station coordinates, tropospheric delays, clocks etc. The purpose of this work was the application of this direct method to a combined VLBI data analysis using the software packages OCCAM (Version 6.1, Gauss-Markov-Model) and DOGSCS (Gerstl et al.; 2001). The theoretical basis and the preparatory steps for the implementation of this approach are presented here.

A coherent fiber link for very long baseline interferometry.

PubMed

Clivati, Cecilia; Costanzo, Giovanni A; Frittelli, Matteo; Levi, Filippo; Mura, Alberto; Zucco, Massimo; Ambrosini, Roberto; Bortolotti, Claudio; Perini, Federico; Roma, Mauro; Calonico, Davide

2015-11-01

We realize a coherent fiber link for application in very long baseline interferometry (VLBI) for radio astronomy and geodesy. A 550-km optical fiber connects the Italian National Metrological Institute (INRIM) to a radio telescope in Italy and is used for the primary Cs fountain clock stability and accuracy dissemination. We use an ultrastable laser frequency- referenced to the primary standard as a transfer oscillator; at the radio telescope, an RF signal is generated from the laser by using an optical frequency comb. This scheme now provides the traceability of the local maser to the SI second, realized by the Cs fountain at the 1.7 × 10(-16) accuracy. The fiber link never limits the experiment and is robust enough to sustain radio astronomical campaigns. This experiment opens the possibility of replacing the local hydrogen masers at the VLBI sites with optically-synthesized RF signals. This could improve VLBI resolution by providing more accurate and stable frequency references and, in perspective, by enabling common- clock VLBI based on a network of telescopes connected by fiber links.

Postglacial Rebound from VLBI Geodesy: On Establishing Vertical Reference

NASA Technical Reports Server (NTRS)

Argus, Donald F.

1996-01-01

Difficulty in establishing a reference frame fixed to the earth's interior complicates the measurement of the vertical (radial) motions of the surface. I propose that a useful reference frame for vertical motions is that found by minimizing differences between vertical motions observed with VLBI [Ma and Ryan] and predictions from postglacial rebound predictions [Peltier]. The optimal translation of the geocenter is 1.7mm/year toward 36degN, 111degE when determined from the motions of 10 VLBI sites. This translation gives a better fit of observations to predictions than does the VLBI reference frame used by Ma and Ryan, but the improvement is statistically insignificant. The root mean square of differences decreases 20% to 0.73 mm/yr and the correlation coefficient increases from 0.76 to 0.87. Postglacial rebound is evident in the uplift of points in Sweden and Ontario that were beneath the ancient ice sheets of Fennoscandia and Canada, and in the subsidence of points in the northeastern U.S., Germany, and Alaska that were around the periphery of the ancient ice sheets.

Troposphere Delay Raytracing Applied in VLBI Analysis

NASA Astrophysics Data System (ADS)

Eriksson, David; MacMillan, Daniel; Gipson, John

2014-12-01

Tropospheric delay modeling error is one of the largest sources of error in VLBI analysis. For standard operational solutions, we use the VMF1 elevation-dependent mapping functions derived from European Centre for Medium Range Forecasting (ECMWF) data. These mapping functions assume that tropospheric delay at a site is azimuthally symmetric. As this assumption does not reflect reality, we have instead determined the raytrace delay along the signal path through the three-dimensional troposphere refractivity field for each VLBI quasar observation. We calculated the troposphere refractivity fields from the pressure, temperature, specific humidity, and geopotential height fields of the NASA GSFC GEOS-5 numerical weather model. We discuss results using raytrace delay in the analysis of the CONT11 R&D sessions. When applied in VLBI analysis, baseline length repeatabilities were better for 70% of baselines with raytraced delays than with VMF1 mapping functions. Vertical repeatabilities were better for 2/3 of all stations. The reference frame scale bias error was 0.02 ppb for raytracing versus 0.08 ppb and 0.06 ppb for VMF1 and NMF, respectively.

A search for AGN activity in Infrared-Faint Radio Sources (IFRS)

NASA Astrophysics Data System (ADS)

Lenc, Emil; Middelberg, Enno; Norris, Ray; Mao, Minnie

2009-04-01

We propose to observe a large sample of radio sources from the ATLAS (Australia Telescope Large Area Survey) source catalogue with the LBA, to determine their compactness. The sample consists of 36 sources with no counterpart in the co-located SWIRE survey (3.6 um to 160 um), carried out with the Spitzer Space Telescope. This rare class of sources, dubber Infrared-Faint Radio Sources (IFRS), is inconsistent with current galaxy evolution models. VLBI observations are an essential way to obtain further clues on what these objects are and why they are hidden from infrared observations. We will measure the flux densities on long baselines to determine their compactness. Only five IFRS have been previously targeted with VLBI observations (resulting in two detections). We propose using single baseline (Parkes-ATCA) eVLBI observations with the LBA at 1 Gbps to maximise sensitivity. With the observations proposed here we will increase the number of VLBI-observed IFRS from 5 to 36, allowing us to draw statistical conclusions about this intriguing new class of objects.

A search for AGN activity in Infrared-Faint Radio Sources (IFRS)

NASA Astrophysics Data System (ADS)

Lenc, Emil; Middelberg, Enno; Norris, Ray; Mao, Minnie

2010-04-01

We propose to observe a large sample of radio sources from the ATLAS (Australia Telescope Large Area Survey) source catalogue with the LBA, to determine their compactness. The sample consists of 36 sources with no counterpart in the co-located SWIRE survey (3.6 um to 160 um), carried out with the Spitzer Space Telescope. This rare class of sources, dubber Infrared-Faint Radio Sources (IFRS), is inconsistent with current galaxy evolution models. VLBI observations are an essential way to obtain further clues on what these objects are and why they are hidden from infrared observations. We will measure the flux densities on long baselines to determine their compactness. Only five IFRS have been previously targeted with VLBI observations (resulting in two detections). We propose using single baseline (Parkes-ATCA) eVLBI observations with the LBA at 1 Gbps to maximise sensitivity. With the observations proposed here we will increase the number of VLBI-observed IFRS from 5 to 36, allowing us to draw statistical conclusions about this intriguing new class of objects.

A Model of Relation between Fluctuation of Double Differential Total Ionospheric Electron Content and Angular Distance of the Two Satellites Observed by Same-beam VLBI

NASA Astrophysics Data System (ADS)

Xiao, Yao; Qing-hui, Liu

2018-01-01

Time delay and phase fluctuation are produced when the signals of a spacecraft are transmitted through the ionosphere of the earth, which give rise to a great influence on the measurement precision of VLBI (Very Long Baseline Interferometry). Using the 1-year same-beam VLBI data of 2 satellites (Rstar and Vstar) in the Japanese lunar exploration project SELENE, we obtained a model of the relation between the fluctuation of double differential total electron content in the ionosphere and the angular distance of the two satellites. For the 6 baselines, the root mean square r of fluctuation (in units of TECU) and the angular distance of the two satellites θ (in units of ∘) has a relation of r = 0.773θ + 0.562, and for the 4 VLBI stations, the relation is r = 0.554θ + 0.399 from the baselines inversion. The results can serve as a reference for the derivation of differential phase delay and for the occultation observation and study of planetary ionospheres.

Toward the ICRF3: Astrometric Comparison of the USNO 2016A VLBI Solution with ICRF2 and Gaia DR1

NASA Astrophysics Data System (ADS)

Frouard, Julien; Johnson, Megan C.; Fey, Alan; Makarov, Valeri V.; Dorland, Bryan N.

2018-06-01

The VLBI USNO 2016A (U16A) solution is part of a work-in-progress effort by USNO toward the preparation of the ICRF3. Most of the astrometric improvement with respect to the ICRF2 is due to the re-observation of the VCS sources. Our objective in this paper is to assess U16A’s astrometry. A comparison with ICRF2 shows statistically significant offsets of size 0.1 mas between the two solutions. While Gaia DR1 positions are not precise enough to resolve these offsets, they are found to be significantly closer to U16A than ICRF2. In particular, the trend for typically larger errors for southern sources in VLBI solutions is decreased in U16A. Overall, the VLBI-Gaia offsets are reduced by 21%. The U16A list includes 718 sources not previously included in ICRF2. Twenty of those new sources have statistically significant radio-optical offsets. In two-thirds of the cases, these offsets can be explained from PanSTARRS images.

Plate motions and deformations from geologic and geodetic data

NASA Technical Reports Server (NTRS)

Jordan, Thomas H.

1989-01-01

The very long baseline interferometry (VLBI) measurements made in the western U.S. since 1979 provide discrete samples of the temporal and spatial deformation field. The interpretation of the VLBI derived rates of deformation requires an examination of geologic information and more densely sampled ground based geodetic data. Triangulation and trilateration data measured on two regional networks, one in the central Mojave Desert and one in the Coast Ranges east of the San Andreas fault, were processed. At the spatial scales spanned by these local geodetic networks, auxiliary geologic and geophysical data were utilized to examine the relation between measured incremental strain and the accommodation of strain seen in local geologic structures, strain release in earthquakes, and principal stress directions inferred from in situ measurements. VLBI data was also processed from stations distributed across the Pacific-North America plate boundary zone in the western U.S. The VLBI data were used to constrain the integrated rate of deformation across portions of the continental plate boundary in California and to provide a tectonic framework to interpret regional geodetic and geologic studies.

Jet outflow and gamma-ray emission correlations in S5 0716+714

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rani, B.; Krichbaum, T. P.; Marscher, A. P.

Here, using millimeter very long baseline interferometry (VLBI) observations of the BL Lac object S5 0716+714 from August 2008 to September 2013, we investigate variations in the core flux density and orientation of the sub-parsec scale jet, i.e. position angle. The γ-ray data obtained by the Fermi Large Area Telescope are used to investigate the high-energy flux variations over the same time period. For the first time in any blazar, we report a significant correlation between the γ-ray flux variations and the position angle variations in the VLBI jet. The cross-correlation analysis also indicates a positive correlation such that themore » mm-VLBI core flux density variations are delayed with respect to the γ-ray flux by 82±32 days. This suggests that the high-energy emission is coming from a region located ≥(3.8±1.9) parsecs upstream of the mm-VLBI core (closer to the central black hole). Lastly, these results imply that the observed inner jet morphology has a strong connection with the observed γ-ray flares.« less

EXPLORATION OF SOURCE FREQUENCY PHASE REFERENCING TECHNIQUES FOR ASTROMETRY AND OBSERVATIONS OF WEAK SOURCES WITH HIGH FREQUENCY SPACE VERY LONG BASELINE INTERFEROMETRY

DOE Office of Scientific and Technical Information (OSTI.GOV)

Rioja, M.; Dodson, R.; Malarecki, J.

2011-11-15

Space very long baseline interferometry (S-VLBI) observations at high frequencies hold the prospect of achieving the highest angular resolutions and astrometric accuracies, resulting from the long baselines between ground and satellite telescopes. Nevertheless, space-specific issues, such as limited accuracy in the satellite orbit reconstruction and constraints on the satellite antenna pointing operations, limit the application of conventional phase referencing. We investigate the feasibility of an alternative technique, source frequency phase referencing (SFPR), to the S-VLBI domain. With these investigations we aim to contribute to the design of the next generation of S-VLBI missions. We have used both analytical and simulationmore » studies to characterize the performance of SFPR in S-VLBI observations, applied to astrometry and increased coherence time, and compared these to results obtained using conventional phase referencing. The observing configurations use the specifications of the ASTRO-G mission for their starting point. Our results show that the SFPR technique enables astrometry at 43 GHz, using alternating observations with 22 GHz, regardless of the orbit errors, for most weathers and under a wide variety of conditions. The same applies to the increased coherence time for the detection of weak sources. Our studies show that the capability to carry out simultaneous dual frequency observations enables application to higher frequencies, and a general improvement of the performance in all cases, hence we recommend its consideration for S-VLBI programs.« less

NASA space geodesy program: Catalogue of site information

NASA Technical Reports Server (NTRS)

Bryant, M. A.; Noll, C. E.

1993-01-01

This is the first edition of the NASA Space Geodesy Program: Catalogue of Site Information. This catalogue supersedes all previous versions of the Crustal Dynamics Project: Catalogue of Site Information, last published in May 1989. This document is prepared under the direction of the Space Geodesy and Altimetry Projects Office (SGAPO), Code 920.1, Goddard Space Flight Center. SGAPO has assumed the responsibilities of the Crustal Dynamics Project, which officially ended December 31, 1991. The catalog contains information on all NASA supported sites as well as sites from cooperating international partners. This catalog is designed to provde descriptions and occupation histories of high-accuracy geodetic measuring sites employing space-related techniques. The emphasis of the catalog has been in the past, and continues to be with this edition, station information for facilities and remote locations utilizing the Satellite Laser Ranging (SLR), Lunar Laser Ranging (LLR), and Very Long Baseline Interferometry (VLBI) techniques. With the proliferation of high-quality Global Positioning System (GPS) receivers and Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) transponders, many co-located at established SLR and VLBI observatories, the requirement for accurate station and localized survey information for an ever broadening base of scientists and engineers has been recognized. It is our objective to provide accurate station information to scientific groups interested in these facilities.

APOD Mission Status and Observations by VLBI

NASA Astrophysics Data System (ADS)

Tang, Geshi; Sun, Jing; Li, Xie; Liu, Shushi; Chen, Guangming; Ren, Tianpeng; Wang, Guangli

2016-12-01

On September 20, 2015, 20 satellites were successfully launched from the TaiYuan Satellite Launch Center by a Chinese CZ-6 test rocket and are, since then, operated in a circular, near-polar orbit at an altitude of 520 km. Among these satellites, a set of four CubSats, named APOD (Atmospheric density detection and Precise Orbit Determination), are intended for atmospheric density in-situ detection and derivation via precise orbit. The APOD satellites, manufactured by DFH Co., carry a number of instruments including a density detector, a dual-frequency GNSS (GPS/BD) receiver, an SLR reflector, and a VLBI S/X beacon. The APOD mission aims at detecting the atmospheric density below 520 km. The ground segment is controlled by BACC (Beijing Aerospace Control Center) including payload operation as well as science data receiving, processing, archiving, and distribution. Currently, the in-orbit test of the nano-satellites and their payloads are completed, and preliminary results show that the precision of the orbit determination is about 10 cm derived from both an overlap comparison and an SLR observation validation. The in-situ detected density calibrated by orbit-derived density demonstrates that the accuracy of atmospheric mass density is approximately 4.191×10^{-14} kgm^{-3}, about 5.5% of the measurement value. Since three space-geodetic techniques (i.e., GNSS, SLR, and VLBI) are co-located on the APOD nano-satellites, the observations can be used for combination and validation in order to detect systematic differences. Furthermore, the observations of the APOD satellites by VLBI radio telescopes can be used in an ideal fashion to link the dynamical reference frames of the satellite with the terrestrial and, most importantly, with the celestial reference frame as defined by the positions of quasars. The possibility of observing the APOD satellites by IVS VLBI radio telescopes will be analyzed, considering continental-size VLBI observing networks and the small telescopes with sufficient speed.

Consistent realization of Celestial and Terrestrial Reference Frames

NASA Astrophysics Data System (ADS)

Kwak, Younghee; Bloßfeld, Mathis; Schmid, Ralf; Angermann, Detlef; Gerstl, Michael; Seitz, Manuela

2018-03-01

The Celestial Reference System (CRS) is currently realized only by Very Long Baseline Interferometry (VLBI) because it is the space geodetic technique that enables observations in that frame. In contrast, the Terrestrial Reference System (TRS) is realized by means of the combination of four space geodetic techniques: Global Navigation Satellite System (GNSS), VLBI, Satellite Laser Ranging (SLR), and Doppler Orbitography and Radiopositioning Integrated by Satellite. The Earth orientation parameters (EOP) are the link between the two types of systems, CRS and TRS. The EOP series of the International Earth Rotation and Reference Systems Service were combined of specifically selected series from various analysis centers. Other EOP series were generated by a simultaneous estimation together with the TRF while the CRF was fixed. Those computation approaches entail inherent inconsistencies between TRF, EOP, and CRF, also because the input data sets are different. A combined normal equation (NEQ) system, which consists of all the parameters, i.e., TRF, EOP, and CRF, would overcome such an inconsistency. In this paper, we simultaneously estimate TRF, EOP, and CRF from an inter-technique combined NEQ using the latest GNSS, VLBI, and SLR data (2005-2015). The results show that the selection of local ties is most critical to the TRF. The combination of pole coordinates is beneficial for the CRF, whereas the combination of Δ UT1 results in clear rotations of the estimated CRF. However, the standard deviations of the EOP and the CRF improve by the inter-technique combination which indicates the benefits of a common estimation of all parameters. It became evident that the common determination of TRF, EOP, and CRF systematically influences future ICRF computations at the level of several μas. Moreover, the CRF is influenced by up to 50 μas if the station coordinates and EOP are dominated by the satellite techniques.

SHARP - V. Modelling gravitationally-lensed radio arcs imaged with global VLBI observations

NASA Astrophysics Data System (ADS)

Spingola, C.; McKean, J. P.; Auger, M. W.; Fassnacht, C. D.; Koopmans, L. V. E.; Lagattuta, D. J.; Vegetti, S.

2018-05-01

We present milliarcsecond (mas) angular resolution observations of the gravitationally lensed radio source MG J0751+2716 (at z = 3.2) obtained with global Very Long Baseline Interferometry (VLBI) at 1.65 GHz. The background object is highly resolved in the tangential and radial directions, showing evidence of both compact and extended structure across several gravitational arcs that are 200 to 600 mas in size. By identifying compact sub-components in the multiple images, we constrain the mass distribution of the foreground z = 0.35 gravitational lens using analytic models for the main deflector [power-law elliptical mass model; ρ(r)∝r-γ, where γ = 2 corresponds to isothermal] and for the members of the galaxy group. Moreover, our mass models with and without the group find an inner mass-density slope steeper than isothermal for the main lensing galaxy, with γ1 = 2.08 ± 0.02 and γ2 = 2.16 ± 0.02 at the 4.2σ level and 6.8σ level, respectively, at the Einstein radius (b1 = 0.4025 ± 0.0008 and b2 = 0.307 ± 0.002 arcsec, respectively). We find randomly distributed image position residuals of about 3 mas, which are much larger that the measurement errors (40 μas on average). This suggests that at the mas level, the assumption of a smooth mass distribution fails, requiring additional structure in the model. However, given the environment of the lensing galaxy, it is not clear whether this extra mass is in the form of sub-haloes within the lens or along the line of sight, or from a more complex halo for the galaxy group.

Implementation and testing of the gridded Vienna Mapping Function 1 (VMF1)

NASA Astrophysics Data System (ADS)

Kouba, J.

2008-04-01

The new gridded Vienna Mapping Function (VMF1) was implemented and compared to the well-established site-dependent VMF1, directly and by using precise point positioning (PPP) with International GNSS Service (IGS) Final orbits/clocks for a 1.5-year GPS data set of 11 globally distributed IGS stations. The gridded VMF1 data can be interpolated for any location and for any time after 1994, whereas the site-dependent VMF1 data are only available at selected IGS stations and only after 2004. Both gridded and site-dependent VMF1 PPP solutions agree within 1 and 2 mm for the horizontal and vertical position components, respectively, provided that respective VMF1 hydrostatic zenith path delays (ZPD) are used for hydrostatic ZPD mapping to slant delays. The total ZPD of the gridded and site-dependent VMF1 data agree with PPP ZPD solutions with RMS of 1.5 and 1.8 cm, respectively. Such precise total ZPDs could provide useful initial a priori ZPD estimates for kinematic PPP and regional static GPS solutions. The hydrostatic ZPDs of the gridded VMF1 compare with the site-dependent VMF1 ZPDs with RMS of 0.3 cm, subject to some biases and discontinuities of up to 4 cm, which are likely due to different strategies used in the generation of the site-dependent VMF1 data. The precision of gridded hydrostatic ZPD should be sufficient for accurate a priori hydrostatic ZPD mapping in all precise GPS and very long baseline interferometry (VLBI) solutions. Conversely, precise and globally distributed geodetic solutions of total ZPDs, which need to be linked to VLBI to control biases and stability, should also provide a consistent and stable reference frame for long-term and state-of-the-art numerical weather modeling.

Astrometry Information Center - Naval Oceanography Portal

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VLBI2010: The Astro-Geo Connection

NASA Technical Reports Server (NTRS)

Porcas, Richard

2010-01-01

VLBI2010 holds out promise for greatly increased precision in measuring geodetic and Earth rotation parameters. As a by-product there will be a wealth of interesting new astronomical data. At the same time, astronomical knowledge may be needed to disentangle the astronomical and geodetic contributions to the measured delays and phases. This presentation explores this astro-geo link.

Physical conditions near red giant and supergiant stars - An interpretation of SiO VLBI maps

NASA Technical Reports Server (NTRS)

Alcock, Charles; Ross, Randy R.

1986-01-01

Understanding the dynamical structure of circumstellar envelopes around cool giant and supergiant stars depends critically on the knowledge of what happens in the 'near zone' of the envelope, within a few stellar radii of the star. One probe with adequate angular resolution to study the near zone is VLBI observation of the SiO masers. It is shown that VLBI maps of VX Sgr establish that the particle density in the SiO masers is very high (about 10 to the 12th/cu cm), indicating that the masers form in dense cloudlets and not in a spherically expanding wind. The implications of these results for the mechanism of mass loss are discussed.

Geodetic VLBI observations at Simeiz station

NASA Astrophysics Data System (ADS)

Volvach, A.; Petrov, L.; Nesterov, N.

Very long baseline interferometry (VLBI) observations under international geodetic programs are carried out at Simeiz station since June 1994. 22-m radiotelescope is equipped by dual-band S/X receivers, hydrogen maser CH-70 and data acquisition terminal Mark-IIIA. Observations are conducted by 24 hours sessions scheduled 6-15 times per year. Observational programs are a part of common efforts for maintenance of terrestrial reference frame, celestial reference frame and monitoring Earth orientation parameters carried out by international community under the auspices of International VLBI Service (IVS). Data are recorded on magnetic tapes which are shipped to correlator centers for further correlation and fringing. Fringed data are archived and are freely available via Internet for scientific analysis after 1-2 months after observations.

Radio interferometric measurements for accurate planetary orbiter navigation

NASA Technical Reports Server (NTRS)

Poole, S. R.; Ananda, M.; Hildebrand, C. E.

1979-01-01

The use of narrowband delta-VLBI to achieve accurate orbit determination is presented by viewing a spacecraft from widely separated stations followed by viewing a nearby quasar from the same stations. Current analysis is examined that establishes the orbit determination accuracy achieved with data arcs spanning up to 3.5 d. Strategies for improving prediction accuracy are given, and the performance of delta-VLBI is compared with conventional radiometric tracking data. It is found that accuracy 'within the fit' is on the order of 0.5 km for data arcs having delta-VLBI on the ends of the arcs and for arc lengths varying from one baseline to 3.5 d. The technique is discussed with reference to the proposed Venus Orbiting Imaging Radar mission.

International VLBI Service for Geodesy and Astrometry: 2000 General Meeting Proceedings

NASA Technical Reports Server (NTRS)

Vandenberg, Nancy R. (Editor); Baver, Karen D. (Editor)

2000-01-01

This volume is the proceedings of the first General Meeting of the International Very Long Base Interferometry (VLBI) Service for Geodesy and Astrometry (IVS), held in Koetzting, Germany, February 21-24, 2000. The content of this volume also appears on the IVS web site at: http://ivscc.gsfc.nasa.gov/publications/gm2000. The goal of the program committee for the General Meeting was to provide an interesting and informative program for a wide cross section of IVS members, including station operators, program managers, and analysts. The program included reports, tutorials, invited and contributed papers, and poster presentations. The tutorial papers should be particularly useful references because each one provides an overview and introduction to a topic relevant to VLBI.

International VLBI Service for Geodesy and Astrometry 2013 Annual Report

NASA Technical Reports Server (NTRS)

Baver, Karen D.; Behrend, Dirk; Armstrong, Kyla L.

2014-01-01

This volume of reports is the 2013 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the permanent components of IVS. The IVS 2013 Annual Report documents the work of the IVS components for the calendar year 2013, our fifteenth year of existence. The reports describe changes, activities, and progress of the IVS. Many thanks to all IVS components who contributed to this Annual Report. With the exception of the first section and the last section, the contents of this Annual Report also appear on the IVS Web site at http://ivscc.gsfc.nasa.gov/publications/ar2013.

International VLBI Service for Geodesy and Astrometry 2012 Annual Report

NASA Technical Reports Server (NTRS)

Baver, Karen D.; Behrend, Dirk; Armstrong, Kyla L.

2013-01-01

This volume of reports is the 2012 Annual Report of the International VLBI Service for Geodesy and Astrometry (IVS). The individual reports were contributed by VLBI groups in the international geodetic and astrometric community who constitute the permanent components of IVS. The IVS 2012 Annual Report documents the work of the IVS components for the calendar year 2012, our fourteenth year of existence. The reports describe changes, activities, and progress ofthe IVS. Many thanks to all IVS components who contributed to this Annual Report. With the exception of the first section and parts of the last section (described below), the contents of this Annual Report also appear on the IVS Web site athttp:ivscc.gsfc.nasa.gov/publications/ar2012

A VLBI variance-covariance analysis interactive computer program. M.S. Thesis

NASA Technical Reports Server (NTRS)

Bock, Y.

1980-01-01

An interactive computer program (in FORTRAN) for the variance covariance analysis of VLBI experiments is presented for use in experiment planning, simulation studies and optimal design problems. The interactive mode is especially suited to these types of analyses providing ease of operation as well as savings in time and cost. The geodetic parameters include baseline vector parameters and variations in polar motion and Earth rotation. A discussion of the theroy on which the program is based provides an overview of the VLBI process emphasizing the areas of interest to geodesy. Special emphasis is placed on the problem of determining correlations between simultaneous observations from a network of stations. A model suitable for covariance analyses is presented. Suggestions towards developing optimal observation schedules are included.

Global geodesy using GPS without fiducial sites

NASA Technical Reports Server (NTRS)

Heflin, Michael; Bertiger, Willy; Blewitt, Geoff; Freedman, Adam; Hurst, Ken; Lichten, Steve; Lindqwister, Ulf; Vigue, Yvonne; Webb, Frank; Yunck, Tom

1992-01-01

Baseline lengths and geocentric radii have been determined from GPS data without the use of fiducial sites. Data from the first GPS experiment for the IERS and Geodynamics (GIG '91) have been analyzed with a no-fiducial strategy. A baseline length daily repeatability of 2 mm + 4 parts per billion was obtained for baselines in the Northern Hemisphere. Comparison of baseline lengths from GPS and the global VLBI solution GLB659 (Caprette et al. 1990) show rms agreement of 2.1 parts per billion. The geocentric radius mean daily repeatability for all sites was 15 cm. Comparison of geocentric radii from GPS and SV5 (Murray et al. 1990) show rms agreement of 3.8 cm. Given n globally distributed stations, the n(n - 1)/2 baseline lengths and n geocentric radii uniquely define a rigid closed polyhedron with a well-defined center of mass. Geodetic information can be obtained by examining the structure of the polyhedron and its change with time.

SWARM: A Compact High Resolution Correlator and Wideband VLBI Phased Array Upgrade for SMA

NASA Astrophysics Data System (ADS)

Weintroub, Jonathan

2014-06-01

A new digital back end (DBE) is being commissioned on Mauna Kea. The “SMA Wideband Astronomical ROACH2 Machine”, or SWARM, processes a 4 GHz usable band in single polarization mode and is flexibly reconfigurable for 2 GHz full Stokes dual polarization. The hardware is based on the open source Reconfigurable Open Architecture Computing Hardware 2 (ROACH2) platform from the Collaboration for Astronomy Signal Processing and Electronics Research (CASPER). A 5 GSps quad-core analog-to-digital converter board uses a commercial chip from e2v installed on a CASPER-standard printed circuit board designed by Homin Jiang’s group at ASIAA. Two ADC channels are provided per ROACH2, each sampling a 2.3 GHz Nyquist band generated by a custom wideband block downconverter (BDC). The ROACH2 logic includes 16k-channel Polyphase Filterbank (F-engine) per input followed by a 10 GbE switch based corner-turn which feeds into correlator-accumulator logic (X-engines) co-located with the F-engines. This arrangement makes very effective use of a small amount of digital hardware (just 8 ROACH2s in 1U rack mount enclosures). The primary challenge now is to meet timing at full speed for a large and very complex FPGA bit code. Design of the VLBI phased sum and recorder interface logic is also in process. Our poster will describe the instrument design, with the focus on the particular challenges of ultra wideband signal processing. Early connected commissioning and science verification data will be presented.

The Spring 1985 high precision baseline test of the JPL GPS-based geodetic system

NASA Technical Reports Server (NTRS)

Davidson, John M.; Thornton, Catherine L.; Stephens, Scott A.; Blewitt, Geoffrey; Lichten, Stephen M.; Sovers, Ojars J.; Kroger, Peter M.; Skrumeda, Lisa L.; Border, James S.; Neilan, Ruth E.

1987-01-01

The Spring 1985 High Precision Baseline Test (HPBT) was conducted. The HPBT was designed to meet a number of objectives. Foremost among these was the demonstration of a level of accuracy of 1 to 2:10 to the 7th power, or better, for baselines ranging in length up to several hundred kilometers. These objectives were all met with a high degree of success, with respect to the demonstration of system accuracy in particular. The results from six baselines ranging in length from 70 to 729 km were examined for repeatability and, in the case of three baselines, were compared to results from colocated VLBI systems. Repeatability was found to be 5:10 to the 8th power (RMS) for the north baseline coordinate, independent of baseline length, while for the east coordinate RMS repeatability was found to be larger than this by factors of 2 to 4. The GPS-based results were found to be in agreement with those from colocated VLBI measurements, when corrected for the physical separations of the VLBI and CPG antennas, at the level of 1 to 2:10 to the 7th power in all coordinates, independent of baseline length. The results for baseline repeatability are consistent with the current GPA error budget, but the GPS-VLBI intercomparisons disagree at a somewhat larger level than expected. It is hypothesized that these differences may result from errors in the local survey measurements used to correct for the separations of the GPS and VLBI antenna reference centers.

International mission planning for space Very Long Baseline Interferometry

NASA Technical Reports Server (NTRS)

Ulvestad, James S.

1994-01-01

Two spacecraft dedicated to Very Long Baseline Interferometry (VLBI) will be launched in 1996 and 1997 to make observations using baselines between the space telescopes and many of the world's ground radio telescopes. The Japanese Institute of Space and Astronautical Science (ISAS) will launch VSOP (VLBI Space Observatory Program) in September 1996, while the Russian Astro Space Center (ASC) is scheduled to launch RadioAstron in 1997. Both spacecraft will observe radio sources at frequencies near 1.7, 4.8, and 22 GHz; RadioAstron will also observe at 0.33 GHz. The baselines between space and ground telescopes will provide 3-10 times the resolution available for ground VLBI at the same observing frequencies. Ground tracking stations on four continents will supply the required precise frequency reference to each spacecraft measure the two-way residual phase and Doppler on the ground-space link, and record 128 Megabit/s of VLBI data downlinked from the spacecraft. The spacecraft data are meaningless without cross-correlation against the data from Earth-bound telescopes, which must take place at special-purpose VLBI correlation facilities. Therefore, participation by most of the world's radio observatories is needed to achieve substantial science return from VSOP and RadioAstron. The collaboration of several major space agencies and the ground observatories, which generally follow very different models for allocation of observing time and for routine operations, leads to great complexity in mission planning and in day-to-day operations. This paper describes some of those complications and the strategies being developed to assure productive scientific missions.

A test of water vapor radiometer-based troposphere calibration using VLBI observations on a 21-kilometer baseline

NASA Technical Reports Server (NTRS)

Linfield, R. P.; Teitelbaum, L. P.; Skjerve, L. J.; Keihm, S. J.; Walter, S. J.; Mahoney, M. J.; Treuhaft, R. N.

1995-01-01

Simultaneous very long baseline interferometry (VLBI) and water vapor radiometer (WVR) measurements on a 21 km baseline showed that calibration by WVRs removed a significant fraction of the effect of tropospheric delay fluctuations for these experiments. From comparison of the residual delay variations within scans and between scans, the total tropospheric contribution t the delay residuals for each of the three 5 to 20 hour sessions was estimated as 1, 17, and 10%, with the first value being uncertain. The observed improvement in rms residual delay from WVR calibration during these three sessions was 4, 16, and 2%, respectively. The improvement is consistent with the estimated 2 to 3 mm path delay precision of current WVRs. The VLBI measurements, of natural radio sources, were conducted in April and May 1993 at Goldstone, California. Dual-frequency (2.3 and 8.4 GHz) observations were employed to remove the effects of charged particles from the data. Measurements with co-pointed WVRs, located within 50 m of the axis of each antenna, were performed to test the ability of the WVRs to calibrate line-of-sight path delays. Factors that made WVR performance assessment difficult included (1) the fact that the level of tropospheric fluctuations was smaller than is typical for Goldstone during these experiments and (2) VLBI delay variations on longer time scales (i.e., over multiple scans) contained uncalibrated instrumental effects (probably a result of slow temperature variations in the VLBI hardware) that were larger than the tropospheric effects.

The New Generation Russian VLBI Network

NASA Technical Reports Server (NTRS)

Finkelstein, Andrey; Ipatov, Alexander; Smolentsev, Sergey; Mardyshkin, Vyacheslav; Fedotov, Leonid; Surkis, Igor; Ivanov, Dmitrij; Gayazov, Iskander

2010-01-01

This paper deals with a new project of the Russian VLBI Network dedicated for Universal Time determinations in quasi on-line mode. The basic principles of the network design and location of antennas are explained. Variants of constructing receiving devices, digital data acquisition system, and phase calibration system are specially considered. The frequency ranges and expected values of noise temperature are given.

Multi-Epoch 8.4GHz VLBI Observations of the Nucleus of Centaurus A

NASA Technical Reports Server (NTRS)

Meier, David L.

1992-01-01

We present the results of several 8.4 GHz VLBI observations of the nucleus of Centaurus A. We fing that the source possesses a classical core-jet structure with the inner portion of the jet expanding at a proper motion of 4.o mas yr or an apparet velocity of 0.26c along the jet.

Rigorous Combination of GNSS and VLBI: How it Improves Earth Orientation and Reference Frames

NASA Astrophysics Data System (ADS)

Lambert, S. B.; Richard, J. Y.; Bizouard, C.; Becker, O.

2017-12-01

Current reference series (C04) of the International Earth Rotation and Reference Systems Service (IERS) are produced by a weighted combination of Earth orientation parameters (EOP) time series built up by combination centers of each technique (VLBI, GNSS, Laser ranging, DORIS). In the future, we plan to derive EOP from a rigorous combination of the normal equation systems of the four techniques.We present here the results of a rigorous combination of VLBI and GNSS pre-reduced, constraint-free, normal equations with the DYNAMO geodetic analysis software package developed and maintained by the French GRGS (Groupe de Recherche en GeÌodeÌsie Spatiale). The used normal equations are those produced separately by the IVS and IGS combination centers to which we apply our own minimal constraints.We address the usefulness of such a method with respect to the classical, a posteriori, combination method, and we show whether EOP determinations are improved.Especially, we implement external validations of the EOP series based on comparison with geophysical excitation and examination of the covariance matrices. Finally, we address the potential of the technique for the next generation celestial reference frames, which are currently determined by VLBI only.

Tropospheric delay ray tracing applied in VLBI analysis

NASA Astrophysics Data System (ADS)

Eriksson, David; MacMillan, D. S.; Gipson, John M.

2014-12-01

Tropospheric delay modeling error continues to be one of the largest sources of error in VLBI (very long baseline interferometry) analysis. For standard operational solutions, we use the VMF1 elevation-dependent mapping functions derived from European Centre for Medium-Range Weather Forecasts data. These mapping functions assume that tropospheric delay at a site is azimuthally symmetric. As this assumption is not true, we have instead determined the ray trace delay along the signal path through the troposphere for each VLBI quasar observation. We determined the troposphere refractivity fields from the pressure, temperature, specific humidity, and geopotential height fields of the NASA Goddard Space Flight Center Goddard Earth Observing System version 5 numerical weather model. When applied in VLBI analysis, baseline length repeatabilities were improved compared with using the VMF1 mapping function model for 72% of the baselines and site vertical repeatabilities were better for 11 of 13 sites during the 2 week CONT11 observing period in September 2011. When applied to a larger data set (2011-2013), we see a similar improvement in baseline length and also in site position repeatabilities for about two thirds of the stations in each of the site topocentric components.

Geodetic measurement of deformation in California. Ph.D. Thesis

NASA Technical Reports Server (NTRS)

Sauber, Jeanne Marie

1988-01-01

The very long baseline interferometry (VLBI) measurements made in the western U.S. since 1979 as part of the NASA Crustal Dynamics Project provide discrete samples of the temporal and spatial deformation field. The interpretation of the VLBI-derived rates of deformation requires an examination of geologic information and more densely sampled ground-based geodetic data. In the first two of three related studies embodying this thesis triangulation and trilateration data measured on two regional networks are processed, one in the central Mojave Desert and one in the Coast Ranges east of the San Andreas fault. At the spatial scales spanned by these local geodetic networks, auxiliary geologic and geophysical data have been utilized to examine the relation between measured incremental strain and the accommodation of strain seen in local geological structures, strain release in earthquakes, and principal stress directions inferred from in situ measurements. In the third study, VLBI data from stations distributed across the Pacific - North American plate boundary zone in the western United States are processed. The VLBI data have been used to constrain the integrated rate of deformation across portions of the continental plate boundary in California and to provide a tectonic framework to interpret regional geodetic and geologic studies.

A possible space VLBI constellation utilizing the stable orbits around the TLPs in the Earth-Moon system.

NASA Astrophysics Data System (ADS)

Liu, Bin; Tang, Jingshi; Hou, Xiyun

2016-07-01

Current studies indicate that there are stable orbits around but far away from the triangular libration points .Two special quasi-periodic orbits around each triangular libration points L4 , L5 in the Earth-Moon sys-tem perturbed by Sun are gain , and the stable orbits discussed in this work are ideal places for space colonies because no orbit control is needed. These stable orbits can also be used as nominal orbits for space VLBI (Very Long Baseline Interferometry) stations. The two stations can also form baselines with stations on the Earth and the Moon, or with stations located around another TLP. Due to the long distance between the stations, the observation precision can be greatly enhanced compared with the VLBI stations on the Earth. Such a VLBI constellation not only can advance the radio astronomy, but also can be used as a navigation system for human activities in the Earth-Moon system and even in the solar system. This paper will focus on the navigation constellation coverage issues, and the orbit determination accuracy problems within the Earth-Moon sys-tem and interplanetary space.

Application of Laser Ranging and VLBI Data to a Study of Plate Tectonic Driving Forces

NASA Technical Reports Server (NTRS)

Solomon, S. C.

1980-01-01

The conditions under which changes in plate driving or resistive forces associated with plate boundary earthquakes are measurable with laser ranging or very long base interferometry were investigated. Aspects of plate forces that can be characterized by such measurements were identified. Analytic solutions for two dimensional stress diffusion in a viscoelastic plate following earthquake faulting on a finite fault, finite element solutions for three dimensional stress diffusion in a viscoelastic Earth following earthquake faulting, and quantitative constraints from modeling of global intraplate stress on the magnitude of deviatoric stress in the lithosphere are among the topics discussed.

The performance of differential VLBI delay during interplanetary cruise

NASA Technical Reports Server (NTRS)

Moultrie, B.; Wolff, P. J.; Taylor, T. H.

1984-01-01

Project Voyager radio metric data are used to evaluate the orbit determination abilities of several data strategies during spacecraft interplanetary cruise. Benchmark performance is established with an operational data strategy of conventional coherent doppler, coherent range, and explicitly differenced range data from two intercontinental baselines to ameliorate the low declination singularity of the doppler data. Employing a Voyager operations trajectory as a reference, the performance of the operational data strategy is compared to the performances of data strategies using differential VLBI delay data (spacecraft delay minus quasar delay) in combinations with the aforementioned conventional data types. The comparison of strategy performances indicates that high accuracy cruise orbit determination can be achieved with a data strategy employing differential VLBI delay data, where the quantity of coherent radio metric data has been greatly reduced.

From truck to optical fibre: the coming-of-age of eVLBI

NASA Astrophysics Data System (ADS)

Szomoru, A.; Biggs, A.; Garrett, M.; van Langevelde, H. J.; Olnon, F.; Paragi, Z.; Parsley, S.; Pogrebenko, S.; Reynolds, C.

Spurred by the advent of disk-based recording systems and the nearly explosive increase of internet bandwidth, eVLBI (Parsley et al. te{parsley}) has undergone a remarkable development over the past two years. From ftp-based transfers of small amounts of astronomical data, through near real-time correlation (disk-buffered at the correlator), it has culminated this spring in the first three telescope real-time correlation at JIVE (Onsala, Westerbork and Jodrell Bank). In this paper we will give a review of this development and the current state of affairs. We will also address the current limitations and the way we may improve both bandwidth and reliability and finally we will discuss the opportunities a true high-bandwidth real-time VLBI correlator will provide. (astro-ph/0412686)

Deep Space Network Radiometric Remote Sensing Program

NASA Technical Reports Server (NTRS)

Walter, Steven J.

1994-01-01

Planetary spacecraft are viewed through a troposphere that absorbs and delays radio signals propagating through it. Tropospheric water, in the form of vapor, cloud liquid, and precipitation, emits radio noise which limits satellite telemetry communication link performance. Even at X-band, rain storms have severely affected several satellite experiments including a planetary encounter. The problem will worsen with DSN implementation of Ka-band because communication link budgets will be dominated by tropospheric conditions. Troposphere-induced propagation delays currently limit VLBI accuracy and are significant sources of error for Doppler tracking. Additionally, the success of radio science programs such as satellite gravity wave experiments and atmospheric occultation experiments depends on minimizing the effect of water vapor-induced propagation delays. In order to overcome limitations imposed by the troposphere, the Deep Space Network has supported a program of radiometric remote sensing. Currently, water vapor radiometers (WVRs) and microwave temperature profilers (MTPs) support many aspects of the Deep Space Network operations and research and development programs. Their capability to sense atmospheric water, microwave sky brightness, and atmospheric temperature is critical to development of Ka-band telemetry systems, communication link models, VLBI, satellite gravity wave experiments, and radio science missions. During 1993, WVRs provided data for propagation model development, supported planetary missions, and demonstrated advanced tracking capability. Collection of atmospheric statistics is necessary to model and predict performance of Ka-band telemetry links, antenna arrays, and radio science experiments. Since the spectrum of weather variations has power at very long time scales, atmospheric measurements have been requested for periods ranging from one year to a decade at each DSN site. The resulting database would provide reliable statistics on daily, monthly, and seasonal variations. Only long-term monitoring will prevent biases from being introduced by an exceptionally wet or dry year. Support for planetary missions included tropospheric calibration for the recent Mars Observer gravity wave experiments and Ka-band link experiment (KaBLE). Additionally, several proposed radio science experiments such as profiling planetary atmospheres using satellite occultations and Ka-band gravitational wave searches require advanced radiometer technology development. Finally, there has been a consistent advanced technology program to advance satellite navigational and tracking capabilities. This year that included an experiment with radiometer based tropospheric calibration for a series of VLBI catalog measurements.

(abstract) Deep Space Network Radiometric Remote Sensing Program

NASA Technical Reports Server (NTRS)

Walter, Steven J.

1994-01-01

Planetary spacecraft are viewed through a troposphere that absorbs and delays radio signals propagating through it. Tropospheric water, in the form of vapor, cloud liquid,and precipitation , emits radio noise which limits satellite telemetry communication link performance. Even at X-band, rain storms have severely affected several satellite experiments including a planetary encounter. The problem will worsen with DSN implementation of Ka-band becausecommunication link budgets will be dominated by tropospheric conditions. Troposphere-induced propagation delays currently limit VLBI accuracy and are significant sources of error for Doppler tracking. Additionally, the success of radio science programs such as satellite gravity wave experiments and atmospheric occultation experiments depends on minimizing the effect of watervapor-induced prop agation delays. In order to overcome limitations imposed by the troposphere, the Deep Space Network has supported a program of radiometric remote sensing. Currently, water vapor radiometers (WVRs) and microwave temperature profilers (MTPs) support many aspects of the Deep Space Network operations and research and development programs. Their capability to sense atmospheric water, microwave sky brightness, and atmospheric temperature is critical to development of Ka-band telemetry systems, communication link models, VLBI, satellite gravity waveexperiments, and r adio science missions. During 1993, WVRs provided data for propagation mode development, supp orted planetary missions, and demonstrated advanced tracking capability. Collection of atmospheric statistics is necessary to model and predict performance of Ka-band telemetry links, antenna arrays, and radio science experiments. Since the spectrum of weather variations has power at very long time scales, atmospheric measurements have been requested for periods ranging from one year to a decade at each DSN site. The resulting database would provide reliable statistics on daily, monthly, and seasonal variations. Only long-term monitoring will prevent biases from being introduced by an exceptionally wet or dry year. Support for planetary missions included tropospheric calibration for the recent Mars Observer gravity wave experiments and Ka-band link experiment (KaBLE). Additionally, several proposed radio science experiments such as profiling planetary atmospheres using satellite occultations and Ka-band gravitational wave searches require advanced radiometer technology development. Finally, there has been a consistent advanced technology program to advance satellite navigational and tracking capabilities. This year that included an experiment with radiometer based tropospheric calibration for a series of VLBI catalog measurements.

The AGN-Starburst connection in COLA galaxies

NASA Astrophysics Data System (ADS)

Hurley, Rossa; Phillips, Chris; Norris, Ray; Appleton, Phil; Conway, John; Parra, Rodrigo

2007-10-01

We propose to observe the COLA-S sample of 107 galaxies to test the hypothesis that VLBI-detectable AGN cores in IR-luminous sources are accompanied by intense compact star-formation activity. To maximise our sensitivity with available resources, we propose single-baseline 1 Gbit/s VLBI observations between Narrabri and Parkes, and will correlate the data in near-real time using the CPSR-II computer at Parkes.

VLBI Observations of ALSEP Transmitters

NASA Technical Reports Server (NTRS)

Counselman, C. C.

1977-01-01

The technique of differential very-long-baseline inteferometry (VLBI) Apollo Lunar Surface Experiments Package was used to measure the relative positions of the (ALSEP) transmitters at the Apollo 12, 14, 15, 16, and 17 lunar landing sites with uncertainties less than 0.005 sec of the geocentric arc. These measurements yielded improved determinations of the selenodetic coordinates of the Apollo landing sites, and of the physical libration of the moon.

Astrometry VLBI in Space (AVS

NASA Technical Reports Server (NTRS)

Altunin, V.; Alekseev, V.; Akim, E.; Eubanks, M.; Kingham, K.; Treuhaft, R.; Sukhanov, K.

1995-01-01

A proposed new space radio astronomy mission for astrometry is described. The Astrometry VLBI (very long baseline) in Space (AVS) nominal mission includes two identical spacecraft, each with a 4-m antenna sending data to a 70-m ground station. The goals of AVS are improving astrometry accuracy to the microarcsecond level and improving the accuracy of the transformation between the inertial radio and optical coordinate reference frames.

Providing hydrogen maser timing stability to orbiting VLBI radio telescope observations by post-measurement compensation of linked frequency standard imperfections

NASA Astrophysics Data System (ADS)

Springett, James C.

1994-05-01

Orbiting VLBI (OVLBI) astronomical observations are based upon measurements acquired simultaneously from ground-based and earth-orbiting radio telescopes. By the mid-1990s, two orbiting VLBI observatories, Russia's Radioastron and Japan's VSOP, will augment the worldwide VLBI network, providing baselines to earth radio telescopes as large as 80,000 km. The challenge for OVLBI is to effectuate space to ground radio telescope data cross-correlation (the observation) to a level of integrity currently achieved between ground radio telescopes. VLBI radio telescopes require ultrastable frequency and timing references in order that long term observations may be made without serious cross-correlation loss due to frequency source drift and phase noise. For this reason, such instruments make use of hydrogen maser frequency standards. Unfortunately, space-qualified hydrogen maser oscillators are currently not available for use on OVLBI satellites. Thus, the necessary long-term stability needed by the orbiting radio telescope may only be obtained by microwave uplinking a ground-based hydrogen maser derived frequency to the satellite. Although the idea of uplinking the frequency standard intrinsically seems simple, there are many 'contaminations' which degrade both the long and short term stability of the transmitted reference. Factors which corrupt frequency and timing accuracy include additive radio and electronic circuit thermal noise, slow or systematic phase migration due to changes of electronic circuit temporal operating conditions (especially temperature), ionosphere and troposphere induced scintillations, residual Doppler-incited components, and microwave signal multipath propagation. What is important, though, is to realize that ultimate stability does not have to be achieved in real-time. Instead, information needed to produce a high degree of coherence in the subsequent cross-correlation operation may be derived from a two-way coherent radio link, recorded and later introduced as compensations adjunct to the VLBI correlation process. Accordingly, this paper examines the technique for stable frequency/time transfer within the OVLBI system, together with a critique of the types of link degradation components which must be compensated, and the figures of merit known as coherence factors.

Providing hydrogen maser timing stability to orbiting VLBI radio telescope observations by post-measurement compensation of linked frequency standard imperfections

NASA Technical Reports Server (NTRS)

Springett, James C.

1994-01-01

Orbiting VLBI (OVLBI) astronomical observations are based upon measurements acquired simultaneously from ground-based and earth-orbiting radio telescopes. By the mid-1990s, two orbiting VLBI observatories, Russia's Radioastron and Japan's VSOP, will augment the worldwide VLBI network, providing baselines to earth radio telescopes as large as 80,000 km. The challenge for OVLBI is to effectuate space to ground radio telescope data cross-correlation (the observation) to a level of integrity currently achieved between ground radio telescopes. VLBI radio telescopes require ultrastable frequency and timing references in order that long term observations may be made without serious cross-correlation loss due to frequency source drift and phase noise. For this reason, such instruments make use of hydrogen maser frequency standards. Unfortunately, space-qualified hydrogen maser oscillators are currently not available for use on OVLBI satellites. Thus, the necessary long-term stability needed by the orbiting radio telescope may only be obtained by microwave uplinking a ground-based hydrogen maser derived frequency to the satellite. Although the idea of uplinking the frequency standard intrinsically seems simple, there are many 'contaminations' which degrade both the long and short term stability of the transmitted reference. Factors which corrupt frequency and timing accuracy include additive radio and electronic circuit thermal noise, slow or systematic phase migration due to changes of electronic circuit temporal operating conditions (especially temperature), ionosphere and troposphere induced scintillations, residual Doppler-incited components, and microwave signal multipath propagation. What is important, though, is to realize that ultimate stability does not have to be achieved in real-time. Instead, information needed to produce a high degree of coherence in the subsequent cross-correlation operation may be derived from a two-way coherent radio link, recorded and later introduced as compensations adjunct to the VLBI correlation process. Accordingly, this paper examines the technique for stable frequency/time transfer within the OVLBI system, together with a critique of the types of link degradation components which must be compensated, and the figures of merit known as coherence factors.

The Effect of Improved Sub-Daily Earth Rotation Models on Global GPS Data Processing

NASA Astrophysics Data System (ADS)

Yoon, S.; Choi, K. K.

2017-12-01

Throughout the various International GNSS Service (IGS) products, strong periodic signals have been observed around the 14 day period. This signal is clearly visible in all IGS time-series such as those related to orbit ephemerides, Earth rotation parameters (ERP) and ground station coordinates. Recent studies show that errors in the sub-daily Earth rotation models are the main factors that induce such noise. Current IGS orbit processing standards adopted the IERS 2010 convention and its sub-daily Earth rotation model. Since the IERS convention had published, recent advances in the VLBI analysis have made contributions to update the sub-daily Earth rotation models. We have compared several proposed sub-daily Earth rotation models and show the effect of using those models on orbit ephemeris, Earth rotation parameters and ground station coordinates generated by the NGS global GPS data processing strategy.

Seismology and space-based geodesy

NASA Technical Reports Server (NTRS)

Tralli, David M.; Tajima, Fumiko

1993-01-01

The potential of space-based geodetic measurement of crustal deformation in the context of seismology is explored. The achievements of seismological source theory and data analyses, mechanical modeling of fault zone behavior, and advances in space-based geodesy are reviewed, with emphasis on realizable contributions of space-based geodetic measurements specifically to seismology. The fundamental relationships between crustal deformation associated with an earthquake and the geodetically observable data are summarized. The response and spatial and temporal resolution of the geodetic data necessary to understand deformation at various phases of the earthquake cycle is stressed. The use of VLBI, SLR, and GPS measurements for studying global geodynamics properties that can be investigated to some extent with seismic data is discussed. The potential contributions of continuously operating strain monitoring networks and globally distributed geodetic observatories to existing worldwide modern digital seismographic networks are evaluated in reference to mutually addressable problems in seismology, geophysics, and tectonics.

Troposphere mapping functions for GPS and very long baseline interferometry from European Centre for Medium-Range Weather Forecasts operational analysis data

NASA Astrophysics Data System (ADS)

Boehm, Johannes; Werl, Birgit; Schuh, Harald

2006-02-01

In the analyses of geodetic very long baseline interferometry (VLBI) and GPS data the analytic form used for mapping of the atmosphere delay from zenith to the line of site is most often a three-parameter continued fraction in 1/sin(elevation). Using the 40 years reanalysis (ERA-40) data of the European Centre for Medium-Range Weather Forecasts for the year 2001, the b and c coefficients of the continued fraction form for the hydrostatic mapping functions have been redetermined. Unlike previous mapping functions based on data from numerical weather models (isobaric mapping functions (Niell, 2000) and Vienna mapping functions (VMF) (Boehm and Schuh, 2004)), the new c coefficients are dependent on the day of the year, and unlike the Niell mapping functions (Niell, 1996) they are no longer symmetric with respect to the equator (apart from the opposite phase for the two hemispheres). Compared to VMF, this causes an effect on the VLBI or GPS station heights that is constant and as large as 2 mm at the equator and that varies seasonally between 4 mm and 0 mm at the poles. The updated VMF, based on these new coefficients and called VMF1 hereinafter, yields slightly better baseline length repeatabilities for VLBI data. The hydrostatic and wet mapping functions are applied in various combinations with different kinds of a priori zenith delays in the analyses of all VLBI International VLBI Service for Geodesy and Astrometry (IVS)-R1 and IVS-R4 24-hour sessions of 2002 and 2003; the investigations concentrate on baseline length repeatabilities, as well as on absolute changes of station heights.

Geodesy by radio interferometry: Water vapor radiometry for estimation of the wet delay

DOE Office of Scientific and Technical Information (OSTI.GOV)

Elgered, G.; Davis, J.L.; Herring, T.A.

1991-04-10

An important source of error in very-long-baseline interferometry (VLBI) estimates of baseline length is unmodeled variations of the refractivity of the neutral atmosphere along the propagation path of the radio signals. The authors present and discuss the method of using data from a water vapor readiometer (WVR) to correct for the propagation delay caused by atmospheric water vapor, the major cause of these variations. Data from different WVRs are compared with estimated propagation delays obtained by Kalman filtering of the VLBI data themselves. The consequences of using either WVR data of Kalman filtering to correct for atmospheric propagation delay atmore » the Onsala VLBI site are investigated by studying the repeatability of estimated baseline lengths from Onsala to several other sites. The lengths of the baselines range from 919 to 7,941 km. The repeatability obtained for baseline length estimates shows that the methods of water vapor radiometry and Kalman filtering offer comparable accuracies when applied to VLBI observations obtained in the climate of the Swedish west coast. The use of WVR data yielded a 13% smaller weighted-root-mean-square (WRMS) scatter of the baseline length estimates compared to the use of a Kalman filter. It is also clear that the best minimum elevation angle for VLBI observations depends on the accuracy of the determinations of the total propagation delay to be used, since the error in this delay increases with increasing air mass. For use of WVR data along with accurate determinations of total surface pressure, the best minimum is about 20{degrees}; for use of a model for the wet delay based on the humidity and temperature at the ground, the best minimum is about 35{degrees}.« less

Early science with the Korean VLBI network: evaluation of system performance

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lee, Sang-Sung; Byun, Do-Young; Kim, Jongsoo

2014-04-01

We report the very long baseline interferometry (VLBI) observing performance of the Korean VLBI Network (KVN). The KVN is the first millimeter-dedicated VLBI network in East Asia. The KVN consists of three 21 m radio telescopes with baseline lengths in a range of 305-476 km. The quasi-optical system equipped on the antennas allows simultaneous observations at 22, 43, 86, and 129 GHz. The first fringes of the KVN were obtained at 22 GHz on 2010 June 8. Test observations at 22 and 43 GHz on 2010 September 30 and 2011 April 4 confirmed that the full cycle of VLBI observationsmore » works according to specification: scheduling, antenna control system, data recording, correlation, post-correlation data processing, astrometry, geodesy, and imaging analysis. We found that decorrelation due to instability in the hardware at times up to 600 s is negligible. The atmosphere fluctuations at KVN baseline are partly coherent, which allows us to extend integration time under good winter weather conditions up to 600 s without significant loss of coherence. The post-fit residuals at KVN baselines do not exhibit systematic patterns, and the weighted rms of the residuals is 14.8 ps. The KVN is ready to image compact radio sources both in snapshot and full-track modes with residual noise in calibrated phases of less than 2 deg at 22 and 43 GHz and with dynamic ranges of ∼300 for snapshot mode and ∼1000 for full-track mode. With simultaneous multi-frequency observations, the KVN can be used to make parsec-scale spectral index maps of compact radio sources.« less

Analysis of the DORIS, GNSS, SLR, VLBI and gravimetric time series at the GGOS core sites

NASA Astrophysics Data System (ADS)

Moreaux, G.; Lemoine, F. G.; Luceri, V.; Pavlis, E. C.; MacMillan, D. S.; Bonvalot, S.; Saunier, J.

2017-12-01

Since June 2016 and the installation of a new DORIS station in Wettzell (Germany), four geodetic sites (Badary, Greenbelt, Wettzell and Yarragadee) are equipped with the four space geodetic techniques (DORIS, GNSS, SLR and VLBI). In line with the GGOS (Global Geodetic Observing System) objective of achieving a terrestrial reference frame at the millimetric level of accuracy, the combination centers of the four space techniques initiated a joint study to assess the level of agreement among these space geodetic techniques. In addition to the four sites, we will consider all the GGOS core sites including the seven sites with at least two space geodetic techniques in addition to DORIS. Starting from the coordinate time series, we will estimate and compare the mean positions and velocities of the co-located instruments. The temporal evolution of the coordinate differences will also be evaluated with respect to the local tie vectors and discrepancies will be investigated. Then, the analysis of the signal content of the time series will be carried out. Amplitudes and phases of the common signals among the techniques, and eventually from gravity data, will be compared. The first objective of this talk is to describe our joint study: the sites, the data, and the objectives. The second purpose is to present the first results obtained from the GGAO (Goddard Geophysical and Astronomic Observatory) site of Greenbelt.

Effect of different tropospheric mapping functions on the TRF, CRF and position time-series estimated from VLBI

NASA Astrophysics Data System (ADS)

Tesmer, Volker; Boehm, Johannes; Heinkelmann, Robert; Schuh, Harald

2007-06-01

This paper compares estimated terrestrial reference frames (TRF) and celestial reference frames (CRF) as well as position time-series in terms of systematic differences, scale, annual signals and station position repeatabilities using four different tropospheric mapping functions (MF): The NMF (Niell Mapping Function) and the recently developed GMF (Global Mapping Function) consist of easy-to-handle stand-alone formulae, whereas the IMF (Isobaric Mapping Function) and the VMF1 (Vienna Mapping Function 1) are determined from numerical weather models. All computations were performed at the Deutsches Geodätisches Forschungsinstitut (DGFI) using the OCCAM 6.1 and DOGS-CS software packages for Very Long Baseline Interferometry (VLBI) data from 1984 until 2005. While it turned out that CRF estimates only slightly depend on the MF used, showing small systematic effects up to 0.025 mas, some station heights of the computed TRF change by up to 13 mm. The best agreement was achieved for the VMF1 and GMF results concerning the TRFs, and for the VMF1 and IMF results concerning scale variations and position time-series. The amplitudes of the annual periodical signals in the time-series of estimated heights differ by up to 5 mm. The best precision in terms of station height repeatability is found for the VMF1, which is 5 7% better than for the other MFs.

Differential tracking data types for accurate and efficient Mars planetary navigation

NASA Technical Reports Server (NTRS)

Edwards, C. D., Jr.; Kahn, R. D.; Folkner, W. M.; Border, J. S.

1991-01-01

Ways in which high-accuracy differential observations of two or more deep space vehicles can dramatically extend the power of earth-based tracking over conventional range and Doppler tracking are discussed. Two techniques - spacecraft-spacecraft differential very long baseline interferometry (S/C-S/C Delta(VLBI)) and same-beam interferometry (SBI) - are discussed. The tracking and navigation capabilities of conventional range, Doppler, and quasar-relative Delta(VLBI) are reviewed, and the S/C-S/C Delta (VLBI) and SBI types are introduced. For each data type, the formation of the observable is discussed, an error budget describing how physical error sources manifest themselves in the observable is presented, and potential applications of the technique for Space Exploration Initiative scenarios are examined. Requirements for spacecraft and ground systems needed to enable and optimize these types of observations are discussed.

Round-Trip System Available to Measure Path Length Variation in Korea VLBI System for Geodesy

NASA Technical Reports Server (NTRS)

Oh, Hongjong; Kondo, Tetsuro; Lee, Jinoo; Kim, Tuhwan; Kim, Myungho; Kim, Suchul; Park, Jinsik; Ju, Hyunhee

2010-01-01

The construction project of Korea Geodetic VLBI officially started in October 2008. The construction of all systems will be completed by the end of 2011. The project was named Korea VLBI system for Geodesy (KVG), and its main purpose is to maintain the Korea Geodetic Datum. In case of the KVG system, an observation room with an H-maser frequency standard is located in a building separated from the antenna by several tens of meters. Therefore KVG system will adopt a so-called round-trip system to transmit reference signals to the antenna with reduction of the effect of path length variations. KVG s round-trip system is designed not only to use either metal or optical fiber cables, but also to measure path length variations directly. We present this unique round trip system for KVG.

Implementation and Testing of VLBI Software Correlation at the USNO

NASA Technical Reports Server (NTRS)

Fey, Alan; Ojha, Roopesh; Boboltz, Dave; Geiger, Nicole; Kingham, Kerry; Hall, David; Gaume, Ralph; Johnston, Ken

2010-01-01

The Washington Correlator (WACO) at the U.S. Naval Observatory (USNO) is a dedicated VLBI processor based on dedicated hardware of ASIC design. The WACO is currently over 10 years old and is nearing the end of its expected lifetime. Plans for implementation and testing of software correlation at the USNO are currently being considered. The VLBI correlation process is, by its very nature, well suited to a parallelized computing environment. Commercial off-the-shelf computer hardware has advanced in processing power to the point where software correlation is now both economically and technologically feasible. The advantages of software correlation are manifold but include flexibility, scalability, and easy adaptability to changing environments and requirements. We discuss our experience with and plans for use of software correlation at USNO with emphasis on the use of the DiFX software correlator.

Crustal movements in Europe observed with EUROPE and IVS-T2 VLBI networks

NASA Astrophysics Data System (ADS)

Zubko, N.; Poutanen, M.

2011-07-01

The comparative analysis of the EUROPE and IVS-T2 geodetic VLBI sessions has been performed. The main purpose of both campaigns is to observe and accurately determine the VLBI station coordinates and their time evolution. In this analysis our interest is to understand the influence of network configuration on the estimated parameters and, also, how much the results of these two campaigns are consistent. We have used the VieVS software developing at Vienna University of Technology to analyze the EUROPE and IVS-T2 sessions of 2002-2009. We have analyzed the difference of crustal movements obtained with these two networks and the effect of network configuration and station selection. The EPN (EUREF permanent GNSS Network) and IGS (International GNSS Service) networks can be used to compare the results.

Very Long Baseline Interferometry: Dependencies on Frequency Stability

NASA Astrophysics Data System (ADS)

Nothnagel, Axel; Nilsson, Tobias; Schuh, Harald

2018-04-01

Very Long Baseline Interferometry (VLBI) is a differential technique observing radiation of compact extra-galactic radio sources with pairs of radio telescopes. For these observations, the frequency standards at the telescopes need to have very high stability. In this article we discuss why this is, and we investigate exactly how precise the frequency standards need to be. Four areas where good clock performance is needed are considered: coherence, geodetic parameter estimation, correlator synchronization, and UT1 determination. We show that in order to ensure the highest accuracy of VLBI, stability similar to that of a hydrogen maser is needed for time-scales up to a few hours. In the article, we are considering both traditional VLBI where extra-galactic radio sources are observed, as well as observation of man-made artificial radio sources emitted by satellites or spacecrafts.

Spacecraft Doppler tracking with a VLBI antenna

NASA Technical Reports Server (NTRS)

Comoretto, G.; Iess, L.; Bertotti, B.; Brenkle, J. P.; Horton, T.

1990-01-01

Preliminary results are reported from Doppler-shift measurements to the Voyager-2 spacecraft at a distance of 26 AU, obtained using the 32-m VLBI antenna at Medicina (Italy) during July and August 1988. The apparatus comprises the el-az antenna, an S-X-band receiver, a hydrogen maser to generate the reference signal, a Mark III VLBI terminal, and a digital tone extractor capable of isolating a tone of known frequency from a noisy signal and giving its phase and amplitude. A signal transmitted in S-band from the NASA Deep Space Network (DSN) station in Australia and retransmitted coherently in X-band by Voyager, was received 7 h 6 min later at Medicina and at the DSN station in Madrid. Sample data are presented graphically and shown to be of generally high quality; further in-depth analysis is under way.

Astrometric "Core-shifts" at the Highest Frequencies

NASA Technical Reports Server (NTRS)

Rioja, Maria; Dodson, Richard

2010-01-01

We discuss the application of a new VLBI astrometric method named "Source/Frequency Phase Referencing" to measurements of "core-shifts" in radio sources used for geodetic observations. We detail the reasons that astrometrical observations of 'core-shifts' have become critical in the era of VLBI2010. We detail how this new method allows the problem to be addressed at the highest frequencies and outline its superior compensation of tropospheric errors.

Fortaleza Station Report for 2012

NASA Technical Reports Server (NTRS)

Kaufmann, Pierre; Pereira de Lucena, A. Macilio; Sombra da Silva, Adeildo

2013-01-01

This is a brief report about the activities carried out at the Fortaleza geodetic VLBI station (ROEN: R´adio Observat´orio Espacial do Nordeste), located in Eus´ebio, CE, Brazil, during the period from January until December 2012. The observing activities were resumed in May after the major maintenance that comprised the azimuth bearing replacement. The total observational experiments consisted of 103 VLBI sessions and continuous GPS monitoring recordings.

Coordinating, Scheduling, Processing and Analyzing IYA09

NASA Technical Reports Server (NTRS)

Gipson, John; Behrend, Dirk; Gordon, David; Himwich, Ed; MacMillan, Dan; Titus, Mike; Corey, Brian

2010-01-01

The IVS scheduled a special astrometric VLBI session for the International Year of Astronomy 2009 (IYA09) commemorating 400 years of optical astronomy and 40 years of VLBI. The IYA09 session is the most ambitious geodetic session to date in terms of network size, number of sources, and number of observations. We describe the process of designing, coordinating, scheduling, pre-session station checkout, correlating, and analyzing this session.

VLBI2020: From Reality to Vision

NASA Technical Reports Server (NTRS)

Titov, Oleg

2010-01-01

The individual apparent motions of distant radio sources are believed to be caused by the effect of intrinsic structure variations of the active galactic nuclei (AGN). However, some cosmological models of the expanded Universe predict that systematic astrometric proper motions of distant quasars do not vanish as the radial distance from the observer to the quasar grows. These systematic effects can even increase with the distance, making it possible to measure them with high-precision astrometric techniques like VLBI. The Galactocentric acceleration of the Solar System barycenter may cause a secular aberration drift with a magnitude of 4 uas/yr. The Solar System motion relative to the cosmic microwave background produces an additional dipole effect, proportional to red shift. We analyzed geodetic VLBI data spanning from 1979 until 2009 to estimate the vector spherical harmonics in the expansion of the vector field of the proper motion of 687 radio sources. The dipole and quadrupole vector spherical harmonics were estimated with an accuracy of 1-5 as/yr. We have shown that over the next decade the geodetic VLBI may approach the level of accuracy on which the cosmological models of the Universe could be tested. Hence, it is important to organize a dedicated observational program to increase the number of measured proper motions to 3000.

Observation model and parameter partials for the JPL VLBI parameter estimation software MODEST, 19 94

NASA Technical Reports Server (NTRS)

Sovers, O. J.; Jacobs, C. S.

1994-01-01

This report is a revision of the document Observation Model and Parameter Partials for the JPL VLBI Parameter Estimation Software 'MODEST'---1991, dated August 1, 1991. It supersedes that document and its four previous versions (1983, 1985, 1986, and 1987). A number of aspects of the very long baseline interferometry (VLBI) model were improved from 1991 to 1994. Treatment of tidal effects is extended to model the effects of ocean tides on universal time and polar motion (UTPM), including a default model for nearly diurnal and semidiurnal ocean tidal UTPM variations, and partial derivatives for all (solid and ocean) tidal UTPM amplitudes. The time-honored 'K(sub 1) correction' for solid earth tides has been extended to include analogous frequency-dependent response of five tidal components. Partials of ocean loading amplitudes are now supplied. The Zhu-Mathews-Oceans-Anisotropy (ZMOA) 1990-2 and Kinoshita-Souchay models of nutation are now two of the modeling choices to replace the increasingly inadequate 1980 International Astronomical Union (IAU) nutation series. A rudimentary model of antenna thermal expansion is provided. Two more troposphere mapping functions have been added to the repertoire. Finally, corrections among VLBI observations via the model of Treuhaft and lanyi improve modeling of the dynamic troposphere. A number of minor misprints in Rev. 4 have been corrected.

The Tropospheric Products of the International VLBI Service for Geodesy and Astrometry

NASA Technical Reports Server (NTRS)

Heinkelmann, Robert; Schwatke, Christian

2010-01-01

The IVS runs two tropospheric products: The IVS tropospheric parameter rapid combination monitors the zenith wet delay (ZWD) and zenith total delay (ZTD) of the rapid turnaround sessions R1 and R4. Goal of the combination is the identification and the exclusion of outliers by comparison and the assessment of the precision of current VLBI solutions in terms of tropospheric parameters. The rapid combination is done on a weekly basis four weeks after the observation files are released on IVS Data Centers. Since tropospheric and geodetic parameters, such as vertical station components, can significantly correlate, the consistency of the ZTD can be a measure of the consistency of the corresponding TRF as well. The ZWD mainly rely on accurate atmospheric pressure data. Thus, besides estimation techniques, modeling and analyst s noise, ZWD reflects differences in the atmospheric pressure data applied to the VLBI analysis. The second product, called tropospheric parameter long-term combination, aims for an accurate determination of climatological signals, such as trends of the atmospheric water vapor observed by VLBI. Therefore, the long-term homogeneity of atmospheric pressure data plays a crucial role for this product. The paper reviews the methods applied and results achieved so far and describes the new maintenance through DGFI.

Observing with Sibling and Twin Telescopes

NASA Astrophysics Data System (ADS)

Plank, Lucia; Lovell, Jim; McCallum, Jamie; Mayer, David

2016-12-01

With the transition to VGOS, co-located radio telescopes will be common at many sites. This can be as a sibling telescope when a VGOS antenna is built next to a legacy one, or as the concept of a twin telescope with two identical VGOS antennas. The co-location of two antennas offers new possibilities in both operation and analysis. The immediate question for observing with sibling/twin telescopes is the applied observing strategy and its realization in the scheduling software. In this contribution we report about our efforts implementing new scheduling modes for sibling and twin telescopes in the Vienna VLBI Software. For the example of the sibling telescope in Hobart, several types of sessions will be discussed: an improved tag-along mode for the 26-m antenna (Ho), a proper implementation of the twin-mode using the antenna with the shorter slewing time, and an astrometric support mode enabling the observation of weak sources with the AuScope array.

Next Generation NASA Initiative for Space Geodesy

NASA Technical Reports Server (NTRS)

Merkowitz, S. M.; Desai, S.; Gross, R. S.; Hilliard, L.; Lemoine, F. G.; Long, J. L.; Ma, C.; McGarry J. F.; Murphy, D.; Noll, C. E.;

Investigation of scale effects in the TRF determined by VLBI

NASA Astrophysics Data System (ADS)

Wahl, Daniel; Heinkelmann, Robert; Schuh, Harald

2017-04-01

The improvement of the International Terrestrial Reference Frame (ITRF) is of great significance for Earth sciences and one of the major tasks in geodesy. The translation, rotation and the scale-factor, as well as their linear rates, are solved in a 14-parameter transformation between individual frames of each space geodetic technique and the combined frame. In ITRF2008, as well as in the current release ITRF2014, the scale-factor is provided by Very Long Baseline Interferometry (VLBI) and Satellite Laser Ranging (SLR) in equal shares. Since VLBI measures extremely precise group delays that are transformed to baseline lengths by the velocity of light, a natural constant, VLBI is the most suitable method for providing the scale. The aim of the current work is to identify possible shortcomings in the VLBI scale contribution to ITRF2008. For developing recommendations for an enhanced estimation, scale effects in the Terrestrial Reference Frame (TRF) determined with VLBI are considered in detail and compared to ITRF2008. In contrast to station coordinates, where the scale is defined by a geocentric position vector, pointing from the origin of the reference frame to the station, baselines are not related to the origin. They are describing the absolute scale independently from the datum. The more accurate a baseline length, and consequently the scale, is estimated by VLBI, the better the scale contribution to the ITRF. Considering time series of baseline length between different stations, a non-linear periodic signal can clearly be recognized, caused by seasonal effects at observation sites. Modeling these seasonal effects and subtracting them from the original data enhances the repeatability of single baselines significantly. Other effects influencing the scale strongly, are jumps in the time series of baseline length, mainly evoked by major earthquakes. Co- and post-seismic effects can be identified in the data, having a non-linear character likewise. Modeling the non-linear motion or completely excluding affected stations is another important step for an improved scale determination. In addition to the investigation of single baseline repeatabilities also the spatial transformation, which is performed for determining parameters of the ITRF2008, are considered. Since the reliability of the resulting transformation parameters is higher the more identical points are used in the transformation, an approach where all possible stations are used as control points is comprehensible. Experiments that examine the scale-factor and its spatial behavior between control points in ITRF2008 and coordinates determined by VLBI only showed that the network geometry has a large influence on the outcome as well. Introducing an unequally distributed network for the datum configuration, the correlations between translation parameters and the scale-factor can become remarkably high. Only a homogeneous spatial distribution of participating stations yields a maximally uncorrelated scale-factor that can be interpreted independent from other parameters. In the current release of the ITRF, the ITRF2014, for the first time, non-linear effects in the time series of station coordinates are taken into account. The present work shows the importance and the right direction of the modification of the ITRF calculation. But also further improvements were found which lead to an enhanced scale determination.

Probing the Innermost Regions of AGN Jets and Their Magnetic Fields with RadioAstron. I. Imaging BL Lacertae at 21 Microarcsecond Resolution

NASA Astrophysics Data System (ADS)

Gómez, José L.; Lobanov, Andrei P.; Bruni, Gabriele; Kovalev, Yuri Y.; Marscher, Alan P.; Jorstad, Svetlana G.; Mizuno, Yosuke; Bach, Uwe; Sokolovsky, Kirill V.; Anderson, James M.; Galindo, Pablo; Kardashev, Nikolay S.; Lisakov, Mikhail M.

2016-02-01

We present the first polarimetric space very long baseline interferometry (VLBI) imaging observations at 22 GHz. BL Lacertae was observed in 2013 November 10 with the RadioAstron space VLBI mission, including a ground array of 15 radio telescopes. The instrumental polarization of the space radio telescope is found to be less than 9%, demonstrating the polarimetric imaging capabilities of RadioAstron at 22 GHz. Ground-space fringes were obtained up to a projected baseline distance of 7.9 Earth diameters in length, allowing us to image the jet in BL Lacertae with a maximum angular resolution of 21 μas, the highest achieved to date. We find evidence for emission upstream of the radio core, which may correspond to a recollimation shock at about 40 μas from the jet apex, in a pattern that includes other recollimation shocks at approximately 100 and 250 μas from the jet apex. Polarized emission is detected in two components within the innermost 0.5 mas from the core, as well as in some knots 3 mas downstream. Faraday rotation analysis, obtained from combining RadioAstron 22 GHz and ground-based 15 and 43 GHz images, shows a gradient in rotation measure and Faraday-corrected polarization vector as a function of position angle with respect to the core, suggesting that the jet in BL Lacertae is threaded by a helical magnetic field. The intrinsic de-boosted brightness temperature in the unresolved core exceeds 3× {10}12 K, suggesting, at the very least, departure from equipartition of energy between the magnetic field and radiating particles.

Update on VLBA Astrometry of Cassini

NASA Astrophysics Data System (ADS)

Jones, Dayton L.; Folkner, William M.; Jacobson, Robert A.; Jacobs, Christopher S.; Romney, Jon; Dhawan, Vivek; Fomalont, Edward B.

2015-01-01

The NRAO Very Long Baseline Array (VLBA) has been used to measure positions of the Cassini spacecraft 2-3 times per year during the decade since it arrived at Saturn. Combining these measurements with fits for Cassini's orbit about Saturn from Doppler tracking by the NASA Deep Space Network provides accurate positions for the Saturn system barycenter in the inertial International Celestial Reference Frame (ICRF) at each observing epoch. These positions in turn help to improve our knowledge of Saturn's orbit and thus the planetary ephemeris on which future interplanetary spacecraft navigation, pulsar timing, and studies of solar system dynamics depend. This observational program will continue to the end of Cassini's mission in 2017, thereby covering as large a fraction of Saturn's orbital period as possible. A multi-year period of accurate astrometry also increases the range of times over which ephemeris improvements can be extrapolated. Our current residuals with respect to JPL's DE430 ephemeris are approximately 0.2 mas in right ascension and 0.3 mas in declination. The primary error sources are residual troposphere delay calibration errors and uncertainties in the ICRF positions of some of our phase reference sources. The reference source position uncertainties are being reduced by continuing VLBI observations. Similar VLBI techniques will be applied to the Juno spacecraft when it begins orbiting Jupiter in 2016, thereby improving the orbit for this planet as well. This work has been carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. Support from the NASA Planetary Astronomy Program is gratefully acknowledged. The VLBA is a facility of the National Radio Astronomy Observatory, which is operated by Associated Universities, Inc, under a cooperative agreement with the National Science Foundation.

The AB Doradus system revisited: The dynamical mass of AB Dor A/C

NASA Astrophysics Data System (ADS)

Azulay, R.; Guirado, J. C.; Marcaide, J. M.; Martí-Vidal, I.; Ros, E.; Tognelli, E.; Jauncey, D. L.; Lestrade, J.-F.; Reynolds, J. E.

2017-10-01

Context. The study of pre-main-sequence (PMS) stars with model-independent measurements of their masses is essential to check the validity of theoretical models of stellar evolution. The well-known PMS binary AB Dor A/C is an important benchmark for this task, since it displays intense and compact radio emission, which makes possible the application of high-precision astrometric techniques to this system. Aims: We aim to revisit the dynamical masses of the components of AB Dor A/C to refine earlier comparisons between the measurements of stellar parameters and the predictions of stellar models. Methods: We observed in phase-reference mode the binary AB Dor A/C, 0.2'' separation, with the Australian Long Baseline Array at 8.4 GHz. The astrometric information resulting from our observations was analyzed along with previously reported VLBI, optical (Hipparcos), and infrared measurements. Results: The main star AB Dor A is clearly detected in all the VLBI observations, which allowed us to analyze the orbital motion of the system and to obtain model-independent dynamical masses of 0.90 ± 0.08 M⊙ and 0.090 ± 0.008 M⊙, for AB Dor A and AB Dor C, respectively. Comparisons with PMS stellar evolution models favor and age of 40-50 Myr for AB Dor A and of 25-120 Myr for AB Dor C. Conclusions: We show that the orbital motion of the AB Dor A/C system is remarkably well determined, leading to precise estimates of the dynamical masses. Comparison of our results with the prediction of evolutionary models support the observational evidence that theoretical models tend to slightly underestimate the mass of the low-mass stars.

GPS Position Time Series @ JPL

NASA Technical Reports Server (NTRS)

Owen, Susan; Moore, Angelyn; Kedar, Sharon; Liu, Zhen; Webb, Frank; Heflin, Mike; Desai, Shailen

2013-01-01

Different flavors of GPS time series analysis at JPL - Use same GPS Precise Point Positioning Analysis raw time series - Variations in time series analysis/post-processing driven by different users. center dot JPL Global Time Series/Velocities - researchers studying reference frame, combining with VLBI/SLR/DORIS center dot JPL/SOPAC Combined Time Series/Velocities - crustal deformation for tectonic, volcanic, ground water studies center dot ARIA Time Series/Coseismic Data Products - Hazard monitoring and response focused center dot ARIA data system designed to integrate GPS and InSAR - GPS tropospheric delay used for correcting InSAR - Caltech's GIANT time series analysis uses GPS to correct orbital errors in InSAR - Zhen Liu's talking tomorrow on InSAR Time Series analysis

Phase Calibration for the Block 1 VLBI System

NASA Technical Reports Server (NTRS)

Roth, M. G.; Runge, T. F.

1983-01-01

Very Long Baseline Interferometry (VLBI) in the DSN provides support for spacecraft navigation, Earth orientation measurements, and synchronization of network time and frequency standards. An improved method for calibrating instrumental phase shifts has recently been implemented as a computer program in the Block 1 system. The new calibration program, called PRECAL, performs calibrations over intervals as small as 0.4 seconds and greatly reduces the amount of computer processing required to perform phase calibration.

Optimizing the African VLBI Network for Astronomy and Geodesy

NASA Astrophysics Data System (ADS)

de Witt, A.; Mayer, D.; MacLeod, G.; Combrinck, L.; Petrov, L.; Nickola, M.

2016-12-01

The African VLBI Network will be a pan-African network of radio telescopes comprised of converted redundant satellite Earth-station antennas and new purpose-built radio telescopes. The first of these antennas, in Ghana, is currently being converted to a radio telescope and current funding is estimated to permit the conversion of two more antennas in Africa. These antennas will initially be equipped with a 5-GHz and 6.7-GHz receiver and the next receiver likely to be fitted is a 1.4-1.7-GHz receiver. While it would be advantageous for the AVN antennas to be able to participate also in geodetic and astrometric VLBI observations, there is no funding currently for this. In this paper we re-visit the scientific justifications for the AVN in an attempt to optimize the AVN for each science case, both astronomical and geodetic.

Estimation of accuracy of earth-rotation parameters in different frequency bands

NASA Astrophysics Data System (ADS)

Vondrak, J.

1986-11-01

The accuracies of earth-rotation parameters as determined by five different observational techniques now available (i.e., optical astrometry /OA/, Doppler tracking of satellites /DTS/, satellite laser ranging /SLR/, very long-base interferometry /VLBI/ and lunar laser ranging /LLR/) are estimated. The differences between the individual techniques in all possible combinations, separated by appropriate filters into three frequency bands, were used to estimate the accuracies of the techniques for periods from 0 to 200 days, from 200 to 1000 days and longer than 1000 days. It is shown that for polar motion the most accurate results are obtained with VLBI anad SLR, especially in the short-period region; OA and DTS are less accurate, but with longer periods the differences in accuracy are less pronounced. The accuracies of UTI-UTC as determined by OA, VLBI and LLR are practically equivalent, the differences being less than 40 percent.

Earth orientation determinations by short duration VLBI observations

NASA Astrophysics Data System (ADS)

Nothnagel, Axel; Zhihan, Qian; Nicolson, George D.; Tomasi, Paolo

1994-03-01

In May 1989 and April 1990 the radio telescopes of the Wettzell Geodetic Fundamental Station in Germany and of the Shanghai Observatory near Seshan in China observed two series of daily VLBI experiments of short duration for precise determination of UT1. In 1990 a few experiments were complemented by the Hartebeesthoek Radio Astronomy Observatory in South Africa and the Medicina telescope of the Bologna Istituto di Radioastronomia in Italy. Employing the South African station together with the east-west baseline formed by the observatories of Seshan and Medicina permitted simultaneous determinations of UT1 and polar motion. Here we report on the results of these observations. Comparing the UT1 results with those of the IRIS Intensive series gives a clear indication of the absolute accuracy of such short duration VLBI measurements which is estimated to be of the order of ±60 µs.

A VLBI experiment using a remote atomic clock via a coherent fibre link

PubMed Central

Clivati, Cecilia; Ambrosini, Roberto; Artz, Thomas; Bertarini, Alessandra; Bortolotti, Claudio; Frittelli, Matteo; Levi, Filippo; Mura, Alberto; Maccaferri, Giuseppe; Nanni, Mauro; Negusini, Monia; Perini, Federico; Roma, Mauro; Stagni, Matteo; Zucco, Massimo; Calonico, Davide

2017-01-01

We describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks. PMID:28145451

Probing the solar corona with very long baseline interferometry.

PubMed

Soja, B; Heinkelmann, R; Schuh, H

2014-06-20

Understanding and monitoring the solar corona and solar wind is important for many applications like telecommunications or geomagnetic studies. Coronal electron density models have been derived by various techniques over the last 45 years, principally by analysing the effect of the corona on spacecraft tracking. Here we show that recent observational data from very long baseline interferometry (VLBI), a radio technique crucial for astrophysics and geodesy, could be used to develop electron density models of the Sun's corona. The VLBI results agree well with previous models from spacecraft measurements. They also show that the simple spherical electron density model is violated by regional density variations and that on average the electron density in active regions is about three times that of low-density regions. Unlike spacecraft tracking, a VLBI campaign would be possible on a regular basis and would provide highly resolved spatial-temporal samplings over a complete solar cycle.

CONT14: Preparation and Prospects

NASA Astrophysics Data System (ADS)

Behrend, Dirk; Thomas, Cynthia; Himwich, Ed; MacMillan, Dan

2014-12-01

In May 2014, a network of seventeen stations at sixteen sites will observe the next continuous VLBI campaign named CONT14. From May 6 through May 20, ten northern hemisphere and seven southern hemisphere stations will record S/X VLBI data for fifteen continuous UT days at a rate of 512 Mbps. This campaign is the continuation of similar campaigns loosely organized every three years. We discuss the preparation work such as allocation of station, media, and correlator resources and the observational procedure that will ensure that the best possible S/X VLBI data set be recorded. With a network size of seventeen stations, CONT14 is a precursor of continuous VGOS-type observing but with the S/X system. In the next several years, continuous VGOS observing on the broadband system will become a reality. With this in mind, we provide an outlook on future prospects of CONT campaigns.

International VLBI Service for Geodesy and Astrometry: 1999 Annual Report

NASA Technical Reports Server (NTRS)

Vandenberg, Nancy R. (Editor)

1999-01-01

This volume of reports is the 1999 Annual Report of the International VLBI Service for Geodesy and Astrometry -IVS. The individual reports were contributed by VLBI groups in the international geodetic community who constitute the components of IVS. The 1999 Annual Report documents the work of the IVS components for the year ending March 1, 1999, the official inauguration date of IVS. As the newest of the space technique services, IVS decided to publish this Annual Report as a reference to our organization and its components. The entire contents of this Annual Report also appear on the IVS website at: http://ivscc.gsfc.nasa.gov/pub/arl999. The IVS 1999 Annual Report will be a valuable reference for information about IVS and its components. This Annual Report will serve as a baseline from which we can measure the anticipated progress of IVS in coming years.

A VLBI experiment using a remote atomic clock via a coherent fibre link.

PubMed

Clivati, Cecilia; Ambrosini, Roberto; Artz, Thomas; Bertarini, Alessandra; Bortolotti, Claudio; Frittelli, Matteo; Levi, Filippo; Mura, Alberto; Maccaferri, Giuseppe; Nanni, Mauro; Negusini, Monia; Perini, Federico; Roma, Mauro; Stagni, Matteo; Zucco, Massimo; Calonico, Davide

2017-02-01

We describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks.

Very-long-baseline interferometry techniques applied to problems of geodesy, geophysics, planetary science, astronomy, and general relativity

DOE Office of Scientific and Technical Information (OSTI.GOV)

Counselman, C.C. III

1973-09-01

Very-long-baseline interferometry (VLBI) techniques have already been used to determine the vector separations between antennas thousands of kilometers apart to within 2 m and the directions of extragalactic radio sources to 0.1'', and to track an artificial satellite of the earth and the Apollo Lunar Rover on the surface of the Moon. The relative loostions of the Apollo Lunar Surface Experiment Package (ALSEP) transmitters on the lunar surface are being measured within 1 m, and the Moon's libration is being messured to 1'' of selenocentric src. Attempts are under way to measure the solar gravitational deflection of radio waves moremore » accurately than previously possible, by means of VLBI. A wide variety of scientific problems is being attacked by VLBI techniques, which may soon be two orders of magnitude more accurate than at present. (auth)« less

A VLBI experiment using a remote atomic clock via a coherent fibre link

NASA Astrophysics Data System (ADS)

Clivati, Cecilia; Ambrosini, Roberto; Artz, Thomas; Bertarini, Alessandra; Bortolotti, Claudio; Frittelli, Matteo; Levi, Filippo; Mura, Alberto; Maccaferri, Giuseppe; Nanni, Mauro; Negusini, Monia; Perini, Federico; Roma, Mauro; Stagni, Matteo; Zucco, Massimo; Calonico, Davide

2017-02-01

We describe a VLBI experiment in which, for the first time, the clock reference is delivered from a National Metrology Institute to a radio telescope using a coherent fibre link 550 km long. The experiment consisted of a 24-hours long geodetic campaign, performed by a network of European telescopes; in one of those (Medicina, Italy) the local clock was alternated with a signal generated from an optical comb slaved to a fibre-disseminated optical signal. The quality of the results obtained with this facility and with the local clock is similar: interferometric fringes were detected throughout the whole 24-hours period and it was possible to obtain a solution whose residuals are comparable to those obtained with the local clock. These results encourage further investigation of the ultimate VLBI performances achievable using fibre dissemination at the highest precision of state-of-the-art atomic clocks.

The Italian VLBI Network: First Results and Future Perspectives

NASA Astrophysics Data System (ADS)

Stagni, Matteo; Negusini, Monia; Bianco, Giuseppe; Sarti, Pierguido

2016-12-01

A first 24-hour Italian VLBI geodetic experiment, involving the Medicina, Noto, and Matera antennas, shaped as an IVS standard EUROPE, was successfully performed. In 2014, starting from the correlator output, a geodetic database was created and a typical solution of a small network was achieved, here presented. From this promising result we have planned new observations in 2016, involving the three Italian geodetic antennas. This could be the beginning of a possible routine activity, creating a data set that can be combined with GNSS observations to contribute to the National Geodetic Reference Datum. Particular care should be taken in the scheduling of the new experiments in order to optimize the number of usable observations. These observations can be used to study and plan future experiments in which the time and frequency standards can be given by an optical fiber link, thus having a common clock at different VLBI stations.

A Recommendation on SLR Ranging to Future Global Navigation Satellite Systems

NASA Astrophysics Data System (ADS)

Labrecque, J. L.; Miller, J. J.; Pearlman, M.

2008-12-01

The multi-agency US Geodetic Requirements Working Group has recommended that Satellite Laser Retro- reflectors be installed on GPS III satellites as a principal component of the Positioning, Navigation, and Timing mandate of the Global Positioning System. The Working Group, which includes NASA, NGA, NOAA, NRL, USGS, and the USNO, echoes the Global Geodetic Observing System recommendation that SLR retro- reflectors be installed on all GNSS satellites. It is further recommended that the retro-reflectors conform to and hopefully exceed the minimum standard of the International Laser Ranging Service for retro-reflector cross sections of 100 million square meters for the HEO GNSS satellites to insure sufficiently accurate ranging by the global network of satellite laser ranging systems. The objective of this recommendation is to contribute to the improvement in the International Terrestrial Reference Frame, and its derivative the WGS84 reference frame, through continuing improvements in the characterization of the GPS orbits and clocks. Another objective is to provide an independent means of assessing the interoperability and accuracy of the GNSS systems and regional augmentation systems. The ranging to GNSS-mounted retro-reflectors will constitute a significant new means of space-based collocation to constrain the tie between the GPS and SLR networks that constitute over 50% of the data from which the ITRF is derived. The recommendation for the installation of SLR retro-reflectors aboard future GPS satellites is one of a number of efforts aimed at improving the accuracy and stability of ITRF. These steps are being coordinated with and supportive of the efforts of the GGOS and its services such at the VLBI2010 initiative, developing a next generation geodetic network, near real-time GPS positioning and EOP determination, and numerous efforts in the improvement of geodetic algorithms for GPS, SLR, VLBI, DORIS, and the determination of the ITRF. If past is prologue, the requirements of accuracy placed upon GNSS systems will continue to evolve at a factor of ten per decade for the lifetime of the GPS III, extending to 2025 and beyond. Global societal priorities such as sea level change measurement already require a factor of ten or more improvement in the accuracy and stability of the ITRF. Increasing accuracy requirements by civilian users for precision positioning and time keeping will certainly continue to grow at an exponential rate. The PNT accuracy of our GNSS systems will keep pace with these societal needs only if we equip the GNSS systems with the capability to identify and further reduce systematic errors.

The Global Space Geodesy Network and the Essential Role of Latin America Sites

NASA Astrophysics Data System (ADS)

Pearlman, M. R.; Ma, C.; Neilan, R.; Noll, C. E.; Pavlis, E. C.; Wetzel, S.

2013-05-01

The improvements in the reference frame and other space geodesy data products spelled out in the GGOS 2020 plan will evolve over time as new space geodesy sites enhance the global distribution of the network, and new technologies are implemented at current and new sites, thus enabling improved data processing and analysis. The goal of 30 globally distributed core sites with VLBI, SLR, GNSS and DORIS (where available) will take time to materialize. Co-location sites with less than the full core complement will continue to play a very important role in filling out the network while it is evolving and even after full implementation. GGOS, through its Call for Participation, bi-lateral and multi-lateral discussions, and work through the scientific Services have been encouraging current groups to upgrade and new groups to join the activity. This talk will give an update on the current expansion of the global network and the projection for the network configuration that we forecast over the next 10 years based on discussions and planning that has already occurred. We will also discuss some of the historical contributions to the reference frame from sites in Latin America and need for new sites in the future.

Measuring rapid ocean tidal earth orientation variations with very long baseline interferometry

NASA Technical Reports Server (NTRS)

Sovers, O. J.; Jacobs, C. S.; Gross, R. S.

1993-01-01

Ocean tidal effects on universal time and polar motion (UTPM) are investigated at four nearly diurnal (K(sub 1), P(sub 1), O(sub 1), and Q(sub 1)) and four nearly semidiurnal (K(sub 2), S(sub 2), M(sub 2), and N(sub 2)) frequencies by analyzing very long baseline interferometry (VLBI) data extending from 1978 to 1992. We discuss limitations of comparisons between experiment and theory for the retograde nearly diurnal polar motion components due to their degeneracy with prograde components of the nutation model. Estimating amplitudes of contributions to the modeled VLBI observables at these eight frequencies produces a statistically highly significant improvement of 7 mm to the residuals of a fit to the observed delays. Use of such an improved UTPM model also reduces the 14-30 mm scatter of baseline lengths about a time-linear model of tectonic motion by 3-14 mm, also withhigh significance levels. A total of 28 UTPM ocean tidal amplitudes can be unambiguously estimated from the data, with resulting UTI and PM magnitudes as large as 21 micro secs and 270 microarc seconds and formal uncertainties of the order of 0.3 micro secs and 5 microarc secs for UTI and PM, respectively. Empirically determined UTPM amplitudes and phases are com1pared to values calculated theoretically by Gross from Seiler's global ocean tide model. The discrepancy between theory and experiment is larger by a factor of 3 for UTI amplitudes (9 micro secs) than for prograde PM amplitudes (42 microarc secs). The 14-year VLBI data span strongly attenuates the influence of mismodeled effects on estimated UTPM amplitudes and phases that are not coherent with the eight frequencies of interest. Magnitudes of coherent and quasi-coherent systematic errors are quantified by means of internal consistency tests. We conclude that coherent systematic effects are many times larger than the formal uncertainties and can be as large as 4 micro secs for UTI and 60 microarc secs for polar motion. On the basis of such ealistic error estimates, 22 of the 31 fitted UTPM ocean tidal amplitudes differ from zero by more than 2 sigma.