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1

Ray-traced troposphere slant delays for precise point positioning

NASA Astrophysics Data System (ADS)

Precise satellite orbits and clock information for global navigation satellite systems (GNSS) allow zero-difference position solutions, also known as precise point positioning (PPP) to be calculated. In recent years numerical weather models (NWM) have undergone an improvement of spatial and temporal resolution. This makes them not only useful for the computation of mapping functions but also allows slant troposphere delays from ray-tracing to be obtained. For this study, such ray-traced troposphere corrections have been applied to code and phase observations of 13 sites from the International GNSS Service (IGS) receiver network, which are located inside the boundaries of the Japanese Meteorological Agency (JMA) meso-scale weather model, covering a period of 4 months. The results from this approach are presented together with a comparison to standard PPP processing results. Moreover the advantages and caveats of the introduction of ray-traced slant delays for precise point positioning are discussed.

Hobiger, T.; Ichikawa, R.; Takasu, T.; Koyama, Y.; Kondo, T.

2008-05-01

2

Network based real-time precise point positioning

NASA Astrophysics Data System (ADS)

Network based real-time precise point positioning system includes two stages, i.e. real-time estimation of satellite clocks based on a reference network and real-time precise point positioning thereafter. In this paper, a satellite- and epoch-differenced approach, adopted from what is introduced by Han et al. (2001), is presented for the determination of satellite clocks and for the precise point positioning. One important refinement of our approach is the implementation of the robust clock estimation. A prototype software system is developed, and data from the European Reference Frame Permanent Network on September 19, 2009 is used to evaluate the approach. Results show that our approach is 3 times and 90 times faster than the epoch-difference approach and the zero-difference approach, respectively, which demonstrates a significant improvement in the computation efficiency. The RMS of the estimated clocks is at the level of 0.1 ns (3 cm) compared to the IGS final clocks. The clocks estimates are then applied to the precise point positioning in both kinematic and static mode. In static mode, the 2-h estimated coordinates have a mean accuracy of 3.08, 5.79, 6.32 cm in the North, East and Up directions. In kinematic mode, the mean kinematic coordinates accuracy is of 4.63, 5.82, 9.20 cm.

Li, Haojun; Chen, Junping; Wang, Jiexian; Hu, Congwei; Liu, Zhiqiang

2010-11-01

3

Precise Point Positioning Based on BDS and GPS Observations

NASA Astrophysics Data System (ADS)

BeiDou Navigation Satellite System (BDS) has obtained the ability applying initial navigation and precise point services for the Asian-Pacific regions at the end of 2012 with the constellation of 5 Geostationary Earth Orbit (GEO), 5 Inclined Geosynchronous Orbit (IGSO) and 4 Medium Earth Orbit (MEO). Till 2020, it will consist with 5 GEO, 3 IGSO and 27 MEO, and apply global navigation service similar to GPS and GLONASS. As we known, GPS precise point positioning (PPP) is a powerful tool for crustal deformation monitoring, GPS meteorology, orbit determination of low earth orbit satellites, high accuracy kinematic positioning et al. However, it accuracy and convergence time are influenced by the quality of pseudo-range observations and the observing geometry between user and Global navigation satellites system (GNSS) satellites. Usually, it takes more than 30 minutes even hours to obtain centimeter level position accuracy for PPP while using GPS dual-frequency observations only. In recent years, many researches have been done to solve this problem. One of the approaches is smooth pseudo-range by carrier-phase observations to improve pseudo-range accuracy. By which can improve PPP initial position accuracy and shorten PPP convergence time. Another sachems is to change position dilution of precision (PDOP) with multi-GNSS observations. Now, BDS has the ability to service whole Asian-Pacific regions, which make it possible to use GPS and BDS for precise positioning. In addition, according to researches on GNSS PDOP distribution, BDS can improve PDOP obviously. Therefore, it necessary to do some researches on PPP performance using both GPS observations and BDS observations, especially in Asian-Pacific regions currently. In this paper, we focus on the influences of BDS to GPS PPP mainly in three terms including BDS PPP accuracy, PDOP improvement and convergence time of PPP based on GPS and BDS observations. Here, the GPS and BDS two-constellation data are collected from BeiDou experimental tracking stations (BETS) built by Wuhan University. And BDS precise orbit and precise clock products are applied by GNSS center, Wuhan University. After an introduction about GPS+BDS PPP mathematical and the error correction modes, we analyze the influence of BDS to GPS PPP carefully with calculating results. The statistics results show that BDS PPP can reach centimeter level and BDS can improve PDOP obviously. Moreover, the convergence time and position stability of GPS+BDS PPP is better than that of GPS PPP.

Gao, ZhouZheng; Zhang, Hongping; Shen, Wenbin

2014-05-01

4

Assessing the Accuracy of the Precise Point Positioning Technique

NASA Astrophysics Data System (ADS)

The Precise Point Positioning (PPP) GPS data processing technique has developed over the past 15 years to become a standard method for growing categories of positioning and navigation applications. The technique relies on single receiver point positioning combined with the use of precise satellite orbit and clock information and high-fidelity error modelling. The research presented here uniquely addresses the current accuracy of the technique, explains the limits of performance, and defines paths to improvements. For geodetic purposes, performance refers to daily static position accuracy. PPP processing of over 80 IGS stations over one week results in few millimetre positioning rms error in the north and east components and few centimetres in the vertical (all one sigma values). Larger error statistics for real-time and kinematic processing are also given. GPS PPP with ambiguity resolution processing is also carried out, producing slight improvements over the float solution results. These results are categorised into quality classes in order to analyse the root error causes of the resultant accuracies: "best", "worst", multipath, site displacement effects, satellite availability and geometry, etc. Also of interest in PPP performance is solution convergence period. Static, conventional solutions are slow to converge, with approximately 35 minutes required for 95% of solutions to reach the 20 cm or better horizontal accuracy. Ambiguity resolution can significantly reduce this period without biasing solutions. The definition of a PPP error budget is a complex task even with the resulting numerical assessment, as unlike the epoch-by-epoch processing in the Standard Position Service, PPP processing involving filtering. An attempt is made here to 1) define the magnitude of each error source in terms of range, 2) transform ranging error to position error via Dilution Of Precision (DOP), and 3) scale the DOP through the filtering process. The result is a deeper understanding of how PPP works, rather than just the performance of the technique determined from estimated station coordinates. From the above analysis, the limitations of PPP and the source of these limitations are isolated, including site displacement modelling, geometric measurement strength, pseudorange noise and multipath, etc. It is argued that new ambiguity resolution and multi-GNSS PPP processing will only partially address these limitations. Improved modelling is required for: site displacement effects, pseudorange noise and multipath, and code and phase biases. As well, more robust undifferenced-phase ambiguity validation and overall stochastic modelling is required.

Bisnath, S. B.; Collins, P.; Seepersad, G.

2012-12-01

5

NASA Astrophysics Data System (ADS)

Precise Point Positioning (PPP) is a technique used to determine highprecision position with a single GNSS receiver. Unlike DGPS or RTK, satellite observations conducted by the PPP technique are not differentiated, therefore they require that parameter models should be used in data processing, such as satellite clock and orbit corrections. Apart from explaining the theory of the PPP technique, this paper describes the available web-based online services used in the post-processing of observation results. The results obtained in the post-processing of satellite observations at three points, with different characteristics of environment conditions, using the CSRS-PPP service, will be presented as the results of the experiment. This study examines the effect of the duration of the measurement session on the results and compares the results obtained by working out observations made by the GPS system and the combined observations from GPS and GLONASS. It also presents the analysis of the position determination accuracy using one and two measurement frequencies

Krzan, Grzegorz; Dawidowicz, Karol; Krzysztof, ?wia?ek

2013-09-01

6

Development of a Real-Time Single-Frequency Precise Point Positioning System and Test Results

Precise Point Positioning (PPP) has received wide attention in the past several years as one of the next generation RTK technologies. PPP can offer great flexibility and cost-saving to field positioning work as it can eliminates the need to deploy base stations. Centimetre to decimetre accurate positioning accuracy has been widely demonstrated for PPP using a dual-frequency GPS receiver. The

Yang Gao; Yufeng Zhang; Kongzhe Chen

2006-01-01

7

Rapid re-convergences to ambiguity-fixed solutions in precise point positioning

Integer ambiguity resolution at a single receiver can be achieved if the fractional-cycle biases are separated from the ambiguity\\u000a estimates in precise point positioning (PPP). Despite the improved positioning accuracy by such integer resolution, the convergence\\u000a to an ambiguity-fixed solution normally requires a few tens of minutes. Even worse, these convergences can repeatedly occur\\u000a on the occasion of loss of

Jianghui Geng; Xiaolin Meng; Alan H. Dodson; Maorong Ge; Felix N. Teferle

2010-01-01

8

Precise Point Positioning for the Efficient and Robust Analysis of GPS Data from Large Networks

NASA Technical Reports Server (NTRS)

Networks of dozens to hundreds of permanently operating precision Global Positioning System (GPS) receivers are emerging at spatial scales that range from 10(exp 0) to 10(exp 3) km. To keep the computational burden associated with the analysis of such data economically feasible, one approach is to first determine precise GPS satellite positions and clock corrections from a globally distributed network of GPS receivers. Their, data from the local network are analyzed by estimating receiver- specific parameters with receiver-specific data satellite parameters are held fixed at their values determined in the global solution. This "precise point positioning" allows analysis of data from hundreds to thousands of sites every (lay with 40-Mflop computers, with results comparable in quality to the simultaneous analysis of all data. The reference frames for the global and network solutions can be free of distortion imposed by erroneous fiducial constraints on any sites.

Zumberge, J. F.; Heflin, M. B.; Jefferson, D. C.; Watkins, M. M.; Webb, F. H.

1997-01-01

9

Precise Point Positioning for the Efficient and Robust Analysis of GPS Data From Large Networks

NASA Technical Reports Server (NTRS)

Networks of dozens to hundreds of permanently operating precision Global Positioning System (GPS) receivers are emerging at spatial scales that range from 10(exp 0) to 10(exp 3) km. To keep the computational burden associated with the analysis of such data economically feasible, one approach is to first determine precise GPS satellite positions and clock corrections from a globally distributed network of GPS receivers. Then, data from the local network are analyzed by estimating receiver specific parameters with receiver-specific data; satellite parameters are held fixed at their values determined in the global solution. This "precise point positioning" allows analysis of data from hundreds to thousands of sites every day with 40 Mflop computers, with results comparable in quality to the simultaneous analysis of all data. The reference frames for the global and network solutions can be free of distortion imposed by erroneous fiducial constraints on any sites.

Zumberge, J. F.; Heflin, M. B.; Jefferson, D. C.; Watkins, M. M.; Webb, F. H.

1997-01-01

10

Incorporation of the GPS satellite ephemeris covariance matrix into the precise point positioning

NASA Astrophysics Data System (ADS)

In GPS positioning, usually the satellite ephemeris are fixed in the observation equations using broadcast or published values. Therefore, to have a realistic covariance matrix for the observations one must incorporate a well-defined covariance matrix of the satellite ephemeris into the observations covariance matrix. Contributions so far have discussed only the variance and covariance of the observations. Precise Point Positioning (PPP) is a technique aimed at processing of measurements from a single (stand-alone) GPS receiver to compute high-accurate position. In this paper, the covariance matrix of the satellite ephemeris and its impact on the position estimates through the PPP are discussed.

Shirazian, Masoud

2013-09-01

11

Real-Time IGS products verification in the context of their use in Precise Point Positioning

NASA Astrophysics Data System (ADS)

Precise Point Positioning (PPP) is a positioning technique of single GNSS receiver which applies high quality products from permanent GNSS observations to utilize the computational potential of global network analysis. Estimated satellite orbits and clocks corrections are introduced into equation system as known parameters. PPP requires the application of precise products, since their quality directly reflects the positioning accuracy. In June 2007 IGS Real-time Pilot Project has started in order to satisfy real-time users, which is especially important for Precise Point Positioning. Currently available streams including precise orbits, clocks and code biases are standardized on RTCM-SC 104 formats and may be used as a substitute for ultra-rapid products. The target combination product performances are 0.3ns for satellite clock accuracy and orbit accuracy at the level of the IGS Ultra predictions with maximum latency of 10s. This study presents the quality assessment of currently available Real-Time IGS products. Long-term test include comparisons of disseminated information with final and high-rate products, stability assessment over time, as well as latency validation of available RTCM streams.

Hadas, Tomasz; Bosy, Jaroslaw; Kaplon, Jan; Sierny, Jan

2013-04-01

12

Precise Point Positioning technique for short and long baselines time transfer

NASA Astrophysics Data System (ADS)

In this work the clock parameters determination of several timing receivers TTS-4 (AOS), ASHTECH Z-XII3T (OP, ORB, PTB, USNO) and SEPTENTRIO POLARX4TR (ORB, since February 11, 2012) by use of the Precise Point Positioning (PPP) technique were presented. The clock parameters were determined for several time links based on the data delivered by time and frequency laboratories mentioned above. The computations cover the period from January 1 to December 31, 2012 and were performed in two modes with 7-day and one-month solution for all links. All RINEX data files which include phase and code GPS data were recorded in 30-second intervals. All calculations were performed by means of Natural Resource Canada's GPS Precise Point Positioning (GPS-PPP) software based on high-quality precise satellite coordinates and satellite clock delivered by IGS as the final products. The used independent PPP technique is a very powerful and simple method which allows for better control of antenna positions in AOS and a verification of other time transfer techniques like GPS CV, GLONASS CV and TWSTFT. The PPP technique is also a very good alternative for calibration of a glass fiber link PL-AOS realized at present by AOS. Currently PPP technique is one of the main time transfer methods used at AOS what considerably improve and strengthen the quality of the Polish time scales UTC(AOS), UTC(PL), and TA(PL). KEY-WORDS: Precise Point Positioning, time transfer, IGS products, GNSS, time scales.

Lejba, Pawel; Nawrocki, Jerzy; Lemanski, Dariusz; Foks-Ryznar, Anna; Nogas, Pawel; Dunst, Piotr

2013-04-01

13

According to the precise positioning needs of the autonomous mobile robot, a three-point precise location measurement method is presented for the mobile robot control. In this location method, the parameters measured on the ultrasonic sensors be re-demarcated and filtered by error threshold under local coordinate system, precise position of mobile robot can be achieved. The performance of three-point precise location

Guo Shuai; Li Guolin; He Yongyi; Li Xianhua

2009-01-01

14

NASA Astrophysics Data System (ADS)

Given the severe effects of the ionosphere on global navigation satellite system (GNSS) signals, single-frequency (SF) precise point positioning (PPP) users can only achieve decimeter-level positioning results. Ionosphere-free combinations can eliminate the majority of ionospheric delay, but increase observation noise and slow down dual-frequency (DF) PPP convergence. In this paper, we develop a regional ionosphere modeling and rapid convergence approach to improve SF PPP (SFPPP) accuracy and accelerate DF PPP (DFPPP) convergence speed. Instead of area model, ionospheric delay is modeled for each satellite to be used as a priori correction. With the ionospheric, wide-lane uncalibrated phase delay (UPD) and residuals satellite DCBs product, the wide-lane observations for DF users change to be high-precision pseudorange observations. The validation of a continuously operating reference station (CORS) network was analyzed. The experimental results confirm that the approach considerably improves the accuracy of SFPPP. For DF users, convergence time is substantially reduced.

Yao, Yibin; Zhang, Rui; Song, Weiwei; Shi, Chuang; Lou, Yidong

2013-10-01

15

Effect of the 24 September 2011 solar radio burst on precise point positioning service

NASA Astrophysics Data System (ADS)

intense solar radio burst occurred on 24 September 2011, which affected the tracking of Global Navigation Satellite Systems' (GNSS) signals by receivers located in the sunlit hemisphere of the Earth. This manuscript presents for the first time the impacts of this radio burst on the availability of Fugro's real-time precise point positioning service for GNSS receivers and on the quality of the L band data link used to broadcast this service. During the peak of the radio burst (12:50-13:20 UT), a reduction in the L band signal-to-noise ratio (SNR) is observed. For some receiver locations, a reset in the position filter is observed, which can be either due to the reduction in the L band SNR or the reduction in the number of tracked GNSS satellites. This reset in the position filter is accompanied by degradation in the positioning accuracy, which is also discussed herein.

Sreeja, V.; Aquino, M.; Jong, Kees; Visser, Hans

2014-03-01

16

In recent years, many national timing laboratories have installed geodetic Global Positioning System receivers together with their traditional GPS/GLONASS Common View receivers and Two Way Satellite Time and Frequency Transfer equipment. Many of these geodetic receivers operate continuously within the International GNSS Service (IGS), and their data are regularly processed by IGS Analysis Centers. From its global network of over 350 stations and its Analysis Centers, the IGS generates precise combined GPS ephemeredes and station and satellite clock time series referred to the IGS Time Scale. A processing method called Precise Point Positioning (PPP) is in use in the geodetic community allowing precise recovery of GPS antenna position, clock phase, and atmospheric delays by taking advantage of these IGS precise products. Previous assessments, carried out at Istituto Nazionale di Ricerca Metrologica (INRiM; formerly IEN) with a PPP implementation developed at Natural Resources Canada (NRCan), showed PPP clock solutions have better stability over short/medium term than GPS CV and GPS P3 methods and significantly reduce the day-boundary discontinuities when used in multi-day continuous processing, allowing time-limited, campaign-style time-transfer experiments. This paper reports on follow-on work performed at INRiM and NRCan to further characterize and develop the PPP method for time transfer applications, using data from some of the National Metrology Institutes. We develop a processing procedure that takes advantage of the improved stability of the phase-connected multi-day PPP solutions while allowing the generation of continuous clock time series, more applicable to continuous operation/monitoring of timing equipment. PMID:19686979

Guyennon, Nicolas; Cerretto, Giancarlo; Tavella, Patrizia; Lahaye, François

2009-08-01

17

Impact of orbit, clock and EOP errors in GNSS Precise Point Positioning

NASA Astrophysics Data System (ADS)

Precise point positioning (PPP; [1]) has gained ever-increasing usage in GNSS carrier-phase positioning, navigation and timing (PNT) since its inception in the late 1990s. In this technique, high-precision satellite clocks, satellite ephemerides and earth-orientation parameters (EOPs) are applied as fixed input by the user in order to estimate receiver/location-specific quantities such as antenna coordinates, troposphere delay and receiver-clock corrections. This is in contrast to "network" solutions, in which (typically) less-precise satellite clocks, satellite ephemerides and EOPs are used as input, and in which these parameters are estimated simultaneously with the receiver/location-specific parameters. The primary reason for increased PPP application is that it offers most of the benefits of a network solution with a smaller computing cost. In addition, the software required to do PPP positioning can be simpler than that required for network solutions. Finally, PPP permits high-precision positioning of single or sparsely spaced receivers that may have few or no GNSS satellites in common view. A drawback of PPP is that the accuracy of the results depend directly on the accuracy of the supplied orbits, clocks and EOPs, since these parameters are not adjusted during the processing. In this study, we will examine the impact of orbit, EOP and satellite clock estimates on PPP solutions. Our primary focus will be the impact of these errors on station coordinates; however the study may be extended to error propagation into receiver-clock corrections and/or troposphere estimates if time permits. Study motivation: the United States Naval Observatory (USNO) began testing PPP processing using its own predicted orbits, clocks and EOPs in Summer 2012 [2]. The results of such processing could be useful for real- or near-real-time applications should they meet accuracy/precision requirements. Understanding how errors in satellite clocks, satellite orbits and EOPs propagate into PPP positioning and timing results allows researchers to focus their improvement efforts in areas most in need of attention. The initial study will be conducted using the simulation capabilities of Bernese GPS Software and extended to using real data if time permits. [1] J.F. Zumberge, M.B. Heflin, D.C. Jefferson, M.M. Watkins and F.H. Webb, Precise point positioning for the efficient and robust analysis of GPS data from large networks, J. Geophys. Res., 102(B3), 5005-5017, doi:10.1029/96JB03860, 1997. [2] C. Hackman, S.M. Byram, V.J. Slabinski and J.C. Tracey, Near-real-time and other high-precision GNSS-based orbit/clock/earth-orientation/troposphere parameters available from USNO, Proc. 2012 ION Joint Navigation Conference, 15 pp., in press, 2012.

Hackman, C.

2012-12-01

18

Precise point positioning for the efficient and robust analysis of GPS data from large networks

Networks of dozens to hundreds of permanently operating precision Global Positioning System (GPS) receivers are emerging at spatial scales that range from 100 to 10 km. To keep the computational burden associated with the analysis of such data economically feasible, one approach is to first determine precise GPS satellite positions and clock corrections from a globally distributed network of

J. F. Zumberge; M. B. Heflin; D. C. Jefferson; M. M. Watkins; F. H. Webb

1997-01-01

19

NASA Astrophysics Data System (ADS)

The Precise Point Positioning (PPP) technique using the un-differenced GPS observations and the precise IGS orbits and satellite clock products is recently a frequently used approach for geocentric coordinate determination. Until now, only the GPS observations are used for PPP mainly due to fact, that for the other GNSS the precise satellite clocks were not generally available. Recently the ESOC GLONASS Data Analysis Centre besides the GLONASS orbits provides regularly also the satellite clocks estimates in 5-minute intervals. In the paper will be introduced the model for computing real-valued ambiguities form code and phase GPS and GLONASS un-differenced observations as well as the procedures for reduction of observed GPS and GLONASS ranges. The models for separate GPS and GLONASS coordinates, clocks and troposphere delays estimates are mutually compared. Finally, the combination of GLONASS and GPS un-differenced data will be demonstrated. The role of additional parameters which are necessary to be introduced for combined solution will be investigated. The results from PPP processing based on separated GNSS observations as well as from their combination in joint adjustment will be discussed. All the procedures mentioned are examined by using the software package ABSOLUTE which is developed for the PPP GNSS processing at the Slovak University of Technology in Bratislava.

Hefty, Jan; Gerhatova, Lubomira

2010-05-01

20

Flight Test Evaluation of Precise Point Positioning Techniques Using Optical Ranging

NASA Astrophysics Data System (ADS)

This article reports on a flight test for the purpose of validating single-vehicle Global Positioning System (GPS) precise point positioning (PPP) of an aircraft using JPL's Global Navigation Satellite System (GNSS)-Inferred Positioning System (GIPSY) software and postprocessed satellite products. The article provides a comparison of a laser ranging device to GPS position estimates relative to a fixed ground station. The range data derived independently from the laser and GPS techniques agree to an average of 6.6 cm (RMS). The flight test was conducted on a Cessna aircraft circling the laser ranging device installed at Table Mountain in Wrightwood, California, at a range of approximately 6 km while the aircraft flew at an altitude of about 4.3 km. An error budget is presented based on the GPS, laser, meteorology, and inertial sensors employed. The survey of the locations of the instruments and associated error is presented. The range error of 6.6 cm RMS is consistent with the error in the instruments and survey.

Williamson, W.; Haines, B.; Wilson, K.; Kovalik, J.; Wright, M.; Meyer, R.; Bar-Sever, Y.

2012-11-01

21

NASA Astrophysics Data System (ADS)

Ambiguity resolution dedicated to a single global positioning system (GPS) station can improve the accuracy of precise point positioning. In this process, the estimation accuracy of the narrow-lane fractional-cycle biases (FCBs), which destroy the integer nature of undifferenced ambiguities, is crucial to the ambiguity-fixed positioning accuracy. In this study, we hence propose the improved narrow-lane FCBs derived from an ambiguity-fixed GPS network solution, rather than the original (i.e. previously proposed) FCBs derived from an ambiguity-float network solution. The improved FCBs outperform the original FCBs by ensuring that the resulting ambiguity-fixed daily positions coincide in nature with the state-of-the-art positions generated by the International GNSS Service (IGS). To verify this improvement, 1 year of GPS measurements from about 350 globally distributed stations were processed. We find that the original FCBs differ more from the improved FCBs when fewer stations are involved in the FCB estimation, especially when the number of stations is less than 20. Moreover, when comparing the ambiguity-fixed daily positions with the IGS weekly positions for 248 stations through a Helmert transformation, for the East component, we find that on 359 days of the year the daily RMS of the transformed residuals based on the improved FCBs is smaller by up to 0.8 mm than those based on the original FCBs, and the mean RMS over the year falls evidently from 2.6 to 2.2 mm. Meanwhile, when using the improved rather than the original FCBs, the RMS of the transformed residuals for the East component of 239 stations (i.e. 96.4% of all 248 stations) is clearly reduced by up to 1.6 mm, especially for stations located within a sparse GPS network. Therefore, we suggest that narrow-lane FCBs should be determined with ambiguity-fixed, rather than ambiguity-float, GPS network solutions.

Geng, Jianghui; Shi, Chuang; Ge, Maorong; Dodson, Alan H.; Lou, Yidong; Zhao, Qile; Liu, Jingnan

2012-08-01

22

NASA Astrophysics Data System (ADS)

High-rate continuous GPS data can provide direct, high-quality measurements of surface wave displacements generated by large earthquakes (Larson et al., 2003; Bock et al., 2004; Larson, 2009). To achieve high precision, differential positioning is often used in the GPS analysis strategy with distant reference stations held fixed. In this presentation, we examine the use of the Precise Point Positioning (PPP) technique to estimate epoch-by-epoch positions at single stations. Specifically, we use the PPP software developed by Natural Resources Canada (Heroux and Kouba, 2001) to analyze high-rate (5 Hz) GPS data collected at stations of the Plate Boundary Observatory (PBO) in southern California at the time of the M7.2 El Mayor-Cucapah Earthquake of April 4, 2010. The hypocenter for this earthquake was located in northern Baja California, approximately 50 km south of Mexicali on the US-Mexico border, at a depth of ~10 km. Large horizontal displacements were observed at a number of PBO GPS sites, with the largest peak-to-peak displacements exceeding 90 cm in the east-west component for 10-sec period waves observed at El Centro, CA (P496), located about 70 km northeast of the epicenter. The PPP technique clearly resolved surface waves with 1 to 2 cm amplitudes at sites more than 800 km away from the epicenter, illustrating that surface waves eventually reach even distant reference sites within the period of interest and can thereby introduce artifacts for differential GPS positioning. Fine-tuning of PPP methodology revealed the following: 1) Since the quality of a PPP solution will not be optimal until the carrier phase ambiguities have converged (tens of minutes), it is best to begin the analyses well before the arrival of seismic waves. To reduce computations, the data for this convergence period need not be high-rate; 2) The use of 5-second precise satellite clock sampling instead of the nominal 30-second clock sampling minimized clock interpolation errors and resulted in improved solutions; 3) Allowing some slow variation in the zenith tropospheric delay over periods of minutes appears to improve the vertical component solution. We conclude that high-rate (5 Hz) PPP can augment observations of ground displacements with periods of 1 s or longer at a resolution of 1 to 2 cm at single stations, both in the near and far fields, avoiding potential bias introduced by motions at a reference site. This makes high-rate PPP analysis of GPS data a useful technique for quantifying longer period ground motions of engineering interest.

Dragert, H.; Henton, J. A.; Lahaye, F.; Kouba, J.; Larson, K. M.; Rogers, G. C.

2010-12-01

23

Effects of non-modeled signal biases in multi-GNSS Precise Point Positioning

NASA Astrophysics Data System (ADS)

Precise Point Positioning (PPP) which is based on processing of un-differenced GNSS phase and code observations using the precise satellites orbits and satellites clocks is suitable for autonomous high quality geocentric coordinate determination without necessity of terrestrial reference sites. Increase of number of broadcasted GNSS signals and the combination of more satellite systems in common adjustment model emphasize the importance of consideration of intra-system and inter-system biases. The complexity of proper bias modeling is underlined by the fact that their origins are both in satellites and receivers. Part of the GNSS signal delays (e.g. system-specific satellite clock offsets, differential code biases, etc.) which are included in global network solution products is modeling predominantly the satellite dependent biases. The multi-GNSS receiver's biases could be evaluated within the individual site processing of un-differenced code and phase observations by addition of set of parameters related to receiver dependent inter-system, inter-code and inter-channel biases. In the paper are presented results of PPP-based estimates of GPS, GLONASS and GIOVE-B inter-system and intra-system biases for several sites with different GNSS instrumentation. Besides the biases estimated as constant parameters during the processed sessions, also the time evolution of the receiver-related biases is considered. All the procedures are examined by using the software package ABSOLUTE developed for the PPP multi-GNSS processing at the Slovak University of Technology in Bratislava.

Hefty, J.; Gerhatova, L.

2012-04-01

24

Precise point positioning using dual-frequency GPS and GLONASS measurements

NASA Astrophysics Data System (ADS)

This thesis presents a comprehensive study on Precise Point Positioning (PPP) using combined GPS/GLONASS dual frequency code and carrier phase observations. The existing PPP technique is implemented using only GPS measurements, which will be restricted from use in some situations such as in urban canyons and open-pit mine areas due to insufficient satellite number. In addition, the positioning accuracy and convergence time of PPP need to be further improved. A good strategy is to integrate GPS and GLONASS. In this research, a combined GPS/GLONASS traditional PPP model and a combined GPS/GLONASS U of C PPP model are developed, including their functional and stochastic models. The combined GPS and GLONASS PPP models have been implemented in a new version of the P3 software package. The performance of the combined GPS and GLONASS PPP is assessed using static data from IGS tracking network and kinematic data from an experiment. Numerical results indicate that the positioning accuracy and convergence time have a significant improvement after adding GLONASS observations. A further improvement can be expected when a full GLONASS constellation is completed in the near future. The stability of the GPS-GLONASS system time difference is investigated in the thesis. Recommendations for future work are also addressed.

Cai, Changsheng

25

NASA Astrophysics Data System (ADS)

A new method for the calibration of regional ionospheric delay based on uncombined precise point positioning (U-PPP) is proposed in this study. The performance of the new method was comparatively validated in terms of its accuracy and robustness with respect to the phase-smoothed pseudorange (PSP) method through two short-baseline experiments. Accuracy of the PPP-derived ionospheric delays was further assessed by interpolating them to a user station to perform single-frequency simulated kinematic PPP. Two 24-hr period datasets of four continuous operation reference system (CORS) stations were analyzed, collected during calm and disturbed ionospheric conditions, respectively. The single-frequency GPS observables from a user station, that were a-priori corrected by the interpolated ionospheric delays, were utilized to implement single-frequency PPP (SF-PPP). The results show that interpolation accuracy is better than 1 dm and, with the proposed method, is less affected by the ionospheric activity; meanwhile, positioning accuracy of SF-PPP was 4~5 cm (horizontal) and better than 1 dm (vertical). For comparison, two reference SF-PPP solutions were also obtained, in which the ionospheric delays are eliminated either by forming semi-combination observations or by using global ionosphere maps (GIM) model values; in both cases the positioning accuracy was only 4~7 dm (horizontal) and 1 m (vertical). These results provide a further demonstration of the performance of PPP-based regional ionospheric calibration in the parameter domain.

Wei, Li; Pengfei, Cheng; Jinzhong, Bei; Hanjiang, Wen; Hua, Wang

2012-08-01

26

Satellite- and Epoch Differenced Precise Point Positioning Based on a Regional Augmentation Network

Precise Point Positioning (PPP) has been demonstrated as a simple and effective approach for user positioning. The key issue in PPP is how to shorten convergence time and improve positioning efficiency. Recent researches mainly focus on the ambiguity resolution by correcting residual phase errors at a single station. The success of this approach (referred to hereafter as NORM-PPP) is subject to how rapidly one can fix wide-lane and narrow-lane ambiguities to achieve the first ambiguity-fixed solution. The convergence time of NORM-PPP is receiver type dependent, and normally takes 15–20 min. Different from the general algorithm and theory by which the float ambiguities are estimated and the integer ambiguities are fixed, we concentrate on a differential PPP approach: the satellite- and epoch differenced (SDED) approach. In general, the SDED approach eliminates receiver clocks and ambiguity parameters and thus avoids the complicated residual phase modeling procedure. As a further development of the SDED approach, we use a regional augmentation network to derive tropospheric delay and remaining un-modeled errors at user sites. By adding these corrections and applying the Robust estimation, the weak mathematic properties due to the ED operation is much improved. Implementing this new approach, we need only two epochs of data to achieve PPP positioning converging to centimeter-positioning accuracy. Using seven days of GPS data at six CORS stations in Shanghai, we demonstrate the success rate, defined as the case when three directions converging to desired positioning accuracy of 10 cm, reaches 100% when the interval between the two epochs is longer than 15 min. Comparing the results of 15 min' interval to that of 10 min', it is observed that the position RMS improves from 2.47, 3.95, 5.78 cm to 2.21, 3.93, 4.90 cm in the North, East and Up directions, respectively. Combining the SDED coordinates at the starting point and the ED relative coordinates thereafter, we demonstrate the performance of RTK PPP with standard deviation of 0.80, 1.34, 0.97 cm in the North, East and Up directions.

Li, Haojun; Chen, Junping; Wang, Jiexian; Wu, Bin

2012-01-01

27

NASA Astrophysics Data System (ADS)

Ionospheric disturbances are characterized as fast and random variability in the ionosphere. Those phenomena are difficult to predict, detect and model. Occurrence of some strong ionospheric disturbances can cause, inter alia degradation and interruption of GNSS signals. Therefore they are especially harmful for real-time applications, as for example Precise Point Positioning (PPP) in real time, where one of the most important requirements is to ensure the high level of reliability. In such applications verification and confirmation of a high trust degree towards the estimated coordinates is a very critical issue. In one of the previous papers (K. Wezka, 2012 -Identification of system performance parameters and their usability) two sets of parameters have been proposed for enhance reliability of the PPP. The first one for data quality control (QC) of the raw GNSS observations and the second one for examination of the quality, robustness and performance of various processing approaches (strategies). To the second group the following parameters has been proposed: accuracy, precision, availability, integrity and convergence time. In consideration of perturbation of GNSS signal resulting from sudden ionospheric disturbances, one of the most important demands is effective autonomous integrity monitoring. The poster presents first preliminary results of the applicability of the proposed parameters in order to ensure the high level of reliability/integrity of GNSS observations and positioning results under the presence of strong ionospheric anomalies. The data-set from continuously operated GNSS station located at high latitude, where ionospheric disturbances occur more frequently, were used for the analysis. Various selected Receiver Autonomous Integrity Monitoring (RAIM) approaches for quality control of the GNSS observables are applied to the data sets recorded under different (low/quite and high) ionospheric activities. Based on those analyses the usability of the proposed parameters is verified.

Wezka, K.; Galas, R.

2013-12-01

28

Employing GPS L5 Carrier-Frequency in Precise Point Positioning

NASA Astrophysics Data System (ADS)

Justine Spits and Marcelo C. Santos Dept. of Geodesy and Geomatics Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3 Tel: (1-506) 453-4698, Email: msantos@unb.ca, jspits@unb.ca Precise Point Positioning (PPP) is a GNSS technique which, in most cases nowadays, makes use of Global Positioning System (GPS) dual-frequency signals. The increasing availability of the new GPS L5 signal brings about the question on how much can PPP benefit if it uses L5 in conjunction with the legacy L1 and L2 signals. This poster discusses this issue. It involves the study of the use of L5 in conjunction with the other GPS signals (L1 and L2) with emphasis on the potentialities associated with the various combinations, such as L1-L5, L2-L5 and L1-L2-L5. These combinations will bring benefits in different ways, for example, for ionospheric delay mitigation, ambiguity resolution, convergence time and accuracy. Simulated L5 data will be used to test the PPP algorithms. Performance will be compared against the current dual-frequency PPP methodology. Justine Spits: Ph.D. (Liège, Belgium); Post-Doctoral Fellow, University of New Brunswick Marcelo. C. Santos: Ph.D. (New Brunswick); Professor, University of New Brunswick

Spits, J.; Santos, M. C.

2012-12-01

29

Errors Analysis in GPS Precise Point Positioning: Impact of Ambiguity Fixing

NASA Astrophysics Data System (ADS)

GNSS geodetic positioning using the classical double-difference approach may have some limitations. For example, fixing ambiguities can be challenging for long baselines, while processing short baseline only give the relative displacement between the two stations. In this context and thanks to the continuous improvement of IGS GNSS orbit and clock products, the Precise Point Positioning (PPP) technique appears in the literature as a powerful alternative. If all local Earth deformations are correctly taken into account, residuals of position time series may be used to assess the processing quality in terms of receiver performance and environment, constellation orbits and clocks error projection, and processing options pertinence. The main limitation of most of the current PPP processing strategies is that ambiguities can not be fixed to integer values. However, Mercier et al. (2008) demonstrated that GPS satellite “electronic” biases can be a priori identified in such a way that using a consistent set of GPS orbits, clocks and biases, phase ambiguities recover their integer nature. The CNES-CLS IGS Analysis Center is being providing such set of data since August 2010. This study evaluate the performance of PPP in front of the nowadays requirements of geodesy. We processed data from several IGS sites in order to compute coordinate series on a daily basis but also at higher frequencies (down to 30 second interval). We investigated both the impact of the processing batch duration from hours to several days and the cut-off elevation angle. Various spurious “non geophysical” signals (random, periodic, jumps...)appeared in our series. Especially artificial "midnight jumps" when adopting the usual 24-hours batch solutions (when satellite passes were cut at 0h). The impact of fixing ambiguities on PPP solutions has been investigated. We demonstrate that most of the artifacts affecting “floating” PPP solutions disappeared when ambiguities were fixed.

Perosanz, F.; Fund, F.; Mercier, F.; Loyer, S.; Capdeville, H.

2010-12-01

30

Triple-frequency GPS precise point positioning with rapid ambiguity resolution

NASA Astrophysics Data System (ADS)

At present, reliable ambiguity resolution in real-time GPS precise point positioning (PPP) can only be achieved after an initial observation period of a few tens of minutes. In this study, we propose a method where the incoming triple-frequency GPS signals are exploited to enable rapid convergences to ambiguity-fixed solutions in real-time PPP. Specifically, extra-wide-lane ambiguity resolution can be first achieved almost instantaneously with the Melbourne-Wübbena combination observable on L2 and L5. Then the resultant unambiguous extra-wide-lane carrier-phase is combined with the wide-lane carrier-phase on L1 and L2 to form an ionosphere-free observable with a wavelength of about 3.4 m. Although the noise of this observable is around 100 times the raw carrier-phase noise, its wide-lane ambiguity can still be resolved very efficiently, and the resultant ambiguity-fixed observable can assist much better than pseudorange in speeding up succeeding narrow-lane ambiguity resolution. To validate this method, we use an advanced hardware simulator to generate triple-frequency signals and a high-grade receiver to collect 1-Hz data. When the carrier-phase precisions on L1, L2 and L5 are as poor as 1.5, 6.3 and 1.5 mm, respectively, wide-lane ambiguity resolution can still reach a correctness rate of over 99 % within 20 s. As a result, the correctness rate of narrow-lane ambiguity resolution achieves 99 % within 65 s, in contrast to only 64 % within 150 s in dual-frequency PPP. In addition, we also simulate a multipath-contaminated data set and introduce new ambiguities for all satellites every 120 s. We find that when multipath effects are strong, ambiguity-fixed solutions are achieved at 78 % of all epochs in triple-frequency PPP whilst almost no ambiguities are resolved in dual-frequency PPP. Therefore, we demonstrate that triple-frequency PPP has the potential to achieve ambiguity-fixed solutions within a few minutes, or even shorter if raw carrier-phase precisions are around 1 mm. In either case, we conclude that the efficiency of ambiguity resolution in triple-frequency PPP is much higher than that in dual-frequency PPP.

Geng, Jianghui; Bock, Yehuda

2013-05-01

31

A Comparison of Real-Time Precise Point Positioning Zenith Total Delay Estimates

NASA Astrophysics Data System (ADS)

The use of observations from Global Navigation Satellite Systems (GNSS) in operational meteorology is increasing worldwide due to the continuous evolution of GNSS. The assimilation of near real-time (NRT) GNSS-derived zenith total delay (ZTD) estimates into local, regional and global scale numerical weather prediction (NWP) models is now in operation at a number of meteorological institutions. The development of NWP models with high update cycles for now-casting and monitoring of extreme weather events in recent years, requires the estimation of ZTD with minimal latencies, i.e. from 5 to 10 minutes, while maintaining an adequate level of accuracy for these. The availability of real-time (RT) observations and products from the IGS RT service and associated analysis centers make it possible to compute precise point positioning (PPP) solutions in RT, which provide ZTD along with position estimates. This study presents a comparison of the RT ZTD estimates from three different PPP software packages (G-Nut/Tefnut, BNC2.7 and PPP-Wizard) to the state-of-the-art IGS Final Troposphere Product employing PPP in the Bernese GPS Software. Overall, the ZTD time series obtained by the software packages agree fairly well with the estimates following the variations of the other solutions, but showing various biases with the reference. After correction of these the RMS differences are at the order of 0.01 m. The application of PPP ambiguity resolution in one solution or the use of different RT product streams shows little impact on the ZTD estimates.

Ahmed, F.; Vaclavovic, P.; Dousa, J.; Teferle, F. N.; Laurichesse, D.; Bingley, R.

2013-12-01

32

Ambiguity resolution in precise point positioning with hourly data for global single receiver

NASA Astrophysics Data System (ADS)

Integer ambiguity resolution (IAR) can improve precise point positioning (PPP) performance significantly. IAR for PPP became a highlight topic in global positioning system (GPS) community in recent years. More and more researchers focus on this issue. Progress has been made in the latest years. In this paper, we aim at investigating and demonstrating the performance of a global zero-differenced (ZD) PPP IAR service for GPS users by providing routine ZD uncalibrated fractional offsets (UFOs) for wide-lane and narrow-lane. Data sets from all IGS stations collected on DOY 1, 100, 200 and 300 of 2010 are used to validate and demonstrate this global service. Static experiment results show that an accuracy better than 1 cm in horizontal and 1-2 cm in vertical could be achieved in ambiguity-fixed PPP solution with only hourly data. Compared with PPP float solution, an average improvement reaches 58.2% in east, 28.3% in north and 23.8% in vertical for all tested stations. Results of kinematic experiments show that the RMS of kinematic PPP solutions can be improved from 21.6, 16.6 and 37.7 mm to 12.2, 13.3 and 34.3 mm for the fixed solutions in the east, north and vertical components, respectively. Both static and kinematic experiments show that wide-lane and narrow-lane UFO products of all satellites can be generated and provided in a routine way accompanying satellite orbit and clock products for the PPP user anywhere around the world, to obtain accurate and reliable ambiguity-fixed PPP solutions.

Zhang, Xiaohong; Li, Pan; Guo, Fei

2013-01-01

33

Real Time Precise Point Positioning: Preliminary Results for the Brazilian Region

NASA Astrophysics Data System (ADS)

GNSS positioning can be carried out in relative or absolute approach. In the last years, more attention has been driven to the real time precise point positioning (PPP). To achieve centimeter accuracy with this method in real time it is necessary to have available the satellites precise coordinates as well as satellites clocks corrections. The coordinates can be used from the predicted IGU ephemeris, but the satellites clocks must be estimated in a real time. It can be made from a GNSS network as can be seen from EUREF Permanent Network. The infra-structure to realize the PPP in real time is being available in Brazil through the Brazilian Continuous Monitoring Network (RBMC) together with the Sao Paulo State GNSS network which are transmitting GNSS data using NTRIP (Networked Transport of RTCM via Internet Protocol) caster. Based on this information it was proposed a PhD thesis in the Univ. Estadual Paulista (UNESP) aiming to investigate and develop the methodology to estimate the satellites clocks and realize PPP in real time. Then, software is being developed to process GNSS data in the real time PPP mode. A preliminary version of the software was called PPP_RT and is able to process GNSS code and phase data using precise ephemeris and satellites clocks. The PPP processing can be accomplished considering the absolute satellite antenna Phase Center Variation (PCV), Ocean Tide Loading (OTL), Earth Body Tide, among others. The first order ionospheric effects can be eliminated or minimized by ion-free combination or parameterized in the receiver-satellite direction using a stochastic process, e.g. random walk or white noise. In the case of ionosphere estimation, a pseudo-observable is introduced in the mathematical model for each satellite and the initial value can be computed from Klobuchar model or from Global Ionospheric Map (GIM). The adjustment is realized in the recursive mode and the DIA (Detection Identification and Adaptation) is used for quality control. In this paper our proposition is to present the mathematical models implemented in the PPP_RT software and some proposal to accomplish the PPP in real time as for example using tropospheric model from Brazilian Numerical Weather Forecast Model (BNWFM) and estimating the ionosphere using stochastic process. GPS data sample from the Brazilian region was processed using the PPP_RT software considering periods under low and high ionospheric activities and the results estimating the ionosphere were compared with the ion-free combination. The PPP results also were analyzed considering the strategy of the troposphere estimation, Hopfield model or using the BNWFM. For the troposphere case, the values from BNWFM can reach similar results when estimating the troposphere. For the ionosphere case, the results have shown that ionosphere estimation can improve the time convergence of the PPP processing what is very important for PPP in real time.

Marques, Haroldo; Monico, João.; Hirokazu Shimabukuro, Milton; Aquino, Marcio

2010-05-01

34

NASA Astrophysics Data System (ADS)

Ambiguity resolution (AR) for a single receiver has been a popular topic in Global Positioning System (GPS) recently. Ambiguity-resolution methods for precise point positioning (PPP) have been well documented in recent years, demonstrating that it can improve the accuracy of PPP. However, users are often concerned about the reliability of ambiguity-fixed PPP solution in practical applications. If ambiguities are fixed to wrong integers, large errors would be introduced into position estimates. In this paper, we aim to assess the correct fixing rate (CFR), i.e., number of ambiguities correctly fixing to the total number of ambiguities correctly and incorrectly fixing, for PPP user ambiguity resolution on a global scale. A practical procedure is presented to evaluate the CFR of PPP user ambiguity resolution. GPS data of the first 3 days in each month of 2010 from about 390 IGS stations are used for experiments. Firstly, we use GPS data collected from about 320 IGS stations to estimate global single-differenced (SD) wide-lane and narrow-lane satellite uncalibrated phase delays (UPDs). The quality of UPDs is evaluated. We found that wide-lane UPD estimates have a rather small standard deviation (Std) between 0.003 and 0.004 cycles while most of Std of narrow-lane estimates are from 0.01 to 0.02 cycles. Secondly, many experiments have been conducted to investigate the CFR of integer ambiguity resolution we can achieve under different conditions, including reference station density, observation session length and the ionospheric activity. The results show that the CFR of PPP can exceed 98.0 % with only 1 h of observations for most user stations. No obvious correlation between the CFR and the reference station density is found. Therefore, nearly homogeneous CFR can be achieved in PPP AR for global users. At user end, higher CFR could be achieved with longer observations. The average CFR for 30-min, 1-h, 2-h and 4-h observation is 92.3, 98.2, 99.5 and 99.7 %, respectively. In order to get acceptable CFR, 1 h is a recommended minimum observation time. Furthermore, the CFR of PPP can be affected by diurnal variation and geomagnetic latitude variation in the ionosphere. During one day at the hours when rapid ionospheric variations occur or in low geomagnetic latitude regions where equatorial electron density irregularities are produced relatively frequently, a significant degradation of the CFR is demonstrated.

Zhang, Xiaohong; Li, Pan

2013-06-01

35

A closer look at the concept of regional clocks for Precise Point Positioning

NASA Astrophysics Data System (ADS)

Under the precondition of at least two successfully tracked signals at different carrier frequencies we may obtain their ionosphere free linear combination. By introducing approximate values for geometric effects like orbits and tropospheric delay as well as an initial bias parameter N per individual satellite we can solve for the satellite clock with respect to the receiver clock. Noting, that residual effects like orbit errors, remaining tropospheric delays and a residual bias parameter map into these parameters, this procedure leaves us with a kind of virtual clock differences. These clocks cover regional effects and are therefore clearly correlated with clocks at nearby station. Therefore we call these clock differences, which are clearly different from clock solutions provided for instance by IGS, the "regional clocks". When introducing the regional clocks obtained from real-time data of a GNSS reference station network we are able to process the coordinates of a nearby isolated station via a PPP .In terms of PPP-convergence time which will be reduced down to 30 minutes or less, this procedure is clearly favorable. The accuracy is quite comparable with state of the art PPP procedures. Nevertheless, this approach cannot compete in fixing times with double-difference approaches but the correlation holds over hundreds of kilometers distance to our master station and the clock differences can easily by obtained, even in real-time. This presentation provides preliminary results of the project RA-PPP. RA-PPP is a research project financed by the Federal Ministry for Transport, Innovation and Technology, managed by the Austrian Research Promotion Agency (FFG) in the course of the 6th call of the Austrian Space Application Program (ASAP). RA-PPP stands for Rapid Precise Point Positioning, which denotes the wish for faster and more accurate algorithms for PPP. The concept of regional clocks which will be demonstrated in detail in this presentation is one out of 4 concepts to be evaluated in this project.

Weber, Robert; Karabatic, Ana; Thaler, Gottfried; Abart, Christoph; Huber, Katrin

2010-05-01

36

Precise Point Positioning (PPP) has become a very hot topic in GNSS research and applications. However, it usually takes about several tens of minutes in order to obtain positions with better than 10 cm accuracy. This prevents PPP from being widely used in real-time kinematic positioning services, therefore, a large effort has been made to tackle the convergence problem. One of the recent approaches is the ionospheric delay constrained precise point positioning (IC-PPP) that uses the spatial and temporal characteristics of ionospheric delays and also delays from an a priori model. In this paper, the impact of the quality of ionospheric models on the convergence of IC-PPP is evaluated using the IGS global ionospheric map (GIM) updated every two hours and a regional satellite-specific correction model. Furthermore, the effect of the receiver differential code bias (DCB) is investigated by comparing the convergence time for IC-PPP with and without estimation of the DCB parameter. From the result of processing a large amount of data, on the one hand, the quality of the a priori ionosphere delays plays a very important role in IC-PPP convergence. Generally, regional dense GNSS networks can provide more precise ionosphere delays than GIM and can consequently reduce the convergence time. On the other hand, ignoring the receiver DCB may considerably extend its convergence, and the larger the DCB, the longer the convergence time. Estimating receiver DCB in IC-PPP is a proper way to overcome this problem. Therefore, current IC-PPP should be enhanced by estimating receiver DCB and employing regional satellite-specific ionospheric correction models in order to speed up its convergence for more practical applications.

Zhang, Hongping; Gao, Zhouzheng; Ge, Maorong; Niu, Xiaoji; Huang, Ling; Tu, Rui; Li, Xingxing

2013-01-01

37

Precise Point Positioning (PPP) has become a very hot topic in GNSS research and applications. However, it usually takes about several tens of minutes in order to obtain positions with better than 10 cm accuracy. This prevents PPP from being widely used in real-time kinematic positioning services, therefore, a large effort has been made to tackle the convergence problem. One of the recent approaches is the ionospheric delay constrained precise point positioning (IC-PPP) that uses the spatial and temporal characteristics of ionospheric delays and also delays from an a priori model. In this paper, the impact of the quality of ionospheric models on the convergence of IC-PPP is evaluated using the IGS global ionospheric map (GIM) updated every two hours and a regional satellite-specific correction model. Furthermore, the effect of the receiver differential code bias (DCB) is investigated by comparing the convergence time for IC-PPP with and without estimation of the DCB parameter. From the result of processing a large amount of data, on the one hand, the quality of the a priori ionosphere delays plays a very important role in IC-PPP convergence. Generally, regional dense GNSS networks can provide more precise ionosphere delays than GIM and can consequently reduce the convergence time. On the other hand, ignoring the receiver DCB may considerably extend its convergence, and the larger the DCB, the longer the convergence time. Estimating receiver DCB in IC-PPP is a proper way to overcome this problem. Therefore, current IC-PPP should be enhanced by estimating receiver DCB and employing regional satellite-specific ionospheric correction models in order to speed up its convergence for more practical applications. PMID:24253190

Zhang, Hongping; Gao, Zhouzheng; Ge, Maorong; Niu, Xiaoji; Huang, Ling; Tu, Rui; Li, Xingxing

2013-01-01

38

NASA Astrophysics Data System (ADS)

recent development of the International Global Navigation Satellite Systems Service Real-Time Pilot Project and the enormous progress in precise point positioning (PPP) techniques provide a promising opportunity for real-time determination of Integrated Water Vapor (IWV) using GPS ground networks for various geodetic and meteorological applications. In this study, we develop a new real-time GPS water vapor processing system based on the PPP ambiguity fixing technique with real-time satellite orbit, clock, and phase delay corrections. We demonstrate the performance of the new real-time water vapor estimates using the currently operationally used near-real-time GPS atmospheric data and collocated microwave radiometer measurements as an independent reference. The results show that an accuracy of 1.0 ~ 2.0 mm is achievable for the new real-time GPS based IWV value. Data of such accuracy might be highly valuable for time-critical geodetic (positioning) and meteorological applications.

Li, Xingxing; Dick, Galina; Ge, Maorong; Heise, Stefan; Wickert, Jens; Bender, Michael

2014-05-01

39

NASA Astrophysics Data System (ADS)

Precise Point Positioning (PPP) is a positioning technique that uses a single GNSS (Global Navigation Satellite System) receiver that requires external information from analysis of global GNSS permanent network, in particular precise orbits and satellite clocks. This technique is commonly used in post-processing mode and gives results comparable to relative positioning. A shortcoming of this technique is the time required for the solution to converge, which is a main limitation for near real-time and real-time applications. The convergence time depends on the quality of GNSS data, on the accuracy of the a priori parameters and on fast ambiguity resolution. Until recently, near real-time and real-time users were limited in the sources of precise products, since only the predicted part of the ultra-rapid products were available. In 2012, the International GNSS Service (IGS) launched the Real-Time Service (RTS), making available a dedicated set of real-time products, known as IGS-RTS. Nevertheless, there is still no standard procedure for handling the troposphere delay. The a priori troposphere delay, as well as mapping functions, has to be derived from an external source and the adjustment model should account for the correction to an apriori value of the delay. Currently, a number of empirical troposphere state models and mapping functions are available for users in real-time. Near-real time model of troposphere delay can also be determined from the analysis of regional GNSS permanent network. In this paper, we make use of the IGS-RTS along with a number of a priori tropospheric models in order the assess how they influence convergence time and estimated position. For this purpose, we use GPS Analysis and Positioning Software (GAPS) for near-real time processing and GNSS-Wroclaw Algorithms for Real-time Positioning (GNSS-WARP) software for real-time processing of GPS only data together with IGS-RTS precise orbits and satellite clocks. As a priori troposphere model we used GPT together with the Saastamoinen formula, UNB3 model and regional near-real time troposphere model from the analysis of a network of permanent GNSS stations. We combine these models with Niell and VMF mapping functions to compute slant troposphere delays, including those of low-elevation satellites.

Hadas, Tomasz; Santos, Marcelo; Garcia, Alex; Bosy, Jaroslaw; Kaplon, Jan

2014-05-01

40

NASA Astrophysics Data System (ADS)

The estimation of total precipitable water vapour (PWV) using kinematic GNSS has been investigated since around 2001, aiming to extend the use of static ground-based GNSS, from which PWV estimates are now operationally assimilated into numerical weather prediction models. To date, kinematic GNSS PWV studies suggest a PWV measurement agreement with radiosondes of 2-3 mm, almost commensurate with static GNSS measurement accuracy, but only shipborne experiments have so far been carried out. As a first step towards extending such sea level-based studies to platforms that operate at a range of altitudes, such as airplanes or land based vehicles, the kinematic GNSS estimation of PWV over an exactly repeated trajectory is considered. A data set was collected from a GNSS receiver and antenna mounted on a carriage of the Snowdon Mountain Railway, UK, which continually ascends and descends through 950 m of vertical relief. Static GNSS reference receivers were installed at the top and bottom of the altitude profile, and derived zenith wet delay (ZWD) was interpolated to the altitude of the train to provide reference values together with profile estimates from the 100 m resolution runs of the Met Office's Unified Model. We demonstrate similar GNSS accuracies as obtained from previous shipborne studies, namely a double difference relative kinematic GNSS ZWD accuracy within 14 mm, and a kinematic GNSS precise point positioning ZWD accuracy within 15 mm. The latter is a more typical airborne PWV estimation scenario i.e. without the reliance on ground-based GNSS reference stations. We show that the kinematic GPS-only precise point positioning ZWD estimation is enhanced by also incorporating GLONASS observations.

Webb, S. R.; Penna, N. T.; Clarke, P. J.; Webster, S.; Martin, I.

2013-12-01

41

NASA Astrophysics Data System (ADS)

We evaluate the impact of mapping functions developed from different atmospheric structures on precise point positioning. In each case the atmospheric structure is derived from the same Numerical Weather Model (NWM). We compared five different structures -- from simpler to more realistic: spherical concentric, spherical osculating, ellipsoidal, gradient, and 3D -- and a state-of-art mapping function, Vienna Mapping Functions Site (VMF1). We used data from IGS station ALGO. Results correspond to comparisons with the IGS (non- cumulative) weekly solution. The spherical concentric model shows a large (cm-level) bias in the north component. The spherical osculating (and ellipsoidal) model shows an improvement in the up component, by almost one order of magnitude, over that of VMF1. The 3D atmosphere model reduces the horizontal bias to less than 1 mm, but there is no apparent improvement in the vertical position, which we attribute to unaccounted non-tidal atmospheric pressure loading. Finally, the gradient atmosphere shows biases with magnitude in between those of the spherical osculating and 3d models.

Nievinski, F. G.; Santos, M. C.

2008-12-01

42

NASA Astrophysics Data System (ADS)

Precise Point Positioning (PPP) is one of the possible approaches for GNSS data processing. As known this technique is faster and more flexible compared to the others which are based on a differenced approach and constitute a reliable methods for accurate positioning of remote GNSS stations, even in some remote area such as Antarctica. Until few years ago one of the major limits of the method was the impossibility to resolve the ambiguity as integer but nowadays many methods are available to resolve this aspect. The first software package permitting a PPP solution was the GIPSY OASIS realized, developed and maintained by JPL (NASA). JPL produce also orbits and files ready to be used with GIPSY. Recently, using these products came possible to resolve ambiguities improving the stability of solutions. PPP permit to estimate position into the reference frame of the orbits (IGS) and when coordinate in others reference frames, such al ITRF, are needed is necessary to apply a transformation. Within his products JPL offer, for each day, a global 7 parameter transformation that permit to locate the survey into the ITRF RF. In some cases it's also possible to create a costumed process and obtain analogous parameters using local/regional reference network of stations which coordinates are available also in the desired reference frame. In this work some tests on accuracy has been carried out comparing different PPP solutions obtained using the same software packages (GIPSY) but considering the ambiguity resolution, the global and regional transformation parameters. In particular two test area have been considered, first one located in Antarctica and the second one in Italy. Aim of the work is the evaluation of the impact of ambiguity resolution and the use of local/regional transformation parameter in the final solutions. Tests shown how the ambiguity resolution improve the precision, especially in the EAST component with a scattering reduction about 8%. And the use of global transformation parameter permit to improve the accuracy of about 59%, 63% and 29% in the three components N E U, but other tests shown how is possible to improve the accuracy of 67% 71% and 53% using regional transformation parameters. Example of the impact of global vs regional parameters transformation in a GPS time series

Gandolfi, S.; Poluzzi, L.; Tavasci, L.

2012-12-01

43

NASA Astrophysics Data System (ADS)

The development of single-receiver integer ambiguity resolution in recent years has made the GPS precise point positioning (PPP) technique a valuable tool in measuring centimeter-level displacements epoch by epoch at a single station. A good application for this technique is identifying ground motions in an earthquake and tsunami early warning system. With a single receiver, PPP with ambiguity resolution can reproduce the positioning accuracy of conventional differential positioning techniques, but does not depend on any nearby reference stations which may also be displaced during an earthquake. As a result, the PPP data processing is more straightforward and efficient, suggesting that onsite displacement estimation can be carried out semi-autonomously at each GPS station and only a small amount of data, i.e. 3D displacements rather than raw measurements in the differential positioning, need to be transmitted to warning centers. Due to these merits and as part of a NASA-sponsored research effort, we have developed an operational real-time PPP system for Western North America, a vast region of tectonic deformation and significant seismic risk. A group of about 75 real-time GPS stations throughout North America and located far from western US coast (>300 km) is employed to estimate satellite-specific corrections (i.e. satellite clocks and fractional-cycle biases) with the predicted satellite orbits provided by the IGS (International GNSS Services). We note that our PPP implementation is challenged by geophysical constraints in North America and so all clients in the zone of deformation are outside the coverage of the reference network, and thus measurement errors originating in the atmosphere, satellite orbits and clocks are less correlated between the reference stations and the PPP clients. Despite this difficulty, the horizontal positioning accuracy at each PPP station is around 1 cm while the vertical better than 5 cm. This accuracy is sufficient to optimally combine the 1-Hz PPP-derived displacements with collocated (100 Hz) accelerometer data using a Kalman filter to estimate total displacement waveforms with millimeter-level accuracy. We also report on the testing of our approach in a simulated real-time environment for the 2006 Mw 6.0 Parkfield and 2010 Mw 7.2 El Mayor-Cucapah earthquakes.

Geng, J.; Bock, Y.; Fang, P.; Haase, J. S.

2012-12-01

44

NASA Astrophysics Data System (ADS)

For earthquake early warning (EEW) and emergency response, earthquake magnitude is the crucial parameter to be determined rapidly and correctly. However, a reliable and rapid measurement of the magnitude of an earthquake is a challenging problem, especially for large earthquakes (M>8). Here, the magnitude is determined based on the GPS displacement waveform derived from real-time precise point positioning (PPP). The real-time PPP results are evaluated with an accuracy of 1 cm in the horizontal components and 2-3 cm in the vertical components, indicating that the real-time PPP is capable of detecting seismic waves with amplitude of 1cm horizontally and 2-3cm vertically with a confidence level of 95%. In order to estimate the magnitude, the unique information provided by the GPS displacement waveform is the horizontal peak displacement amplitude. We show that the empirical relation of Gutenberg (1945) between peak displacement and magnitude holds up to nearly magnitude 9.0 when displacements are measured with GPS. We tested the proposed method for three large earthquakes. For the 2010 Mw 7.2 El Mayor-Cucapah earthquake, our method provides a magnitude of M7.18±0.18. For the 2011 Mw 9.0 Tohoku-oki earthquake the estimated magnitude is M8.74±0.06, and for the 2010 Mw 8.8 Maule earthquake the value is M8.7±0.1 after excluding some near-field stations. We therefore conclude that depending on the availability of high-rate GPS observations, a robust value of magnitude up to 9.0 for a point source earthquake can be estimated within 10s of seconds or a few minutes after an event using a few GPS stations close to the epicenter. The rapid magnitude could be as a pre-requisite for tsunami early warning, fast source inversion, and emergency response is feasible.

Fang, Rongxin; Shi, Chuang; Song, Weiwei; Wang, Guangxing; Liu, Jingnan

2014-05-01

45

NASA Astrophysics Data System (ADS)

In order to improve the performance of precise point positioning (PPP), this paper presents a new data processing scheme to shorten the convergence time and the observation time required for a reliable ambiguity-fixing. In the new scheme, L1 and L2 raw observations are used and the slant ionospheric delays are treated as unknown parameters. The empirical spatial and temporal constraints and the ionospheric delays derived from a real-time available ionospheric model are all considered as pseudo-observations into the estimation for strengthening the solution. Furthermore, we develop a real-time computational procedure for generating uncalibrated phase delays (UPDs) on L1 and L2 frequencies. The PPP solution is first carried out on all reference stations based on the proposed scheme, undifferenced float ambiguities on L1 and L2 frequencies can be directly obtained from the new scheme. The L1 and L2 UPDs are then generated and broadcasted to users in real-time. This data product and also the performance of the new PPP scheme are evaluated. Our results indicate that the new processing scheme considering ionospheric characteristics can reduce the convergence time by about 30 % for float kinematic solutions. The observation time for a reliable ambiguity-fixing is shortened by 25 % compared to that of the traditional ambiguity-fixed kinematic solution. When the new method is used for static reference stations, the observation time for ambiguity-fixing is about 10 min in static mode and only 5 min if the coordinates are fixed to well-known values.

Li, Xingxing; Ge, Maorong; Zhang, Hongping; Wickert, Jens

2013-05-01

46

NASA Astrophysics Data System (ADS)

Using single frequency GPS measurements from the CHAMP satellite it was demonstrated before that during the solar maximum the LEO orbit can be determined with an accuracy below 10 cm RMS (1D). The first order ionosphere effect is removed by linear combination of pseudorange and carrier phase measurements using the so-called LP linear combination. The main limitation of this POD approach is the high noise of the pseudorange measurements. By forming the LP linear combination, noise in the pseudorange measurements is reduced by 50%, and by estimating half-cycle phase ambiguities an additional reduction can be expected in all systematic effects (e.g. multipath, group delay variations, etc.). Furthermore, thanks to the high quality GPS measurements from the GRACE mission, it was demonstrated that the orbit of a LEO satellite can be determined using single frequency measurements with an accuracy of 5 cm RMS (1D). This is very close to the cm-level accuracy of the LEO orbits based on dual-frequency carrier phase measurements. Here we show that based on the latest gravity field models short term perturbations can very accurately be modelled in the numerical integration, considerably reducing the number of empirical parameters. This allows to average errors in the pseudorange measurements over a longer period of time and further increase the accuracy of GRACE orbit down to 3 cm RMS (1D) and beyond. However, at this level of accuracy systematic errors in the code to carrier coherence play a crucial role, especially in terms of signal group delay variations of GPS satellite and LEO antenna. Here we demonstrate a novel approach in the code to carrier calibration based on a geometry and ionosphere free linear combination. We show that it is possible to separate the group delay variations of the GPS satellite antenna from the group delay variations of a LEO or a ground antenna. This leads to very accurate mean group delay maps of the GPS satellite antennas. Validation of the code to carrier coherence calibration is carried out with the independent measurements from a high-gain steerable ground antenna. Here we show how code to phase coherence calibration improves LEO orbit determination based on single frequency measurements and compare our results with the ground precise point positioning based on single-frequency GPS measurements. A typical performance curve relating convergence time and accuracy is derived for PPP, as well as POD of the GRACE satellites during the solar minimum and maximum.

Svehla, Drazen; Escobar, Diego A.; Dow, John M.

47

Ultra-precision positioning assembly

An apparatus and method is disclosed for ultra-precision positioning. A slide base provides a foundational support. A slide plate moves with respect to the slide base along a first geometric axis. Either a ball-screw or a piezoelectric actuator working separate or in conjunction displaces the slide plate with respect to the slide base along the first geometric axis. A linking device directs a primary force vector into a center-line of the ball-screw. The linking device consists of a first link which directs a first portion of the primary force vector to an apex point, located along the center-line of the ball-screw, and a second link for directing a second portion of the primary force vector to the apex point. A set of rails, oriented substantially parallel to the center-line of the ball-screw, direct movement of the slide plate with respect to the slide base along the first geometric axis and are positioned such that the apex point falls within a geometric plane formed by the rails. The slide base, the slide plate, the ball-screw, and the linking device together form a slide assembly. Multiple slide assemblies can be distributed about a platform. In such a configuration, the platform may be raised and lowered, or tipped and tilted by jointly or independently displacing the slide plates.

Montesanti, Richard C. (San Francisco, CA); Locke, Stanley F. (Livermore, CA); Thompson, Samuel L. (Pleasanton, CA)

2002-01-01

48

NASA Astrophysics Data System (ADS)