Cheney, Robert E.
Since altimetry data are not really old enough to use the term data archaeology, Mr. Cheney referred to the stewardship of these data. He noted that it is very important to document the basis for an altimetry data set as the algorithms and corrections used to arrive at the Geophysical Data Record (GDR) have been improving and are continuing to improve the precision of sea level data derived from altimetry. He noted that the GEOSAT Exact Repeat Mission (ERM) data set has recently been reprocessed by his organization in the National Ocean Service of NOAA and made available to the scientific community on CD/ROM disks by the National Oceanographic Data Center of the U.S. (NODC). The new data set contains a satellite orbit more precise by an order of magnitude together with an improved water vapor correction. A new, comprehensive GDR Handbook has also been prepared.
Glenn, N. F.; Shrestha, R.; Li, A.; Spaete, L.
Numerous studies have demonstrated the utility of ground and airborne LiDAR data to quantify ecosystem structure. In addition, data from satellite-based laser altimetry (e.g. ICESat's GLAS instrument) have been used to estimate vegetation heights, aboveground carbon, and topography in forested areas. With the upcoming ICESAT-2 satellite scheduled to launch in 2017, we have the potential to map vegetation characteristics and dynamics in other ecosystems, including semiarid and low-height ecosystems, at global and regional scales. The ICESat-2 satellite will include the Advanced Topographic Laser Altimeter System (ATLAS) with a configuration of 6 laser beams with 532 nm wavelength and photon counting detectors. We will demonstrate the potential of ICESat-2 to provide estimates of vegetation structure and topography in a dryland ecosystem by simulating the configuration of the ATLAS mission. We will also examine how airborne LiDAR can be used together with ICESat-2 and other satellite data to achieve estimates of aboveground carbon. We will explore how these data may be used for future monitoring and quantification of spatial and temporal changes in aboveground carbon and topography.
Leão, J. O.; Silva, J. S.; Research Team Of Rhasa
Satellite altimetry is now a mature tool to collect water levels over large and medium-size rivers. The present study is dedicated to the determination of bankfull discharge using satellite altimetry and imagery to determine hydrological parameters such as the phase of the bankfull discharge at the crossing of the river bed with the ground track of the altimetry missions or the surface slope of the river. We applied the methodology to the two major tributaries of the Amazon river, namely the Rio Madeira and the Rio Negro. The results are a significzant difference in the values of bankfull discharge in the two basins, together with significant difference in along-course slopes, reflecting the difference in geomorphological context between the two watersheds.
Lu, Xiaomei; Hu, Yongxiang; Trepte, Charles; Liu, Zhaoyan
A super-resolution laser altimetry technique has been proposed to provide improved lidar altimetry from Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observation (CALIPSO) lidar data, and it is applicable to other similar atmospheric profiling lidar with low-pass filters. To achieve high altimetry resolution, the new technique relies on an empirical relationship between the peak signal ratio and the distance between land surface and the peak signal range bin center, which is directly derived from the CALIPSO lidar measurements and does not require the CALIPSO's transient response. The CALIPSO surface elevation results in Northern America retrieved by the new technique agree with the National Elevation Database high resolution elevation maps, and the comparisons suggest that the precision of the technique is much better than 1.4 m. The preliminary data product of land surface elevation retrieved by the new technique from CALIPSO lidar measurements is available to the altimetry community for evaluation.
Kurtz, Nathan T.; Markus, Thorsten; Cavalieri, Donald J.; Sparling, Lynn C.; Krabill, William B.; Gasiewski, Albin J.; Sonntag, John G.
Combinations of sea ice freeboard and snow depth measurements from satellite data have the potential to provide a means to derive global sea ice thickness values. However, large differences in spatial coverage and resolution between the measurements lead to uncertainties when combining the data. High resolution airborne laser altimeter retrievals of snow-ice freeboard and passive microwave retrievals of snow depth taken in March 2006 provide insight into the spatial variability of these quantities as well as optimal methods for combining high resolution satellite altimeter measurements with low resolution snow depth data. The aircraft measurements show a relationship between freeboard and snow depth for thin ice allowing the development of a method for estimating sea ice thickness from satellite laser altimetry data at their full spatial resolution. This method is used to estimate snow and ice thicknesses for the Arctic basin through the combination of freeboard data from ICESat, snow depth data over first-year ice from AMSR-E, and snow depth over multiyear ice from climatological data. Due to the non-linear dependence of heat flux on ice thickness, the impact on heat flux calculations when maintaining the full resolution of the ICESat data for ice thickness estimates is explored for typical winter conditions. Calculations of the basin-wide mean heat flux and ice growth rate using snow and ice thickness values at the 70 m spatial resolution of ICESat are found to be approximately one-third higher than those calculated from 25 km mean ice thickness values.
Marks, K. M.; Smith, W. H.
We have examined three factors influencing the use of satellite altimeter data to map seamounts and guyots in the deep ocean: (1) the resolution of seamount and guyot gravity anomalies by altimetry; (2) the non-linearity of the relationship between gravity and bathymetry; and (3) the homogeneity of the mass density within the seamount or guyot. When altimeter data are used to model the marine gravity anomaly field the result may have limited resolution due to noise levels in the altimeter data, track spacing of the satellite profiles, inclination angles of the orbits, and filters used to combine and interpolate the data (Sandwell and Smith, JGR, 1997). We compared the peak-to-trough amplitude of gravity anomalies in Sandwell and Smith`'s version 15.1 field to peak-to-trough amplitudes measured by gravimeters on board ships. The satellite gravity field amplitudes match ship measurements well over seamounts and guyots having volumes exceeding ~2000 km3. Over smaller volume seamounts, where the anomalies have most of their power at quite short wavelengths, the satellite field under-estimates the anomaly amplitude. If less filtering could be done, or a new mission with a lower noise level were flown, more of the anomalies associated with small seamounts might be resolved. Smith and Sandwell (Science, 1997) predicted seafloor topography from altimetric gravity assuming that the density of seafloor topography is nearly constant over ~100 km distances, and that the relationship between gravity and topography may be approximated by a liner filter over those distances. In fact, the true theoretical relationship is non-linear (Parker, Geophys. J. R. astr. Soc, 1972); it can be expressed as an N-th order expansion, with the N=1 term representing a linear filter and the N>1 terms accounting for higher-order corrections. We find that N=2 is a sufficient approximation at both seamounts and guyots. Constant density models of large volume guyots do not fit the observed gravity
Kosuth, P.; Cazenave, A.; Blitzkow, D.
Due to extremely poor road infrastructure and resulting difficulties in direct topographic levelling, Amazon river basin lacks a consistent topographical referential. Space based altimetric techniques appear to be the only feasible alternative to establish such a referential. Extensive processing of Topex/Poseidon satellite radar altimeter data upon continental open water bodies of the Amazon Basin over 1993-2000 period has been realised. Due to relatively poor reliability of Topex/Poseidon data at river low stage, processing focussed on maximum annual water levels. Such maximum annual water levels have been determined for more than 150 intersections between satellite ground traces and rivers, accuracy being improved by correlation analysis with continuous water level time series at closest gauging station. Results have been translated to a geoidal referential using EGM96 geoid model. Annual upper enveloppes of rivers longitudinal water profiles have been interpolated, allowing to quantify maximum annual water levels at existing gauging stations. Confrontation between satellite determined and field measured maximum water levels at these stations allowed to quantify the geoidal altitude of more than 80 stations with a decimetric accuracy (<0.5m). This method has been checked and validated through internal consistency analysis, hydrological consistency analysis and confrontation with bi-frequency GPS positionning measurement results at 22 stations. Mean difference between geoidal altitudes determined by satellite radar altimetry and bi-frequency GPS positionning for these 22 stations is +0.29 m +/- 0.63 m, GPS positionning results being lower than satellite radar altimetry ones. Till now about 30 000 km of Amazon Basin rivers over Brazil, Bolivia, Peru and Ecuador benefit from these altimetric references. This opens way for improved understanding of Amazon river dynamics and enlightens possible improvements in applying satellite radar altimetry techniques over
Lee, C.; Seo, K.; Scambos, T.
We present a novel method for estimating the elevation change on the Antarctic ice shelves using laser altimetry data from the Ice Cloud and land Elevation Satellite (ICESat; 2003-2009). Unlike the conventional crossover or repeat-track analysis fixed on the geodetic position, we estimate the elevation change rate at points fixed on the surface of moving ice, i.e. in the Lagrangian coordinate system. The ICESat ground tracks are relocated into the Lagrangian coordinate system based on the velocity field from the interferometric synthetic aperture radar (InSAR) and then the elevation change rate is measured from their crossover differences. The thickness change rates converted from the elevation change rates through a hydrostatic formula are applied to the mass conservation equation in the Lagrangian coordinate system, in order to derive the basal melt rate. In Lagrangian approach, the crossover difference is less affected by the small-scale surface relief on the moving ice, which causes the large uncertainty of elevation change rate in the conventional (Eulerian) crossover analysis. The basal melt derived from the mass conservation equation is also less sensitive to the noises of gridded ice thicknesses in the Largrangian approach than in the Eulerian approach. Our analysis provides a reliable map of basal melt rate and thickness change rate in the Antarctic ice shelves, which is a snapshot for the ICESat period. The highest ice thinning rates, accompanying strongest basal melts, are observed in the small ice shelves along the Amundsen Sea coast. In the Ross Ice Shelf, the ice thickness change is mainly controlled by the shutdown of Kamb Ice Stream. The ice thinning is dominant in the Filchner-Ronne Ice Shelf and strong within 150 km from the ice front.
Csatho, B.; Schenk, T.; Babonis, G.; Nagarajan, S.; Krabill, W.
The main objective of the ICESat satellite laser altimetry mission is to determine the mass balance of polar ice sheets and their contributions to current sea level changes. By measuring surface topography, ICESat also provides important boundary conditions for ice sheet and atmospheric modeling. To quantify surface elevation changes, to investigate their causes, and to improve predictive ice sheet models, accurate elevation changes on a seasonal, annual and inter-annual basis at scales of drainage basins and outlet glaciers are essential. Determining the spatial and temporal distribution of surface changes from repeat satellite laser altimetry remains a challenging problem, mainly because the footprints of repeat missions do not precisely overlap. We have developed a new method that is based on fitting analytical functions to laser points within repeat tracks or cross-over areas for estimating the ice sheet surface topography. The mathematical model of the change detection algorithm is based on the assumption that for a small surface area, e.g. 1 km by 1 km, only the absolute elevation changes over time but not the shape of the surface patch. Therefore, laser points of all time epochs of a small surface patch contribute to the shape parameters, and the laser points of each time period determine the absolute elevation of the surface patch at that period. The least squares adjustment delivers the surface elevation changes together with statistical information that is extremely helpful in judging how significant the elevation changes and the derived volume changes are. We demonstrate the feasibility of the proposed approach by reconstructing surface and volume changes of the Jakobshavn drainage basin in west Greenland. The accuracy of the surface change estimates derived from repeat ICESat measurements is verified by using NASA's Airborne Topographic Mapper (ATM) airborne laser altimetry. We then combine repeat ICESat and ATM laser altimetry and stereo satellite
Storm surges can cause catastrophic damage to properties and loss of life in coastal communities. Thus it is important to enhance our capabilities of observing and forecasting storm surges for mitigating damage and loss. In this presentation we show examples of observing storm surges around the world using nadir satellite altimetry, during Hurricane Sandy, Igor, and Isaac, as well as other cyclone events. The satellite observations are evaluated against tide-gauge observations and discussed for dynamic mechanisms. We also show the potential of a new wide-swath altimetry mission, the Surface Water and Ocean Topography (SWOT), for observing storm surges.
Sandwell, D.T. )
Publications related to geophysical applications of Seasat and Geosat altimetry are reviewed for the period 1987-1990. Problems discussed include geoid and gravity errors, regional geoid heights and gravity anomalies, local gravity field/flexure, plate tectonics, and gridded geoid heights/gravity anomalies. 99 refs.
Cohen, Steven C.; Degnan, John J., III; Bufton, Jack L.; Garvin, James B.; Abshire, James B.
The Geoscience Laser Altimetry/Ranging System (GLARS), a combined laser ranging and altimetry system capable of subcentimeter position determinations of retroflector targets and subdecimeter profiling of topography, is described. The system uses advanced but currently available state-of-the-art components. Laboratory, field, and numerical experiments have indicated the suitability of GLARS as an instrument for Eos and other space platforms.
The use of satellite altimetry for hydrological applications, either it is basin management or hydrological modeling really started with the 21st century. Before, during two decades, the efforts were concentrated on the data processing until a precision of a few decimeters could be achieved. Today, several web sites distribute hundreds of series spread over hundeds of rivers runing in the major basins of the world. Among these, the Amazon basin has been the most widely studied. Satellite altimetry is now routinely used in this transboundary basin to predict discharges ranging over 4 orders of magnitude. In a few years, satellite altimetry should evolve dramatically. This year, we should see the launchs of Jason-3 and that of Sentinel-3A operating in SAR mode. With SAR, the accuracy and resolution of a growing number of measurements should be improved. In 2020, SWOT will provide a full coverage that will join in a unique framework all the previous and forthcoming missions. These technical and thematical evolutions will be illustrated by examples taken in the Amazon and Congo basin.
Li, JunLi; Fang, Hui; Yang, Liao
Lakes in arid regions of Central Asia act as essential components of regional water cycles, providing sparse but valuable water resource for the fragile ecological environments and human lives. Lakes in Central Asia are sensitive to climate change and human activities, and great changes have been found since 1960s. Mapping and monitoring these inland lakes would improve our understanding of mechanism of lake dynamics and climatic impacts. ICESat/GLAS satellite laser altimetry provides an efficient tool of continuously measuring lake levels in these poorly surveyed remote areas. An automated mapping scheme of lake level changes is developed based on GLAS altimetry products, and the spatial and temporal characteristics of 9 typical lakes in Central Asia are analyzed to validate the level accuracies. The results show that ICESat/GLAS has a good performance of lake level monitoring, whose patterns of level changes are the same as those of field observation, and the max differences between GLAS and field data is 3cm. Based on the results, it is obvious that alpine lakes are increasing greatly in lake levels during 2003-2009 due to climate change, while open lakes with dams and plain endorheic lakes decrease dramatically in water levels due to human activities, which reveals the overexploitation of water resource in Central Asia.
Krabill, W. B.; Brooks, R. L.
Radar altimeter measurements from the GEOS-3 and SEASAT satellites are being evaluated to assess their potential contribution to terrain mapping. The primary evaluation area is the San Joaquin Valley of southern California; 40,000/sq km of the Valley have been mapped at a contour interval of 10 m from the satellite altimeter measurements. The accuracy of the altimeter derived terrain elevations is being assessed by comparison with 1:24,000 and digitized 1:250,000 maps and by intercomparisons at the crossover altimeter intersections. Comparisons of the altimeter derived elevations with historical maps archived at the U.S. Geological Survey confirms the USGS 1926-1972 subsidence contours for this area. Preliminary results from a similar analysis in the Houston-Galveston area of subsidence also demonstrates a capability of measuring land subsidence by satellite altimetry.
Smith, B. E.; Shean, D. E.; Joughin, I. R.
The Amundsen coast of Antarctica is at present the site of some of the largest ice-sheet contributions to global sea-level change. NASA's Ice Bridge program has made detailed ice-thickness and altimetry surveys on the principal glaciers in this region, including Pine Island, Thwaites, Pope and Smith, focusing attention on the grounding zones in each case, where warming ocean waters have led to thinning of the seaward ends of these glaciers. This has led to accelerated discharge in three ways: by directly removing ice from the glaciers, by steepening the surface profiles near the grounding zone, and by thinning the ice towards the flotation point, reducing the area in contact with the bed and removing basal shear stress. By combining data from IceSat and Ice Bridge laser altimetry with Worldview stereophotogrammetry, we have developed maps of elevation change throughout the basins of these glaciers. These, together with Ice Bridge and AGASEA ice-thickness measurements, allow us to estimate ice freeboard at any time between 2003 and 2011 for any point for which we have an ice thickness measurement. Based on this, we have mapped grounding line positions for the major surveyed glaciers in the area. These maps show extensive grounding-line retreat in Pine Island and Pope glaciers through 2010, and substantial thinning near the Thwaites ice shelf bringing further ice into flotation. Extrapolation of current thinning rates, under the assumption that their spatial pattern and magnitude will remain constant, gives one scenario for the near future rate of grounding line retreat. Under this scenario, the current rate grounding line retreat can continue, at least locally, for the next 20-30 years, with slower rates of retreat thereafter as the grounding line reaches ice with larger surface slopes. Alternately, if the thinning pattern follows the grounding line, with peak thinning rates in the 10-20 km immediately upstream of the grounding line, the initial phase of the retreat
The Geoscience Laser Altimeter System (GLAS) is planned for launch on ICESat in 2002, into a 600 km altitude, near polar orbit from Vandenberg, California. The sys- tem is designed to operate up to five years in orbit. GLAS is under development by NASA Goddard and it will be delivered to the spacecraft contractor, Ball Aerospace, for mating and testing with the spacecraft bus. The GLAS instrument will transmit both near infrared (1064 nm) and green (532 nm) pulses using a diode-pumped, Q- switched Nd:YAG laser. The 1064 wavelength will be used for surface altimetry, in- cluding dense clouds, and the 532 wavelength will be used for atmospheric backscat- ter measurements. The altitude measurement will produce elevation time series of the Greenland and Antarctic ice sheets, which will enable determination of present-day elevation change and mass balance. Other applications of the altimetry channel in- clude precise measurements of land topography and vegetation canopy heights, sea ice roughness and thickness, and ocean surface elevations. The atmospheric channel will provide information on the vertical distribution of clouds and aerosols. The laser pulse energy at 1064 nm is about 75 mJ with a width of about 5 ns and the pulse has a divergence of about 0.11 mrad, which illuminates a spot on the surface with a 66 m diameter. Three lasers are available (two are required for lifetime requirements and the third provides redundancy). The pulse echo is captured with a 1 m telescope mounted on the rigid GLAS optical bench. A Si analog detector receives the return pulse and an A/D converter digitizes the pulse with a 1 GHz sampling rate. Two detectors and two digitizers are available for redundancy. Unlike wide pulse radar altimeters, accurate knowledge of the laser beam direction is required for the laser altimeter. The pointing will be determined with the assistance of an innovative system of CCD cameras that will measure the direction of each laser pulse with respect to
Sirota, A. M.; Lebedev, S. A.; Burykin, S. N.; Timokhin, E. N.; Chernyshkov, P. P.
Satellite altimetry data provide good possibility to reveal the zones of high dynamic activity e g oceanic currents and fronts mesoscale features etc The four oceanic region were considered Irminger Sea Mid Atlantic Ridge North Atlantic Canary Upwelling Region Eastern Central Atlantic and Southeastern Pacific Both satellite altimetry data TOPEX Poseidon ERS -1 2 and in situ measurements oceanographic surveys demonstrated good correlation between these two different types of data in revealing of dynamic features at the ocean surface The main dynamic features in the regions are Sub-Polar Front and North Atlantic Current Irminger Sea and Mid Atlantic Ridge Canary Current and coastal upwelling Eastern Central Atlantic Sub-Tropical Front and South Pacific Current Southeastern Pacific Analysis of distribution abundance and biological state of various fish species revealed the links between organisms and their dynamic environmental conditions in the considered regions Variability of the distribution and abundance of rock grenadier over Mid Atlantic Ridge is closely connected to variations of Sub-Polar Front location Distribution of fishery grounds in the Irminger Sea coincides with dynamic heterogeneities at the sea surface elevation field Distribution of small pelagic fish in Canary Upwelling Region is influenced by mesoscale features of Canary Current and coastal upwelling Sub-Tropical Front meandering and eddies in Southeast Pacific influence significantly horse mackerel distribution Thus the peculiarities of dynamic features of the ocean
Pioneer works using satellite altimetry over rivers started two decades ago. Next decade, we should have SWOT, the first mission to monitor all the water bodies on Earth larger than (250 m x 250 m). Over these three decades, radar altimetry for hydrology will have evolved significantly. In the past decade, ESA's ENVISAT has turned to be the most useful altimetry mission for hydrology. The major improvement brought by ENVISAT has been to propose various estimates of the radar "range" (the distance between the sensor and reflecting surface) in the raw data distributed. Owing to this choice in ranges, typical rms error for series computed with the ice-1 algorithm for the ENVISAT or Jason-2 data is in the range of 20-40 cm, which is a factor 2 to 4 better than it was previously with the standard -ocean- tracking algorithm, with the T/P mission for instance. Before ENVISAT, it has long been considered that altimetry could work only over wide rivers or large lakes. When the contrast in backscatter between the river surface and the surrounding ground was favorable, valuable time series have been recovered over reaches as narrow as a few tens of meters. All the past missions, including ENVISAT, were working in the Ku band in Low Resolution mode (LR), in opposite to the delay Doppler (DD), SAR, mode, which should be the most common technology in the near-future missions. SAR mode is currently tested with Cryosat-2, launched in2010. With AltiKa, to be launched in February this year, a new band will be tested, the Ka band. In 2014, ESA should launch Sentinel-3A, the first of a series of four SAR satellites. Thus, in the middle of the decade, we should have the most favorable situation ever encountered, with 2 to 3 SAR altimeters (Sentinel-3A from 2014, Sentinel-3B from 2016, Jason-CS from 2017), and in LR mode (Jason 2 & 3 and AltiKa). Next decade, SWOT will embark a Ka band wide swath (120 km) interferometric altimeter. It will cover the Earth continents twice every 22 days
Bellaieche, G.; Aguttes, J. P.
Association of the mini satellite concept and oceanic altimetry missions is discussed. Mission definition and most constraining requirements (mesoscale for example) demonstrate mini satellites to be quite adequate for such missions. Progress in altimeter characteristics, orbit determination, and position reporting allow consideration of oceanic altimetry missions using low Earth orbit satellites. Satellite constellation, trace keeping and orbital period, and required payload characteristics are exposed. The mission requirements covering Sun synchronous orbit, service area, ground system, and launcher characteristics as well as constellation maintenance strategy are specified. Two options for the satellite, orbital mechanics, propulsion, onboard power and stabilizing subsystems, onboard management, satellite ground linkings, mechanical and thermal subsystems, budgets, and planning are discussed.
Farrell, Sinead L.; Markus, Thorsten; Kwok, Ron; Connor, Laurence
With the conclusion of the science phase of the Ice, Cloud and land Elevation Satellite (ICESat) mission in late 2009, and the planned launch of ICESat-2 in late 2015, NASA has recently established the IceBridge program to provide continuity between missions. A major goal of IceBridge is to obtain a sea-ice thickness time series via airborne surveys over the Arctic and Southern Oceans. Typically two laser altimeters, the Airborne Topographic Mapper (ATM) and the Land, Vegetation and Ice Sensor (LVIS), are utilized during IceBridge flights. Using laser altimetry simulations of conventional analogue systems such as ICESat, LVIS and ATM, with the multi-beam system proposed for ICESat-2, we investigate differences in measurements gathered at varying spatial resolutions and the impact on sea-ice freeboard. We assess the ability of each system to reproduce the elevation distributions of two seaice models and discuss potential biases in lead detection and sea-surface elevation, arising from variable footprint size and spacing. The conventional systems accurately reproduce mean freeboard over 25km length scales, while ICESat-2 offers considerable improvements over its predecessor ICESat. In particular, its dense along-track sampling of the surface will allow flexibility in the algorithmic approaches taken to optimize the signal-to-noise ratio for accurate and precise freeboard retrieval.
Weiffenbach, G. C.
The parameters necessary for obtaining a 10 cm accuracy for GEOS-C satellite altimetry are outlined. These data include oceanographic parameters, instrument calibration, pulse propagation, sea surface effects, and optimum design.
Anderson, Allen Joel; Sandwell, David T.
A detailed gravity field map of the mid Arctic Ocean, spreading ridge system was produced on the basis of ERS-1 satellite altimetry data. Areas of special concern, the Barents and Kara Seas, and areas surrounding the islands of Svalbard, Frans Josef Land and Novoya Zemlya are reviewed. ERS-1 altimetry covers unique Arctic and Antarctic latitudes above 72 degrees. Before ERS-1 it was not possible to study these areas with satellite altimetry. Gravity field solutions for the Barents Sea, portions of the Arctic Ocean and the Norwegian sea are shown. The largest gravity anomalies occur along the Greenland fracture zone as well as along transform faults near Svalbard.
Wade, Ian P.; Heywood, Karen J.
The PRIME cruise to the North Atlantic during June/July 1996 surveyed and sampled an extremely vigorous and deep-reaching eddy with a significant barotropic component. Although it exhibited anticyclonic flow and featured a warm core at depth, it had been capped at some point during its lifetime, so appeared as a cold feature in the upper 500 m. Satellite-derived sea-surface temperatures (SST) showed it to have moved little during the few weeks prior to the cruise. In this paper we discuss the origin of the PRIME eddy including where and when it is likely to have formed. Consistently large amounts of cloud cover restrict the use of SST imagery to track such features. Altimetry provides a better method to trace this eddy back in time and space since microwave radiation is not significantly affected by cloud cover. Sea-level anomaly (SLA) data from the TOPEX/POSEIDON and European Remote Sensing (ERS) satellites were used. Results show that the eddy remained almost stationary in the Iceland Basin since first being detected in late 1995 and that it almost certainly formed locally, probably as a result of an instability in the current flow around the northwest of the Hatton Bank. Comparisons between satellite SLAs and hydrographic estimates of sea-surface elevation confirm that the eddy had a substantial barotropic flow. Both the altimeter data and the sea-surface height derived from the acoustic Doppler current profiler agree that the PRIME eddy had a sea-surface elevation of about 20 cm and that its diameter was about 120 km.
Smith, R. G.; Salloway, M. K.; Berry, P. A. M.; Dowson, M.; Hahn, S.; Wagner, W.; Egibo, A.; Benveniste, J.
Soil surface moisture is a key scientific parameter; however, it is extremely difficult to measure remotely, particularly in arid and semi-arid terrain. This paper outlines the development of a novel methodology to generate soil moisture estimates in these regions from multi-mission satellite radar altimetry. Key to this approach is the development of detailed DRy Earth ModelS (DREAMS), which encapsulate the detailed and intricate surface brightness variations over the Earth's land surface, resulting from changes in surface roughness and composition. These DREAMS are complicated to build and require multiple stages of processing and manual intervention. However, this approach obviates the requirement for detailed ground truth to populate theoretical models, facilitating derivation of surface soil moisture estimates over arid regions, where detailed survey data are generally not available. DREAMS have been produced over a number of deserts worldwide and a selection are presented in this paper. An overview of the SMALT processing scheme, covering the progression of the data from altimeter sigma0 through to final soil moisture estimate, is included along with example SMALT products. In order to validate these products comparisons with other remote sensing techniques and in-situ data have been performed over a number of desert regions. SMALT products are made freely available to the scientific community through the website http://tethys.eaprs.cse.dmu.ac.uk/SMALT
Sirota, A.; Lebedev, S.; Burykin, S.; Timokhin, E.; Chernyshkov, P.
The three oceanic regions were considered: Mid Atlantic Ridge (North Atlantic), Canary Upwelling Region (Eastern Central Atlantic), and Southeastern Pacific. Both satellite altimetry data (TOPEX/Poseidon, ERS -1, 2) and in situ measurements (oceanographic surveys) demonstrated good correlation between these two different types of data in revealing of dynamic features at the ocean surface. The main dynamic features in the regions are: Sub-Polar Front and North Atlantic Current (Mid Atlantic Ridge), Canary Current and coastal upwelling (Eastern Central Atlantic), Sub- Tropical Front and South Pacific Current (Southeastern Pacific). Analysis of distribution, abundance and biological state of various fish species revealed the links between organisms and their dynamic environmental conditions in the considered regions. Variability of the distribution and abundance of rock grenadier over Mid Atlantic Ridge is closely connected to variations of Sub- Polar Front location. Distribution of small pelagic fish in Canary Upwelling Region is influenced by mesoscale features of Canary Current and coastal upwelling. Sub- Tropical Front meandering and eddies in Southeast Pacific influence significantly horse mackerel distribution.
Ocean circulation is a critical factor in determining the Earth's climate. Satellite altimetry has been proven a powerful technique for measuring the height of the sea surface for the study of global ocean circulation dynamics. A major objective of my research is to investigate the utility of altimeter data for ocean circulation studies. The 6 years' data record of TOPEX/POSEIDON have been analyzed to study the spatial and temporal characteristics of large-scale ocean variability. A major result obtained in 1998 is the discovery of large-scale oscillations in sea level with a period of 25 days in the Argentine Basin of the South Atlantic Ocean (see diagram). They exhibit a dipole pattern with counterclockwise rotational propagation around the Zapiola Rise (centered at 45S and 317E), a small seamount in the abyssal plain of the basin. The peak-to-trough amplitude is about 10 cm over a distance of 500-1000 km. The amplitude of these oscillations has large seasonal-to-interannual variations. The period and rotational characteristics of these oscillations are remarkably similar to the observations made by two current meters deployed near the ocean bottom in the region. What TOPEX/POSEIDON has detected apparently are manifestations of the movement of the entire water column (barotropic motion). The resultant transport variation is estimated to be about 50 x 10(exp 6) cubic M/S, which is about 50% of the total water transport in the region. Preliminary calculations suggest that these oscillations are topographically trapped waves. A numerical model of the South Atlantic is used to investigate the nature of and causes for these waves. A very important property of sea surface height is that it is directly related to the surface geostrophic velocity, which is related to deep ocean circulation through the density field. Therefore altimetry observations are not only useful for determining the surface circulation but also for revealing information about the deep ocean. Another
Pearlman, Michael R.
Satellite Laser Ranging (SLR) is currently providing precision orbit determination for measurements of: 1) Ocean surface topography from satellite borne radar altimetry, 2) Spatial and temporal variations of the gravity field, 3) Earth and ocean tides, 4) Plate tectonic and regional deformation, 5) Post-glacial uplift and subsidence, 6) Variations in the Earth's center-of-mass, and 7) Variations in Earth rotation. SLR also supports specialized programs in time transfer and classical geodetic positioning, and will soon provide precision ranging to support experiments in relativity.
Smith, David E.; Zuber, Maria T.
Laser altimeters are presently operating on spacecraft at Mars (MOLA), at the asteroid 433 Eros (NLR), and an earlier system operated at the Moon (Clementine) several years ago. These systems have all advanced our understanding of the evolution of the primary body and several more laser altimeter systems will be launched in the next several years around Earth and other planets to address a wide range of scientific problems. Laser technology for precision altimetry and atmospheric lidar is still in its infancy but the promise of the technology and its demonstrated results already show that laser altimetry/lidar will play an important role in future space observations. To date, lasers have mapped the Moon, Mars, and an asteroid but in a short while they will help measure the planetary librations of Mercury, the tidal distortions of Europa, and tree heights, upper atmosphere winds and the icecaps of planet Earth. Major areas of interest for the immediate future are the development of long-life lasers that can withstand the rigors of long planetary missions in extreme thermal and radiation environments and continue to operate successfully for many years.
Yu, Anthony W.; Krainak, Michael A,; Harding, David J.; Abshire, James B.; Sun, Xiaoli; Cavanaugh, John; Valett, Susan
In this paper we will discuss our eighteen-month progress of a three-year Instrument Incubator Program (IIP) funded by NASA Earth Science Technology Office (ESTO) on swath mapping laser altimetry system. This paper will discuss the system approach, enabling technologies and instrument concept for the swath mapping laser altimetry.
The Arctic sea ice cover is most sensitive to climate change and variability, mainly due to the ice-albedo feedback effect. With an increase in the average temperature across the Arctic during the past few decades, sea ice has been melting rapidly. The decline in the sea ice extent was estimated as 10% per decade since satellite observations began in 1979. Sea ice thickness is an important parameter that moderates the heat exchange between the ocean and the atmosphere, extent of sea ice deformation and sea ice circulation in the Arctic Ocean. In addition, sea ice thermodynamics and dynamics depend on the thickness of the sea ice cover. In order to estimate the trend in the sea ice volume, both the extent and thickness must be known. Hence, it is important to measure the sea ice freeboard (a representative fraction of the thickness) distribution in the Arctic Ocean. In this thesis, the total ice freeboards (height of the snow/ice surface above the sea level) were derived from satellite laser altimetry. NASA's Ice Cloud and Land Elevation Satellite (ICESat) carries a Geoscience Laser Altimetry System (GLAS) onboard, and provides dense coverage of snow (or sea ice) surface heights in the Arctic Ocean up to 86° N. The total freeboard height at each ICESat footprint location was computed by removing the instantaneous sea surface height from the ice/snow surface height. In this study, the instantaneous sea surface heights were modeled using a combination of geodetic and oceanographic models. In order to improve the accuracy of the freeboard estimation, an accuracy assessment of the ocean tide models (one of the component models in the sea surface height estimation) in the Arctic Ocean was performed. The Arctic Ocean Tide Inverse Model (AOTIM-5) was found to have the best accuracy in the Arctic Ocean and was, therefore, used in the sea ice freeboard estimation. It was also shown that the present generation of ocean tide models have ignored the ice-tide interaction
Novak, P.; Sprlak, M.; Hamackova, E.
This contribution presents new integral-based estimators for evaluation of gravitational gradients at satellite altitudes from ground values of the disturbing gravitational potential derived from satellite sea surface altimetry. The estimators are based on surface integral equations of Green's type. The respective tensor-valued Green's function is derived in both spectral and spatial forms and its spatio-spectral properties are studied and discussed. Computer implementation of the new apparatus is based on truncated spherical integration due to spatially limited altimetry data with truncation errors evaluated by a spherical harmonic series from available global gravitational models. The algorithm was validated using synthetic data in closed-loop tests which were also used for propagation of data errors through spatially restricted surface integration. Moreover, the effect of omission and commission errors associated with global gravitational models used for evaluation of truncation errors were also estimated. These studies prove that spatially restricted altimetry data with the 10 cm white noise and truncation errors derived from GRACE-based global gravitational models result in estimation of satellite gravitational gradients with the 1 mE level accuracy. The new estimators were applied for validation of actual satellite gravitational gradients measured by the GOCE gradiometer. As input sea surface altimetry data DTU10MSS (corrected by mean dynamic ocean topography) and the GRACE-based global gravitational model GGM05S were used. Gravitational gradients estimated by the new apparatus were compared with GOCE observations and respective differences were spectrally analyzed. Results of the analyses show a large potential of the new algorithms in connection with available altimetry data for validation and calibration of satellite gravitational gradients.
Andersen, O. B.; Cheng, Y.; Deng, X.; Steward, M.; Gharineiat, Z.
The combination of the coarse temporal sampling by satellite altimeters in the deep ocean with the high temporal sampling at sparsely located tide gauges along the coast has been used to improve the forecast of high water for the North Sea along the Danish Coast and for the northeast coast of Australia. For both locations we have tried to investigate the possibilities and limitations of the use of satellite altimetry to capture high frequency signals (surges) using data from the past 20 years. The two regions are chosen to represent extra-tropical and tropical storm surge conditions. We have selected several representative high water events on the two continents based on tide gauge recordings and investigated the capability of satellite altimetry to capture these events in the sea surface height data. Due to the lack of recent surges in the North Sea we focused on general high water level and found that in the presence of two or more satellites we could capture more than 90% of the high water sea level events. In the Great Barrier Reef section of the northeast Australian coast, we have investigated several large tropical cyclones; one of these being Cyclone Larry, which hit the Queensland coast in March 2006 and caused both loss of lives as well as huge devastation. Here we demonstrate the importance of integrating tide gauges with satellite altimetry for forecasting high water at the city of Townsville in northeast Australia.
Kallenbach, R.; Koch, C.; Christensen, U.; Hilchenbach, M.; Michaelis, H.; Kracht, D.
Laser altimetry is a powerful tool to map planetary surfaces. In addition to the static topography, time-dependent variations such as libration and tidal elevation can be extracted from laser altimeter data in order to investigate the internal structure of the planetary body. In the frame of the BepiColombo Laser Altimeter project, simulations on the extraction of the tidal amplitude on Mercury's surface due to the solar gravitation have been carried out. Based on these results, we evaluate the instrument requirements for a laser altimeter that orbits Jupiter's moon Europa. The tidal bulges of Europa's ice crust should be as high as 30 m, if there is a subsurface ocean, but less than 1 m, if there is solid ice all the way down to the bedrock. The measurement precision achievable with an altimeter applying a miniaturized diode laser-pumped Nd:YAG laser and a single photon counting technique is explored, and the potentials of the integration of the laser altimeter with a high-resolution camera are discussed.
Kaula, W. M.; Schubert, G.; Lingenfelter, R. E.; Sjogren, W. L.; Wollenhaupt, W. R.
About 4.5 revolutions of laser altimetry were obtained by Apollo 15. This altimetry indicates a 2-km displacement of the center of mass from the center of figure toward the earthside. The terrae are quite rough, with frequent changes of 1 km or more in successive altitudes at about 33-km intervals. The mean altitude of terrae above maria is about 3 km with respect to the center of mass, indicating a thickness of about 24 km for a high-alumina crust. The maria are extremely level, with elevations varying not more than plus or minus 150 m about the mean over some stretches of 200 to 600 km. However, different maria have considerably different mean elevations. The largest unanticipated feature found is a 1400 km wide depression centered at about 180 deg longitude, and 2 km deep with respect to a 1737-km sphere (about 6 km deep with respect to the surrounding terrae). This basin has the appearance of typical terrae, although there are indications of a ring structure of about 600-km radius in the Orbiter photography. Altitudes across circum-Orientale features suggest that Mare Orientale is also a deep basin. The data appear to corroborate a model of early large-scale differentiation of a crust, followed a considerable time later by short intense episodes of mare filling with low viscosity lavas.
Parsons, C. L.
The results of three generations of satellite-borne radar altimetry experiments are summarized. The diverse measurements possible from this instrument are shown to be directly applicable to studies of the importance of the oceans in climate. The radar altimeter has unique value for investigations seeking knowledge of the interconnections between ocean dynamics, heat and momentum transfer across the air-sea interface, sea ice extent, and polar ice sheet thickness.
Mather, R S; Rizos, C; Coleman, R
The Geodynamics Experimental Ocean Satellite (GEOS-3) radar altimeter has provided some information on the dynamic sea-surface topography of the global oceans. Regional studies of the densely surveyed Sargasso Sea indicate that the average nontidal variability of the oceans is +/- 28 centimeters. Sea-surface highs and lows determined from GEOS-3 altimetry correlate favorably with eddy structures inferred from Nimbus-6 infrared imagery. PMID:17778877
Todorova, S.; Hobiger, T.; Schuh, H.
The ionosphere is a dispersive medium for the observables of all space geodetic techniques operating in the microwave band such as the Global Positioning System GPS and satellite altimetry missions When the signals pass trough the ionosphere both their group and phase velocity are disturbed The effect is in first approximation proportional to the Slant Total Electron Content STEC along the ray path Thus observations carried out on two distinct frequencies can be used to obtain information about the TEC values This study aims at the development of precise Global Ionosphere Maps GIMs by combination of various space geodetic techniques As a basis for the combined model global ionosphere maps from GPS data in two hours intervals have been created The inhomogeneous distribution of the GPS stations can be partly compensated by adding satellite altimetry data The combination is done by applying a least-squares adjustment Gauss-Markov model on each set of observations and then combining the normal equations by adding the relevant matrices The integrated ionosphere model is expected to be more accurate and reliable than the results derived by the two individual methods Some first tests of the combination of GPS GIMs with altimetry data will be shown
Mather, R. S.; Rizos, C.; Morrison, T.
Techniques are described for determining the height of Mean Sea Level (MSL) at coastal sites from satellite altimetry. Such information is of value in the adjustment of continental leveling networks. Numerical results are obtained from the 1977 GEOS-3 altimetry data bank at Goddard Space Flight Center using the Bermuda calibration of the altimeter. Estimates are made of the heights of MSL at the leveling datums for Australia and a hypothetical Galveston datum for central North America. The results obtained are in reasonable agreement with oceanographic estimates obtained by extrapolation. It is concluded that all gravity data in the Australian bank AUSGAD 76 and in the Rapp data file for central North America refer to the GEOS-3 altimeter geoid for 1976.0 with uncertainties which do not exceed + or - 0.1 mGal.
Brooks, R.L.; Campbell, W.J.; Ramseier, R.O.; Stanley, H.R.; Zwally, H.J.
The surface elevation of the southern Greenland ice sheet and surface features of the ice flow are obtained from the radar altimeter on the GEOS 3 satellite. The achieved accuracy in surface elevation is ???2 m. As changes in surface elevation are indicative of changes in ice volume, the mass balance of the present ice sheets could be determined by repetitive mapping of the surface elevation and the surface could be monitored to detect surging or significant changes in ice flow. ?? 1978 Nature Publishing Group.
Lazarewicz, A.P.; Schwank, D.C.
The topography of the marine geoid (and corresponding sea surface) contains characteristic local features caused by sediments. These features can be successfully detected and located using matched filters to process single tracks of satellite altimeter data. Comparison of detected seamount features with the SNYBAPS and Scripps bathymetric data bases can reveal uncharted seamounts. This technique has been applied to 33 Seasat tracks in a region of the western Pacific bounded by 0 to 15 degrees North and 160 and 165 degrees East. From this analysis, we find three uncharted seamounts in this region. In all three cases, a detailed examination of the bathymetry shows no known bathymetric feature consistent with the detected signature. The method used to estimate the size and location of these uncharted seamounts is discussed.
Cartwright, David E.; Ray, Richard D.
As a novel approach to computing the flux of tidal power into shelf areas, tidal maps of an oceanic area near the Patagonian Shelf are derived from 11 months of altimetry records from the GEOSAT satellite. Power fluxes are computed from the maps through Laplace's tidal equations. Flux vectors for M2 clearly show a convergence on to the southern portion of the shelf sea and their total is nearly twice the loss computed by Miller for that area. A decrease of 'quality factor' with frequency from M2 to S2 is in keeping with Webb's hypothesis of shelf resonances at frequencies a little higher than the tidal band.
Most of the kinetic energy of ocean circulation is contained in the ubiquitous mesoscale eddies. Their prominent signatures in sea surface height have rendered satellite altimetry highly effective in observing global ocean eddies. Our knowledge of ocean eddy dynamics has grown by leaps and bounds since the advent of satellite altimetry in the early 1980s. A brief review of recent progress will be presented. Notwithstanding the tremendous progress made from the existing observations, the limited resolution has prevented us from studying variability at wavelengths shorter than 50-100 km, where important eddy processes take place, ranging from energy dissipation to mixing and transport of water properties that are critical to understanding the roles of ocean in climate. The technology of radar interferometry promises to make wide-swath measurement of sea surface height at a resolution that is able to resolve eddy structures down to 10-25 km. This approach holds the potential to meet the challenge of extending the observations to the submesoscale and set a standard for future altimetric measurement of the ocean.
Csatho, B. M.; Schenk, A. F.; van der Veen, C. J.; Krabill, W. B.
The ability to predict rates of global climatic change, melting ice, and rising seas through the next century relies on an accurate understanding and modeling of glacier and ice-sheet behavior. To quantify ice sheet mass balance, investigate dynamic behavior and to improve predictive ice-sheet models, accurate seasonal, annual and inter-annual elevation changes are of paramount importance. Starting in 1978 an ever-increasing fleet of satellites are monitoring the polar ice sheets. These measurements as well as elevations from NASA’s Airborne Topographic Mapper (ATM) laser altimeter campaigns provide estimates of ice sheet volume changes and mass balance. However, comparison of mass balance estimates derived from these data reveal some glaring differences. The bias between different estimates can be attributed to various factors, for example uncertainties in firn-compaction rates, preferential sampling of local high points by radar altimetry or errors introduced by the interpolation of sparse laser altimetry observations. Moreover change detection methods have significant difficulty to estimate changes over rugged, steep slopes, especially when repeat measurements not perfectly overlap. We have developed a new, comprehensive method, called Surface Elevation Reconstruction And Change detection (SERAC), which determines surface changes by a simultaneous reconstruction of surface topography. The method is based on fitting analytical functions to laser points within repeat tracks or cross-over areas for estimating the ice sheet surface topography. The mathematical model of the change detection algorithm is based on the assumption that for a small surface area, e.g. 1 km by 1 km, only the absolute elevation changes over time but not the shape of the surface patch. Therefore, laser points of all time epochs of a small surface patch contribute to the shape parameters, and the laser points of each time period determine the absolute elevation of the surface patch at that
Alizadeh, M. Mahdi; Schuh, Harald
Among different systems remote sensing the ionosphere, space geodetic techniques have turned into a promising tool for monitoring and modeling the ionospheric parameters. Due to the fact that ionosphere is a dispersive medium, the signals travelling through this medium provide information about the parameters of the ionosphere in terms of Total Electron Content (TEC) or electron density along the ray path. The classical input data for development of Global Ionosphere Maps (GIM) of the Vertical Total Electron Content (VTEC) is obtained from the dual-frequency Global Navigation Satellite Systems (GNSS) ground-based observations. Nevertheless due to the fact that GNSS ground stations are in-homogeneously distributed with poor coverage over the oceans (namely southern Pacific and southern Atlantic) and also parts of Africa, the precision of VTEC maps are rather low in these areas. From long term analyses it is believed that the International GNSS Service (IGS) VTEC maps have an accuracy of 1-2 TECU in areas well covered with GNSS receivers; conversely, in areas with poor coverage the accuracy can be degraded by a factor of up to five. On the other hand dual-frequency satellite altimetry missions (such as Jason-1&2) provide direct VTEC values exactly over the oceans, and furthermore the Low Earth Orbiting (LEO) satellites such as the Formosat-3/COSMIC (F/C) provide about a great number of globally distributed occultation measurements per day, which can be used to obtain VTEC values. Combining these data with the ground-based data improves the accuracy and reliability of the VTEC maps by closing of observation gaps that arise when using ground-based data only. In this approach an essential step is the evaluation and calibration of the different data sources used for the combination procedure. This study investigates the compatibility of calibrated TEC observables derived from GNSS dual-frequency data, recorded at global ground-based station networks, with space-based TEC
Andersen, Ole B.; Cheng, Y.; Deng, X.; Steward, M.; Gharinerat, Z.
Integrating coarse temporal sampling by the satellite altimeter in the deep ocean with the high temporal sampling at tide gauges in sparse location along the coast has been used to improve the forecast of high water in the North Sea along the Danish Coast and storm surges along the Northeast coast of Australia. Along with satellite altimetric data, we have tried to investigate high frequency signals (surges) using data from the past 20 years to investigate existence of ability to capture surges in the regions. We have selected several representative high water events on the two continents based on tide gauge recordings and investigated the capability of the satellite altimeters to capture these in the sea surface height. On the European coast we find that when two or more satellites are available we capture more than 90% of the extreme sea level events. In the Great Barrier Reef section of the Northeast Australia, we have investigated several large cyclones causing much destruction when they hit the coast. One of these being the Cyclone Larry, which hit the Queensland coast in March 2006 and caused both losses of lives as well as huge devastation. Here we demonstrate the importance of integrating tide gauges with satellite altimetry for forecasting high water at the city of Townville in North East Australia.
Maillard, Philippe; Calmant, Stéphane
Radar-based satellite altimetry is a well recognized measuring technique with good precision for oceanographic applications. For continental hydrology, its use is complicated by a number of factors such as river width, satellite crossing angle and noise from the river banks or islands. These factors make precision vary significantly. The satellite crossing points can be made into virtual gauging stations that can complement the existing network of in situ stations. This article describes a series of spatially explicit processing to correct or exclude altimetry measurements not related to the water level. While some processing take advantage of a priori information such as the centerline of the river, other processing are based on pattern recognition to characterize the shape described by the sequence of points. These problems are dealt with by fitting a second degree polynomial curve to the sequence of points and characterizing its shape. The correction is applied by determining a weight for each point in the crossing sequence of measurements. These processing approaches have been combined into a single tool called VHSTOOL. The method is tested on a 1000 km stretch of the S˜ao Francisco River in Brazil. Data from Envisat cover the 2003-2010 period while the recently launched Altika sensor provided data for a few months in 2013. Results show that the average accuracy of 60 cm obtained (45 cm by removing outliers) is comparable to that of completely manual methods. Altika measurements could not be validated since no recent in situ data was available but initial evaluation suggests increased details should bring some improvements over Envisat data.
Webb, C. E.; Neumann, T.; Markus, T.
Despite technical challenges encountered after its launch in 2003, NASA's original Ice, Cloud and land Elevation Satellite (ICESat) produced a rich topographic record, and provided our first large-scale assessments of elevation change and mass balance of the polar ice sheets. The lessons learned from this mission, combined with the availability of new technologies, have guided the design and development of the follow-on ICESat-2 mission and its Advanced Topographic Laser Altimeter System (ATLAS). Scheduled for launch in 2017, ICESat-2 will operate year-round, at a lower orbit inclination, extending coverage to +/- 88 degrees latitude, and at a lower altitude, yielding 1,387 revolutions in a 91-day repeat ground track. The ATLAS instrument uses photon-counting detectors to record surface returns from six laser beams, grouped into three pairs, yielding denser spatial coverage and enabling direct measurements of local slopes. As a result, ICESat-2 will provide a more detailed view of the Earth's surface. Here, we discuss the mission design and concepts of operations. We focus primarily on the strategies being developed for collecting altimetry data over different surfaces, including the ice sheets, sea ice, oceans, vegetation and other scientific targets of opportunity.
Schenk, T.; Csatho, B. M.; Duncan, K.
During the last two decades surface elevation data have been gathered over the Greenland Ice Sheet (GrIS) from a variety of different sensors including spaceborne and airborne laser altimetry, such as NASA's Ice Cloud and land Elevation Satellite (ICESat), Airborne Topographic Mapper (ATM) and Laser Vegetation Imaging Sensor (LVIS), as well as from stereo satellite imaging systems, most notably from Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Worldview. The spatio-temporal resolution, the accuracy, and the spatial coverage of all these data differ widely. For example, laser altimetry systems are much more accurate than DEMs derived by correlation from imaging systems. On the other hand, DEMs usually have a superior spatial resolution and extended spatial coverage. We present in this paper an overview of the SERAC (Surface Elevation Reconstruction And Change detection) system, designed to cope with the data complexity and the computation of elevation change histories. SERAC simultaneously determines the ice sheet surface shape and the time-series of elevation changes for surface patches whose size depends on the ruggedness of the surface and the point distribution of the sensors involved. By incorporating different sensors, SERAC is a true fusion system that generates the best plausible result (time series of elevation changes) a result that is better than the sum of its individual parts. We follow this up with an example of the Helmheim gacier, involving ICESat, ATM and LVIS laser altimetry data, together with ASTER DEMs.
Abdalati, Waleed; Zwally, H. Jay; Bindschadler, Robert; Csatho, Bea; Farrell, Sinead Louise; Fricker, Helen Amanda; Harding, David; Kwok, Ronald; Lefsky, Michael; Markus, Thorsten; Marshak, Alexander; Neumann, Thomas; Palm, Stephen; Schutz, Bob; Smith, Ben; Spinhirne, James; Webb, Charles
Satellite and aircraft observations have revealed that remarkable changes in the Earth s polar ice cover have occurred in the last decade. The impacts of these changes, which include dramatic ice loss from ice sheets and rapid declines in Arctic sea ice, could be quite large in terms of sea level rise and global climate. NASA's Ice, Cloud and Land Elevation Satellite-2 (ICESat-2), currently planned for launch in 2015, is specifically intended to quantify the amount of change in ice sheets and sea ice and provide key insights into their behavior. It will achieve these objectives through the use of precise laser measurements of surface elevation, building on the groundbreaking capabilities of its predecessor, the Ice Cloud and Land Elevation Satellite (ICESat). In particular, ICESat-2 will measure the temporal and spatial character of ice sheet elevation change to enable assessment of ice sheet mass balance and examination of the underlying mechanisms that control it. The precision of ICESat-2's elevation measurement will also allow for accurate measurements of sea ice freeboard height, from which sea ice thickness and its temporal changes can be estimated. ICESat-2 will provide important information on other components of the Earth System as well, most notably large-scale vegetation biomass estimates through the measurement of vegetation canopy height. When combined with the original ICESat observations, ICESat-2 will provide ice change measurements across more than a 15-year time span. Its significantly improved laser system will also provide observations with much greater spatial resolution, temporal resolution, and accuracy than has ever been possible before.
Desai, Shailen; Wahr, John; Beckley, Brian
Satellite altimeter sea surface height observations include the geocentric displacements caused by the pole tide, namely the response of the solid Earth and oceans to polar motion. Most users of these data remove these effects using a model that was developed more than 20 years ago. We describe two improvements to the pole tide model for satellite altimeter measurements. Firstly, we recommend an approach that improves the model for the response of the oceans by including the effects of self-gravitation, loading, and mass conservation. Our recommended approach also specifically includes the previously ignored displacement of the solid Earth due to the load of the ocean response, and includes the effects of geocenter motion. Altogether, this improvement amplifies the modeled geocentric pole tide by 15 %, or up to 2 mm of sea surface height displacement. We validate this improvement using two decades of satellite altimeter measurements. Secondly, we recommend that the altimetry pole tide model exclude geocentric sea surface displacements resulting from the long-term drift in polar motion. The response to this particular component of polar motion requires a more rigorous approach than is used by conventional models. We show that erroneously including the response to this component of polar motion in the pole tide model impacts interpretation of regional sea level rise by ± 0.25 mm/year.
Paris, A.; Calmant, S.; Paiva, R. C.; Collischonn, W.; Silva, J. S.; Bonnet, M.; Seyler, F.
The Amazonian basin is the largest hydrological basin all over the world. In the recent past years, the basin has experienced an unusual succession of extreme draughts and floods, which origin is still a matter of debate. Yet, the amount of data available is poor, both over time and space scales, due to factor like basin's size, access difficulty and so on. One of the major locks is to get discharge series distributed over the entire basin. Satellite altimetry can be used to improve our knowledge of the hydrological stream flow conditions in the basin, through rating curves. Rating curves are mathematical relationships between stage and discharge at a given place. The common way to determine the parameters of the relationship is to compute the non-linear regression between the discharge and stage series. In this study, the discharge data was obtained by simulation through the entire basin using the MGB-IPH model with TRMM Merge input rainfall data and assimilation of gage data, run from 1998 to 2010. The stage dataset is made of ~800 altimetry series at ENVISAT and JASON-2 virtual stations. Altimetry series span between 2002 and 2010. In the present work we present the benefits of using stochastic methods instead of probabilistic ones to determine a dataset of rating curve parameters which are consistent throughout the entire Amazon basin. The rating curve parameters have been computed using a parameter optimization technique based on Markov Chain Monte Carlo sampler and Bayesian inference scheme. This technique provides an estimate of the best parameters for the rating curve, but also their posterior probability distribution, allowing the determination of a credibility interval for the rating curve. Also is included in the rating curve determination the error over discharges estimates from the MGB-IPH model. These MGB-IPH errors come from either errors in the discharge derived from the gage readings or errors in the satellite rainfall estimates. The present
Potter, R. C. H.; Laxon, S. W.; Peacock, N.
The spatial and temporal variability of Antarctic sea-ice extent and thickness are re- quired by the climate modelling community to understand the complex coupling be- tween sea ice and ocean-atmosphere. Recent investigations have provided estimates of Arctic sea ice thickness determined from satellite radar altimetry, which have been validated using Upward Looking Sonar data from submarines. In the present study, we aim to explore the potential for estimation of the thickness of Antarctic sea ice. In the boreal summer, the Arctic Ocean is pre-dominantly inhabited by old, multiyear sea-ice that may survive through each melt season and subsequently refreeze and in- crease in thickness under autumn cooling and winter growth. However, aside from the Weddell Sea, the Antarctic sea ice is subject to almost complete seasonal melt/freeze with the formation of pre-dominant first year ice. To estimate ice thickness from mea- surements of ice elevation it is necessary to establish the vertical origin of the radar echo's received over snow covered ice. Assuming reflection originates at the snow/ice interface, the ice freeboard can be estimated. The freeboard is converted to ice thick- ness using fixed densities for ice and seawater and a recently generated Antarctic Snow depth climatology. However, the developed altimetry techniques are designed for rela- tively thick Arctic sea ice and therefore may not be directly applicable to the relatively thin non-compacted first year Antarctic sea ice. In particular, snow loading owing to high precipitation rates is likely to be much larger compared to ice thickness. In ex- treme conditions this may lead to negative freeboard. Nevertheless, data on Antarctic ice thickness is even more sparse than the Arctic and hence information on thickness would be of significant value. We present preliminary results of Antarctic ice thickness estimates and validation using ULS data.
Roohi, Shirzad; Sneeuw, Nico
Urmia lake is a UNESCO protected area with more than a hundred small rocky islands. It is home to several species of birds and animals. Located in northwestern Iran, it is the largest lake in the Middle East and the third largest salty water lake on earth. It has a surface area of approximately 5200 km², and an average depth of 16 m. Unfortunately during the recently years Urmia lake has been shrinking. If the drought process continues at the current rate it would be disappear in the near future. The main factors that speeds up the drought rate of the lake, are dam construction on the main rivers which feeds the lake, evaporation and lack of precipitation during recent years as well as irrigation. The construction of a causeway in the middle of the lake also affects the natural ecosystem of the lake. The case of Urmia lake and similar cases in other parts of the word emphasize the role of new technology such as satellite altimetry in better management of water resource and monitoring such critical situations. In this research we show the current situation and recent past of the lake from processing 10 years of Envisat satellite radar altimetry data. For internal validation of the result, water level time series were built from ascending and descending tracks separately and for external validation in-situ gauge measurements were used. Internal and external comparisons indicates the result are consistent, i.e there is no bias and systematic error in Envisat data. The RMSE between ascending and descending tracks is several centimeters and between satellite and gauge data is 1m. Water level time series analysis shows that there is a declining rate of 0.3 m/year in the water level but after 2005 it seems to have accelerated. This rate increases the salinity of lake and expands receding shoreline rapidly so the lake bed will reveal fast because the lake is shallow especially in the south part. Following this research we are investigating to find the best re-tracker in
Space-age technologies have made satellite remote sensing a powerful new tool to study the Earth on a global scale. However, the opacity of the ocean to electromagnetic sensing has limited spaceborne measurements to the properties of the surface layer of the ocean (such as sea surface temperature and color). The radar altimetric measurement of the height of the sea surface relative to the geoid, the dynamic topography of the ocean, is a very useful quantity for studying the circulation of the ocean. The ability of measuring dynamic topography from space makes satellite altimetry a uniquely useful remote sensing technique because dynamic topography reflects oceanic processes not only at the surface but at depths as well. A simple analysis shows that a one centimeter tilt in the dynamic topography is associated with a mass transport of 1-7 Sv (1Sv= 1 million tons per second) in the open ocean depending on the vertical distribution of current velocity. Such a magnitude is an appreciable fraction of the transport of the Florida Current (circa 30 Sv), for instance. TOPEX/POSEIDON has demonstrated the capability of measuring the time variation of sea level with accuracy approaching to 2 cm when the data are averaged over boxes with several hundred kilometers on each side. The data set has been used for studying ocean circulation phenomena with a wide range of scales, ranging from fast-changing barotropic variability to seasonal and interannual variability such as El Nino and La Nina. The long record of precise measurement of global sea level has also showed great promise for monitoring the variation of mean sea level, an effective indicator of global climate change. Continuation of satellite altimetry missions with capability matching or better than that of TOPEX/POSEIDON should be included as a key component of a Global Ocean Observing System. NASA and CNES have committed to continuing the measurement of TOPEX/POSEIDON with a series of follow-on missions called Jason
Koblinsky, C. J.; Clarke, R. T.; Brenner, A. C.; Frey, H.
Quantitative assessment of water levels and river discharge is often made difficult by large distances, limited access, and low population densities in remote areas. Satellite altimetry provides a repetitive remote sensing approach to determining river levels at a number of locations within a river system, providing the orbital repeat cycle is short enough in time, the ground track maintains a stable repeat over previous locations, and the return power of the altimeter signal can be readily identified and located. The U.S. Navy's Geosat radar altimeter mission between 1985 and 1989 provided the first altimeter measurements with sufficient precision and extended duration to examine the utility of such measurements for long-term monitoring of inland waters. These measurements have been examined over the Amazon basin. Satellite observations are retrieved at four locations that overlap with river gauge measurements. A technique is developed to isolate radar return signals from the river. Two years of satellite measurements are compared with the river gauge retrievals. The overall level of comparison is 0.7 m rms when the technique is applied manually, and 1.2 m rms when an automated version of the method is applied. At one location the average difference is 0.2 m rms. This level of accuracy may not be useful for routine hydrological measurements. However, there are a variety of difficulties that are specific to the Geosat altimeter measurement over rough terrain. Present altimeter satellites, ERS 1 (launched June 1991) and TOPEX/Poseidon (launched August 1992), correct many of these problems. This study suggests that the prospect for obtaining useful measurements of river level from space is promising.
The trend of sea level change and its geographic pattern present a powerful indicator of the overall extent of climate change as well as pose a long-term threat to the world's heavily populated coastal zones. The global and direct measurement of sea level from satellite altimetry over the past two decades, as a key part of a global observing system, has enabled quantitative determination of sea level change and its relation to natural and human-induced causes. Major results from the data record will be reviewed to highlight the challenges in distinguishing between natural variability and long-term trends from human activities. As long-term climate data records from satellite observations are inevitably to be built from successive missions with progressively changing technologies, a major challenge is concerted effort in cross-calibration to ensure consistency between new measurements with existing records. As the desire of increasing spatial resolution to resolve energetic small-scale variability dictates the development of high-resolution wide-swath altimeter, the need for thoughtful design of a system that is able to demonstrate proper transition of technologies in terms of providing consistent global sea level data record will be discussed.
Glazman, Roman E.; Srokosz, Meric A.
For a well-developed sea at equilibrium with a constant wind, the energy-containing range of the wavenumber spectrum for wind-generated gravity waves is approximated by a generalized power law involving the angular spread function and mu, interpreted as a fractal codimension of a small surface patch. Dependence of mu on the wave age is estimated, and the 'Phillips constant', beta, along with the low-wavenumber boundary, k0, of the inertial subrange are analyzed on the basis of the wave action and energy conservation principles. The resulting expressions are employed to evaluate various non-Gaussian statistics of a weakly nonlinear sea surface, which determine the sea state bias in satellite altimetry. The locally accelerated decay of the spectral density function in a high-wavenumber dissipation subrange is pointed out as an important factor of wave dynamics and the geometrical optics treatment of the sea state bias. The analysis is carried out in the approximation of a unidirectional wave field and confined to the case of a well-developed sea.
Laxon, S; McAdoo, D
The derivation of a marine gravity field from satellite altimetry over permanently ice-covered regions of the Arctic Ocean provides much new geophysical information about the structure and development of the Arctic sea floor. The Arctic Ocean, because of its remote location and perpetual ice cover, remains from a tectonic point of view the most poorly understood ocean basin on Earth. A gravity field has been derived with data from the ERS-1 radar altimeter, including permanently ice-covered regions. The gravity field described here clearly delineates sections of the Arctic Basin margin along with the tips of the Lomonosov and Arctic mid-ocean ridges. Several important tectonic features of the Amerasia Basin are clearly expressed in this gravity field. These include the Mendeleev Ridge; the Northwind Ridge; details of the Chukchi Borderland; and a north-south trending, linear feature in the middle of the Canada Basin that apparently represents an extinct spreading center that "died" in the Mesozoic. Some tectonic models of the Canada Basin have proposed such a failed spreading center, but its actual existence and location were heretofore unknown. PMID:17752757
Sulistioadi, Y. B.; Tseng, K.-H.; Shum, C. K.; Hidayat, H.; Sumaryono, M.; Suhardiman, A.; Setiawan, F.; Sunarso, S.
Remote sensing and satellite geodetic observations are capable of hydrologic monitoring of freshwater resources. Although satellite radar altimetry has been used in monitoring water level or discharge, its use is often limited to monitoring large rivers (>1 km) with longer interval periods (>1 week) because of its low temporal and spatial resolutions (i.e., satellite revisit period). Several studies have reported successful retrieval of water levels for small rivers as narrow as 40 m. However, processing current satellite altimetry signals for such small water bodies to retrieve water levels accurately remains challenging. Physically, the radar signal returned by water bodies smaller than the satellite footprint is most likely contaminated by non-water surfaces, which may degrade the measurement quality. In order to address this scientific challenge, we carefully selected the waveform shapes corresponding to the range measurement resulting from standard retrackers for the European Space Agency's (ESA's) Envisat (Environmental Satellite) radar altimetry. We applied this approach to small (40-200 m in width) and medium-sized (200-800 m in width) rivers and small lakes (extent <1000 km2) in the humid tropics of Southeast Asia, specifically in Indonesia. This is the first study that explored the ability of satellite altimetry to monitor small water bodies in Indonesia. The major challenges in this study include the size of the water bodies that are much smaller than the nominal extent of the Envisat satellite footprint (e.g., ~250 m compared to ~1.7 km, respectively) and slightly smaller than the along-track distance (i.e., ~370 m). We addressed this challenge by optimally using geospatial information and optical remote sensing data to define the water bodies accurately, thus minimizing the probability of non-water contamination in the altimetry measurement. Considering that satellite altimetry processing may vary with different geographical regions, meteorological
Kouraev, A. V.; Legresy, B.; Remy, F.
We presen t resu lts of application of radar altimetry and other satellite techniqu es for studies of th e Austfonna ice cap (Svalbard). We assess spatial and temporal d ata availab ility over Austfonna. Th en we d iscuss temporal variab ility of ENVISA T altimetr ic and radio metric measures. We also discuss potential of combination of altimetry and Dig ital Elev ation Models (D EM) for DEM improvement and interpretation of altimeter measurements.
Schröder, Ludwig; Richter, Andreas; Fedorov, Denis; Knöfel, Christoph; Ewert, Heiko; Dietrich, Reinhard; Matveev, Aleksey Yu.; Scheinert, Mirko; Lukin, Valery
Satellite altimetry is a unique technique to observe the contribution of the Antarctic ice sheet to global sea-level change. To fulfill the high quality requirements for its application, the respective products need to be validated against independent data like ground-based measurements. Kinematic GNSS provides a powerful method to acquire precise height information along the track of a vehicle. Within a collaboration of TU Dresden and Russian partners during the Russian Antarctic Expeditions in the seasons from 2001 to 2013 we recorded several such profiles in the region of the subglacial Lake Vostok, East Antarctica. After 2006 these datasets also include observations along seven continental traverses with a length of about 1600km each between the Antarctic coast and the Russian research station Vostok (78° 28' S, 106° 50' E). After discussing some special issues concerning the processing of the kinematic GNSS profiles under the very special conditions of the interior of the Antarctic ice sheet, we will show their application for the validation of NASA's laser altimeter satellite mission ICESat and of ESA's ice mission CryoSat-2. Analysing the height differences at crossover points, we can get clear insights into the height regime at the subglacial Lake Vostok. Thus, these profiles as well as the remarkably flat lake surface itself can be used to investigate the accuracy and possible error influences of these missions. We will show how the transmit-pulse reference selection correction (Gaussian vs. centroid, G-C) released in January 2013 helped to further improve the release R633 ICESat data and discuss the height offsets and other effects of the CryoSat-2 radar data. In conclusion we show that only a combination of laser and radar altimetry can provide both, a high precision and a good spatial coverage. An independent validation with ground-based observations is crucial for a thorough accuracy assessment.
Gopalapillai, G. S.; Mourad, A. G.
A technique for deriving mean gravity anomalies from dense altimetry data was developed. A combination of both deterministic and statistical techniques was used. The basic mathematical model was based on the Stokes' equation which describes the analytical relationship between mean gravity anomalies and geoid undulations at a point; this undulation is a linear function of the altimetry data at that point. The overdetermined problem resulting from the excessive altimetry data available was solved using Least-Squares principles. These principles enable the simultaneous estimation of the associated standard deviations reflecting the internal consistency based on the accuracy estimates provided for the altimetry data as well as for the terrestrial anomaly data. Several test computations were made of the anomalies and their accuracy estimates using GOES-3 data.
Jasinski, Michael F.; Stoll, Jeremy
Satellite retrievals of snow depth and water equivalent (SWE) are critical for monitoring watershed scale processes around the world. However, the problem is especially challenging in mountainous regions where complex heterogeneities limit the utility of low resolution satellite sensors. The Geoscience Laser Altimeter Sensor (GLAS) aboard the Ice, Cloud, and land Elevation Satellite (ICESat) collected surface elevation data along near-repeat reference transects over land areas from 2003-2009. Although intended for monitoring ice caps and sea ice, the seven year global GLAS data base has provided unprecedented opportunity to test the capability of satellite lidar technology for estimating snow depth over land. GLAS single track and low repeat frequency does not provide data sufficient for operational estimates. However, its comparatively small footprint size of -65 m and its database of seasonal repeat observations during both snow and no-snow conditions have been sufficient to evaluate the potential of spacebased lidar altimetry for estimating snow depth. Recent analysis of ICESat elevations in the Uinta Mountains in NE Utah provide encouraging results for watershed scale estimates of snow depth. Research reported here focuses on the sensitivity of several versions of an ICESat snow depth algorithm to a range of landscape types defined by vegetation cover, slope and roughness. Results are compared to available SNOTEL data.
Schutz, B. E.
The Geoscience Laser Altimeter System (GLAS) was launched on ICESat in January 2003 into a 600 km altitude, near polar orbit from Vandenberg, California. The GLAS instrument has been developed by NASA Goddard and it was mated to the spacecraft bus, built by Ball Aerospace, in June 2003. GLAS laser-1 was activated in orbit on February 20, 2003 and elevation profiles of the Greenland and Antarctic ice sheets, as well as land and ocean profiles, have been produced using the 1064 nm wavelength. The laser pulse has a divergence of about 0.11 mrad, which illuminates a spot on the surface with a 66 m diameter. The 170 m spot separation on the surface is determined by the ICESat orbital motion and the 40 Hz laser pulse repetition rate. Unlike wide pulse radar altimeters, accurate knowledge of the laser beam direction is required for the laser altimeter to produce accurate surface profiles. The laser pointing direction is determined with the assistance of an innovative system of CCD cameras plus calibration/validation methodologies. The combination of the laser pulse round trip time of flight and the pointing determination system provides an altitude vector. Determination of the direction of the altitude vector has an accuracy requirement of about 1.5 arcsec (or about 4.5 m on surface). In addition to the instrumentation required to produce the altitude vector, the position of a GLAS reference point with respect to the center of mass of the Earth is required. The ICESat BlackJack GPS receiver from NASA JPL has enabled determination of the radial component of the orbit to a few centimeter accuracy level. Laser-1 operated in orbit with a ground track that was controlled to within 800 m of an 8-day repeat reference track. During laser-1 operation and after the laser ceased firing on March 29, experiments were conducted to assess and improve the accuracy of off-nadir pointing, a unique capability to enhance the science return. This paper will provide an assessment of the
Poehls, K. A.; Kaula, W. M.; Schubert, G.; Sandwell, D.
Using statistical analysis, geoidal admittance (the relationship between the ocean geoid and seafloor topography) obtained from GEOS-3 altimetry was compared to various model admittances. Analysis of several altimetry tracks in the Pacific Ocean demonstrated a low coherence between altimetry and seafloor topography except where the track crosses active or recent tectonic features. However, global statistical studies using the much larger data base of all available gravimetry showed a positive correlation of oceanic gravity with topography. The oceanic lithosphere was modeled by simultaneously inverting surface wave dispersion, topography, and gravity data. Efforts to incorporate geoid data into the inversion showed that the base of the subchannel can be better resolved with geoid rather than gravity data. Thermomechanical models of seafloor spreading taking into account differing plate velocities, heat source distributions, and rock rheologies were discussed.
Wong, L.; Matthews, E.; Downs, W.
Radar altimetry and satellite-to-satellite (SST) range and range rate tracking measurements were used to infer the exterior gravitational field of the earth and the structure of the geoid from GEOS-C metric data. Under the SST analysis, a direct point-by-point estimate of gravity disturbance by means of a recursive filter with backward smoothing was attempted but had to be forsaken because of poor convergence. The adopted representation consists of a more or less uniform grid of discrete masses at a depth of approximately 400 km from the earth's surface. The layer is superimposed on a spherical harmonics model. The procedure for smoothing the altimetry and inferring the fine-structured gravity field over the Atlantic test area is described. The local disturbances are represented by means of a density layer. The altimeter height biases were first estimated by a least squares adjustment at orbital crossover points. After taking out the bias, long wavelength contributions from GEM-6 as well as a calibration correction were subtracted. The residual heights were then represented by a mass distribution beneath the earth's surface.
Steinbrügge, G.; Stark, A.; Hussmann, H.; Sohl, F.; Oberst, J.
Invaluable information about the interior of icy satellites orbiting close to the giant planets can be gained by monitoring the response of the satellite's surfaces to external tidal forces. Due to its geodetic accuracy, laser altimetry is the method of choice to measure time-dependent radial surface displacements from orbit. We present an instrument performance model with special focus on the capabilities to determine the corresponding tidal Love number h2 and apply the model to the Ganymede Laser Altimeter (GALA) on board of the Jupiter Icy Moons Explorer (JUICE). Based on the instrument and spacecraft performance, we derive the range error and the measurement capabilities of the GALA instrument to determine the amplitude of the tide induced radial displacement of Ganymede's surface using the cross-over technique. We find that h2 of Ganymede can be determined with an accuracy of better than 2% by using data acquired during the nominal mission. Furthermore, we show that this accuracy is sufficient to confirm the presence of a putative subsurface water ocean and, additionally, to constrain the thickness of the overlaying ice shell to ± 20km.
Okeowo, M. A.; Lee, H.; Hossain, F.
Until now, processing satellite radar altimetry data over inland water bodies on a large scale has been a cumbersome task primarily due to contaminated measurements from their surrounding topography. It becomes more challenging if the size of the water body is small and thus the number of available high-rate measurements from the water surface is limited. A manual removal of outliers is time consuming which limits a global generation of reservoir elevation profiles. This has limited a global study of lakes and reservoir elevation profiles for monitoring storage changes and hydrologic modeling. We have proposed a new method to automatically generate a time-series information from raw satellite radar altimetry without user intervention. With this method, scientist with little knowledge of altimetry can now independently process radar altimetry for diverse purposes. The method is based on K-means clustering, backscatter coefficient and statistical analysis of the dataset for outlier detection. The result of this method will be validated using in-situ gauges from US, Indus and Bangladesh reservoirs. In addition, a sensitivity analysis will be done to ascertain the limitations of this algorithm based on the surrounding topography, and the length of altimetry track overlap with the lake/reservoir. Finally, a reservoir storage change will be estimated on the study sites using MODIS and Landsat water classification for estimating the area of reservoir and the height will be estimated using Jason-2 and SARAL/Altika satellites.
Baup, F.; Frappart, F.; Maubant, J.
This study presents an approach to determine the volume of water in small lakes (<100 ha) by combining satellite altimetry data and high-resolution (HR) images. The lake being studied is located in the south-west of France and is only used for agricultural irrigation purposes. The altimetry satellite data are provided by RA-2 sensor on board Envisat, and the high-resolution images (<10 m) are obtained from optical (Formosat-2) and synthetic aperture radar (SAR) sensors (Terrasar-X and Radarsat-2) satellites. The altimetry data (data are obtained every 35 days) and the HR images (45) have been available since 2003 and 2010, respectively. In situ data (for the water levels and volumes) going back to 2003 have been provided by the manager of the lake. Three independent approaches are developed to estimate the lake volume and its temporal variability. The first two approaches are empirical and use synchronous ground measurements of the water volume and the satellite data. The results demonstrate that altimetry and imagery can be effectively and accurately used to monitor the temporal variations of the lake (R2altimetry = 0.97, RMSEaltimetry = 5.2%, R2imagery = 0.90, and RMSEimagery = 7.4%). The third method combines altimetry (to measure the lake level) and satellite images (of the lake surface) to estimate the volume changes of the lake and produces the best results (R2 = 0.99) of the three methods, demonstrating the potential of future Sentinel and SWOT missions to monitor small lakes and reservoirs for agricultural and irrigation applications.
Kudryavtseva, Nadia; Soomere, Tarmo; Giudici, Andrea
Currently, three sources of wave data are available for the research community, namely, buoys, modelling, and satellite altimetry. The buoy measurements provide high-quality time series of wave properties but they are deployed only in a few locations. Wave modelling covers large domains and provides good results for the open sea conditions. However, the limitation of modelling is that the results are dependent on wind quality and assumptions put into the model. Satellite altimetry in many occasions provides homogeneous data over large sea areas with an appreciable spatial and temporal resolution. The use of satellite altimetry is problematic in coastal areas and partially ice-covered water bodies. These limitations can be circumvented by careful analysis of the geometry of the basin, ice conditions and spatial coverage of each altimetry snapshot. In this poster, for the first time, we discuss a validation of 30 years of multi-mission altimetry covering the whole Baltic Sea. We analysed data from RADS database (Scharroo et al. 2013) which span from 1985 to 2015. To assess the limitations of the satellite altimeter data quality, the data were cross-matched with available wave measurements from buoys of the Swedish Meteorological and Hydrological Institute and Finnish Meteorological Institute. The altimeter-measured significant wave heights showed a very good correspondence with the wave buoys. We show that the data with backscatter coefficients more than 13.5 and high errors in significant wave heights and range should be excluded. We also examined the effect of ice cover and distance from the land on satellite altimetry measurements. The analysis of cross-matches between the satellite altimetry data and buoys' measurements shows that the data are only corrupted in the nearshore domain within 0.2 degrees from the coast. The statistical analysis showed a significant decrease in wave heights for sea areas with ice concentration more than 30 percent. We also checked and
Yu, A. W.; Krainak, M. A.; Harding, D. J.; Abshire, J. B.; Sun, X.; Betin, A.; Hastings, T.; Filgas, D. M.; Stultz, R. D.; Wang, J.; Bailey, S.; Jack, M.
NASA Goddard Space Flight Center is in the process of completing the first year effort in a three-year Instrument Incubator Program (IIP) to develop and demonstrate technologies for a next-generation, efficient, swath mapping, space altimeter for Earth science. Our approach will ultimately allows for simultaneous measurements of 5-m spatial resolution topography and vegetation vertical structure with decimeter vertical precision in an elevation-imaging swath several km wide from a 400 km altitude Earth orbit to meet the goals of the Lidar Surface Topography (LIST) mission. Our IIP objective is to develop a highly efficient laser altimeter system that can be housed in a compact instrument providing data products that vastly exceed other instruments in the same class. The ultimate goal of a >15% wall plug efficient laser system coupled with a highly sensitive detector array is essential to realizing the ambitious global elevation mapping goals of the LIST mission. The key attributes of the LIST mission, as described in the NRC Earth Science Decadal Survey report, are: (1) a medium cost mission to be launched by NASA between 2016-2020; (2) a single-instrument payload carrying an imaging lidar at low Earth orbit; (3) one-time global mapping of land, ice sheet and glacier topography and vegetation structure through the duration of the mission; (4) observe topography and vegetation structure change through time in selected areas; and (5) achieve 5 m horizontal resolution, 0.1 m vertical precision, and absolute vertical accuracy for ground surface topography including where covered by vegetation. LIST is recommended as a third tiered mission with launch date no earlier than 2018. To achieve the IIP goals, we will use a single laser with beam dividing network to split a single beam into sixteen channels for mapping purpose. Backscatter from the surface is collected with a telescope and the spots from the swath are imaged onto a sensitive detector array. The output from
Keller, Kristian; Casassa, Gino; Rivera, Andrés; Forsberg, Rene; Gundestrup, Niels
The first airborne laser altimetry measurements of a glacier in South America are presented. Data were collected in November of 2001 over Glaciar Tyndall, Torres del Paine National Park, Chilean Patagonia, onboard a Twin Otter airplane of the Chilean Air Force. A laser scanner with a rotating polygon-mirror system together with an Inertial Navigation System (INS) were fixed to the floor of the aircraft, and used in combination with two dual-frequency GPS receivers. Together, the laser-INS-GPS system had a nominal accuracy of 30 cm after data processing. On November 23rd, a total of 235 km were flown over the ablation area of Glaciar Tyndall, with 5 longitudinal tracks with a mean swath width of 300 m, which results in a point spacing of approximately 2 m both along and across track. A digital elevation model (DEM) generated using the laser altimetry data was compared with a DEM produced from a 1975 map (1:50,000 scale — Instituto Geográfico Militar (IGM), Chile). A mean thinning of - 3.1 ± 1.0 m a - 1 was calculated for the ablation area of Glaciar Tyndall, with a maximum value of - 7.7 ± 1.0 m a - 1 at the calving front at 50 m a.s.l. and minimum values of between - 1.0 and - 2.0 ± 1.0 m a - 1 at altitudes close to the equilibrium line altitude (900 m a.s.l.). The thinning rates derived from the airborne survey were similar to the results obtained by means of ground survey carried out at ˜ 600 m of altitude on Glaciar Tyndall between 1975 and 2002, yielding a mean thinning of - 3.2 m a - 1 [Raymond, C., Neumann, T.A., Rignot, E., Echelmeyer, K.A., Rivera, A., Casassa, G., 2005. Retreat of Tyndall Glacier, Patagonia, over the last half century. Journal of Glaciology 173 (51), 239-247.]. A good agreement was also found between ice elevation changes measured with laser data and previous results obtained with Shuttle Radar Topography Mission (SRTM) data. We conclude that airborne laser altimetry is an effective means for accurately detecting glacier elevation
Khaki, Mehdi; Sneeuw, Nico
Human civilization has always been in evolution by having direct access to water resources throughout history. Water, with its qualitative and quantitative effects, plays an important role in economic and social developments. Iran with an arid and semi-arid geographic specification is located in Southwest Asia. Water crisis has appeared in Iran as a serious problem. In this study we're going to use various data sources including satellite radar altimetry and satellite gravimetry to monitor and investigate water resources in Iran. Radar altimeters are an invaluable tool to retrieve from space vital hydrological information such as water level, volume and discharge, in particular from regions where the in situ data collection is difficult. Besides, Gravity Recovery and Climate Experiment (GRACE) provide global high resolution observations of the time variable gravity field of the Earth. This information is used to derive spatio-temporal changes of the terrestrial water storage body. This study isolates the anthropogenic perturbations to available water supplies in order to quantify human water use as compared to available resources. Long-term monitor of water resources in Iran is contain of observing freshwaters, lakes and rivers as well as exploring ground water bodies. For these purposes, several algorithms are developed to quantitatively monitor the water resources in Iran. The algorithms contain preprocessing on datasets, eliminating biases and atmospheric corrections, establishing water level time series and estimating terrestrial water storage considering impacts of biases and leakage on GRACE data. Our primary goal in this effort is to use the combination of satellite radar altimetry and GRACE data to study on water resources as well as methods to dealing with error sources include cross over errors and atmospheric impacts.
Fubara, D. M. J.; Mourad, A. G.
Theory deficiencies, data, and potential computational procedures that make the physical determination of the ocean geoid with true scale, shape, and absolute orientation of an elusive target are outlined. Satellite altimetry potential, in combination with adequate ground support and sea truth to resolve accurate global marine geoid and other peripheral benefits associated with ocean physics, are stated. Results are given in tabular form.
Farrell, S. L.; Newman, T.; Richter-Menge, J.; Haas, C.; Petty, A.; McAdoo, D. C.; Connor, L. N.
Over the last two decades altimeters on satellite and aircraft platforms have revolutionized our understanding of Arctic sea ice mass balance. Satellite laser and radar altimeters provide unique measurements of sea ice elevation, from which ice thickness may be derived, across basin scales and interdecadal time periods. Meanwhile airborne altimetry, together with high-resolution digital imagery, provides a range of novel observations that describe key features of the ice pack including its snow cover, surface morphology and deformation characteristics. We provide an update on current Arctic sea ice thickness conditions based on IceBridge measurements, discussing these in the context of previously observed decadal change. Fundamental to the goal of understanding interannual variability, and monitoring long-term trends in sea ice volume, is the accurate characterization of measurement uncertainty. This is particularly true when linking observations from different sensors. We discuss recent advances in tracking and quantifying the major components of the altimetric sea ice thickness error budget. We pay particular attention to two major components of the error: freeboard and snow loading uncertainty. We describe novel measurement techniques that are helping to reduce measurement uncertainty and allowing, for the first time, quantification of errors with respect to ice type.
Carabajal, C. C.; Boy, J.-P.
We have used a set of Ground Control Points (GCPs) derived from altimetry measurements from the Ice, Cloud and land Elevation Satellite (ICESat) to evaluate the quality of the 30 m posting ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) Global Digital Elevation Model (GDEM) V3 elevation products produced by NASA/METI for Greenland and Antarctica. These data represent the highest quality globally distributed altimetry measurements that can be used for geodetic ground control, selected by applying rigorous editing criteria, useful at high latitudes, where other topographic control is scarce. Even if large outliers still remain in all ASTER GDEM V3 data for both, Greenland and Antarctica, they are significantly reduced when editing ASTER by number of scenes (N≥5) included in the elevation processing. For 667,354 GCPs in Greenland, differences show a mean of 13.74 m, a median of -6.37 m, with an RMSE of 109.65 m. For Antarctica, 6,976,703 GCPs show a mean of 0.41 m, with a median of -4.66 m, and a 54.85 m RMSE, displaying smaller means, similar medians, and less scatter than GDEM V2. Mean and median differences between ASTER and ICESat are lower than 10 m, and RMSEs lower than 10 m for Greenland, and 20 m for Antarctica when only 9 to 31 scenes are included.
Brenner, Anita; Frey, Herb; DiMarzio, John; Tsaoussi, Lucia
The results of the surface topography mapping of South America during the ERS-1 geodetic mission are presented. The altimeter waveforms, the range measurement, and the internal and Doppler range corrections were obtained. The atmospheric corrections and solid tides were calculated. Comparisons between Shuttle laser altimetry and ERS-1 altimetry grid showed good agreement. Satellite radar altimetry data can be used to improve the topographic knowledge of regions for which only poor elevation data currently exist.
Rudenko, Sergei; Dettmering, Denise; Esselborn, Saskia; Fagiolini, Elisa; Schöne, Tilo
We have extended backwards from 2001 to 1979 the current release 05 (RL05) of the Gravity Recovery and Climate Experiment (GRACE) Atmospheric and Oceanic De-aliasing Level-1B (AOD1B) product and studied the impact of this and a previous release 04 (RL04) of the AOD1B product on precise orbits of five altimetry satellites (ERS-1, ERS-2, TOPEX/Poseidon, Envisat and Jason-1) for the time span 1991-2012, as compared to the case when no AOD1B product is used. We have found that using AOD1B RL05 product reduces root mean square (RMS) fits of satellite laser ranging (SLR) observations by about 1.0-6.4 per cent, 2-d arc overlaps in radial, cross-track and along-track directions by about 1.3-12.0, 0.3-10.0 and 2.0-10.0 per cent, respectively, for various satellites tested, as compared to the case without AOD1B product. Using AOD1B RL05 product instead of RL04 one reduces SLR RMS fits by 0.1-0.7 per cent, 2-d arc overlaps in radial, cross-track and along-track directions by 0.1-0.6, 0.1-1.3 and 0.2-1.2 per cent, respectively, for the satellite orbits tested. The multi-mission crossover analysis shows that the application of an AOD1B product reduces the scatter of radial errors by 0.4-2.8 per cent for the satellite missions studied. At the regions with the most pronounced changes the use of the AOD1B products improves the consistency between the sea level as measured by the TOPEX and ERS-2 missions and by the Jason-1 and Envisat missions by 5 to 10 per cent (globally by about 2 per cent). The results of our study show that extended AOD1B RL05 product performs better than the AOD1B RL04 and improves orbits of altimetry satellites and consistency of sea level products.
Rowlands, D. D.; Pavlis, D. E.; Lemoine, F. G.; Neumann, G. A.; Luthcke, S. B.
Altimetry from the Mars Observer Laser Altimeter (MOLA), an instrument on board the Mars Global Surveyor (MGS) spacecraft, has been analyzed for the period of the MGS Science Phasing Orbit-1 (SPO-1) mission phase. Altimeter ranges have been used to improve significantly the orbit and attitude knowledge of the spacecraft by the use of crossover constraint equations derived from short passes of the MOLA data. These constraint equations differ from traditional crossover constraints and exploit the small footprint associated with laser altimetry. The rationale for using this technique with laser altimetry over sloping terrain is laid out and evidence of the resulting benefit is presented.
Satellite radar altimetry is today considered a mature technique in open ocean. The data stream from the various satellite missions are routinely used for a number of applications. In the last decade, significant research has been carried out into overcoming the problems to extend the capabilities of radar altimeters to the coastal zone, with the aim to integrate the altimeter-derived measurements of sea level, wind speed and significant wave height into coastal ocean observing systems. More/better (and new) datasets are being produced. Moreover, the advent of new satellite missions, both nadir-viewing (e.g., Sentinel-3) and wide-swath (e.g. SWOT), should globally improve both quantity and quality of coastal altimetry data. In this talk, after a brief review of the challenges in coastal altimetry and description of the new products, we showcase some application examples how the new products can be exploited, and we discuss directions for a global coastal altimetry dataset as an asset for long term monitoring of sea level and sea state in the coastal ocean.
Zhang, Jiqun; Xu, Kaiqin; Yang, Yonghui; Qi, Lianhui; Hayashi, Seiji; Watanabe, Masataka
Although satellite radar altimetry was developed and optimized for open oceans, it has been used to monitor variations in the level of inland water-bodies such as lakes and rivers. Here, for the first time, we have further used the altimetry-derived variation of water level for estimating the fluctuation of water storage as an addition to the present in situ water storage estimation systems to be used in remote areas and in emergency situation such as in the events flooding monitoring and for studying the effect of climate change. Lake Dongting, the second largest lake in China, influenced frequently by flooding, was, therefore, chosen to demonstrate the potential of the technique. By using the concept of an "assumed reference point", we converted Topex/Poseidon satellite altimetry data on water level variations in Lake Dongting to "water level" data. The "water level" time-series data and in situ water storage were used to establish a rating curve. From the rating curve, we converted data on "water level" derived from seven years (1993-1999) of Topex/Poseidon data to actual water storage in Lake Dongting. The result reveals that the seasonal and annual fluctuations of water storage occurred during the 1990s with a more frequent flooding at the late 1990s' especially the flooding in whole catchment level in 1998 and 1999. The study supports the usefulness of satellite altimetry for dense and continuous monitoring of the temporal variations in water dynamic in moderate to large lakes. PMID:16502025
Anderson, Allen Joel; Sandwell, David T.; Marquart, Gabriele; Scherneck, Hans-Georg
An overall review of the Arctic Geodynamics project is presented. A composite gravity field model of the region based upon altimetry data from ERS-1, Geosat, and Seasat is made. ERS-1 altimetry covers unique Arctic and Antarctic latitudes above 72 deg. Both areas contain large continental shelf areas, passive margins, as well as recently formed deep ocean areas. Until ERS-1 it was not possible to study these areas with satellite altimetry. Gravity field solutions for the Barents sea, portions of the Arctic ocean, and the Norwegian sea north of Iceland are shown. The gravity anomalies around Svalbard (Spitsbergen) and Bear island are particularly large, indicating large isostatic anomalies which remain from the recent breakup of Greenland from Scandinavian. Recently released gravity data from the Armed Forces Topographic Service of Russia cover a portion of the Barents and Kara seas. A comparison of this data with the ERS-1 produced gravity field is shown.
Atwood, D.; Guritz, R.; Muskett, R.; Lingle, C.; Sauber, J.
High quality geodetic ground control is time-consuming and costly to acquire in remote regions, where logistical operations are difficult to support. Hence, there is a strong interest in establishing new sources of ground control points that can be used in conjunction with Interferometric SAR (InSAR) for producing accurate digital elevation models (DEMs). In January 2003, NASA launched the Geoscience Laser Altimeter System (GLAS) into high polar orbit onboard the Ice, Cloud, and land Elevation Satellite (ICESat). A major objective of this spaceborne laser altimeter system, with orbital coverage extending from 86° N to 86° S, is to provide elevation measurements of the Earth's topography with unprecedented accuracy. The intent of our project is to assess the accuracy of ICESat elevation data and evaluate its utility as ground control for topographic mapping. Our study area lies near Barrow, Alaska; 15,650 sq. km of coastal plain adjacent to the Arctic Ocean, characterized by vast expanses of tundra, lakes, and arctic wetlands of such low relief as to be nearly devoid of terrain features. Accuracy of the ICESat elevation measurements is assessed through comparison with differential GPS (DGPS) data, acquired along ICESat ground tracks crossing our study area. Using DGPS as the reference, ICESat yields a mean offset of -0.04 ± 0.15 m for fast static measurements on frozen tundra lakes and 0.22 ± 0.96 m for two kinematic DGPS profiles along the ICESat ground track. These results suggests that ICESat-derived elevations on the Arctic coastal plain are more than sufficiently accurate for use as ground control in DEM generation. The only clear limitation of the ICESat data is the non-uniform distribution of the ICESat tracks within the 33 day near-repeat sub-cycle. Although the coverage is poor at equatorial latitudes, track separation in the Arctic is on the order of tens of kilometers because of orbital convergence at the Poles. To test whether these data can be used
Sulistioadi, Y. B.; Shum, C. K.; Jasinski, M. F.; Hidayat, H.
This study presents results of hydrologic monitoring of a poorly gauged Upper Mahakam Sub Watershed in Kalimantan, Indonesia, using satellite radar altimetry data and a rainfall-runoff model. The study area is part of Mahakam Watershed that drains rugged and rolling terrain of 20,000 km2 dominated by rain forest with patchy farmland with precipitation of about 2,000 mm/year. The Hydrologic Engineering Corps - Hydrologic Modeling System (HEC-HMS) is used to simulate discharges using parameters determined from various geospatial data, including soil type, land cover and digital elevation model. Due to the limited in situ meteorological, water level and discharge data, a modified Thiessen polygon method is used to spatially model the Tropical Rainfall Measuring Mission (TRMM) data to match the location of field meteorological stations. The challenge for employing ESA's Environmental Satellite (Envisat) altimeter includes the limited spatial and temporal resolutions, e.g. the narrower river width compared to the satellite's ground footprint and the 35 days repeat period for the altimeter ground track . To mitigate the spatial limitation, or tracker biases causing the radar altimeter return waveforms to deviate from the expected waveform model, we selected Envisat altimetry water level data based on standard over-water waveform shapes for each of the 18 Hz averaged return signals. Results indicate that the use of Envisat altimetry is a viable approach for estimating water level of medium-sized river (200-800 m width). In addition, contrary to results from previous studies, the Ice-1 waveform retracker is not necessarily the best among the four standard radar waveform retrackers for Envisat altimetry for this study region. Further, although there is good comparison between HEC-HMS simulated and observed discharges, results indicate that satellite altimetry provided better estimates of water level than those inferred from HEC-HMS simulated discharges and rating curves.
Herzfeld, Ute C.; Lingle, Craig S.; Lee, Li-Her
The potential of satellite radar altimetry for high-resolution mapping of Antarctic ice streams is evaluated, using retracked and slope-corrected data from the Lambert Glacier and Amery Ice Shelf area, East Antarctica, acquired by Geosat during the Exact Repeat Mission (ERM), 1986-89. The map area includes lower Lambert Glacier north of 72.18 deg S, the southern Amery Ice Shelf, and the grounded inland ice sheet on both sides. The Geosat ERM altimetry is found to provide substantially more complete coverage than the 1978 Seasat altimetry, due to improved tracking. Variogram methods are used to estimate the noise levels in the data as a function of position throughout the map area. The spatial structure in the data is quantified by constructing experimental variograms using altimetry from the area of the grounding zone of Lambert Glacier, which is the area chiefly of interest in this topographically complex region. Kriging is employed to invert the along-track height measurements onto a fine-scale 3 km grid. The unsmoothed along-track Geosat ERM altimetry yields spatially continuous maps showing the main topographic features of lower Lambert Glacier, upper Amery Ice Shelf and the adjacent inland ice sheet. The probable position of the grounding line of Lambert Glacier is identified from a break in slope at the grounded ice/floating ice transition. The approximate standard error of the kriged map is inferred from the data noise levels.
Fredenslund Levinsen, Joanna; Smith, Ben; Sørensen, Louise S.; Forsberg, René
When estimating elevation changes of ice-covered surfaces from radar altimetry, it is important to correct for slope-induced errors. They cause the reflecting point of the pulse to move up-slope and thus return estimates in the wrong coordinates. Slope-induced errors can be corrected for by introducing a Digital Elevation Model (DEM). In this work, such a DEM is developed for the Greenland Ice Sheet using a combination of Envisat radar and ICESat laser altimetry. If time permits, CryoSat radar altimetry will be included as well. The reference year is 2010 and the spatial resolution 2.5 x 2.5 km. This is in accordance with the results obtained in the ESA Ice Sheets CCI project showing that a 5 x 5 km grid spacing is reasonable for ice sheet-wide change detection (Levinsen et al., 2013). Separate DEMs will be created for the given data sets, and the geostatistical spatial interpolation method collocation will be used to merge them, thus adjusting for potential inter-satellite biases. The final DEM is validated with temporally and spatially agreeing airborne lidar data acquired in the NASA IceBridge and ESA CryoVex campaigns. The motivation for developing a new DEM is based on 1) large surface changes presently being observed, and mainly in margin regions, hence necessitating updated topography maps for accurately deriving and correcting surface elevation changes, and 2) although radar altimetry is subject to surface penetration of the signal into the snowpack, data is acquired continuously in time. This is not the case with e.g. ICESat, where laser altimetry data were obtained in periods of active lasers, i.e. three times a year with a 35-day repeat track. Previous DEMs e.g. have 2007 as the nominal reference year, or they are built merely from ICESat data. These have elevation errors as small as 10 cm, which is lower than for Envisat and CryoSat. The advantage of an updated DEM consisting of combined radar and laser altimetry therefore is the possibility of
A laser satellite power system (SPS) converts solar power captured by earth-orbiting satellites into electrical power on the earth's surface, the satellite-to-ground transmission of power being effected by laser beam. The laser SPS may be an alternative to the microwave SPS. Microwaves easily penetrate clouds while laser radiation does not. Although there is this major disadvantage to a laser SPS, that system has four important advantages over the microwave alternative: (1) land requirements are much less, (2) radiation levels are low outside the laser ground stations, (3) laser beam sidelobes are not expected to interfere with electromagnetic systems, and (4) the laser system lends itself to small-scale demonstration. After describing lasers and how they work, the report discusses the five lasers that are candidates for application in a laser SPS: electric discharge lasers, direct and indirect solar pumped lasers, free electron lasers, and closed-cycle chemical lasers. The Lockheed laser SPS is examined in some detail. To determine whether a laser SPS will be worthy of future deployment, its capabilities need to be better understood and its attractiveness relative to other electric power options better assessed. First priority should be given to potential program stoppers, e.g., beam attenuation by clouds. If investigation shows these potential program stoppers to be resolvable, further research should investigate lasers that are particularly promising for SPS application.
Baup, F.; Frappart, F.; Maubant, J.
This study presents an approach to determining the volume of water in small lakes (<100 ha) by combining satellite altimetry data and high-resolution (HR) images. In spite of the strong interest in monitoring surface water resources on a small scale using radar altimetry and satellite imagery, no information is available about the limits of the remote-sensing technologies for small lakes mainly used for irrigation purposes. The lake being studied is located in the south-west of France and is only used for agricultural irrigation purposes. The altimetry satellite data are provided by an RA-2 sensor onboard Envisat, and the high-resolution images (<10 m) are obtained from optical (Formosat-2) and synthetic aperture radar (SAR) antenna (Terrasar-X and Radarsat-2) satellites. The altimetry data (data are obtained every 35 days) and the HR images (77) have been available since 2003 and 2010, respectively. In situ data (for the water levels and volumes) going back to 2003 have been provided by the manager of the lake. Three independent approaches are developed to estimate the lake volume and its temporal variability. The first two approaches (HRBV and ABV) are empirical and use synchronous ground measurements of the water volume and the satellite data. The results demonstrate that altimetry and imagery can be effectively and accurately used to monitor the temporal variations of the lake (R2ABV = 0.98, RMSEABV = 5%, R2HRBV = 0.90, and RMSEABV = 7.4%), assuming a time-varying triangular shape for the shore slope of the lake (this form is well adapted since it implies a difference inferior to 2% between the theoretical volume of the lake and the one estimated from bathymetry). The third method (AHRBVC) combines altimetry (to measure the lake level) and satellite images (of the lake surface) to estimate the volume changes of the lake and produces the best results (R2AHRBVC = 0.98) of the three methods, demonstrating the potential of future Sentinel and SWOT missions to
Iglesias, Isabel; Lázaro, Clara; Joana Fernandes, M.; Bastos, Luísa
Presently, satellite altimetry record is long enough to appropriately study inter-annual signals in sea level anomaly and ocean surface circulation, allowing the association of teleconnection patterns of low-frequency variability with the response of sea level. The variability of the Atlantic Ocean at basin-scale is known to be complex in space and time, with the dominant mode occurring on annual timescales. However, interannual and decadal variability have already been documented in sea surface temperature. Both modes are believed to be linked and are known to influence sea level along coastal regions. The analysis of the sea level multiannual variability is thus essential to understand the present climate and its long-term variability. While in the open-ocean sea level anomaly from satellite altimetry currently possesses centimetre-level accuracy, satellite altimetry measurements become invalid or of lower accuracy along the coast due to the invalidity of the wet tropospheric correction (WTC) derived from on-board microwave radiometers. In order to adequately analyse long-term changes in sea level in the coastal regions, satellite altimetry measurements can be recovered by using an improved WTC computed from recent algorithms that combine wet path delays from all available observations (remote sensing scanning imaging radiometers, GNSS stations, microwave radiometers on-board satellite altimetry missions and numerical weather models). In this study, a 20-year (1993-2013) time series of multi-mission satellite altimetry (TOPEX/Poseidon, Jason-1, OSTM/Jason-2, ERS-1/2, ENVISAT, CryoSat-2 and SARAL), are used to characterize the North Atlantic (NA) long-term variability on sea level at basin-scale and analyse its response to several atmospheric teleconnections known to operate on the NA. The altimetry record was generated using an improved coastal WTC computed from either the GNSS-derived path Delay or the Data Combination methodologies developed by University of
Richter, T. G.; Kempf, S. D.; Holt, J. W.; Morse, D. L.; Blankenship, D. D.; Peters, M. E.
In response to an NSF-OPP proposal from Lamont Doherty Earth Observatory (R. Bell and M. Studinger) to study Lake Vostok, a team from the University of Texas Institute for Geophysics (UTIG) conducted the first comprehensive aerogeophysical survey of Lake Vostok during the 2000/01 austral summer. A Twin Otter was instrumented for measurements of gravity, magnetics, ice thickness, and surface elevation. The survey grid was 165 x 330 km (line spacing 7.5 km with 11.25 km and 22.5 km tie-lines), augmented by 12 regional lines extending 180 - 440 km from the primary grid. The remote polar location, high altitude, and extreme cold presented significant technical and physiological challenges, but the survey was completed successfully in 36 flights over 26 days, and has resulted in excellent geophysical data sets. We describe here the acquisition and reduction of the gravity field and ice-surface elevation data sets. Gravimetry and laser altimetry both require high-quality, precise positioning for use in data reduction. Three carrier-phase GPS receivers were operated in parallel aboard the aircraft, with an identical suite at the surface camp. All GPS data sets were reduced using two different software packages -- K&RS and GIPSY-OASIS. K&RS produced the most accurate positions but is inappropriate for long baselines. While GIPSY-OASIS yields positions in circumstances unfavorable to K&RS (i.e., long baselines and lines without closure), it was about half as accurate as K&RS and was insufficient for achieving the desired accuracy of 1-2 mGal in the reduced gravity data. Gravity was measured with a Bell Aerospace BGM-3 marine gravimeter provided by the Naval Oceanographic Office and modified for airborne use. GPS data are used to correct for inertial accelerations induced by aircraft movement. Up to 21 GPS solutions were available for each line. Selection was made through correlation of the high-frequency accelerations recorded by the gravity meter and those derived from the
Zhang, Changyou; Blais, J. A. R.
One-year average satellite altimetry data from the Exact Repeat Missions (ERM) of GEOSAT have been used to determine marine gravity disturbances in the Labrador Sea region using the inverse Hotine approach with FFT techniques. The derived satellite gravity information has been compared to shipboard gravity as well as gravity information derived by least-squares collocation (LSC), GEMT3 and OSU91A geopotential models in the Orphan Knoll area. The RMS and mean differences between satellite and shipboard gravity disturbances are about 8.0 and 2.8 mGal, respectively. There is no significantly difference between the results obtained using FFT and LSC.
Dhomps, A.-L.; Guinehut, S.; Le Traon, P.-Y.; Larnicol, G.
Differences and complementarities between Sea Level Anomalies (SLA) deduced from altimeter measurements and dynamic height anomalies (DHA) calculated from Argo in situ temperature (T) and salinity (S) profiles are globally analyzed. Compared to previous studies, Argo data allows a much better spatial coverage of all oceans and particularly the Southern Ocean, the use of salinity measurements and the use of a deeper reference level. The use of time series along the Argo float trajectories also provides a means to describe the vertical structure of the ocean both for the low frequency and the mesoscale part of the circulation. The comparison shows the very good consistency between Argo and altimeter observations. Correlations range from 0.9 in low latitudes to 0.3 in high latitudes where the contributions of deep baroclinic and barotropic signals are the largest. The study underlines the large influence of salinity observations on the consistency between altimetry and hydrographic observations. SLA/DHA consistency is thus improved by 35% (relative to the SLA minus DHA signal) by using measured S profiles instead of climatology data. The use of a deep reference level also significantly improves the correlation at mid and high latitudes. The role of seasonal signals on the correlation and regression analysis between altimeter and Argo observations is also analyzed. As they are mainly associated with the heating/cooling of surface layers, removing these large scale signals significantly reduces the correlation and impacts the geographical structure of the Argo/altimetry regression coefficients. These results emphasize the need to separate the different time and space scales in order to improve the merging of the two data sets. The study of seasonal to interannual SLA minus DHA signals finally reveals interesting signals related to deep ocean circulation variations. Future work is, however, needed to understand the observed differences and relate them to different
Gavrikov, A. V.; Krinitsky, M. A.; Grigorieva, V. G.
A new database of ocean wave parameters has been created based on satellite altimetry observations. The basis was data from the European Space Agency project GlobWave (www.globwave.org), which was transformed to suit upcoming requirements for global wave analysis. The new database contains additional wave characteristics (altimetry wind speed estimated using different parametric models, steepness, period, and some quality control parameters). It provides up-to-date tools for mass data preprocessing. The new database makes it possible to optimize wave field diagnostics on regional and global scales. Using the Envisat and Jason-1 satellite missions as an the example, we demonstrate the specific features of using the initial GlobalWave data set and the modified database.
Cotton, P. D.; Menard, Y.
The GAMBLE th ematic network brought together European exper ts in ocean altimetry to consider future developmen ts in satellite altimetry. The aim was to provide r ecommendations for research activ ities, and future altimeter missions, th at w ere necessary to support and build on present developments in operational o ceanography and to main tain o cean monitoring programmes. This paper reviews user r equiremen ts for operational products th at rely on altimeter d ata, and assess how well presen t and planned activities satisfy those requ irements. It provides recommendations for future missions required to form a cost-eff ectiv e, robust, operational satellite altimeter measuremen t system.  provides a full discussion.
Hurkmans, R.T.W.L.; Bamber, J.L.; Sorensen, L. S.; Joughin, I. R.; Davis, C. H.; Krabill, W. B.
Estimation of ice sheet mass balance from satellite altimetry requires interpolation of point-scale elevation change (dHdt) data over the area of interest. The largest dHdt values occur over narrow, fast-flowing outlet glaciers, where data coverage of current satellite altimetry is poorest. In those areas, straightforward interpolation of data is unlikely to reflect the true patterns of dHdt. Here, four interpolation methods are compared and evaluated over Jakobshavn Isbr, an outlet glacier for which widespread airborne validation data are available from NASAs Airborne Topographic Mapper (ATM). The four methods are ordinary kriging (OK), kriging with external drift (KED), where the spatial pattern of surface velocity is used as a proxy for that of dHdt, and their spatiotemporal equivalents (ST-OK and ST-KED).
Fu, L L; Chelton, D B
Sea level measurements by the Seasat altimeter were used to study the temporal variability of the Antarctic Circumpolar Current between July and October 1978. Large-scale zonal coherence in the cross-stream sea level difference was observed, indicating a general increase in the surface geostrophic velocity of the current around the Southern Ocean. The result demonstrates the power of satellite altimetry to monitor the variability of large-scale ocean currents. PMID:17749887
Blair, J. Bryan; Hofton, Michelle A.; Smith, David E. (Technical Monitor)
Laser altimeters provide a precise and accurate method for mapping topography at fine horizontal and vertical scales. A laser altimeter provides range by measuring the roundtrip flight time of a short pulse of laser light from the laser altimeter instrument to the target surface. The range is then combined with laser beam pointing knowledge and absolute position knowledge to provide an absolute measurement of the surface topography. Newer generations of laser altimeters measure the range by recording the shape and time of the outgoing and received laser pulses. The shape of the return pulse can also provide unique information about the vertical structure of material such as vegetation within each laser footprint. Distortion of the return pulse is caused by the time-distributed reflections adding together and representing the vertical distribution of surfaces within the footprint. Larger footprints (10 - 100m in diameter) can support numerous target surfaces and thus provide the potential for producing complex return pulses. Interpreting the return pulse from laser altimeters has evolved from simple timing between thresholds, range-walk corrections, constant-fraction discriminators, and multi-stop time interval units to actual recording of the time varying return pulse intensity - the return waveform. Interpreting the waveform can be as simple as digitally thresholding the return pulse, calculating a centroid, to fitting one or more gaussian pulse-shapes to the signal. What we present here is a new technique for using the raw recorded return pulse as a raw observation to detect centimeter-level vertical topographic change using large footprint airborne and spaceborne laser altimetry. We use the correlation of waveforms from coincident footprints as an indication of the similarity in structure of the waveforms from epoch to epoch, and assume that low correlation is an indicator of vertical structure or elevation change. Thus, using vertically and horizontally
Zwally, H. Jay; Busalacchi, Antonioa J. (Technical Monitor)
A major uncertainty in predicting sea level rise is the sensitivity of ice sheet mass balance to climate change, as well as the uncertainty in present mass balance. Since the annual water exchange is about 8 mm of global sea level equivalent, the +/- 25% uncertainty in current mass balance corresponds to +/- 2 mm/yr in sea level change. Furthermore, estimates of the sensitivity of the mass balance to temperature change range from perhaps as much as - 10% to + 10% per K. Although the overall ice mass balance and seasonal and inter-annual variations can be derived from time-series of ice surface elevations from satellite altimetry, satellite radar altimeters have been limited in spatial coverage and elevation accuracy. Nevertheless, new data analysis shows mixed patterns of ice elevation increases and decreases that are significant in terms of regional-scale mass balances. In addition, observed seasonal and interannual variations in elevation demonstrate the potential for relating the variability in mass balance to changes in precipitation, temperature, and melting. From 2001, NASA's ICESat laser altimeter mission will provide significantly better elevation accuracy and spatial coverage to 86 deg latitude and to the margins of the ice sheets. During 3 to 5 years of ICESat-1 operation, an estimate of the overall ice sheet mass balance and sea level contribution will be obtained. The importance of continued ice monitoring after the first ICESat is illustrated by the variability in the area of Greenland surface melt observed over 17-years and its correlation with temperature. In addition, measurement of ice sheet changes, along with measurements of sea level change by a series of ocean altimeters, should enable direct detection of ice level and global sea level correlations.
Uebbing, Bernd; Forootan, Ehsan; Kusche, Jürgen; Braakmann-Folgmann, Anne
Soil moisture represents an important component of the terrestrial water cycle that controls., evapotranspiration and vegetation growth. Consequently, knowledge on soil moisture variability is essential to understand the interactions between land and atmosphere. Yet, terrestrial measurements are sparse and their information content is limited due to the large spatial variability of soil moisture. Therefore, over the last two decades, several active and passive radar and satellite missions such as ERS/SCAT, AMSR, SMOS or SMAP have been providing backscatter information that can be used to estimate surface conditions including soil moisture which is proportional to the dielectric constant of the upper (few cm) soil layers . Another source of soil moisture information are satellite radar altimeters, originally designed to measure sea surface height over the oceans. Measurements of Jason-1/2 (Ku- and C-Band) or Envisat (Ku- and S-Band) nadir radar backscatter provide high-resolution along-track information (~ 300m along-track resolution) on backscatter every ~10 days (Jason-1/2) or ~35 days (Envisat). Recent studies found good correlation between backscatter and soil moisture in upper layers, especially in arid and semi-arid regions, indicating the potential of satellite altimetry both to reconstruct and to monitor soil moisture variability. However, measuring soil moisture using altimetry has some drawbacks that include: (1) the noisy behavior of the altimetry-derived backscatter (due to e.g., existence of surface water in the radar foot-print), (2) the strong assumptions for converting altimetry backscatters to the soil moisture storage changes, and (3) the need for interpolating between the tracks. In this study, we suggest a new inversion framework that allows to retrieve soil moisture information from along-track Jason-2 and Envisat satellite altimetry data, and we test this scheme over the Australian arid and semi-arid regions. Our method consists of: (i
Griggs, J.; Bamber, J. L.
The mass balance of the Antarctic and Greenland is required to assess their contribution to sea level rise as well as evaluate their sensitivities to variable future forcings. There is general agreement that the ice sheets are losing mass and that loss may be increasing. However, the range of estimates and the uncertainty in those estimates is in many cases, larger than the signal measured, particularly in a regional sense. Cryosat-2 will improve on the legacy satellite measurements from ERS-1 and -2 by using its interferometric model to determine elevation on steep slopes and through it's greater across - track resolution. The new technique will overcome many of the limitations of previous radar altimeters but elevations will still suffer from variable penetration in the firn and errors due to short-wavelength roughness. Building on previous work comparing and combining laser and radar altimeter data, we will assess the uncertainty in elevation due to these limitations. We use NASA Operation Ice Bridge airborne laser altimetry to assess biases in absolute elevation and elevation rates from Cryosat-2 data. We focus on the first 6 months of released data from the mission so that variability in penetration observed can be attributed to seasonal temperature, melt and accumulation variations can be assessed. Unfortunately, coincident airborne data is not currently available so we will assess the impact of the time difference between the datasets as well as presenting comparisons to older, longer time period, NASA ICESat satellite altimetry. The project aims to fully quantify biases and develop algorithms to correct for them and here we present our first comparisons. This will allow us to determine the likely improvement in mass balance estimates from Cryosat-2 as compared to legacy datasets.
Marcos, Marta; Woppelmann, Guy
Vertical ground displacements at tide gauge sites were estimated from the differenced time series of monthly satellite altimetry sea level anomalies minus tide gauge. We have used the time series of satellite altimetry that are routinely processed and distributed by four major data providers (three gridded and one along-track products) together with monthly tide gauge records from the datum controlled data set of the Permanent Service for Mean Sea Level (PSMSL). Differenced time series were built using three variants of altimetric time series. Each resulting record was analyzed assuming a combination of white noise and power-law noise of a priori unknown spectral index. The rate uncertainties, computed taking into account the noise content in the differenced time series, will be discussed. In particular, in the context of the departures from the white noise (expected only if both the satellite altimeter and the tide gauge were recording mostly the same sea level signals and their instrumental errors were negligible) and its amplitude. The most suitable altimetric product in terms of correlation and variance reduction at tide gauges, among those investigated, will be identified. Rates of vertical land motion computed with Global Positioning System (GPS) and rates obtained from the combination of altimetry and tide gauge records will be finally compared for those stations where both measurements are available.
Zwally, H. Jay; Zukor, Dorothy J. (Technical Monitor)
A major uncertainty in predicting sea level rise is the sensitivity of ice sheet mass balance to climate change, as well as the uncertainty in present mass balance. Since the annual water exchange is about 8 mm of global sea level equivalent, the 20% uncertainty in current mass balance corresponds to 1.6 mm/yr in sea level change. Furthermore, estimates of the sensitivity of the mass balance to temperature change range from perhaps as much as - 10% to + 10% per K. A principal purpose of obtaining ice sheet elevation changes from satellite altimetry has been estimation of the current ice sheet mass balance. Limited information on ice sheet elevation change and their implications about mass balance have been reported by several investigators from radar altimetry (Seasat, Geosat, ERS-1&2). Analysis of ERS-1&2 data over Greenland for 7 years from 1992 to 1999 shows mixed patterns of ice elevation increases and decreases that are significant in terms of regional-scale mass balances. Observed seasonal and interannual variations in ice surface elevation are larger than previously expected because of seasonal and interannUal variations in precipitation, melting, and firn compaction. In the accumulation zone, the variations in firn compaction are modeled as a function of temperature leaving variations in precipitation and the mass balance trend. Significant interannual variations in elevation in some locations, in particular the difference in trends from 1992 to 1995 compared to 1995 to 1999, can be explained by changes in precipitation over Greenland. Over the 7 years, trends in elevation are mostly positive at higher elevations and negative at lower elevations. In addition, trends for the winter seasons (from a trend analysis through the average winter elevations) are more positive than the corresponding trends for the summer. At lower elevations, the 7-year trends in some locations are strongly negative for summer and near zero or slightly positive for winter. These
Cohen, S. C.; Degnan, J. J.; Bufton, J. L.; Garvin, J. B.; Abshire, J. B.
The Geoscience Laser Altimetry Ranging System (GLARS) is a highly precise distance measurement system to be used for making extremely accurate geodetic observations from a space platform. It combines the attributes of a pointable laser ranging system making observations to cube corner retroreflectors placed on the ground with those of a nadir looking laser altimeter making height observations to ground, ice sheet, and oceanic surfaces. In the ranging mode, centimeter-level precise baseline and station coordinate determinations will be made on grids consisting of 100 to 200 targets separated by distances from a few tens of kilometers to about 1000 km. These measurements will be used for studies of seismic zone crustal deformations and tectonic plate motions. Ranging measurements will also be made to a coarser, but globally distributed array of retroreflectors for both precise geodetic and orbit determination applications. In the altimetric mode, relative height determinations will be obtained with approximately decimeter vertical precision and 70 to 100 meter horizontal resolution. The height data will be used to study surface topography and roughness, ice sheet and lava flow thickness, and ocean dynamics. Waveform digitization will provide a measure of the vertical extent of topography within each footprint. The planned Earth Observing System is an attractive candidate platform for GLARS since the GLAR data can be used both for direct analyses and for highly precise orbit determination needed in the reduction of data from other sensors on the multi-instrument platform. (1064, 532, and 355 nm)Nd:YAG laser meets the performance specifications for the system.
Tapley, B. D.; Sandwell, D. T.
All seamount signatures apparent in the SEASAT altimeter profiles were located and digitized. In addition to locating the seamount signatures, their amplitudes were also estimated. The second phase consisted of determining what basic characteristics of a seamount can be extracted from a single vertical deflection profile. Seven seamounts that had both good bathymetric coverage and good satellite altimeter coverage were used to test a simple flexural model. A method was developed to combine satellite altimeter profiles from several different satellites to construct a detailed and accurate geoid.
The LAGEOS I (Laser Geodynamics Satellite) was developed and launched by the Marshall Space Flight Center on May 4, 1976 from Vandenberg Air Force Base, California . The two-foot diameter satellite orbited the Earth from pole to pole and measured the movements of the Earth's surface.
Dhomps, A.-L.; Guinehut, S.; Le Traon, P.-Y.; Larnicol, G.
Differences, similarities and complementarities between Sea Level Anomalies (SLA) deduced from altimeter measurements and dynamic height anomalies (DHA) calculated from Argo in situ temperature (T) and salinity (S) profiles are globally analyzed. SLA and DHA agree remarkably well and, compared to previous studies, Argo dataset allows an improvement in the coherence between SLA and DHA. Indeed, Argo data provides a much better spatial coverage of all oceans and particularly the Southern Ocean, the use of an Argo mean dynamic height, the use of measured salinity profiles (versus climatological salinity), and the use of a deeper reference level (1000 m versus 700 m). The large influence of Argo salinity observations on the consistency between altimetry and hydrographic observations is particularly demonstrated with an improvement of 35% (relative to the SLA minus DHA signal) by using measured salinity profiles instead of climatological data. The availability of observations along the Argo float trajectories also provides a means to describe the sea level variability of the global ocean both for the low frequency and the mesoscale part of the circulation. Results indicate that sea level variability is dominated by baroclinic signal at seasonal to inter-annual periods for all latitudes. In the tropics, sea level variability is baroclinic for meso-scale to interannual periods and at high latitudes, sea level variability is barotropic with also deep baroclinic signals (i.e. influence of deep temperature and salinity signals) for intra seasonal and mesoscale periods. These results emphasize the need to separate the different time and space scales in order to improve the merging of the two data sets. The qualitative study of seasonal to interannual SLA minus DHA signals finally reveals signals related to deep ocean circulation variations and basin-scale barotropic signals. Future work is, however, needed to understand the observed differences and relate them to different
Calmant, S.; Paris, A.; Paiva, R. C.; Collischonn, W.; Santos da Silva, J.; Bonnet, M.
In a basin such as the Amazon basin, many parts of the basin are devoid of measurements, whatever it is rain or stage / discharge measurements. This specificity is even more dramatic since this occurs in the -Andean- upstream part of the rivers, where the largest rainfalls are encountered. Therefore, it is almost impossible to ascertain the quality of model outputs such as discharge series in these areas. In the present study, we present a methodology to check for the likelihood of discharge series by comparing the discharge values to stage values gained by satellite altimetry. An iterative refinement is searched until a plausible rating curve is found at the location of each altimetry series. A case study is presented for the Japura - Caqueta river, a Brazil-Colombia transboundary river for which satellite altimetry is the only source of information in the Colombian -upstream- part of the basin. Noteworthy, the computation of the rating curves implies the joint tuning of the Manning coefficients and mean depth of the cross sections.
Kaula, W. M.; Schubert, G.; Lingenfelter, R. E.; Sjogren, W. L.; Wollenhaupt, W. R.
Weighted mean laser altimetry data from Apollo 15, 16, and 17 tracks were analyzed, yielding a mean lunar radius of 1737.7 km and an offset of center-of mass from center of figure of 2.55 km toward 24 deg E. Weighted mean elevations with respect to a 1738 km radius sphere for various terrain types are: (1) farside terrae +1.8 km, (2) nearside terrae -1.4 km, (3) ringed maria -4.0 km, and (4) other maria -2.3 km. Comparison of gravity and topography data indicates that there is a variation in density in the outer parts of the moon and that the moon has a crust which is equivalent to at least 60 km of material of 2.95 grams per cu cm density. This result and moment-of-inertia data are consistent with a lunar interior model with a uniform density gradient in the mantle to the bottom of the lithosphere, constant density in the asthenosphere, and no core.
Csatho, Beata M; Schenk, Anton F; van der Veen, Cornelis J; Babonis, Gregory; Duncan, Kyle; Rezvanbehbahani, Soroush; van den Broeke, Michiel R; Simonsen, Sebastian B; Nagarajan, Sudhagar; van Angelen, Jan H
We present a new record of ice thickness change, reconstructed at nearly 100,000 sites on the Greenland Ice Sheet (GrIS) from laser altimetry measurements spanning the period 1993-2012, partitioned into changes due to surface mass balance (SMB) and ice dynamics. We estimate a mean annual GrIS mass loss of 243 ± 18 Gt ⋅ y(-1), equivalent to 0.68 mm ⋅ y(-1) sea level rise (SLR) for 2003-2009. Dynamic thinning contributed 48%, with the largest rates occurring in 2004-2006, followed by a gradual decrease balanced by accelerating SMB loss. The spatial pattern of dynamic mass loss changed over this time as dynamic thinning rapidly decreased in southeast Greenland but slowly increased in the southwest, north, and northeast regions. Most outlet glaciers have been thinning during the last two decades, interrupted by episodes of decreasing thinning or even thickening. Dynamics of the major outlet glaciers dominated the mass loss from larger drainage basins, and simultaneous changes over distances up to 500 km are detected, indicating climate control. However, the intricate spatiotemporal pattern of dynamic thickness change suggests that, regardless of the forcing responsible for initial glacier acceleration and thinning, the response of individual glaciers is modulated by local conditions. Recent projections of dynamic contributions from the entire GrIS to SLR have been based on the extrapolation of four major outlet glaciers. Considering the observed complexity, we question how well these four glaciers represent all of Greenland's outlet glaciers. PMID:25512537
In this study, we demonstrate three environmental-related applications employing altimetry and remote sensing satellites, and exemplify the prospective usage underlying the current progressivity in mechanical and data analyzing technologies. Our discussion starts from the improved waveform retracking techniques in need for altimetry measurements over coastal and inland water regions. We developed two novel auxiliary procedures, namely the Subwaveform Filtering (SF) method and the Track Offset Correction (TOC), for waveform retracking algorithms to operationally detect altimetry waveform anomalies and further reduce possible errors in determination of the track offset. After that, we present two demonstrative studies related to the ionospheric and tropospheric compositions, respectively, as their variations are the important error sources for satellite electromagnetic signals. We firstly compare the total electron content (TEC) measured by multiple altimetry and GNSS sensors. We conclude that the ionosphere delay measured by Jason-2 is about 6-10 mm shorter than the GPS models. On the other hand, we use several atmospheric variables to study the climate change over high elevation areas. Five types of satellite data and reanalysis models were used to study climate change indicators. We conclude that the spatial distribution of temperature trend among data products is quite different, which is probably due to the choice of various time spans. Following discussions about the measuring techniques and relative bias between data products, we applied our improved altimetry techniques to three environmental science applications with helps of remote sensing imagery. We first manifest the detectability of hydrological events by satellite altimetry and radiometry. The characterization of one-dimensional (along-track) water boundary using former Backscattering Coefficient (BC) method is assisted by the two-dimensional (horizontal) estimate of water extent using the Moderate
Brunt, Kelly M.; Fricker, Helen A.; Padman, Laurie; Scambos, Ted A.; O'Neel, Shad
We use laser altimetry from the Ice, Cloud, and land Elevation Satellite (ICESat) to map the grounding zone (GZ) of the Ross Ice Shelf, Antarctica, at 491 locations where ICESat tracks cross the grounding line (GL). Ice flexure in the GZ occurs as the ice shelf responds to short-term sea-level changes due primarily to tides. ICESat repeat-track analysis can be used to detect this region of flexure since each repeated pass is acquired at a different tidal phase; the technique provides estimates for both the landward limit of flexure and the point where the ice becomes hydrostatically balanced. We find that the ICESat-derived landward limits of tidal flexure are, in many places, offset by several km (and up to ∼60 km) from the GL mapped previously using other satellite methods. We discuss the reasons why different mapping methods lead to different GL estimates, including: instrument limitations; variability in the surface topographic structure of the GZ; and the presence of ice plains. We conclude that reliable and accurate mapping of the GL is most likely to be achieved when based on synthesis of several satellite datasets
Csatho, B. M.; Schenk, A.; Nagarajan, S.; Babonis, G. S.
Investigations of ice sheet mass balance and the changing dynamics of outlet glaciers have been hampered by the lack of comprehensive data. In recent years, this situation has been remedied. Satellite laser altimetry data from the Ice Cloud and land Elevation Satellite mission (ICESat), combined with airborne laser altimetry, provide accurate measurements of surface elevation changes, and surface velocities derived from various satellite platforms yield crucial information on changing glacier dynamics. Taken together, a rich and diverse data set is emerging that allows for characterizing the spatial and temporal evolution of ice sheets and outlet glaciers. In particular, it enables quantitative studies of outlet glaciers undergoing rapid and complex changes. Although airborne and laser altimetry have been providing precise measurements of ice sheet topography since the early 1990s, determining detailed and accurate spatial and temporal distribution of surface changes remains a challenging problem. We have developed a new, comprehensive method, called Surface Elevation Reconstruction And Change detection (SERAC), which estimates surface changes by a simultaneous reconstruction of surface topography from fused multisensor data. The mathematical model is based on the assumption that for a small surface area, only the absolute elevation changes over time but not the shape of the surface patch. Therefore, laser points of all time epochs contribute to the shape parameters; points of each time period determine the absolute elevation of the surface patch at that period. This method provides high-resolution surface topography, precise changes and a rigorous error estimate of the quantities. By using SERAC we combined ICESat and ATM laser altimetry data to determine the evolution of surface change rates of the whole Greenland Ice Sheet between 2003 and 2009 on a high-resolution grid. Our reconstruction, consistent with GRACE results, shows ice sheet thinning propagating
Urban, Timothy James
Satellite altimeter data spanning twenty-five years from GEOS-3, SEASAT, GEOSAT, ERS-1, TOPEX, and ERS-2 have been gathered, improved, validated, and integrated. Satellite data were updated with TOPEX-level corrections where possible. Wet troposphere and ionosphere altimeter corrections were evaluated globally, along-track, and zonally. Global mean sea level (GMSL) trend adjustments were made to ERS-1 and ERS-2 to correct for radiometer drifts with respect to TOPEX, improving their GMSL comparisons. IRI-95 model ionosphere corrections were evaluated against TOPEX dual-frequency measurements. New orbits were computed with improved accuracy for GEOS-3 (20 to 30 cm), SEASAT (10 to 15 cm), and GEOSAT (7 to 9 cm). An adaptive sequential filter was utilized to remove residual one-cycle-per-revolution radial orbit error. Optimal state noise compensation parameters for the filter were determined using a genetic algorithm. The application of the filter reduced single-satellite internal crossover differences. Relative altimeter biases between TOPEX and the other missions were determined using tide gauges. Relative biases were estimated for GEOS-3 (173 cm), SEASAT (37 and 26 cm, for 17-day and 3-day repeats), GEOSAT (7.9 and 8.9 cm, for GM and ERM), ERS-1 (-44.7 and -48.0 cm, for Phases A-F and Phase G), and ERS-2 (-9.0 cm). These biases are consistent with several other recent bias determinations and calibration campaigns. GMSL trends estimated for GEOSAT (1985--1988), ERS-1 (1991--1996), TOPEX (1993--1999), and ERS-2 (1995--1997), are -3.4 +/- 2.4, 2.3 +/- 1.1, 3.2 +/- 0.6, and 6.0 +/- 2.2 mm/year, respectively. GEOS-3 MSL exhibits a large slope (˜40 cm/year) and does not provide truly global coverage, and therefore cannot be used for global analyses. The short duration of the two SEASAT missions (July to October 1978) prevent GMSL trend analysis. GMSL from ERS-1, TOPEX, and ERS-2 were integrated into a single time series having an estimated mean sea level trend of 3
Basic features of sea surface topography are reviewed, to show those oceanographic results which may be of value to a geodetic satellite program: (1) the shape and magnitude of the large scale features of the mean sea surface, relative to a level surface; (2) the position and magnitude of the slopes across the western boundary currents, from a variety of data; (3) an estimate of the position of the geoid, tied into the U.S. leveling network; and (4) a documented change of 60 to 70 cm in mean sea level, with respect to the geoid, between the U.S. east and west coasts.
Dooley, R. P.; Nathanson, F. E.; Brooks, L. W.
Pulse compression techniques are studied which are applicable to a satellite altimeter having a topographic resolution of + 10 cm. A systematic design procedure is used to determine the system parameters. The performance of an optimum, maximum likelihood processor is analysed, which provides the basis for modifying the standard split-gate tracker to achieve improved performance. Bandwidth considerations lead to the recommendation of a full deramp STRETCH pulse compression technique followed by an analog filter bank to separate range returns. The implementation of the recommended technique is examined.
Carabajal, C. C.; Boy, J.
A global set of Ground Control Points (GCPs) from altimetry measurements from the Ice, Cloud and land Elevation Satellite (ICESat) has been produced with the support of the NASA's Earth Surface and Interior Program. The highest quality altimetry measurements that can be used for ground control have been selected by applying rigorous editing criteria. This database represents a key means to establishing a much-needed global topography reference frame to aid solid Earth application studies, particularly useful at high latitudes, where other topographic control is scarce. ICESat GCPs were used to characterize and quantify spatially varying elevation biases in Digital Elevation Models (DEMs) in the polar regions, assessing the horizontal and vertical accuracy of valuable topographic datasets produced by sensors like ASTER (Advanced Spaceborne Thermal Emission and Reflection Radiometer) missions, and datasets like GMTED2010 (Global Multi-resolution Terrain Elevation Data), developed by the USGS (United States Geological Survey) and NGA (National Geospatial-Intelligence Agency), a large improvement over the global GTOPO30 dataset. We have analyzed error statistics globally and per continent, in conjunction with MODIS (Moderate Resolution Imaging Spectro-Radiometer) and MERIS (Medium Resolution Imaging Spectrometer) land cover products, relief, topography and other DEM and altimetry specific parameters, and will present the results of these evaluations.
Radebach, Alexander; Donges, Jonathan F.; Donner, Reik V.; Barbosa, Susana; Lange, Holger; Kurths, Jürgen
Since the advent of the satellite era, global sea-level altimetry data sets are available. To study complex oceanographic processes and their coupling to atmospheric dynamics it is necessary to advance beyond analyzing global mean sea-level rise or local trends. We apply a wide range of methods from linear and nonlinear time series analysis for investigating the complex dynamics of observed sea-level altimetry time series at different locations around the globe. Employing this toolkit, linear and nonlinear autodependencies (autocorrelation and auto-mutual information functions), deterministic structure (recurrence quantification and recurrence network analysis), time-reversibility characteristics (visibility graph analysis) and the relative importance of stochastic vs. deterministic dynamics (complexity-entropy plane) are studied. Combining the complimentary information from all metrics, consistent spatial patterns of sea-level dynamics are detected. Classical statistical properties such as variance, skewness and Shannon entropy of the probability distribution of sea-level reveal the special importance of western boundary currents as well as parts of the Antarctic Circumpolar Current as regions of particularly complex sea-level dynamics. In turn, the nonlinear dynamics characteristics present a somewhat different pattern exhibiting particularly high complexity in the tropics as well as the Gulf Stream and Kuroshio regions. Notably, these are also the areas where missing values due to atmospheric processes are most prominent. Further research is required to fully disentangle the dynamic complexity of sea-level from potential artifacts in the underlying altimetry data.
Escudier, Romain; Renault, Lionel; Pascual, Ananda; Brasseur, Pierre; Chelton, Dudley; Beuvier, Jonathan
Three different eddy detection and tracking methods are applied to the outputs of a high-resolution simulation in the Western Mediterranean Sea in order to extract mesoscale eddy characteristics. The results are compared with the same eddy statistics derived from satellite altimetry maps over the same period. Eddy radii are around 30 km in altimetry maps whereas, in the model, they are around 20 km. This is probably due to the inability of altimetry maps to resolve the smaller mesoscale in the region. About 30 eddies are detected per day in the basin with a very heterogeneous spatial distribution and relatively short lifespans (median life around 13 days). Unlike other areas of the open ocean, they do not have a preferred direction of propagation but appear to be advected by mean currents. The number of detected eddies seems to present an annual cycle when separated according to their lifespan. With the numerical simulation, we show that anticyclones extend deeper in the water column and have a more conic shape than cyclones.
Ray, Richard D.; Zaron, E. D.
Temporal variability of the internal tide is inferred from a 17-year combined record of Topex/Poseidon and Jason satellite altimeters. A global sampling of along-track sea-surface height wavenumber spectra finds that non-stationary variance is generally 25% or less of the average variance at wavenumbers characteristic of mode-l tidal internal waves. With some exceptions the non-stationary variance does not exceed 0.25 sq cm. The mode-2 signal, where detectable, contains a larger fraction of non-stationary variance, typically 50% or more. Temporal subsetting of the data reveals interannual variability barely significant compared with tidal estimation error from 3-year records. Comparison of summer vs. winter conditions shows only one region of noteworthy seasonal changes, the northern South China Sea. Implications for the anticipated SWOT altimeter mission are briefly discussed.
Fu, Lee-Lueng; Vazquez, Jorge; Parke, Michael E.
The nearly continuous 3.5 years of altimeter data in the western North Atlantic Ocean from the GEOS 3 mission (April 1975 to November 1978) have been used to study the seasonal variability of the Gulf Stream. The differences between altimetric measurements of sea surface height made at satellite ground track intersections, called crossovers, are utilized to construct time series of sea level variations. The results indicate that the Gulf Stream in the region off Cape Hatteras has a pronounced seasonal variability. The peak-to-peak amplitude of the seasonal cycle in terms of cross-stream sea level difference is about 15 cm, with a maximum in April and a minimum in December. The result is in good agreement with historic hydrographic observations and recent direct measurements of the Gulf Stream. The mechanisms responsible for the observed seasonal variability are discussed.
Rudenko, Sergei; Gruber, Christian
This study makes use of current GFZ monthly and daily gravity field products from 2002 to 2014 based on radial basis functions (RBF) instead of time variable gravity field modeling for precise orbit determination of altimetry satellites. Since some monthly solutions are missing in the GFZ GRACE RL05a solution and in order to reach a better quality for the precise orbit determination, daily generated RBF solutions obtained from Kalman filtered GRACE data processing and interpolated in case of gaps have been used. Moreover, since the geopotential coefficients of low degrees are better determined using SLR observations to geodetic satellites like Lageos, Stella, Starlette and Ajisai than from GRACE observations, these terms are co-estimated in the RBF solutions by using apriori SLR-derived values up to degree and order 4. Precise orbits for altimetry satellites Envisat (2002-2012), Jason-1 (2002-2013) and Jason-2 (2008-2014) are then computed over the given time intervals using this approach and compared with the orbits obtained when using other models such as EIGEN-6S4. An analysis of the root-mean-square values of the observation fits of SLR and DORIS observations and the orbit arcs overlaps will allow us to draw a conclusion on the quality of the RBF solution and to use these new trajectories for sea level trend estimates and geophysical application.
Ray, Richard D.; Byrne, Deidre A.
Seafloor pressure records, collected at 11 stations aligned along a single ground track of the Topex/Poseidon and Jason satellites, are analyzed for their tidal content. With very low background noise levels and approximately 27 months of high-quality records, tidal constituents can be estimated with unusually high precision. This includes many high-frequency lines up through the seventh-diurnal band. The station deployment provides a unique opportunity to compare with tides estimated from satellite altimetry, point by point along the satellite track, in a region of moderately high mesoscale variability. That variability can significantly corrupt altimeter-based tide estimates, even with 17 years of data. A method to improve the along-track altimeter estimates by correcting the data for nontidal variability is found to yield much better agreement with the bottom-pressure data. The technique should prove useful in certain demanding applications, such as altimetric studies of internal tides.
Dung Tran, Tuan; Ho, Thi Huong Mai
The study area is bordered on the East China Sea, the Philippine Sea, and the Australian-Indo plate in the Northeast, in the East and in the South, respectively. It is a large area with the diversely complicated conditions of geological structure. In spite of over the past many years of investigation, marine geological structure in many places have remained poorly understood because of a thick seawater layer as well as of the sensitive conflicts among the countries in the region. In recent years, the satellite altimeter technology allows of enhancement the marine investigation in any area. The ocean surface height is measured by a very accurate radar altimeter mounted on a satellite. Then, that surface can be converted into marine gravity anomaly or bathymetry by using the mathematical model. It is the only way to achieve the data with a uniform resolution in acceptable time and cost. The satellite altimetry data and its variants are essential for understanding marine geological structure. They provide a reliable opportunity to geologists and geophysicists for studying the geological features beneath the ocean floor. Also satellite altimeter data is perfect for planning the more detailed shipboard surveys. Especially, it is more meaningful in the remote or sparsely surveyed regions. In this paper, the authors have effectively used the satellite altimetry and shipboard data in combination. Many geological features, such as seafloor spreading ridges, fault systems, volcanic chains as well as distribution of sedimentary basins are revealed through the 2D, 3D model methods of interpretation of satellite-shipboard-derived data and the others. These results are improved by existing boreholes and seismic data in the study area.
Rowlands, D. D.; Pavlis, D. E.; Lemoine, F. G.; Neumann, G. A.; Luthcke, S. B.
Altimetry from the Mars Observer Laser Altimeter (MOLA) which is carried on board Mars Global Surveyor (MGS) has been analyzed for the period of the MOS mission known as Science Phasing Orbit 1 (SPO-1). We have used these altimeter ranges to improve orbit and attitude knowledge for MGS. This has been accomplished by writing crossover constraint equations that have been derived from short passes of MOLA data. These constraint equations differ from traditional Crossover constraints and exploit the small foot print associated with laser altimetry.
Velpuri, N. M.; Senay, G. B.; Asante, K. O.
Lake Turkana is one of the largest desert lakes in the world and is characterized by high degrees of inter- and intra-annual fluctuations. The hydrology and water balance of this lake have not been well understood due to its remote location and unavailability of reliable ground truth datasets. Managing surface water resources is a great challenge in areas where in-situ data are either limited or unavailable. In this study, multi-source satellite-driven data such as satellite-based rainfall estimates, modelled runoff, evapotranspiration, and a digital elevation dataset were used to model Lake Turkana water levels from 1998 to 2009. Due to the unavailability of reliable lake level data, an approach is presented to calibrate and validate the water balance model of Lake Turkana using a composite lake level product of TOPEX/Poseidon, Jason-1, and ENVISAT satellite altimetry data. Model validation results showed that the satellite-driven water balance model can satisfactorily capture the patterns and seasonal variations of the Lake Turkana water level fluctuations with a Pearson's correlation coefficient of 0.90 and a Nash-Sutcliffe Coefficient of Efficiency (NSCE) of 0.80 during the validation period (2004-2009). Model error estimates were within 10% of the natural variability of the lake. Our analysis indicated that fluctuations in Lake Turkana water levels are mainly driven by lake inflows and over-the-lake evaporation. Over-the-lake rainfall contributes only up to 30% of lake evaporative demand. During the modelling time period, Lake Turkana showed seasonal variations of 1-2 m. The lake level fluctuated in the range up to 4 m between the years 1998-2009. This study demonstrated the usefulness of satellite altimetry data to calibrate and validate the satellite-driven hydrological model for Lake Turkana without using any in-situ data. Furthermore, for Lake Turkana, we identified and outlined opportunities and challenges of using a calibrated satellite-driven water
Birkett, C. M.
Although their primary priorities are aimed at ocean and ice studies, satellite radar altimeters can be successful at detecting surface water height variations over inland water targets. In particular, the ability to monitor stage variations in lakes, inland seas, rivers, wetlands and flood plains has been demonstrated. The results demonstrate how sub-monthly, seasonal, and interannual variations can be monitored with accuracy's ranging from 3-4cm rms (lakes) to greater than 10cm rms (rivers/wetlands). The instruments sample the surface at pre-defined geographical and temporal resolutions, have day/nighttime operational capability and are not hindered by the presence of clouds. These are keen advantages where traditional ground-based gauges are lacking or where there is little or slow dissemination of data. This presentation gives a review of past and present capabilities and demonstrates various applications within a number of interdisciplinary projects. Note will be made of the contribution towards the determination of surface water fluxes and dynamics in the Amazon Basin, its use as a validation tool within the Global Rainforest Mapping Project, and its capability to monitor both the effects of large-scale climatic and small-scale flooding events. Such studies now pose new questions regarding the future potential to a) reduce the minimum target size observable, b) improve the accuracy of the height measurements, c) to have global information on a near-real time basis, and d) to have higher-level products such as lake volume and river discharge. In lieu of these requirements, a number of dedicated water observing instruments are being discussed under the `HYRDA-SAT' concept. In addition, focus is now on the forthcoming radar altimeters missions Jason-1 and ENVISAT, and on the new lidar mission, ICESAT.
Mitchell, J. L.; Teague, W. J.; Jacobs, G. A.; Hurlburt, H. E.
Altimeter data from the Geosat Exact Repeat Mission (ERM) are analyzed with the aid of a simulation from an eddy-resolving primitive equation model of the North Pacific basin in the region of the Kuroshio and Kuroshio Extension. The model domain covers the Pacific Ocean north of 20°S and has a resolution of 0.125° latitude and 0.176° longitude. The model is synoptically driven by daily 1000-mbar winds from the European Centre for Medium-Range Weather Forecasts (ECMWF) which encompass the Geosat time period. Model output is sampled along Geosat ground tracks for the period of the ERM. Additionally, the model and the Geosat data are compared with climatological hydrography and satellite IR frontal position analyses. Analyses compared include maps of sea surface height (SSH) mean and variability, eddy kinetic energy (EKE), seasonal transport anomaly, and time-longitude plots of SSH anomaly. The model simulation provides annual mean SSH fields for 1987 and 1988 which reproduce the four quasi-permanent meanders seen in hydrographic climatology (cyclonic at 138°E and anticyclonic at 144°E, 150°E, and 160°E). These are linked to the bottom topography. In the model simulation, Geosat altimeter data, and climatology, we observe four peaks in SSH variability associated with meander activity and two peaks in EKE, with the strongest about 3200 cm2 s-2 along the mean Kuroshio path in the Geosat data. The local maxima in SSH variability tend to occur where relatively strong, topographically steered meridional abyssal currents intersect the zonally oriented Kuroshio Extension. Westward propagation of SSH anomalies at phase speeds of 2 to 3 cm s-1 in the region east of 155°E is observed in the model simulation and Geosat observations. A late summer maximum in the upper ocean transport anomaly of the Kuroshio Extension is inferred from changes in the cross-stream differential in SSH from the simulation and Geosat observations.
Challenor, P.; Woolf, D.; Gommenginger, C.; Srokosz, M.; Cotton, D.; Carter, D.; Sykes, N.
Before the advent of radar altimeters our understanding of the world's climate was based on a few instruments moored off the coats of Europe, Japan and North America and visual observations taken from merchant ships. This information was patchy and in many cases of poor quality. It was difficult to relate what was happening at one location with another. The advent of the radar altimeter has changed all that. We had a series of satellite instruments that made consistent measurements of significant wave height across the globe. Initial work concentrated on simply mapping the wave climate and investigating means and seasonal variation. However with longer records came the ability to look at inter-annual variability. It had been known since the late 70's that wave heights measured at a few sites around the North East Atlantic had shown a dramatic increase. It was only the combination of spatial and temporal sampling from the altimeter that allowed us to discover the extent of the changes and how they related to the North Atlantic Oscillation index. Of course it is not only the mean wave conditions that are important, extreme waves are of vital interest to naval architects and the designers of offshore structures. Being able to estimate extreme waves from altimeter data enables us for the first time to establish what extreme conditions might be in any part of the world. At present we do not exploit the spatial nature of the altimeter data in our estimation of the extremes but this is an active research area. Research into further uses of altimeter data for looking at the wave climate continues apace. Algorithms for new parameters such as wave period are being developed. Real time applications are appearing. For such applications space- time sampling is an issue and people are coming up with innovative ideas using constellations of cheap altimeters. In this paper we review the progress made in the study of wave climate using the radar altimeter. In addition we look into
Velpuri, N.M.; Senay, G.B.; Asante, K.O.
Lake Turkana is one of the largest desert lakes in the world and is characterized by high degrees of interand intra-annual fluctuations. The hydrology and water balance of this lake have not been well understood due to its remote location and unavailability of reliable ground truth datasets. Managing surface water resources is a great challenge in areas where in-situ data are either limited or unavailable. In this study, multi-source satellite-driven data such as satellite-based rainfall estimates, modelled runoff, evapotranspiration, and a digital elevation dataset were used to model Lake Turkana water levels from 1998 to 2009. Due to the unavailability of reliable lake level data, an approach is presented to calibrate and validate the water balance model of Lake Turkana using a composite lake level product of TOPEX/Poseidon, Jason-1, and ENVISAT satellite altimetry data. Model validation results showed that the satellitedriven water balance model can satisfactorily capture the patterns and seasonal variations of the Lake Turkana water level fluctuations with a Pearson's correlation coefficient of 0.90 and a Nash-Sutcliffe Coefficient of Efficiency (NSCE) of 0.80 during the validation period (2004-2009). Model error estimates were within 10% of the natural variability of the lake. Our analysis indicated that fluctuations in Lake Turkana water levels are mainly driven by lake inflows and over-the-lake evaporation. Over-the-lake rainfall contributes only up to 30% of lake evaporative demand. During the modelling time period, Lake Turkana showed seasonal variations of 1-2m. The lake level fluctuated in the range up to 4m between the years 1998-2009. This study demonstrated the usefulness of satellite altimetry data to calibrate and validate the satellite-driven hydrological model for Lake Turkana without using any in-situ data. Furthermore, for Lake Turkana, we identified and outlined opportunities and challenges of using a calibrated satellite-driven water balance
Babonis, G. S.; Csatho, B.; Schenk, T.
During the past few decades the Greenland and Antarctic ice sheets have lost ice at accelerating rates, caused by increasing surface temperature. The melting of the two big ice sheets has a big impact on global sea level rise. If the ice sheets would melt down entirely, the sea level would rise more than 60 m. Even a much smaller rise would cause dramatic damage along coastal regions. In this paper we report about a major upgrade of surface elevation changes derived from laser altimetry data, acquired by NASA's Ice, Cloud and land Elevation Satellite mission (ICESat) and airborne laser campaigns, such as Airborne Topographic Mapper (ATM) and Land, Vegetation and Ice Sensor (LVIS). For detecting changes in ice sheet elevations we have developed the Surface Elevation Reconstruction And Change detection (SERAC) method. It computes elevation changes of small surface patches by keeping the surface shape constant and considering the absolute values as surface elevations. We report about important upgrades of earlier results, for example the inclusion of local ice caps and the temporal extension from 1993 to 2014 for the Greenland Ice Sheet and for a comprehensive reconstruction of ice thickness and mass changes for the Antarctic Ice Sheets.
Powell, Wesley; Hicks, Edward; Pinchinat, Maxime; Dabney, Philip; McGarry, Jan; Murray, Paul
Single-photon-counting laser altimetry is a new measurement technique offering significant advantages in vertical resolution, reducing instrument size, mass, and power, and reducing laser complexity as compared to analog or threshold detection laser altimetry techniques. However, these improvements come at the cost of a dramatically increased requirement for onboard real-time data processing. Reconfigurable computing has been shown to offer considerable performance advantages in performing this processing. These advantages have been demonstrated on the Multi-KiloHertz Micro-Laser Altimeter (MMLA), an aircraft based single-photon-counting laser altimeter developed by NASA Goddard Space Flight Center with several potential spaceflight applications. This paper describes how reconfigurable computing technology was employed to perform MMLA data processing in real-time under realistic operating constraints, along with the results observed. This paper also expands on these prior results to identify concepts for using reconfigurable computing to enable spaceflight single-photon-counting laser altimeter instruments.
Chen, Nan; Han, Guoqi; Yang, Jingsong; Chen, Dake
Hurricane Sandy made landfall to the northeast of Atlantic City, New Jersey at 23:30 UTC on 29 October 2012 and caused large storm surges and devastating flooding along the New Jersey and New York coasts. Here we combine sea surface height measurements from the HaiYang-2A (HY-2A) satellite altimeter with coastal tide-gauge data to study the features of the Hurricane Sandy storm surges. The HY-2A altimeter captured the cross-shelf profile of surge at the time of Sandy's peak surge, with a surge magnitude of about 1.83 m at the coast and a cross-shelf decaying scale of 68 km. The altimetric surge magnitude agrees approximately with tide-gauge estimate of 1.73 m at nearby Montauk. Further analysis suggests that continental shelf waves were generated during the passage of Sandy. The continental shelf wave observed by altimetry has a propagating speed of 6.5 m/s. The post landfall free shelf wave at Atlantic City observed by tide gauges has a propagating phase speed of 6.8 m/s and cross-shelf e-folding scale of 75 km. In contrast, the post landfall sea level oscillation at Montauk is not associated with a continental shelf wave. The study indicates that satellite altimetry is capable of observing and useful for understanding features of storm surges, complementing existing coastal tide gauges.
Kosuth, P.; Blitzkow, D.; Cochonneau, G.
Satellite radar altimetry (Topex/Poseidon T/P) was used to establish a consistent altimetric reference network over the Amazon basin. A methodolody was developed that uses radar altimetry to quantify maximum annual water levels (referred to the geoid) at intersections between the satellite ground tracks and the river network and derive, through spatial interpolation, maximum annual water levels at hydrometric stations. Comparison with maximum annuyal readings at gauges allowed the determination of local orthometric heights at these stations. Altimetric levelling from Topex/Poseidon measurements has been realized for 97 hydrometric stations along 27 740 km of the Amazon hydrographic network. Validation has been realized both by checking the overall hydraulic consistency of longitudinal river profiles at low and high river stages and by comparing, for 23 hydrometric stations, orthometric heights obtrained from T/P measurements with values obtained from bi-frequency GPS positioning. These results are of major importance for the study of Amazon river flow dynamics and sediment transport.
Shum, C.; Kuo, C.; Mitrovica, J. X.
Glacial isostatic adjustment (GIA) of the solid Earth due to deglaciation since the last Ice Age is characterized by its viscous rebound as a result of relaxation of the shear stresses inside the Earth. GIA uplift (in the form of 3-D crustal motion and the ensuing geoid change due to redistribution of mass in the solid Earth) has been recently measured with long-term GPS (e.g., the BIFROST project). In this paper, we used more than 50 long-term (1860-2000) water level gauges located around the Great Lakes, and satellite altimetry measurements (TOPEX/ POSEIDON and Geosat, 9-15 year data span) to measure the vertical motion of the region. Preliminary results indicate that Lake Superior, Lake Michigan, Lake Huron, Lake Erie, and Lake Ontario, are uplifting at a rate of 1.8, 0.9, 1.4, -0.5, and 1.0 mm/yr, respectively. The uncertainty of the measurement is primarily due to the error in satellite altimetry due to its relatively short data span. The results are compared with available GIA models, including ICE-4G, and Mitrovica-Milne 2001 models, as well as relative vertical motion measured using water level gauges [Manville et al., 2001]. Analysis also includes the examination of GIA models using different estimates of mantle thickness and upper and lower mantle viscosity. Results using the vertical measurement in an inverse geophysical solution will be reported.
Ray, Richard D.; Eanes, Richard J.; Lemoine, Frank G.
The phase lag by which the earth's body tide follows the tidal potential is estimated for the principal lunar semidiurnal tide M(sub 2). The estimate results from combining recent tidal solutions from satellite tracking data and from Topex/Poseidon satellite altimeter data. Each data type is sensitive to the body-tide lag: gravitationally for the tracking data, geometrically for the altimetry. Allowance is made for the lunar atmospheric tide. For the tidal potential Love number kappa(sub 2) we obtain a lag epsilon of 0.20 deg +/- 0.05 deg, implying an effective body-tide Q of 280 and body-tide energy dissipation of 110 +/- 25 gigawatts.
Song, Tony Y.; Ji, Chen; Fu, L. -L.; Zlotnicki, Victor; Shum, C. K.; Yi, Yuchan; Hjorleifsdottir, Vala
The 26 December 2004 Indian Ocean tsunami was the first earthquake tsunami of its magnitude to occur since the advent of both digital seismometry and satellite radar altimetry. Both have independently recorded the event from different physical aspects. The seismic data has then been used to estimate the earthquake fault parameters, and a three-dimensional ocean-general-circulation-model (OGCM) coupled with the fault information has been used to simulate the satellite-observed tsunami waves. Here we show that these two datasets consistently provide the tsunami source using independent methodologies of seismic waveform inversion and ocean modeling. Cross-examining the two independent results confirms that the slip function is the most important condition controlling the tsunami strength, while the geometry and the rupture velocity of the tectonic plane determine the spatial patterns of the tsunami.
Sun, Xiaoli; Cavanaugh, John F.; Neumann, Gregory A.; Smith, David E..; Zubor, Maria T.
The Mercury Laser Altimeter onboard MESSENGER involves unique design elements that deal with the challenges of being in orbit around Mercury. The Mercury Laser Altimeter (MLA) is one of seven instruments on NASA's MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. MESSENGER was launched on 3 August 2004, and entered into orbit about Mercury on 18 March 2011 after a journey through the inner solar system. This involved six planetary flybys, including three of Mercury. MLA is designed to map the topography and landforms of Mercury's surface. It also measures the planet's forced libration (motion about the spin axis), which helps constrain the state of the core. The first science measurements from orbit taken with MLA were made on 29 March 2011 and continue to date. MLA had accumulated about 8.3 million laser ranging measurements to Mercury's surface, as of 31 July 2012, i.e., over six Mercury years (528 Earth days). Although MLA is the third planetary lidar built at the NASA Goddard Space Flight Center (GSFC), MLA must endure a much harsher thermal environment near Mercury than the previous instruments on Mars and Earth satellites. The design of MLA was derived in part from that of the Mars Orbiter Laser Altimeter on Mars Global Surveyor. However, MLA must range over greater distances and often in off-nadir directions from a highly eccentric orbit. In MLA we use a single-mode diode-pumped Nd:YAG (neodymium-doped yttrium aluminum garnet) laser that is highly collimated to maintain a small footprint on the planet. The receiver has both a narrow field of view and a narrow spectral bandwidth to minimize the amount of background light detected from the sunlit hemisphere of Mercury. We achieve the highest possible receiver sensitivity by employing the minimum receiver detection threshold.
Stendardo, Ilaria; Rhein, Monika; Klein, Birgit; Roessler, Achim
Salinity distribution in the North Atlantic is affected by changes in the circulation and freshwater fluxes. Changes in salinity are introduced into the ocean's interior by vertical processes like subduction or convection, and transported along circulation pathways. At a given location and depth, salinity could vary by water mass changes due to changes in the freshwater flux, or by vertical migration of density surfaces caused either by wind-driven changes of ocean ventilation or by thermodynamic processes, like poleward migration of isopycnals as a result of surface warming. Changes in the wind driven circulation with a consequence shift of the subpolar front, that separates the fresher subpolar from the saline subtropical gyre, also have a marked influence on upper ocean salinity in the subpolar North Atlantic. Due to the lack of temporal and spatial resolution of salinity observations, salinity anomalies in the last century could only be studied by 5-year means. Thanks to the Argo program, the temporal and spatial resolution of salinity and temperature profiles since early 2000 have significantly improved, allowing to calculate even monthly means. To further improve temporal and spatial resolution of salinity, Argo profiles are combined with altimetry data and a "Transfer function", the Gravest Empirical Mode (GEM), is calculated. The GEM technique exploits the relationship between T/S profiles and dynamic height in order to parameterize salinity data as a function of dynamic height from the satellite altimetry. This technique gives the opportunity to extend the investigation of the salinity variability, with extremely high temporal (daily) and spatial (1/4°) resolution, back to 1993, the beginning of the altimetry data. This method was tested on several regions of the North Atlantic and it works particularly well for some of them, for example in the regions where the North Atlantic Current plays an important role. Within these regions salinity variability in
Chernyshkov, P. P.; Sirota, A. M.; Lebedev, S. A.
Because of North Atlantic fish stocks are declined the problem of new fishery region searching or bygone fishery grounds return to come into particular prominence. One of the such region is open part of the southeastern Pacific, which was the traditional fishery ground for horse mackerel fishery in 80-90s. In September 2002 fishery and oceanographic expedition had been sent in the region. The main purposes of the expedition are investigation of biologic state of fish species and oceanographic conditions in the region. These investigations were supported with data of satellite observations. Processing of satellite data and mapping were carried out in the AtlantNIRO. To improve the spatial coverage beyond that available from in situ data from CTD survey, satellite SST and surface dynamic topography data were being prepared and transmitted to the vessel. Data source for SST, derived from AVHRR, was the NOAA Satellite Active Archive. Surface dynamic topography derived from the TOPEX/POSEIDON Quick-Look GDR v.1 data. Preliminary results of analysis of synoptic variability of that region as well as comparison of in situ observation with satellite data are presented. Efficiency of use altimetry products for support of exploratory researches is estimated.
de C. Abreu, Luiza Gontijo Álvares; Maillard, Philippe
Different satellite missions have instruments to measure the water level variation of oceans and some of these instruments are being used in continental water applications with satisfying results. Altimeters on-board the Envisat and SARAL(Altika) satellites are consistently used to measure the water level in continental water bodies. Recent studies on satellite altimetry combined with satellite imagery have shown the great potential of this technique to estimate the water volume of rivers, lakes, wetlands and reservoirs and its temporal variation in response to climate and other environmental variables. A consistent monitoring of water level variations in reservoirs is crucial to the development policies and implementation of actions regarding the distribution and use of the stored water resource. The Trés Marias reservoir is located within the São Francisco river basin, known as the national integration" river, which provides water flow to the semi-arid region of Brazil. This study presents a method to combine satellite altimetry and imagery of the lake's surface to estimate volume changes and create a model from which volume changes could be computed from either the altimetry or the lake's surface area. Our intention with this study is to evaluate the method and its precision, and the possibility to apply it in other areas, such as wetlands and other lakes where in situ measurements are not available. Moreover, data of monitoring stations usually have an arbitrary altitude reference and are not available for the general public; the data from the satellite altimetry has the advantage of being of global reference (geoid) and compatible with the establishment of a worldwide lake and reservoir database. We combined Envisat and SARAL/Altika altimetry data from 2007-2014 period with Landsat imagery from the same time frame. The data was corrected using a novel processing technique resulting in a relative precision of 0.24 m (RMSE).
Vigo, Isabel M.; Sánchez Reales, José M.; Trottini, Mario
In this work we study for the first time absolute Surface Geostrophic Currents (SGC) variations using only satellite data. The proposed approach combines 18 years altimetry data, which provide reliable measurements of the Absolute Sea Level (ASL) height with a GOCE geoid model to obtain a Dynamic Topography with an unprecedented precision and accuracy. Our proposal allows overcoming the main limitations of existing approaches based solely on altimetry data (that suffer the lack of an independent reference to derive ASL maps), and approximations based on in-situ data (which are characterized by a sparse and non homogeneous coverage in time and space). Features of the SGC annual variations are also addressed. As a result of our study we provide a new climatology of absolute SGC in the form of a 52 weeks data set of surface current fields, gridded at a quarter degree longitude and latitude resolution resolving spatial scales as short as 140 km. For presentation, this data set is averaged monthly and the results, presented as monthly climatology, are compared with a climatology based on in-situ observations from drifter data.
Sánchez-Reales, J. M.; Vigo, M. I.; Trottini, M.
We have studied, for the first time, variations in absolute surface geostrophic currents (SGC) using satellite data only. The proposed approach combines 18 years' altimetry data, which provide reliable measurements of absolute sea level (ASL), with a gravity field and steady-state ocean circulation explorer geoid model to obtain dynamic topography, and achieves unprecedented precision and accuracy. Our proposal overcomes the main limitations of existing approaches based solely on altimetry data (which suffer from lack of an independent reference for derivation of ASL maps), and approximations based on in-situ data (which are characterized by a sparse and inhomogeneous coverage in time and space). Features of annual variations of SGC are also addressed. As a result of our study we provide new absolute SGC climatology in the form of a 52-week data set of surface current fields, gridded at quarter degree longitude and latitude resolution and resolving spatial scales as short as 140 km. For presentation, this data set is averaged monthly and the results, presented as monthly climatology, are compared with climatology based on in-situ observations from drifter data.
Satellite altimetry has enabled the study of global oceanic mesoscale variability with increasing accuracy and resolution for the past three decades. The combination of the series of precision missions beginning with TOPEX/Poseidon and the series of missions beginning with ERS-1 has created a data record of sea surface height measurement from at least two simultaneously operating altimeters. This 19-year record has fundamentally expanded our knowledge about the dynamics of ocean circulation, in particular at the mesoscale. The progress made to date from the data record will be briefly reviewed, with emphasis on the remaining open questions. Spectral analysis of the existing altimeter data suggests that the spatial resolution is about 150 km in wavelength in space-time gridded data, and about 70-100 km in along-track data. The unresolved short scales, however, have important roles in the energy balance of ocean dynamics as well as the transport and dissipation of many properties of the ocean such as heat and dissolved chemicals. The prospect of the technique of radar interferometry for making high-resolution wide-swath measurement of sea surface height will be discussed with an update on the development of the SWOT (Surface Water and Ocean Topography) Mission, which is being jointly developed by NASA and CNES with contributions from the Canadian Space Agency. SWOT is being designed for applications in both oceanography and land surface hydrology and setting a standard for the next-generation altimetry missions.
Lawver, L. A.; Gahagan, L. M.
Tight-fit plate reconstructions are produced using a global database constrained by marine magnetic anomalies tied to a consistent timescale, paleomagnetic poles, seafloor age dates based on drilling results, and fracture zone and transform fault lineations picked from ship-track and satellite altimetry data. Where a prominent steep gradient in the satellite altimetry data is present near the continental-ocean transition, it is used as a proxy to the continental shelf break [CSB]. Continental block outlines are based on digitization of the steep gradient. In some places, notably off Namibia, there is a very close correlation between that gradient and the ocean-continent boundary deduced from seismic refraction and reflection data. In other regions, there may be some stretched continental crust oceanward of the steep gradient but for reconstruction purposes we assume the crust to be predominantly continental landward of the boundary and oceanic, seaward of the line. Good matchs for conjugate CSBs are found in many places world-wide along passive margins and these will be highlighted. Particularly good matches are observed between the cratonic edges of East Antarctica as determined by sub-ice topographic highs seen along the margins of East Antarctica with respect to Madagascar, Sri Lanka, the southern half of the eastern margin of India, and the region of Australia between 124° E and 133° E along the Great Australian Bight (GAB). There are overlaps of the reconstructed conjugate CSBs, with one overlap between India and East Antarctica (70° E to 85° E) and one between East Antarctica and the western section of the GAB (105° E to 120° E). These two overlaps are coincident with the outer margins of the Lambert Graben - Prydz Bay Basin and the Aurora Subglacial Basin, respectively. It is known that there are substantial glacially-derived sediments prograded off the continental margin onto oceanic crust at Prydz Bay where there may be as much as 200 km in width of
Paris, Adrien; Dias de Paiva, Rodrigo; Santos da Silva, Joecila; Medeiros Moreira, Daniel; Calmant, Stephane; Garambois, Pierre-André; Collischonn, Walter; Bonnet, Marie-Paule; Seyler, Frederique
In this study, rating curves (RCs) were determined by applying satellite altimetry to a poorly gauged basin. This study demonstrates the synergistic application of remote sensing and watershed modeling to capture the dynamics and quantity of flow in the Amazon River Basin, respectively. Three major advancements for estimating basin-scale patterns in river discharge are described. The first advancement is the preservation of the hydrological meanings of the parameters expressed by Manning's equation to obtain a data set containing the elevations of the river beds throughout the basin. The second advancement is the provision of parameter uncertainties and, therefore, the uncertainties in the rated discharge. The third advancement concerns estimating the discharge while considering backwater effects. We analyzed the Amazon Basin using nearly one thousand series that were obtained from ENVISAT and Jason-2 altimetry for more than 100 tributaries. Discharge values and related uncertainties were obtained from the rain-discharge MGB-IPH model. We used a global optimization algorithm based on the Monte Carlo Markov Chain and Bayesian framework to determine the rating curves. The data were randomly allocated into 80% calibration and 20% validation subsets. A comparison with the validation samples produced a Nash-Sutcliffe efficiency (Ens) of 0.68. When the MGB discharge uncertainties were less than 5%, the Ens value increased to 0.81 (mean). A comparison with the in situ discharge resulted in an Ens value of 0.71 for the validation samples (and 0.77 for calibration). The Ens values at the mouths of the rivers that experienced backwater effects significantly improved when the mean monthly slope was included in the RC. Our RCs were not mission-dependent, and the Ens value was preserved when applying ENVISAT rating curves to Jason-2 altimetry at crossovers. The cease-to-flow parameter of our RCs provided a good proxy for determining river bed elevation. This proxy was validated
Hofton, M. A.; Minster, J.-B.; Ridgway, J. R.; Williams, N. P.; Blair, J. B.; Rabine, D. L.; Bufton, J. L.
The topography of the Long Valley caldera, California, was sampled using airborne laser altimetry in 1993, 1995, and 1997 to test the feasibility of using airborne laser altimetry for monitoring deformation of volcanic origin. Results show the laser altimeters are able to resolve subtle topographic features such as a gradual slope and to detect small transient changes in lake elevation. Crossover and repeat pass analyses of laser tracks indicate decimeter-level vertical precision is obtained over flat and low-sloped terrain for altimeter systems performing waveform digitization. Comparisons with complementary, ground-based CPS data at a site close to Bishop airport indicate that the laser and GPS-derived elevations agree to within the error inherent in the measurement and that horizontal locations agree to within the radius of the laser footprint. A comparison of the data at two sites, one where no change and the other where the maximum amount of vertical uplift is expected, indicates approximately 10 cm of relative uplift occurred 1993-1997, in line with predictions from continuous CPS measurements in the region. Extensive terrain mapping flights during the 1995 and 1997 missions demonstrate some of the unique abilities of laser altimetry; the straightforward creation of high resolution, high accuracy digital elevation models of overflown terrain, and the ability to determine ground topography in the presence of significant ground cover such as dense tree canopies. These capabilities make laser altimetry an attractive technique for quantifying topographic change of volcanic origin, especially in forested regions of the world where other remote sensing instruments have difficulty detecting the underlying topography.
Hofton, M. A.; Minster, J.-B.; Ridgway, J. R.; Williams, N. P.; Blair, J.-B.; Rabine, D. L.; Bufton, J. L.
The topography of the Long Valley caldera, California, was sampled using airborne laser altimetry in 1993, 1995, and 1997 to test the feasibility of using airborne laser altimetry for monitoring deformation of volcanic origin. Results show the laser altimeters are able to resolve subtle topographic features such as a gradual slope and to detect small transient changes in lake elevation. Crossover and repeat pass analyses of laser tracks indicate decimeter-level vertical precision is obtained over flat and low-sloped terrain for altimeter systems performing waveform digitization. Comparisons with complementary, ground-based GPS data at a site close to Bishop airport indicate that the laser and GPS-derived elevations agree to within the error inherent in the measurement and that horizontal locations agree to within the radius of the laser footprint. A comparison of the data at two sites, one where no change and the other where the maximum amount of vertical uplift is expected, indicates approximately 10 cm of relative uplift occurred 1993-1997, in line with predictions from continuous GPS measurements in the region. Extensive terrain mapping flights during the 1995 and 1997 missions demonstrate some of the unique abilities of laser altimetry; the straightforward creation of high resolution, high accuracy digital elevation models of overflown terrain, and the ability to determine ground topography in the presence of significant ground cover such as dense tree canopies. These capabilities make laser altimetry an attractive technique for quantifying topographic change of volcanic origin, especially in forested regions of the world where other remote sensing instruments have difficulty detecting the underlying topography.
Baek, S.; Kwoun, Oh-Ig; Braun, Andreas; Lu, Zhiming; Shum, C.K.
We present a digital elevation model (DEM) of King Edward VII Peninsula, Sulzberger Bay, West Antarctica, developed using 12 European Remote Sensing (ERS) synthetic aperture radar (SAR) scenes and 24 Ice, Cloud, and land Elevation Satellite (ICESat) laser altimetry profiles. We employ differential interferograms from the ERS tandem mission SAR scenes acquired in the austral fall of 1996, and four selected ICESat laser altimetry profiles acquired in the austral fall of 2004, as ground control points (GCPs) to construct an improved geocentric 60-m resolution DEM over the grounded ice region. We then extend the DEM to include two ice shelves using ICESat profiles via Kriging. Twenty additional ICESat profiles acquired in 2003-2004 are used to assess the accuracy of the DEM. After accounting for radar penetration depth and predicted surface changes, including effects due to ice mass balance, solid Earth tides, and glacial isostatic adjustment, in part to account for the eight-year data acquisition discrepancy, the resulting difference between the DEM and ICESat profiles is -0.57 ?? 5.88 m. After removing the discrepancy between the DEM and ICESat profiles for a final combined DEM using a bicubic spline, the overall difference is 0.05 ?? 1.35 m. ?? 2005 IEEE.
Townsend, W. F.
Current NASA plans for altimetry and scatterometry of the oceans using spaceborne instrumentation are outlined. The data of interest covers geostrophic and wind-driven circulation, heat content, the horizontal heat flux of the ocean, and the interactions between atmosphere and ocean and ocean and climate. A proposed TOPEX satellite is to be launched in 1991, carrying a radar altimeter to measure the ocean surface topography. Employing dual-wavelength operation would furnish ionospheric correction data. Multibeam instruments could also be flown on the multiple-instrument polar orbiting platforms comprising the Earth Observation System. A microwave radar scatterometer, which functions on the basis of Bragg scattering of microwave energy off of wavelets, would operate at various view angles and furnish wind speeds accurate to 1.5 m/sec and directions accurate to 20 deg.
Madzak, Matthias; Böhm, Sigrid; Böhm, Johannes; Bosch, Wolfgang; Schuh, Harald
A new model for Earth rotation variations based on ocean tide models is highly desirable in order to close the gap between geophysical Earth rotation models and geodetic observations. We have started a project, SPOT (Short Period Ocean Tidal variations in Earth Rotation), with the goal to develop a new model of short period Earth rotation variations based on one of the best currently available empirical ocean tide models obtained from satellite altimetry. We employ the EOT11a model which is an upgrade of EOT08a, developed at DGFI, Munich. As EOT11a does not provide the tidal current velocities which are fundamental contributors to Earth rotation excitation, the calculation of current velocities from the tidal elevations is one of three main areas of research in project SPOT. The second key aspect is the conversion from ocean tidal angular momentum to the corresponding ERP variations using state-of-the-art transfer functions. A peculiar innovation at this step will be to consider the Earth's response to ocean tidal loading based on a realistic Earth model, including an anelastic mantle. The third part of the project deals with the introduction of the effect of minor tides. Ocean tide models usually only provide major semi-diurnal and diurnal tidal terms and the minor tides have to be inferred through admittance assumptions. Within the proposed project, selected minor tidal terms and the corresponding ERP variations shall be derived directly from satellite altimetry data. We determine ocean tidal angular momentum of four diurnal and five sub-daily tides from EOT11a and apply the angular momentum approach to derive a new model of ocean tidal Earth rotation variations. This poster gives a detailed description of project SPOT as well as the status of work progress. First results are presented as well.
Cazenave, A. A.; Llovel, W.; Becker, M.; Cretaux, J.
Global change in land water storage and its effect on sea level is estimated over a 6-year time span (mid-2002 to mid-2008) using satellite altimetry and space gravimetry data from GRACE. Satellite altimetry allows determination of surface water volume change while GRACE data provide vertically-integrated water storage change. The 32 largest river basins are considered as well as lakes not included in the 32 basins (Caspian and Aral seas). We focus on the year to year variability and construct a combined water storage time series that we further express in equivalent sea level time series. The mean trend in total water storage adjusted over this 6-year time span is positive and amounts to 114 +/- 24 km3/yr (net water storage excess). Most of the positive contribution arises from the Amazon and Siberian basins (Lena and Yenisei), followed by the Orinoco, Ob, Nile, Niger, Zambezi, Tocantins and Volga. The largest negative contributions (water deficit) come from the Mississippi, Yukon, Eyre, Brahmaputra, Ganges, Eyre, Murray and Mekong basins. Lakes volume change is slightly negative over the 2002-2008 time span (~ -16 km3/yr). Expressed in terms of equivalent sea level, total water volume change over 2002-2008 leads to a small negative contribution to sea level of -0.27 +/- 0.07 mm/yr. The time series for each basins clearly show that year to year variability dominates so that the value estimated in this study cannot be considered as representative of a long-term trend. Another interesting results of the study is the significant correlation (0.7) between (detrended) year-to- year variability in sea level (corrected for thermal expansion) and GRACE-based land water storage contribution.
Zheng, Jiajia; Ke, Changqing; Shao, Zhude; Li, Fei
Lake level and volume are sensitive to climate change, and their changes can affect the sustainable utilization of regional water resources. Satellite radar/laser altimetry has effectively been used for monitoring water-level changes in recent years. In this study, satellite altimetry data and optical images were used to assess the changes in water level, area, and volume of Hulun Lake in north-eastern China. We derived a time series of lake levels for nearly two decades (1992 to 2010) from the altimetry data of two satellite sensors (Topex/Poseidon and Envisat RA-2); additionally, lake surface extent was extracted from Landsat TM/ETM+ images during the same period. The results indicate that the water level, area, and volume of Hulun Lake decreased over the past two decades. The water level shows a significant decrease (-0.36 m/year) of a total of -5.21 m from 1992 to 2010, specifically including a slight decrease (-0.4 m) during 1992 to 1999 and a sudden drop (-4.81 m) during 2000 to 2010. There has also been a consistent and significant reduction in lake area (-355.35 km2) and volume (-12.92 km3). An integrated examination on changes in temperature, evaporation, precipitation, and runoff during 1992 to 2010 shows that the main changes in the Hulun Lake area are correlated with increasing temperature (0.47°C/year) and evaporation (13.61 mm/year), as well as decreasing precipitation (-6.58 mm/year) and runoff (-1.04×108 m3/year). Thus, we infer that climate warming is likely the main cause of the changes in water level, area, and volume of Hulun Lake. In addition, anthropogenic factors accelerate the degradation of the Hulun Lake wetland to some extent.
Lebedev, S.; Sirota, A.
Analyse of structure and time-space variability of Subantarctic front and South Pacific ocean current in Pacific ocean southeast part (20--45S, 70--110W) are based on the sea surface dynamic heights calculated by the TOPEX/Poseidon satellite altimetry data and gradients of the sea surface temperature for time period 1992-2003. The sea surface dynamic heights constructed by the superposition of sea level anomaly distributions over the climatic dynamic topography and temperature gradients at the ocean surface on the basis of the satellite Multi-Channel Sea Surface Temperature (MCSST) data. Comparison calculations results of the Subantarctic front and the Southern Pacific current position on basin the satellite data with the data of the research ship ``Atlantida'' measurements (November - December 2002) has shown good data fit. The analysis of scads fishery distribution in a southeast part of Pacific Ocean (the drag-net and the acoustic data of fish accumulation) and synoptic variability of the sea surface dynamic heights has revealed, that the distribution of the most dense of the fish accumulation is connected to dynamic heterogeneities, which are on northern peripherals of the Subantarctic front.
Kuo, C.; Shum, C.; Alsdorf, D. E.; Lee, H.; Yang, T.; Tseng, K.
In the framework of developing Global Geodetic Observing Systems (GGOS) data sets for hydrologic research and application, here we present a study to potentially exploit a recent new and innovative use of satellite radar altimetry data from abundant historic, present and future missions, including Geosat GM/ERM, ERS-1/-2, TOPEX/Poseidon, GFO, Envisat, Jason-1/-2/-3, CryoSat-2, AltiKa, HY-2, and Sentinel-3. Satellite altimetry has demonstrated its ability to measure accurate, long-term climate records such as the evolutions of sea-level and inland hydrologic water level changes. In particular, recent studies have demonstrated the ability of radar altimetry, via waveform retracking and other innovative processing, to monitor water level variations of small water bodies like rivers (as narrow as 100-200 m, approaching the along-track spatial sampling limit of a 20-Hz altimeter measurement onboard of a spacecraft with speed of ~7.5 km/sec) and small lakes over flat terrains. In this contribution, we address an additional challenge to examine the feasibility of the use of contemporary pulse-limited nadir radar altimetry to observe water level variations over mountainous or terrains with relatively steep gradients, or surfaces with varying seasonal land-covers. In this case, the returned radar waveforms from satellite radar altimetry, if the altimeter measurements remain locked, are much noisier over these complex and steep terrains or rough surfaces. This study will present results on the use of retracked Envisat altimetry data (i.e., not using the retracked heights already available on the GDR) via various waveform retracking algorithms over the Tseng Wen Reservoir, Taiwan, and evaluating the accuracy of retracked measurements by comparing to the available in situ water gauge records. The Tseng Wen Reservoir is a relatively small reservoir (12 km x 2 km, the width of the reservoir crossed by Envisat is 1.5~2 km), located in Chiayi County, Taiwan, with an elevation of
Safren, H. G.; Bufton, J. L.
A computer program, written in FORTRAN, is described which uses a microcomputer to interactively process and plot laser altimetry data taken with a laser altimeter currently under development at the Goddard Space Flight Center. The program uses a plot routine written for a particular microcomputer, so that the program could only be implemented on a different computer by replacing the plot routine. The altimetry data are taken from an aircraft flying over mountainous terrain. The program unpacks the raw data, processes it into along-track distance and ground height and creates plots of the terrain profile. A zoom capability is provided to expand the plot to show greater detail, along either axis, and provision is made to interactively edit out spurious data points.
Safren, H. G.; Bufton, J. L.
A computer program, written in FORTRAN, is described which uses a microcomputer to interactively process and plot laser altimetry data taken with a laser altimeter currently under development at the Goddard Space Flight Center. The program uses a plot routine written for a particular microcomputer, so that the program could only be implemented on a different computer by replacing the plot routine. The altimetry data are taken from an aircraft flying over mountainous terrain. The program unpacks the raw data, processes it into along-track distance and ground height and creates plots of the terrain profile. A zoom capability is provided to expand the plot to show greater detail, along either axis, and provision is made to interactively edit out spurious data points.
Lee, Hyongki; Kim, Jin-woo; Lu, Zhong; Jung, Hahn Chul; Shum, C. K.; Alsdorf, Doug
Wetland loss in Louisiana has been accelerating due primarily to anthropogenic and nature processes, and is being advocated as a problem with national importance. Accurate measurement or modeling of wetland-wide water level changes, its varying extent, its storage and discharge changes resulting in part from sediment loads, erosion and subsidence are fundamental to assessment of hurricane-induced flood hazards and wetland ecology. Here, we use innovative method to integrate interferometric SAR (InSAR) and satellite radar altimetry for measuring absolute or geocentric water level changes and applied the methodology to remote areas of swamp forest in coastal Louisiana. Coherence analysis of InSAR pairs suggested that the HH polarization is preferred for this type of observation, and polarimetric analysis can help to identi:fy double-bonnce backscattering areas in the wetland. Envisat radar altimeter-measured 18- Hz (along-track sampling of 417 m) water level data processed with regional stackfile method have been used to provide vertical references for water bodies separated by levees. The high-resolution (approx.40 m) relative water changes measured from ALOS PALSAR L-band and Radarsat-l C-band InSAR are then integrated with Envisat radar altimetry to obtain absolute water level. The resulting water level time series were validated with in situ gauge observations within the swamp forest. Furthermore, we compare our water elevation changes with 2D flood modeling from LISFLOOD hydrodynamic model. Our study demonstrates that this new technique allows retrospective reconstruction and concurrent monitoring of water conditions and flow dynamics in wetlands, especially those lacking gauge networks.
Farzaneh, Saeed; Sharifi, Mohammad Ali
The upper part of the Earth atmosphere is important for ground-based and satellite radio communication and navigation. It consists of free electrons and ions, mainly ionized by solar radiation. Free electrons, in the ionosphere, have a strong impact on the propagation of radio waves. When the signals pass through the ionosphere, both their group and phase velocity are disturbed. The effect is in first approximation proportional to TEC along the signal path and inversely proportional to the frequency squared. Several space geodetic techniques such as satellite altimetry, LEO satellite and Very Long Baseline Interferometry (VLBI) can be used for modeling the total electron content. In this study the combination of the data from ground GPS observation over west part of USA and from the altimetry mission Jason-2 is performed on the normal equation level in least-square producer and the least-squares variance component estimation is applied to consider the different accuracy levels of the observations. The integrated ionosphere model is expected to be more accurate and reliable than the results derived by the ground GPS observation over the oceans. Keywords: Slepian function, LS-VCE, satellite altimetry, GPS, local ionospheric modeling
Asadzadeh Jarihani, Abdollah; Callow, Nik
Many large rivers (especially those in remote and dryland regions) have sparse or no hydrological gauging data. We explore whether altimetry satellites can provide a remote sensing approach to measure discharge at "virtual" gauging stations, where altimetry tracks intercept large rivers. Based on our evaluation of the Topex/Poseidon (T/P), Jason 1 &2, Envisat, GFO and ICESat platforms on permanent terrestrial water bodies (Lake Argyle and Lake Eildon in Australia), we found that some satellites (ICESat (operational 2003-2009) and Jason-2 "PISTACH" (Prototype Innovant de Système de Traitement pour les Applications Côtières et l'Hydrologie data (operational 2008 to present)) could deliver high accuracy data (RMSE vertical accuracies of 10-25cm.) for large channel and floodplain flows where the inundated area was at least 1km wide. For six locations where Jason-2 tracks passed over the Cooper Creek and Diamantina River (Lake Eyre Basin), we installed water pressure loggers to evaluate the accuracy of Jason-2 PISTACH data during the 2011/12 wet season. While altimetry platforms are impacted by "edge" affects (e.g. trees), these large rivers where the inundated floodplain width can extend up to 65km is highly suited to virtual gauging by Jason-2 altimetry data. Across the six study sites , which included 49 satellite passes across the 2011-12 flood, we find a R2 between 0.90 to 0.98 (mean = 0.955) at the gauges we installed. The altimetry satellites provide continuous water level data at the resolution of every 10 days and reproduces the hydrograph from the water level loggers. The offset path (i.e. non-perpendicular to river channel) also gives important hydrodynamic data on downstream water surface slope. We conclude that altimetry satellite data across the legacy archive available from ICESat, and the Jason-2 data since 2008 is able to provide high accuracy (and near real-time) water level data that can be used for direct data assimilation and model calibration of
Tourian, M. J.; Tarpanelli, A.; Elmi, O.; Qin, T.; Brocca, L.; Moramarco, T.; Sneeuw, N.
Limitations of satellite radar altimetry for operational hydrology include its spatial and temporal sampling as well as measurement problems caused by local topography and heterogeneity of the reflecting surface. In this study, we develop an approach that eliminates most of these limitations to produce an approximately 3 day temporal resolution water level time series from the original typically (sub)monthly data sets for the Po River in detail, and for Congo, Mississippi, and Danube Rivers. We follow a geodetic approach by which, after estimating and removing intersatellite biases, all virtual stations of several satellite altimeters are connected hydraulically and statistically to produce water level time series at any location along the river. We test different data-selection strategies and validate our method against the extensive available in situ data over the Po River, resulting in an average correlation of 0.7, Root-Mean-Square Error of 0.8 m, bias of -0.4 m, and Nash-Sutcliffe Efficiency coefficient of 0.5. We validate the transferability of our method by applying it to the Congo, Mississippi, and Danube Rivers, which have very different geomorphological and climatic conditions. The methodology yields correlations above 0.75 and Nash-Sutcliffe coefficients of 0.84 (Congo), 0.34 (Mississippi), and 0.35 (Danube).
Le Traon, P. Y.
The launch of the US/French mission Topex/Poseidon (T/P) (CNES/NASA) in August 1992 was the start of a revolution in oceanography. For the first time, a very precise altimeter system optimized for large scale sea level and ocean circulation observations was flying. T/P alone could not observe the mesoscale circulation. In the 1990s, the ESA satellites ERS-1/2 were flying simultaneously with T/P. Together with my CLS colleagues, we demonstrated that we could use T/P as a reference mission for ERS-1/2 and bring the ERS-1/2 data to an accuracy level comparable to T/P. Near real time high resolution global sea level anomaly maps were then derived. These maps have been operationally produced as part of the SSALTO/DUACS system for the last 15 yr. They are now widely used by the oceanographic community and have contributed to a much better understanding and recognition of the role and importance of mesoscale dynamics. Altimetry needs to be complemented with global in situ observations. In the end of the 90s, a major international initiative was launched to develop Argo, the global array of profiling floats. This has been an outstanding success. Argo floats now provide the most important in situ observations to monitor and understand the role of the ocean on the earth climate and for operational oceanography. This is a second revolution in oceanography. The unique capability of satellite altimetry to observe the global ocean in near real time at high resolution and the development of Argo were essential to the development of global operational oceanography, the third revolution in oceanography. The Global Ocean Data Assimilation Experiment (GODAE) was instrumental in the development of the required capabilities. This paper provides an historical perspective on the development of these three revolutions in oceanography which are very much interlinked. This is not an exhaustive review and I will mainly focus on the contributions we made together with many colleagues and
Le Traon, P. Y.
The launch of the French/US mission Topex/Poseidon (T/P) (CNES/NASA) in August 1992 was the start of a revolution in oceanography. For the first time, a very precise altimeter system optimized for large-scale sea level and ocean circulation observations was flying. T/P alone could not observe the mesoscale circulation. In the 1990s, the ESA satellites ERS-1/2 were flying simultaneously with T/P. Together with my CLS colleagues, we demonstrated that we could use T/P as a reference mission for ERS-1/2 and bring the ERS-1/2 data to an accuracy level comparable to T/P. Near-real-time high-resolution global sea level anomaly maps were then derived. These maps have been operationally produced as part of the SSALTO/DUACS system for the last 15 yr. They are now widely used by the oceanographic community and have contributed to a much better understanding and recognition of the role and importance of mesoscale dynamics. Altimetry needs to be complemented with global in situ observations. At the end of the 90s, a major international initiative was launched to develop Argo, the global array of profiling floats. This has been an outstanding success. Argo floats now provide the most important in situ observations to monitor and understand the role of the ocean on the earth climate and for operational oceanography. This is a second revolution in oceanography. The unique capability of satellite altimetry to observe the global ocean in near-real-time at high resolution and the development of Argo were essential for the development of global operational oceanography, the third revolution in oceanography. The Global Ocean Data Assimilation Experiment (GODAE) was instrumental in the development of the required capabilities. This paper provides an historical perspective on the development of these three revolutions in oceanography which are very much interlinked. This is not an exhaustive review and I will mainly focus on the contributions we made together with many colleagues and
Ray, Richard D.; Chao, Benjamin F. (Technical Monitor)
Satellite altimetry has opened a surprising new avenue to observing internal tides in the open ocean. The tidal surface signatures are very small, a few cm at most, but in many areas they are robust, owing to averaging over many years. By employing a simplified two dimensional wave fitting to the surface elevations in combination with climatological hydrography to define the relation between the surface height and the current and pressure at depth, we may obtain rough estimates of internal tide energy fluxes. Initial results near Hawaii with Topex/Poseidon (T/P) data show good agreement with detailed 3D (three dimensional) numerical models, but the altimeter picture is somewhat blurred owing to the widely spaced T/P tracks. The resolution may be enhanced somewhat by using data from the ERS-1 (ESA (European Space Agency) Remote Sensing) and ERS-2 satellite altimeters. The ERS satellite tracks are much more closely spaced (0.72 deg longitude vs. 2.83 deg for T/P), but the tidal estimates are less accurate than those for T/P. All altimeter estimates are also severely affected by noise in regions of high mesoscale variability, and we have obtained some success in reducing this contamination by employing a prior correction for mesoscale variability based on ten day detailed sea surface height maps developed by Le Traon and colleagues. These improvements allow us to more clearly define the internal tide surface field and the corresponding energy fluxes. Results from throughout the global ocean will be presented.
Ray, Richard D.
The most convincing estimates of mantle Q at periods of many hours have historically come from extrapolating seismic and free-oscillation estimates via some assumed frequency dependence, sometimes contrained by estimates from the Chandler Wobble. At the semidiurnal tidal period, direct estimates of Q have been difficult to obtain because of the dominating signals of the ocean tides, which account for more than 95!k of the tidal energy dissipation. But knowledge of the ocean tides has been rapidly improving, primarily owing to satellite altimetry, and in 1996 we reported (NATURE, 381, 595-7) an estimate of solid-earth tidal energy dissipation and mantle Q based on combining satellite altimeter measurements with tracking observations of tidally induced satellite orbit perturbations. Tidal estimates from both reveal a small systematic difference in the quadrature component of the degree-2, order-2 spherical harmonic coefficients, which we attribute to a small lag in the earth's body tide. The formalism accounts for this lag in both the altimeter and tracking solutions and also accounts for a very small contribution from the lunar atmospheric tide. Since this original report, both altimeter and tracking estimates have improved. Recent solutions for the body-tide lag at the M2 period are 0.20 +/- 0.09 degrees, implying an energy dissipation of 100 +/- 50 gigawatts and a solid-earth Q of 300. Further new solutions will be discussed, as will the prospects for significantly reducing error bars and for obtaining estimates from other tides in the diurnal band.
Abshire, James B.; Mcgarry, Jan F.; Pacini, Linda K.; Blair, J. Bryan; Elman, Gregory C.
A numerical simulator of a pulsed, direct detection laser altimeter has been developed to investigate the performance of space-based laser altimeters operating over surfaces with various height profiles. The simulator calculates the laser's optical intensity waveform as it propagates to and is reflected from the terrain surface and is collected by the receiver telescope. It also calculates the signal and noise waveforms output from the receiver's optical detector and waveform digitizer. Both avalanche photodiode and photomultiplier detectors may be selected. Parameters of the detected signal, including energy, the 50 percent rise-time point, the mean timing point, and the centroid, can be collected into histograms and statistics calculated after a number of laser firings. The laser altimeter can be selected to be fixed over the terrain at any altitude. Alternatively, it can move between laser shots to simulate the terrain profile measured with the laser altimeter.
Connor, L. N.; McAdoo, D. C.; Laxon, S.; Ridout, A.
Over the past decade, satellite altimetry has emerged as a valuable tool for taking sea ice monitoring from traditional extent measurements (ie. passive microwave) into the third dimension - estimates of sea ice thickness and volume. Thickness estimates are fundamental to improved understanding of polar dynamics and climate modeling. Several studies have now demonstrated the use of both microwave (ERS-2, Envisat/RA-2, CryoSat-2) and laser (ICESat/GLAS) altimeters for determining sea ice thickness. Sea ice, however, is complex and the task of precisely determining its thickness from satellite measurements remains a challenge. Understanding and validating radar returns from sea ice is key to meeting this challenge. Several satellite validation underflights, conducted over Arctic sea ice between 2006 and 2011 using NASA's P-3 and DC-8 aircraft, are evaluated with the goal of understanding and cataloguing particular features, benefits, and caveats of using radar altimeters to measure sea ice. The underflights include ~1000 km Envisat tracks north of the Canadian Archipelago flown in 2006, 2009, 2010, and 2011, and 700 km CryoSat-2 tracks in the northern Arctic Ocean during 2010, 2011, and 2012. All but the 2006 flight were part of Operation Ice Bridge (OIB). Airborne data collected during these flights include data from two laser altimeters, Ku-band and snow thickness radar altimeters, high-resolution digital photography, and gravimetry. Out-and-back flight tracks combined with georegistered digital photography allow a quantitative assessment of lead "snagging" and off-ranging found in radar altimeter measurements over sea ice. Particular attention is given to the measurement of lead elevations with radar altimetry and its impact on sea ice freeboard estimates.
Davis, A. B.; Varnai, T.; Marshak, A.
The primary goal of NASA's current ICESat and future ICESat2 missions is to map the altitude of the Earth's land ice with high accuracy using laser altimetry technology, and to measure sea ice freeboard. Ice however is a highly transparent optical medium with variable scattering and absorption properties. Moreover, it is often covered by a layer of snow with varying depth and optical properties largely dependent on its age. We describe a modeling framework for estimating the potential altimetry bias caused by multiple scattering in the layered medium. We use both a Monte Carlo technique and an analytical diffusion model valid for optically thick media. Our preliminary numerical results are consistent with estimates of the multiple scattering delay from laboratory measurements using snow harvested in Greenland, namely, a few cm. Planned refinements of the models are described.
Paris, Adrien; Paiva, Rodrigo C. D.; Santos da Silva, Joecila; Medeiros Moreira, Daniel; Calmant, Stéphane; Collischonn, Walter; Bonnet, Marie-Paule; Seyler, Frédérique
The Amazonian basin is the largest hydrological basin all over the world. Over the past few years, it has experienced an unusual succession of extreme droughts and floods, which origin is still a matter of debate. One of the major issues in understanding such events is to get discharge series distributed over the entire basin. Satellite altimetry can be used to improve our knowledge of the hydrological stream flow conditions in the basin, through rating curves. Rating curves are mathematical relationships between stage and discharge at a given place. The common way to determine the parameters of the relationship is to compute the non-linear regression between the discharge and stage series. In this study, the discharge data was obtained by simulation through the entire basin using the MGB-IPH model with TRMM Merge input rainfall data and assimilation of gage data, run from 1998 to 2009. The stage dataset is made of ~900 altimetry series at ENVISAT and Jason-2 virtual stations, sampling the stages over more than a hundred of rivers in the basin. Altimetry series span between 2002 and 2011. In the present work we present the benefits of using stochastic methods instead of probabilistic ones to determine a dataset of rating curve parameters which are hydrologicaly meaningful throughout the entire Amazon basin. The rating curve parameters have been computed using an optimization technique based on Markov Chain Monte Carlo sampler and Bayesian inference scheme. This technique provides an estimate of the best value for the parameters together with their posterior probability distribution, allowing the determination of a credibility interval for calculated discharge. Also the error over discharges estimates from the MGB-IPH model is included in the rating curve determination. These MGB-IPH errors come from either errors in the discharge derived from the gage readings or errors in the satellite rainfall estimates. The present experiment shows that the stochastic approach
Khaki, M.; Forootan, E.; Sharifi, M. A.; Awange, J.; Kuhn, M.
Satellite radar altimetry observations are used to derive short wavelength gravity anomaly fields over the Persian Gulf and the Caspian Sea, where in situ and ship-borne gravity measurements have limited spatial coverage. In this study the retracking algorithm `Extrema Retracking' (ExtR) was employed to improve sea surface height (SSH) measurements that are highly biased in the study regions due to land contaminations in the footprints of the satellite altimetry observations. ExtR was applied to the waveforms sampled by the five satellite radar altimetry missions: TOPEX/POSEIDON, JASON-1, JASON-2, GFO and ERS-1. Along-track slopes have been estimated from the improved SSH measurements and used in an iterative process to estimate deflections of the vertical, and subsequently, the desired gravity anomalies. The main steps of the gravity anomaly computations involve estimating improved SSH using the ExtR technique, computing deflections of the vertical from interpolated SSHs on a regular grid using a biharmonic spline interpolation and finally estimating gridded gravity anomalies. A remove-compute-restore algorithm, based on the fast Fourier transform, has been applied to convert deflections of the vertical into gravity anomalies. Finally, spline interpolation has been used to estimate regular gravity anomaly grids over the two study regions. Results were evaluated by comparing the estimated altimetry-derived gravity anomalies (with and without implementing the ExtR algorithm) with ship-borne free air gravity anomaly observations, and free air gravity anomalies from the Earth Gravitational Model 2008 (EGM2008). The comparison indicates a range of 3-5 mGal in the residuals, which were computed by taking the differences between the retracked altimetry-derived gravity anomaly and the ship-borne data. The comparison of retracked data with ship-borne data indicates a range in the root-mean-square-error (RMSE) between approximately 1.8 and 4.4 mGal and a bias between 0
Portelli, C.; Ousley, G. W., Sr.
The Laser Geodynamic Satellite 2 (LAGEOS 2) is nearly identical to the LAGEOS 1 satellite, which was launched by NASA in 1976. However, LAGEOS 2 is completely passive, and is equipped with fused silian corner reflectors for ranging with ground-based lasers. The addition of LAGEOS 2 will provide the GSFC laser network with significantly increased satellite tracking opportunities, because LAGEOS 1 is at a 110-degree inclination and LAGEOS 2 will be at a 52-degree inclination. The flight profile is given, and information is presented in tabular form on the following topics: Deep Space Network support, frequency assignments, telemetry, tracking, and tracking support responsibility.
Kuragano, Tsurane; Fujii, Yosuke; Kamachi, Masafumi
This study evaluates capability of the Argo observation network for monitoring ocean variation, especially for eddy-scale variation, by using an optimum interpolation (OI) procedure. Sea surface dynamic height anomalies (DHAs) are derived from Argo temperature and salinity profile data, and DHA fields are obtained by the OI based on the space-time correlation scales estimated from along-track sea level anomaly (SLA) data by satellite altimetry. The DHA fields are compared with the SLA fields derived from the same OI applied to the along-track SLA data. The results show that the equatorial Kelvin waves and tropical instability waves are well captured by Argo floats. Eddies are also monitored effectively in the subtropical western North Pacific. The OI results of DHA do not agree well with those of SLA in the high latitudes. A simple test of the space-time OI analysis shows that more than six data in the e-folding domain, where the correlation coefficient of ocean variation is above e-1, are required for the reliable analysis with 99% confidence level. Argo floats provide sufficient number of observations for the reliable analysis in the low latitudes and some areas in the North Pacific. Two to three times more Argo data would be required in most of midlatitudes and much more in high latitudes for capturing eddy-scale variation.
Ray, R. D.; Beckley, B. D.; Lemoine, F. G.
A somewhat unorthodox method for determining vertical crustal motion at a tide-gauge location is to difference the sea level time series with an equivalent time series determined from satellite altimetry, To the extent that both instruments measure an identical ocean signal, the difference will be dominated by vertical land motion at the gauge. We revisit this technique by analyzing sea level signals at 28 tide gauges that are colocated with DORIS geodetic stations. Comparisons of altimeter-gauge vertical rates with DORIS rates yield a median difference of 1.8 mm/yr and a weighted root-mean-square difference of2.7 mm/yr. The latter suggests that our uncertainty estimates, which are primarily based on an assumed AR(l) noise process in all time series, underestimates the true errors. Several sources of additional error are discussed, including possible scale errors in the terrestrial reference frame to which altimeter-gauge rates are mostly insensitive, One of our stations, Male, Maldives, which has been the subject of some uninformed arguments about sea-level rise, is found to have almost no vertical motion, and thus is vulnerable to rising sea levels. Published by Elsevier Ltd. on behalf of COSPAR.
A method is presented in satellite altimetry that attempts to simultaneously determine the geoid and sea surface topography with minimum wavelengths of about 500 km and to reduce the radial orbit error caused by geopotential errors. The modeling of the radial orbit error is made using the linearized Lagrangian perturbation theory. Secular and second order effects are also included. After a rather extensive validation of the linearized equations, alternative expressions of the radial orbit error are derived. Numerical estimates for the radial orbit error and geoid undulation error are computed using the differences of two geopotential models as potential coefficient errors, for a SEASAT orbit. To provide statistical estimates of the radial distances and the geoid, a covariance propagation is made based on the full geopotential covariance. Accuracy estimates for the SEASAT orbits are given which agree quite well with already published results. Observation equations are develped using sea surface heights and crossover discrepancies as observables. A minimum variance solution with prior information provides estimates of parameters representing the sea surface topography and corrections to the gravity field that is used for the orbit generation. The simulation results show that the method can be used to effectively reduce the radial orbit error and recover the sea surface topography.
Fu, L. L.
Progress on the applications of satellite altimetry from SEASAT and Geos-3 to the study of oceanic mesoscale variability and general circulation is reviewed. The major conclusion for the applications to mesoscale variability is that an optimally designed altimetric mission with a lifetime of several years will improve our knowledge of the global mesoscale variability to an extent unattainable by any other practical means. The proposed Topex mission will allow one to view the global oceanic variability in such a wide range of periods and wavelengths: from 20 days to 3 to 5 years; from 50 to 10,000 km. However, the goal of determining the general circulation cannot be achieved by a single altimetric mission, because a highly accurate geoid needs to be determined independently. The scenario of the combination of Topex with Gravsat, a gravity mission that will give accurate geoid information, will allow the global general circulation to be determined at scales as small as 100 km. Areas of research needing to be performed with existing altimeter data are also discussed.
Lee, H.; Shum, C.; Lu, Z.; Alsdorf, D.; Ibaraki, M.; Braun, A.; Kuo, C.
Coastal estuaries, which connect coastal ocean, wetlands and coastal land region, play important roles in ecological environments. Wetlands typically occur in low-lying areas on the edges of lakes, and rivers, or in coastal areas protected from waves and are found in a variety of climates on every continent except Antarctica. Wetlands not only provide habitat for thousands of aquatic/terrestrial plant and animal species but also control floods by holding water much like a sponge by absorbing and reducing the velocity of storm-water. Human activities have so many negative impacts on wetlands and they became main contributing factors to the wetlands losses. For example, Louisiana's wetlands have lost more than 100 km2 of its area per year (Walker et al., 1987; Bourne, 2000). The loss of Louisiana wetlands as a result of ecological erosion or geological subsidence potentially have had significant impacts in slowing down storm surge from the devastating Hurricane Katrina. The ability to quantitatively measure accurate wetland water level changes in Louisiana has impacts on ecology and natural hazards mitigation including improved storm surge modeling resulting from hurricanes. Interferometric synthetic aperture radar (InSAR) has been proven to be useful to measure centimeter-scale water level changes over Amazon flood plain and Everglades wetland using L-band SAR imagery. This is based on the fact that flooded forests permit double-bounce returns, which allow InSAR coherence to be maintained. Furthermore, ERS-1/2 C-band InSAR data have been used to demonstrate its feasibility to monitor water level changes over Louisiana wetlands. In addition, satellite radar altimetry has been used to measure inland water level variation over large river basins. In this study, we use the decadal (1992-2002) Topex/Poseidon (T/P) measurements from cycle 9 to 364 to detect water level changes of Louisiana's and Everglades' wetlands, where the water surfaces are calm or vegetated, causing
Andersen, O. B.; Krogh, P. E.; Michailovsky, C.; Bauer-Gottwein, P.; Christiansen, L.; Berry, P.; Garlick, J.
Space-borne and ground-based time-lapse gravity observations provide new data for water balance monitoring and hydrological model calibration in the future. The HYDROGRAV project (www.hydrograv.dk) will explore the utility of time-lapse gravity surveys for hydrological model calibration and terrestrial water storage monitoring. Merging remote sensing data from GRACE with other remote sensing data like satellite altimetry and also ground based observations are important to hydrological model calibration and water balance monitoring of large regions and can serve as either supplement or as vital information in un-gauged regions. A system of GRACE custom designed Mass Concentration blocks (Mascons) have been designed to model time-variable gravity changes for the largest basins in Southern Africa (Zambezi, Okavango, Limpopo and Orange) covering an area of 9 mill km2 with a resolution of 1 by 1.25 degree. Satellite altimetry have been used to derive high resolution point-wise river height in some of the un-gauged rivers in the region by using dedicated retracking to recovers nearly un-interrupted time series over these rivers. First result from the HYDROGRAV project analyzing GRACE derived mass change from 2002 to 2008 along with in-situ gravity time-lapse observations and radar altimetry monitoring of surface water for the southern Africa river basins will be presented.
Birkett, C. M.; Mertes, L. A.; Dunne, T.; Costa, M. H.; Jasinski, M. J.
Satellite radar altimetry has the ability to monitor variations in surface water height (or stage) for large lakes, wetlands, rivers, and their floodplains. As part of an international programme a complete altimetric analysis of the Amazon Basin is being undertaken based on the ERS and TOPEX/POSEIDON (T/P) satellite missions. Here, an updated and more rigorous evaluation of the T/P data is presented for the first 7.5 years of the mission. For an initial study group of 230 targets, stage variability can be observed for 30-50%, the range reflecting the clarity of the noted seasonality. An assessment of the instrument performance confirms that the minimum river width attainable is set at 1 km in the presence of some inundated floodplain. This does allow observation of many of the large tributaries, but in the outer regions, mountainous terrain additionally places severe limitations. With ground-based stage measurements, validation exercises show that the overall 1992-1999 results can be accurate to 0.4 m rms, but that this is highly variable (>0.13 m) depending of target width and season. The Solimoes and Amazon are particularly well observed with seasonal altimetric amplitudes <13 m, and variations in peak level timing from May to July. The gradient of the main stem is also estimated ranging from 1.4 cm/km for downstream reaches to 3.8 cm/km for more upstream regions. The seasonal variability of this gradient is also explored, noting that downstream of the confluence of the Negro river, a hysteresis characteristic is in effect, suggesting a kinematic nature to the peak-flow flood wave. All the altimetric results demonstrate that the T/P mission is successfully contributing to this long-term surface water dynamics programme.
DeGrandpre, K. G.; Freymueller, J. T.; Kinsman, N.
Western Alaska is a remote region populated by small communities situated in low-lying coastal environments that are sensitive to variations in local relative sea level (RSL). RSL is the measurement of sea level relative to the local ground surface. Quantification of RSL variation requires measuring vertical velocities for both tectonic motion (onshore component) and the ocean surface (offshore component). Tide gauges in conjunction with tidal benchmarks record RSL, but in Western Alaska these datums are of short duration and too sparsely distributed both temporally and spatially to be able to accurately project RSL trends. Satellite altimetry is not suited for near shore estimates, but is used in this study because of the limited tide gauge coverage both spatially and temporally. During the summers of 2013 and 2014, campaign GPS surveys of geodetic benchmarks were undertaken to produce statistically significant velocity measurements of the tectonic component of sea level change for the Seward Peninsula, Yukon-Kuskokwim Delta, and Alaska Peninsula. Occupations of tidal benchmarks were also collected to compare historic tidal records from the mid-1900s to more recent data. Preliminary results from the GPS survey suggest regional subsidence of approximately 1-2 mm/yr of the Seward Peninsula, which supports one of the current glacial isostatic adjustment (GIA) models available for Western Alaska. The vertical velocity of the tectonic component and the satellite derived mean sea level trend will be coupled to produce a model of RSL change in Western Alaska that will be used to aid local communities in the development of adaptation strategies for changing coastal environments.
Csatho, B. M.; Schenk, A. F.; Duncan, K.; Babonis, G. S.; Sonntag, J. G.; Krabill, W. B.; van den Broeke, M. R.; van Angelen, J.; Blair, J. B.; Hofton, M. A.
Comprehensive monitoring of the Greenland Ice Sheet (GrIS) by satellite observations has revealed increasing mass loss since the late 1990s. Dynamic processes have been responsible for as much as half of this estimated loss, including ice flow adjustments to past climate variations and contemporary atmospheric and oceanic forcings. Dynamical processes act on different spatial and temporal scales and can cause non-linear changes, even on short, sub-decadal time scales. Quantitative investigation of these processes is imperative for improving ice sheet models and sea-level predictions. Our 1992-2011 altimetry record has shown that dynamic thinning substantially contributes to mass loss. The large spatial and temporal variations of dynamic mass loss and widespread intermittent thinning indicated the complexity of ice sheet response to climate forcing and points to the need of continuing monitoring of the GrIS at high spatial resolution. Airborne Topographic Mapper (ATM) and Laser Vegetation and Ice Sensor (LVIS) airborne laser altimetry measurements, acquired by NASA's IceBridge mission, allowed us to extend the altimetry record to 2013. We generated a record of ice thickness and mass change of the GrIS spanning the period of 1992-2013, reconstructed at several thousand locations using the Surface Elevation Reconstruction and Change detection (SERAC) approach. Elevation changes are corrected for Glacial Isostatic Adjustment and partitioned into climate and ice dynamics induced components. We present the evolution of ice dynamics and climate induced mass loss of the major GrIS drainage basins in 2003-2013 to investigate their contributions to sea-level change. The detailed record of outlet glacier elevation change is consistent with the propagation of dynamic thinning or thickening initiated at lower elevations. We focus our attention to SE and NE Greenland. In SE Greenland we investigate if thinning continued on fast flowing SE Greenland glaciers (e.g., Koge Bugt, A
Morris, Robin D.; vonToussaint, Udo; Cheeseman, Peter C.; Clancy, Daniel (Technical Monitor)
The need for accurate geometric and radiometric information over large areas has become increasingly important. Laser altimetry is one of the key technologies for obtaining this geometric information. However, there are important application areas where the observing platform has its orbit constrained by the other instruments it is carrying, and so the spatial resolution that can be recorded by the laser altimeter is limited. In this paper we show how information recorded by one of the other instruments commonly carried, a high-resolution imaging camera, can be combined with the laser altimeter measurements to give a high resolution estimate both of the surface geometry and its reflectance properties. This estimate has an accuracy unavailable from other interpolation methods. We present the results from combining synthetic laser altimeter measurements on a coarse grid with images generated from a surface model to re-create the surface model.
Wollenhaupt, W. R.; Sjogren, W. L.
The laser altimeter measures precise altitudes of the command and service module above the lunar surface and can function either with the metric (mapping) camera or independently. In the camera mode, the laser altimeter ranges at each exposure time, which varies between 20 and 28 sec (i.e., 30 to 43 km on the lunar surface). In the independent mode, the laser altimeter ranges every 20 sec. These altitude data and the spacecraft attitudes that are derived from simultaneous stellar photography are used to constrain the photogrammetric reduction of the lunar surface photographs when cartographic products are generated. In addition, the altimeter measurements alone provide broad-scale topographic relief around the entire circumference of the moon. These data are useful in investigating the selenodetic figure of the moon and may provide information regarding gravitational anomalies on the lunar far side.
Siegel, David A.; McGillicuddy, Dennis J., Jr.; Fields, Erik A.
Satellite altimetry and hydrographic observations are used to characterize the mesoscale eddy field in the Sargasso Sea near Bermuda and to address the role of physical processes on the supply of new nutrients to the euphotic zone. The observed sea level anomaly (SLA) field is dominated by the occurrence of westward propagating features with SLA signatures as large as 25 cm, Eulerian temporal scales of roughly a month, lifetimes of several months, spatial scales of approximately 200 km, and a propagation of approximately 5 cm/s . Hydrographic estimates of dynamic height anomaly (referenced to 4000 dbar) are well correlated with satellite SLA (r(sup 2) = 0.65), and at least 85% of the observed dynamic height variability is associated with the first baroclinic mode of motion. This allows us to apply the satellite observations to remotely estimate isopycnal displacements and the flux of nutrients into the euphotic zone due to eddy pumping. Eddy pumping is the process by which mesoscale eddies induce isopycnal displacements that lift nutrient- replete waters into the euphotic zone, driving new primary production. A kinematic approach to the estimation of the eddy pumping results in a flux of 0.24+/-0.1 mol N/sq m/yr (including a scale estimate for the small contribution due to 18 deg water eddies). This flux is more than an order of magnitude larger than the diapycnal diffusive flux as well as scale estimates for the vertical transport due to isopycnal mixing along sloping isopycnal surfaces. Eddy pumping and wintertime convection are the two dominant mechanisms transporting new nutrients into the euphotic zone, and the sum of all physical new nutrient supply fluxes effectively balances previous geochemical estimates of annual new production for this site. However, if biological transports (e.g., nitrogen fixation, etc.) are significant, the new nitrogen supply budget will be in excess of geochemical new production estimates. This suggests that the various physical and
Siegel, David A.; McGillicuddy, Dennis J., Jr.; Fields, Erik A.
Satellite altimetry and hydrographic observations are used to characterize the mesoscale eddy field in the Sargasso Sea near Bermuda and to address the role of physical processes on the supply of new nutrients to the euphotic zone. The observed sea level anomaly (SLA) field is dominated by the occurrence of westward propagating features with SLA signatures as large as 25 cm, Eulerian temporal scales of roughly a month, lifetimes of several months, spatial scales of approximately 200 km, and a propagation of approximately 5 cm/s. Hydrographic estimates of dynamic height anomaly (referenced to 4000 dbar) are well correlated with satellite SLA (r(exp 2) = 0.65), and at least 85% of the observed dynamic height variability is associated with the first baroclinic mode of motion. This allows us to apply the satellite observations to remotely sensed estimate isopycnal displacements and the flux of nutrients into the euphotic zone due to eddy pumping. Eddy pumping is the process by which mesoscale eddies induce isopycnal displacements that lift nutrient-replete waters into the euphotic zone, driving new primary production. A kinematic approach to the estimation of the eddy pumping results in a flux of 0.24 +/- 0.1 mol N/sq m (including a scale estimate for the small contribution due to 18 deg water eddies). This flux is more than an order of magnitude larger than the diapycnal diffusive flux as well as scale estimates for the vertical transport due to isopycnal mixing along sloping isopycnal surfaces. Eddy pumping and wintertime convection are the two dominant mechanisms transporting new nutrients into the euphotic zone, and the sum of all physical new nutrient supply fluxes effectively balances previous geochemical estimates of annual new production for this site. However, if biological transports (e.g., nitrogen fixation, etc.) are significant, the new nitrogen supply budget will be in excess of geochemical new production estimates. This suggests that the various physical
Lebedev, S. A.; Sirota, A. M.
Ten-day sea level anomalies (SLA) charts, based on the TOPEX/Poseidon (T/P) altimetry data for 1992-2003, as well as corresponding charts of sea surface dynamic heights constructed by the superposition of SLA distributions over the climatic dynamic topography, were used to study main oceanic currents in the region 45°S-20°S, 110°W-70°W. Spatial and seasonal variability of the South Pacific Current has been investigated based on the maps of dynamic topography (DT) gradients. Also maps of the temperature gradients at the ocean surface were used to study the mesoscale variability related to the Subtropical Front (STF) in the Southeastern Pacific. The analysis allowed to distinguish the zones with different degree of variability in the current and front positions. The variability is minimal at 99°W, where the current is most intensive. Westward of 105°W, the RMS of the STF position may reach 3° of latitude. This is accompanied by a pronounced spectral peak with a period of 350 days in its temporal variability. Eastward of 105°W, there is no dominant peak in the frequency spectra describing the variability of the current and front. Comparison of the satellite derived front and current positions with in situ data acquired during R/V "Atlantida" expedition in November-December 2002 shows a good correspondence. Zones of high dynamic topography gradients and sea surface temperatures gradients coincide within the physical errors of the method. The analysis of pelagic fish distribution patterns in the Southeastern Pacific Ocean based on acoustic survey data and synoptic variability of the dynamic topography reveals that most dense fish concentrations relate to dynamic heterogeneities, which are located at the northern periphery of the Subtropical Front.
Neumann, G. A.; Lemoine, F. G.; Torrence, M. H.
More than 72,000 laser ranges to the Moon, 16 million to the asteroid 433 Eros, and 600 million to Mars have provided the cartographic context for future exploration. In the case of Mars, the accuracy of geolocated and crossover-corrected Mars Orbiter Laser Altimeter approaches 1 m vertically and 100 m horizontally, while Eros is known at best to 10 m. Our nearest neighbor is by far the least accurately measured topographically, owing in part to the lack of altimetric crossovers. We present advances in the registration of altimetric profiles and images on Mars and Eros. In the former case, pointing knowledge is the limiting factor, while for the NEAR-Shoemaker spacecraft, orbital knowledge was uncertain. Incorporation of altimetry in the orbit determination step improves both data kinds. Photometry, as implemented in the MOLA instrument and the Mercury Laser Altimeter to be flown on MESSENGER, provides an addditional geometric constraint.
Carret, Alice; Cazenave, Anny; Meyssignac, Benoît; Prandi, Pierre; Ablain, Michael; Andersen, Ole; Blazquez, Alejandro
Studying sea level variations in the Arctic region is challenging because of data scarcity. Here we present results of the sea level budget in the Arctic (up to 82°N) during the altimetry era. We first investigate closure of the sea level budget since 2002 using altimetry data from Envisat and Cryosat for estimating sea level, temperature and salinity data from the ORAP5 reanalysis and GRACE space gravimetry to estimate the steric and mass components. Two altimetry sea level data sets are considered (from DTU and CLS), based on Envisat waveforms retracking. Regional sea level trends seen in the altimetric map, in particular over the Beaufort Gyre and along the eastern coast of Greenland are of steric origin. However, in terms of regional average, the steric component contributes very little to the observed sea level trend, suggesting a dominant mass contribution in the Arctic region. This is confirmed by GRACE-based ocean mass time series that agree very well with the altimetry-based sea level time series. Direct estimate of the mass component is not possible prior to GRACE. Thus we estimated the mass contribution over the whole altimetry era from the difference between altimetry-based sea level and the ORAP5 steric component. Finally we compared altimetry-based coastal sea level with tide gauge records available along Norwegian, Greenland and Siberian coastlines and investigated whether the Arctic Oscillation that was the main driver of coastal sea level in the Arctic during the past decades still plays a dominant role or if other factors (e.g., of anthropogenic origin) become detectable.
Egbert, Gary; Ray, Richard
At the dawn of the era of high-precision altimetry, before the launch of TOPEX/Poseidon, ocean tides were properly viewed as a source of noise--tidal variations in ocean height would represent a very substantial fraction of what the altimeter measures, and would have to be accurately predicted and subtracted if altimetry were to achieve its potential for ocean and climate studies. But to the extent that the altimetry could be severely contaminated by tides, it also represented an unprecedented global-scale tidal data set. These new data, together with research stimulated by the need for accurate tidal corrections, led to a renaissance in tidal studies in the oceanographic community. In this paper we review contributions of altimetry to tidal science over the past 20 years, emphasizing recent progress. Mapping of tides has now been extended from the early focus on major constituents in the open ocean to include minor constituents, (e.g., long-period tides; non-linear tides in shelf waters, and in the open ocean), and into shallow and coastal waters. Global and spatially local estimates of tidal energy balance have been refined, and the role of internal tide conversion in dissipating barotropic tidal energy is now well established through modeling, altimetry, and in situ observations. However, energy budgets for internal tides, and the role of tidal dissipation in vertical ocean mixing remain controversial topics. Altimetry may contribute to resolving some of these important questions through improved mapping of low-mode internal tides. This area has advanced significantly in recent years, with several global maps now available, and progress on constraining temporally incoherent components. For the future, new applications of altimetry (e.g., in the coastal ocean, where barotropic tidal models remain inadequate), and new mission concepts (studies of the submesoscale with SWOT, which will require correction for internal tides) may bring us full circle, again pushing
Passaro, Marcello; Benveniste, Jérôme; Cipollini, Paolo; Fenoglio-Marc, Luciana
For more than two decades, it has been possible to map the Significant Wave Height (SWH) globally through Satellite Altimetry. SWH estimation is possible because the shape of an altimetric waveform, which usually presents a sharp leading edge and a slowly decaying trailing edge, depends on the sea state: in particular, the higher the sea state, the longer the rising time of the leading edge. The algorithm for SWH also depends on the width of the point target response (PTR) function, which is usually approximated by a constant value that contributes to the rising time. Particularly challenging for SWH detection are coastal data and low sea states. The first are usually flagged as unreliable due to land and calm water interference in the altimeter footprint; the second are characterized by an extremely sharp leading edge that is consequently poorly sampled in the digitalized waveform. ALES, a new algorithm for reprocessing altimetric waveforms, has recently been validated for sea surface height estimation (Passaro et al. 2014). The aim of this work is to check its validity also for SWH estimation in a particularly challenging area. The German Bight region presents both low sea state and coastal issues and is particularly suitable for validation, thanks to the extended network of buoys of the Bundesamt für Seeschifffahrt und Hydrographie (BSH). In-situ data include open sea, off-shore and coastal sea conditions, respectively at the Helgoland, lighthouse Alte Weser and Westerland locations. Reprocessed data from Envisat, Jason-1 and Jason-2 tracks are validated against those three buoys. The in-situ validation is applied both at the nearest point and at points along-track. The skill metrics is based on bias, standard deviation, slope of regression line, scatter index, number of cycles with correlation larger than 90%. The same metrics is applied to the altimeter data obtained by standard processing and the validation results are compared. Data are evaluated at high
Baki Iz, H.; Shum, C. K.; Chen, Y. Q.; Dai, C. L.
The selenocentric radial footprint radial distances of 8.8 million Chang'E-1, and 8.5 million SELENE laser altimetry measurements were used used to estimate improved triaxial, biaxial and spherical lunar figure parameters together with their geometric centers with respect to the center of mass of the Moon. The estimated equatorial semi-major, minor, and polar axes of the triaxial ellipsoid's shape parameters from the Chang'E-1 and SELENE solutions differ by 143 m, 3 m and 49 m and -186 m, -3 m, and -52 m in their geometric center positions with respect to the lunar center of mass. The new missions' laser altimetry measurements data reveals a more spherical lunar figure and are in better agreement with each other compared to the ULCN 2005's geometrically best fitting lunar figure. The RMS misclosures calculated for Chang'E-1 and SELENE data using all the estimated parameters were used to assess the goodness of fit of each solution. The estimates for geometrically best fitting solution from Chang'E-1's data give the smallest RMS misclosures for both Chang'E-1 and SELENE data. Overall, the spherical harmonic topographical models' RMS misclosures are larger than those best fitting solutions.
Siry, J. W.
A satellite-to-satellite tracking experiment is planned between ATS-F and GEOS-C with a range accuracy of 2-meters and a range rate accuracy of 0.035 centimeters per second for a 10-second integration time. This experiment is planned for 1974. It is anticipated that it will improve the spatial resolution of the satellite geoid by half an order of magnitude to about 6 degrees. Longer integration times should also permit a modest increase in the acceleration resolution. Satellite altimeter data will also be obtained by means of GEOS-C. An overall accuracy of 5-meters in altitude is the goal. The altimeter, per se, is expected to have an instrumental precision of about 2 meters, and an additional capability to observe with a precision of about 0.2 meters for limited periods.
Ablain, M.; Cazenave, A.; Larnicol, G.; Balmaseda, M.; Cipollini, P.; Faugère, Y.; Fernandes, M. J.; Henry, O.; Johannessen, J. A.; Knudsen, P.; Andersen, O.; Legeais, J.; Meyssignac, B.; Picot, N.; Roca, M.; Rudenko, S.; Scharffenberg, M. G.; Stammer, D.; Timms, G.; Benveniste, J.
Sea level is one of the 50 Essential Climate Variables (ECVs) listed by the Global Climate Observing System (GCOS) in climate change monitoring. In the past two decades, sea level has been routinely measured from space using satellite altimetry techniques. In order to address a number of important scientific questions such as "Is sea level rise accelerating?", "Can we close the sea level budget?", "What are the causes of the regional and interannual variability?", "Can we already detect the anthropogenic forcing signature and separate it from the internal/natural climate variability?", and "What are the coastal impacts of sea level rise?", the accuracy of altimetry-based sea level records at global and regional scales needs to be significantly improved. For example, the global mean and regional sea level trend uncertainty should become better than 0.3 and 0.5 mm year-1, respectively (currently 0.6 and 1-2 mm year-1). Similarly, interannual global mean sea level variations (currently uncertain to 2-3 mm) need to be monitored with better accuracy. In this paper, we present various data improvements achieved within the European Space Agency (ESA) Climate Change Initiative (ESA CCI) project on "Sea Level" during its first phase (2010-2013), using multi-mission satellite altimetry data over the 1993-2010 time span. In a first step, using a new processing system with dedicated algorithms and adapted data processing strategies, an improved set of sea level products has been produced. The main improvements include: reduction of orbit errors and wet/dry atmospheric correction errors, reduction of instrumental drifts and bias, intercalibration biases, intercalibration between missions and combination of the different sea level data sets, and an improvement of the reference mean sea surface. We also present preliminary independent validations of the SL_cci products, based on tide gauges comparison and a sea level budget closure approach, as well as comparisons with ocean
Birkett, C.; Mertes, L.; Dunne, T.; Costa, M.; Jasinski, M.
Satellite radar altimetry has the ability to monitor variations in surface water height (stage) for large wetlands, rivers, and associated floodplains. A clear advantage is the provision of data where traditional gauges are absent. As part of an international program a complete altimetric analysis of the Amazon Basin is being undertaken. Here, an updated and more rigorous evaluation of the TOPEX/POSEIDON (T/P) dataset is presented for the first ~7.5 years of the mission. With an initial study group of 230 targets, height variability at many ungauged locations can be observed for 30-50%, the range reflecting the clarity of the variations in lieu of instrument limitations. An assessment of the instrument performance confirms that the minimum river width attainable is ~1 km in the presence of some inundated floodplain. This constraint does allow observation of the main stem (Solimões/Amazon) and the larger tributaries, but rugged terrain in the vicinity of the target additionally places severe limitations on data retrieval. First-order validation exercises with the deduced 1992-1999 time series of stage fluctuations reveal accuracies ranging from tens of centimeters to several metres (mean ~1.1 m rms). Altimetric water levels in the Solimões and Amazon are particularly well defined with amplitudes <13 m and variations in peak level timing from May to July. The water-surface gradient of the main stem is found to vary both spatially and temporally, with values ranging from 1.5 cm/km downstream, to 4.0 cm/km for more upstream reaches. In agreement with ground-based estimates, the seasonal variability of the gradients reveals that the hysteresis characteristic of the flood wave varies along the main stem and the derived altimetric velocity of this flood wave is estimated to be ~0.35 m/s. Overall, the altimetric results demonstrate that the T/P mission is successfully monitoring the transient flood waves of this continental-scale basin.
Birkett, C. M.; Mertes, L. A. K.; Dunne, T.; Costa, M. H.; Jasinski, M. J.
Satellite radar altimetry has the ability to monitor variations in surface water height (stage) for large wetlands, rivers, and associated floodplains. A clear advantage is the provision of data where traditional gauges are absent. As part of an international program, a complete altimetric analysis of the Amazon Basin is being undertaken. Here, an updated and more rigorous evaluation of the TOPEX/POSEIDON (T/P) data set is presented for the first ˜7.5 years of the mission. With an initial study group of 230 targets, height variability at many ungauged locations can be observed for 30-50%, the range reflecting the clarity of the variations in lieu of instrument limitations. An assessment of the instrument performance confirms that the minimum river width attainable is ˜1 km in the presence of some inundated floodplain. This constraint does allow observation of the main stem (Solimões/Amazon) and the larger tributaries, but rugged terrain in the vicinity of the target additionally places severe limitations on data retrieval. First-order validation exercises with the deduced 1992-1999 time series of stage fluctuations reveal accuracies ranging from tens of centimeters to several meters (mean ˜1.1 m rms). Altimetric water levels in the Solimões and Amazon are particularly well defined with amplitudes <13 m and variations in peak-level timing from May to July. The water-surface gradient of the main stem is found to vary both spatially and temporally, with values ranging from 1.5 cm/km downstream to 4.0 cm/km for more upstream reaches. In agreement with ground-based estimates, the seasonal variability of the gradients reveals that the hysteresis characteristic of the flood wave varies along the main stem and the derived altimetric velocity of this flood wave is estimated to be ˜0.35 m/s. Overall, the altimetric results demonstrate that the T/P mission is successfully monitoring the transient flood waves of this continental-scale river basin.
Müller, D.; Matthews, K. J.; Sandwell, D. T.
Radar altimetry measurements of the ocean surface topography from two satellites have recently been used to construct a new global marine gravity model that is twice as accurate as previous models. The model reveals previously invisible abyssal hill (AH) fabric in many parts of the ocean basins, placing valuable additional constraints on tectonic events reflected in changes in the orientation of linear AHs, and thus in spreading direction. AH fabric, if dated via marine magnetic anomalies, puts much tighter temporal constraints on changes in seafloor spreading directions than fracture zones, which, depending on their offset, often take many millions of years to adjust to major plate motion events. The new data also reveal previously unmapped microplates in the Pacific and Indian oceans. They preferentially form in spreading corridors where spreading rates were very high, reaching plate tectonic speed limits, or in response to plate reorganization stresses. The mapping of previously unknown or poorly mapped ridge propagation events during the Cretaceous Normal Superchron (CNS), leading to pseudofaults and extinct ridges, is relevant for interpreting marine magnetic anomaly sequences during the CNS in terms of magnetic field variability. The new grid provides breathtakingly detailed views of individual fault structures, previously only mapped via expensive seismic surveys, in the North Falkland Basin. Here narrow vertical gravity gradient highs and lows can be shown to correspond to seismically imaged horsts and grabens bounded by normal faults. The new gravity field allows us to create a detailed regional fault map outside of existing seismic coverage. The fault network that emerges illustrates that this eastern region of the Falkland Plateau is characterised by broadly distributed faulting, reflecting a wide rift that typically occurs in regions of higher than normal heat flow with relatively thick crust, where local crustal buoyancy effects dominate localising
Fu, L. -L.; Cheney, R. E.
Altimetric measurement of the height of the sea surface from space provides global observation of the world's oceans. The last eight years have witnessed a rapid growth in the use of altimetry data from the study of the ocean circulations, thanks to the multiyear data from the Geosat Mission.
The focus of the paper is the variability of the North Atlantic Ocean. Large-scale low-frequency variability of the North Atlantic has recently been studied based on model simulations, as well as analysis of altimetry data in conjuction with in-situ data. These studies have suggested significant interannual to decadal changes in the North Atlantic in the 1990's.
Yamamoto, K.; Fukuda, Y.; Nakaegawa, T.; Taniguchi, M.
A project to assess the effects of human activities on the subsurface environment in Asian developing cities has been in progress (Research Institute for Humanity and Nature, Japan, 2009). Bangkok, Thailand is one of the study cities in this project. Using GRACE satellite gravity data, we previously recovered landwater mass variation over the Chao Phraya river basin, where Bangkok is located on downstream. However, mainly because of insufficient spatial resolution of the GRACE data then released, it was difficult to distinguish mass variation over the Chao Phraya basin with the ones of the neighboring Mekong, Irrawaddy and Salween river basins. Recently, some new versions of GRACE data sets have been available, and thus we estimated again the mass variations over these basins using version 2 of CNS/GRGS data set. The result shows that mass variations of the each basin could be distinguished due to improvement of the spatial resolution of the data. One of the interesting things is that a negative interannual mass trend is observed only over the Chao Phraya river basin, while the other basins show positive trend values. One of our concerns was which of the landwater components were decreasing. Because GRACE can only detect total terrestrial water storage, we further used satellite altimeter data to separate surface- and groundwater components. EnviSat data were mainly used as satellite altimetry data in this study, because the mission period is overlapping with GRACE mission and the ground track separation is relatively small. River water levels were recovered from satellite altimetry data, and converted to river water storage. Estimated river water storage was subtracted from the GRACE data. Thus, interannual surface- and groundwater trends were discussed separately. Another concern is whether the landwater decrease is caused by meteorological factors or factors of human activities. Thus, we also compared above results with global hydrological simulation model and
Le Traon, P. Y.
The launch of the US/French mission Topex/Poseidon (T/P) (CNES/NASA) in August 1992 was the start of a revolution in oceanography. For the first time, a very precise altimeter system optimized for large scale sea level and ocean circulation observations was flying. Topex/Poseidon revolutionized our vision and understanding of the ocean. It provided new views of the large scale seasonal and interannual sea level and ocean circulation variations. T/P alone could not observe the mesoscale circulation. In the 1990s, the ESA satellites ERS-1/2 were flying simultaneously with T/P. The ERS-1/2 orbit was well adapted for mesoscale circulation sampling but the orbit determination and altimeter performance were much less precise than for T/P. We demonstrated that we could use T/P as a reference mission for ERS-1/2 and bring the ERS-1/2 data to an accuracy level comparable to T/P. This was an essential first step for the merging of T/P and ERS-1/2. The second step required the development of a global optimal interpolation method. Near real time high resolution global sea level anomaly maps were then derived. These maps have been operationally produced as part of the SSALTO/DUACS system for the last 15 years. They are now widely used by the oceanographic community and have contributed to a much better understanding and recognition of the role and importance of mesoscale dynamics. The unique capability of satellite altimetry to observe the global ocean in near real time at high resolution was essential to the development of global ocean forecasting, a second revolution in oceanography. The Global Ocean Data Assimilation Experiment (GODAE) (1998-2008) was phased with the T/P and ERS-1/2 successors (Jason-1 and ENVISAT) and was instrumental in the development of global operational oceanography capabilities. Europe played a leading role in GODAE. In 1998, the global in-situ observing system was inadequate for the global scope of GODAE. This led to the development of Argo, an
Troitskaya, Yuliya; Lebedev, Sergey; Soustova, Irina; Rybushkina, Galina; Papko, Vladislav; Baidakov, Georgy; Panyutin, Andrey
One of the recent applications of satellite altimetry originally designed for measurements of the sea level  is associated with remote investigation of the water level of inland waters: lakes, rivers, reservoirs [2-7]. The altimetry data re-tracking algorithms developed for open ocean conditions (e.g. Ocean-1,2)  often cannot be used in these cases, since the radar return is significantly contaminated by reflection from the land. The problem of minimization of errors in the water level retrieval for inland waters from altimetry measurements can be resolved by re-tracking satellite altimetry data. Recently, special re-tracking algorithms have been actively developed for re-processing altimetry data in the coastal zone when reflection from land strongly affects echo shapes: threshold re-tracking, The other methods of re-tracking (threshold re-tracking, beta-re-tracking, improved threshold re-tracking) were developed in [9-11]. The latest development in this field is PISTACH product , in which retracking bases on the classification of typical forms of telemetric waveforms in the coastal zones and inland water bodies. In this paper a novel method of regional adaptive re-tracking based on constructing a theoretical model describing the formation of telemetric waveforms by reflection from the piecewise constant model surface corresponding to the geography of the region is considered. It was proposed in [13, 14], where the algorithm for assessing water level in inland water bodies and in the coastal zone of the ocean with an error of about 10-15 cm was constructed. The algorithm includes four consecutive steps: - constructing a local piecewise model of a reflecting surface in the neighbourhood of the reservoir; - solving a direct problem by calculating the reflected waveforms within the framework of the model; - imposing restrictions and validity criteria for the algorithm based on waveform modelling; - solving the inverse problem by retrieving a tracking point
Zuber,Maria T.; Smith, David E.; Phillips, Roger J.; Solomon, Sean C.; Neumann, Gregory A.; Hauck, Steven A., Jr.; Peale, Stanton J.; Barnouin, Oliver S.; Head, James W.; Johnson, Catherine L.; Lemoine, Frank G.; Mazarico, Erwan; Sun, Xiaoli; Torrence, Mark H.; Freed, Andrew M.; Klimczak, Christian; Margot, Jean-Luc; Oberst, Juergen; Perry, Mark E.; McNutt, Ralph L., Jr.; Balcerski, Jeffrey A.; Michel, Nathalie; Talpe, Matthieu J.; Yang, Di
Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury s topography occurred after the earliest phases of the planet s geological history.
Zuber, Maria T; Smith, David E; Phillips, Roger J; Solomon, Sean C; Neumann, Gregory A; Hauck, Steven A; Peale, Stanton J; Barnouin, Olivier S; Head, James W; Johnson, Catherine L; Lemoine, Frank G; Mazarico, Erwan; Sun, Xiaoli; Torrence, Mark H; Freed, Andrew M; Klimczak, Christian; Margot, Jean-Luc; Oberst, Jürgen; Perry, Mark E; McNutt, Ralph L; Balcerski, Jeffrey A; Michel, Nathalie; Talpe, Matthieu J; Yang, Di
Laser altimetry by the MESSENGER spacecraft has yielded a topographic model of the northern hemisphere of Mercury. The dynamic range of elevations is considerably smaller than those of Mars or the Moon. The most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains. Within this lowland is a broad topographic rise that experienced uplift after plains emplacement. The interior of the 1500-km-diameter Caloris impact basin has been modified so that part of the basin floor now stands higher than the rim. The elevated portion of the floor of Caloris appears to be part of a quasi-linear rise that extends for approximately half the planetary circumference at mid-latitudes. Collectively, these features imply that long-wavelength changes to Mercury's topography occurred after the earliest phases of the planet's geological history. PMID:22438510
Chang, Sheng-Qi; Huang, Yong; Li, Pei-Jia; Hu, Xiao-Gong; Fan, Min
Accurate altimetric measurement not only can be applied to the calculation of a topography model but also can be used to improve the quality of the orbit reconstruction in the form of crossovers. Altimetry data from the Chang'E-1 (CE-1) laser altimeter are analyzed in this paper. The differences between the crossover constraint equation in the form of height discrepancies and in the form of minimum distances are mainly discussed. The results demonstrate that the crossover constraint equation in the form of minimum distances improves the CE-1 orbit precision. The overlap orbit performance has increased ∼ 30% compared to the orbit using only tracking data. External assessment using the topography model also shows orbit improvement. The results will be helpful for recomputing ephemeris and improving the CE-1 topography model.
Calmant, S.; Seyler, F.; Bonnet, M.; Santos da Silva, J.; Leon, J. G.; Medeiros, D. M.; Roux, E.
Slope of the river is a widely used parameter for discharge estimation. In poorly monitored basins, SRTM have been used to determine river slope (Le Favour et Alsdorf, 2005). Also, SRTM is expected to constrain long wavelength slope in future altimetry mission, such as SWOT. It is then important to assess the quality of SRTM data over river surface, floodplains and wetlands, in particular in case of dense vegetated cover of the river banks, in order to evaluate if such data can reach modeling requirements. We present two types of analysis : river longitudinal profiles and river cross sections extracted from SRTM compared with altitudes computed from altimetry data (ENVISAT, T/P, ICESAT, GPS surveys).
Schwatke, C.; Dettmering, D.; Bosch, W.; Seitz, F.
Satellite altimetry has been designed for sea level monitoring over open ocean areas. However, for some years, this technology has also been used to retrieve water levels from reservoirs, wetlands and in general any inland water body, although the radar altimetry technique has been especially applied to rivers and lakes. In this paper, a new approach for the estimation of inland water level time series is described. It is used for the computation of time series of rivers and lakes available through the web service "Database for Hydrological Time Series over Inland Waters" (DAHITI). The new method is based on an extended outlier rejection and a Kalman filter approach incorporating cross-calibrated multi-mission altimeter data from Envisat, ERS-2, Jason-1, Jason-2, TOPEX/Poseidon, and SARAL/AltiKa, including their uncertainties. The paper presents water level time series for a variety of lakes and rivers in North and South America featuring different characteristics such as shape, lake extent, river width, and data coverage. A comprehensive validation is performed by comparisons with in situ gauge data and results from external inland altimeter databases. The new approach yields rms differences with respect to in situ data between 4 and 36 cm for lakes and 8 and 114 cm for rivers. For most study cases, more accurate height information than from other available altimeter databases can be achieved.
Coyle, D. Barry; Lindauer, Steven J., II; Kay, Richard B.
Since the late 1980's, NASA has developed several small, all-solid state lasers of low repetition rates for use as transmitters in prototype LIDAR and raster scanned altimetry retrieval systems. Our early laser transmitters were developed for high resolution airborne altimetry which employed cavity dumping techniques to produce a pulse shape with a 1 ns rise time. The first such laser was the SUMR (Sub-millimeter resolution) transmitter which used a side pumped, D-shaped half-rod of Nd:YAG for the oscillator active media and produced approximately 3 ns pulses of 100 micro-J energy at a 40 Hz repetition rate. (Coyle and Blair, 1993; Coyle et al., 1995) After several upgrades to improve rep rate and pulse energy, the final version produced 1.2 mJ pulses at 120 Hz with a 3.7 ns pulse width. The laser has become known as SPLT (Sharp Pulsed Laser Transmitter), and has flown successfully on a variety of airborne altimetry missions. (Coyle and Blair, 1995; Blair et al., 1994) From building these systems, we have accrued valuable experience in delivering field-deployable lasers and have become aware of the advantages and disadvantages of employing new technologies. For example, even though the laser's main operating environment is in a "cold" aircraft during flight, the laser must still operate in very warm temperatures. This is important if the mission is based in the desert or a tropical climate since ground calibration data from stationary targets must be gathered before and after each data flight. Because conductive cooling is much more convenient than closed loop water flow, achieving the highest possible laser efficiency is becoming a high priority when designing a flight laser. This is especially true for lasers with higher pulse energies and repetition rates which are needed for high altitude scanning altimeters and LIDARs.
Bufton, Jack; Blair, Bryan; Cavanaugh, John; Garvin, James
The Shuttle Laser Altimeter (SLA) is a Hitchhiker experiment now being integrated for first flight on STS-72 in November 1995. Four Shuttle flights of the SLA are planned at a rate of about a flight every 18 months. They are aimed at the transition of the Goddard Space Flight Center airborne laser altimeter and lidar technology to low Earth orbit as a pathfinder for operational space-based laser remote sensing devices. Future alser altimeter sensors such as the Geoscience Laser Altimeter System (GLAS), an Earth Observing System facility instrument, and the Multi-Beam Laser Altimeter (MBLA), the land and vegetation laser altimeter for the NASA TOPSAT (Topography Satellite) Mission, will utilize systems and approaches being tested with SLA. The SLA Instrument measures the distance from the Space Shuttle to the Earth's surface by timing the two-way propagation of short (approximately 10 na noseconds) laser pulses. laser pulses at 1064 nm wavelength are generated in a laser transmitter and are detected by a telescope equipped with a silicon avalanche photodiode detector. The SLA data system makes the pulse time interval measurement to a precision of about 10 nsec and also records the temporal shape of the laser echo from the Earth's surface for interpretation of surface height distribution within the 100 m diam. sensor footprint. For example, tree height can be determined by measuring the characteristic double-pulse signature that results from a separation in time of laser backscatter from tree canopies and the underlying ground. This is accomplished with a pulse waveform digitizer that samples the detector output with an adjustable resolution of 2 nanoseconds or wider intervals in a 100 sample window centered on the return pulse echo. The digitizer makes the SLA into a high resolution surface lidar sensor. It can also be used for cloud and atmospheric aerosol lidar measurements by lengthening the sampling window and degrading the waveform resolution. Detailed test
Bufton, Jack; Blair, Bryan; Cavanaugh, John; Garvin, James
The Shuttle Laser Altimeter (SLA) is a Hitchhiker experiment now being integrated for first flight on STS-72 in November 1995. Four Shuttle flights of the SLA are planned at a rate of about a flight every 18 months. They are aimed at the transition of the Goddard Space Flight Center airborne laser altimeter and lidar technology to low Earth orbit as a pathfinder for operational space-based laser remote sensing devices. Future alser altimeter sensors such as the Geoscience Laser Altimeter System (GLAS), an Earth Observing System facility instrument, and the Multi-Beam Laser Altimeter (MBLA), the land and vegetation laser altimeter for the NASA TOPSAT (Topography Satellite) Mission, will utilize systems and approaches being tested with SLA. The SLA Instrument measures the distance from the Space Shuttle to the Earth's surface by timing the two-way propagation of short (approximately 10 na noseconds) laser pulses. laser pulses at 1064 nm wavelength are generated in a laser transmitter and are detected by a telescope equipped with a silicon avalanche photodiode detector. The SLA data system makes the pulse time interval measurement to a precision of about 10 nsec and also records the temporal shape of the laser echo from the Earth's surface for interpretation of surface height distribution within the 100 m diam. sensor footprint. For example, tree height can be determined by measuring the characteristic double-pulse signature that results from a separation in time of laser backscatter from tree canopies and the underlying ground. This is accomplished with a pulse waveform digitizer that samples the detector output with an adjustable resolution of 2 nanoseconds or wider intervals in a 100 sample window centered on the return pulse echo. The digitizer makes the SLA into a high resolution surface lidar sensor. It can also be used for cloud and atmospheric aerosol lidar measurements by lengthening the sampling window and degrading the waveform resolution. Detailed test
Tong, Lanjuan; Guan, Hui; Wang, Zhilin
The characteristic of satellite communication link was firstly described and four application modes were put forward. By comparison, it is suggested that microwave link is used in satellite-to-ground communication and laser link is used in inter-satellite communication. Secondly the condition and composition of laser link establishment was analyzed and laser link model was set up, and the principle and composition of APT system was described. Finally, based on STK and MATLAB platform, the process of inter-satellite laser link establishment was designed, and setting the scene of TDRS capturing and tracking user's satellite as an example, simulation was realized and demonstrated.
Noréus, J. P.; Nyborg, M. R.; Hayling, K. L.
The gravity anomaly field in the Gulf of Bothnia has been investigated using (1) in situ high-precision measurements conducted on the sea ice during cold winters, and (2) gravity anomaly profiles computed from collinear satellite radar altimeter data from the Geosat ERM and the Topex/Poseidon missions. The in situ measurements were obtained from a collaboration between the Finnish Geodetic Institute, the Geological Survey of Sweden (SGU) and the National Survey of Sweden (LMV), and were processed with the geostatistical method called kriging. These data were used to calibrate the altimetric gravity. Altimetry generally resolves features of 20 km wavelength or longer, and in some cases detects shorter features when a sampling interval of 10 Hz is used. The precision of the along-track one-dimensional altimetric profiles corresponds to a gravity uncertainty of 2-3 mGal, and comparison with in situ measured gravity show 4 mGal discrepancy. The precision of the in situ measurements is better. However, depending on the sampling distance, the estimation uncertainty interior the in situ data areas may be up to 5 mGal between neighbouring data points. In regions with in situ data gaps, the estimation uncertainty of the in situ gravity measurements is rapidly increasing to a maximum of 9 mGal. An improved estimation uncertainty of 4-9 mGal was obtained in the same data gap regions with the support of satellite altimetry. Altimetric gravity is therefore used to estimate the gravity field in such regions, and to spatially characterize the gravity field in the Gulf of Bothnia.
Ablain, M.; Cazenave, A.; Larnicol, G.; Balmaseda, M.; Cipollini, P.; Faugère, Y.; Fernandes, M. J.; Henry, O.; Johannessen, J. A.; Knudsen, P.; Andersen, O.; Legeais, J.; Meyssignac, B.; Picot, M.; Roca, M.; Rudenko, S.; Scharffenberg, M. G.; Stammer, D.; Timms, G.; Benveniste, J.
Sea level is one of the 50 Essential Climate Variables (ECVs) listed by the Global Climate Observing System (GCOS) in climate change monitoring. In the last two decades, sea level has been routinely measured from space using satellite altimetry techniques. In order to address a number of important scientific questions such as: "Is sea level rise accelerating?", "Can we close the sea level budget?", "What are the causes of the regional and interannual variability?", "Can we already detect the anthropogenic forcing signature and separate it from the internal/natural climate variability?", and "What are the coastal impacts of sea level rise?", the accuracy of altimetry-based sea level records at global and regional scales needs to be significantly improved. For example, the global mean and regional sea level trend uncertainty should become better than 0.3 and 0.5 mm year-1, respectively (currently of 0.6 and 1-2 mm year-1). Similarly, interannual global mean sea level variations (currently uncertain to 2-3 mm) need to be monitored with better accuracy. In this paper, we present various respective data improvements achieved within the European Space Agency (ESA) Climate Change Initiative (ESA CCI) project on "Sea Level" during its first phase (2010-2013), using multi-mission satellite altimetry data over the 1993-2010 time span. In a first step, using a new processing system with dedicated algorithms and adapted data processing strategies, an improved set of sea level products has been produced. The main improvements include: reduction of orbit errors and wet/dry atmospheric correction errors, reduction of instrumental drifts and bias, inter-calibration biases, intercalibration between missions and combination of the different sea level data sets, and an improvement of the reference mean sea surface. We also present preliminary independent validations of the SL_cci products, based on tide gauges comparison and sea level budget closure approach, as well as comparisons
Carter, David L.
I will be participating in the International Workshop on Laser Ranging. I will be presenting to the International Laser Ranging Service (ILRS) general body meeting on the recent accomplishments and status of the NASA Satellite Laser Ranging (SLR) Network. The recent accomplishments and NASA's future plans will be outlined and the benefits to the scientific community will be addressed. I am member of the ILRS governing board, the Missions working group, and the Networks & Engineering working group. I am the chairman of the Missions Working and will be hosting a meeting during the week of the workshop. I will also represent the NASA SLR program at the ILRS governing board and other working group meetings.
Armitage, Thomas W. K.; Bacon, Sheldon; Ridout, Andy L.; Thomas, Sam F.; Aksenov, Yevgeny; Wingham, Duncan J.
Arctic sea surface height (SSH) is poorly observed by radar altimeters due to the poor coverage of the polar oceans provided by conventional altimeter missions and because large areas are perpetually covered by sea ice, requiring specialized data processing. We utilize SSH estimates from both the ice-covered and ice-free ocean to present monthly estimates of Arctic Dynamic Ocean Topography (DOT) from radar altimetry south of 81.5°N and combine this with GRACE ocean mass to estimate steric height. Our SSH and steric height estimates show good agreement with tide gauge records and geopotential height derived from Ice-Tethered Profilers. The large seasonal cycle of Arctic SSH (amplitude ˜5 cm) is dominated by seasonal steric height variation associated with seasonal freshwater fluxes, and peaks in October-November. Overall, the annual mean steric height increased by 2.2 ± 1.4 cm between 2003 and 2012 before falling to circa 2003 levels between 2012 and 2014 due to large reductions on the Siberian shelf seas. The total secular change in SSH between 2003 and 2014 is then dominated by a 2.1 ± 0.7 cm increase in ocean mass. We estimate that by 2010, the Beaufort Gyre had accumulated 4600 km3 of freshwater relative to the 2003-2006 mean. Doming of Arctic DOT in the Beaufort Sea is revealed by Empirical Orthogonal Function analysis to be concurrent with regional reductions in the Siberian Arctic. We estimate that the Siberian shelf seas lost ˜180 km3 of freshwater between 2003 and 2014, associated with an increase in annual mean salinity of 0.15 psu yr-1. Finally, ocean storage flux estimates from altimetry agree well with high-resolution model results, demonstrating the potential for altimetry to elucidate the Arctic hydrological cycle.
Zwally, H. Jay
NASA's Ice, Cloud and Land Elevation Satellite (ICESat) has been measuring elevations of the Antarctic ice sheet and sea-ice freeboard elevations with unprecedented accuracy. Since February 20,2003, data has been acquired during three periods of laser operation varying from 36 to 54 days, which is less than the continuous operation of 3 to 5 years planned for the mission. The primary purpose of ICESat is to measure time-series of ice-sheet elevation changes for determination of the present-day mass balance of the ice sheets, study of associations between observed ice changes and polar climate, and estimation of the present and future contributions of the ice sheets to global sea level rise. ICESat data will continue to be acquired for approximately 33 days periods at 3 to 6 month intervals with the second of ICESat's three lasers, and eventually with the third laser. The laser footprints are about 70 m on the surface and are spaced at 172 m along-track. The on-board GPS receiver enables radial orbit determinations to an accuracy better than 5 cm. The orbital altitude is around 600 km at an inclination of 94 degrees with a 8-day repeat pattern for the calibration and validation period, followed by a 91 -day repeat period for the rest of the mission. The expected range precision of single footprint measurements was 10 cm, but the actual range precision of the data has been shown to be much better at 2 to 3 cm. The star-tracking attitude-determination system should enable footprints to be located to 6 m horizontally when attitude calibrations are completed. With the present attitude calibration, the elevation accuracy over the ice sheets ranges from about 30 cm over the low-slope areas to about 80 cm over areas with slopes of 1 to 2 degrees, which is much better than radar altimetry. After the first period of data collection, the spacecraft attitude was controlled to point the laser beam to within 50 m of reference surface tracks over the ice sheets. Detection of ice
Garcia, D.; Vigo, I.; Chao, B. F.; Del Río, J.
More than a decade of altimetry data h ave been used to develop thr ee studies in the Med iterranean Sea (and Black sea wh en feasible). Firstly, sea level v ariations (SLV) data from 49 tide g auge ( TG) stations along th e North Med iterranean and Black Sea coast have been compared to close SLV measurements from altimetry. From those compar isons, vertical crustal motions at th e TG sites have b een inf erred. Secondly , in terannual SLV have been explored in the Mediterran ean and Black Sea. A kink of tendency in mid-1999 is observed in sev eral regions of the Med iterranean and in the Black Sea. Thirdly, th e steric and eustatic terms of the annual SLV of the Mediterr anean has been inferred from time- variab le gravity (TVG) data and temperature (T) and salinity (S) profiles. It has been inf erred that the SLV are main ly driven by steric SLV, wh ich are par tially offset by w ater mass changes.
Smith, David E.; Zuber, M. T.; Lemoine, F. G.; Rowlands, D. D.
Many of the scientific investigations of the Mars Global Surveyor (MGS) mission require high precision orbital information and some are limited entirely by its quality. These include the laser altimeter (MOLA) the Mars gravity field and atmospheric occultation investigations by radio science, and the planetary dynamics and celestial mechanics investigations. The precision of the orbits can usually be assessed by comparing overlapping orbits for a given period; but these results tend to reflect the repeatability rather than the accuracy. The re-constructed orbits from the doppler and range tracking data on MGS are (to date) at the few meter level radially, and a few hundreds of meters horizontally, using the best gravity models, presently available. With the laser altimeter on MGS we have a mechanism to measure the quality and to actually make significant improvements in the orbital accuracy by incorporating the altimetry data as a tracking datatype. By adding the altimeter measurements at orbital cross-over locations we have been able to reduce die radial error to 1 meter of less on average and have reduced the along track and out of plane error by almost 2 orders of magnitude down to a few meters. It is apparent that the altimeter observation provides a geometric strength to the orbit that it is not possible to obtain from the present doppler and the range data alone. We discuss the results obtained for the first year of the MGS mapping orbit. This work is supported by the NASA Mars Program.
Sanchez-Roman, Antonio; Ruiz, Simón; Pascual, Ananda; Guinehut, Stéphanie; Mourre, Baptiste
The existing Argo network provides essential data in near real time to constrain monitoring and forecasting centers and strongly complements the observations of the ocean surface from space. The comparison of Sea Level Anomalies (SLA) provided by satellite altimeters with in-situ Dynamic Heights Anomalies (DHA) derived from the temperature and salinity profiles of Argo floats contribute to better characterize the error budget associated with the altimeter observations. In this work, performed in the frame of the E-AIMS FP7 European Project, we focus on the Argo observing system in the Mediterranean Sea and its impact on SLA fields provided by satellite altimetry measurements in the basin. Namely, we focus on the sensitivity of specific SLA gridded merged products provided by AVISO in the Mediterranean to the reference depth (400 or 900 dbar) selected in the computation of the Argo Dynamic Height (DH) as an integration of the Argo T/S profiles through the water column. This reference depth will have impact on the number of valid Argo profiles and therefore on their temporal sampling and the coverage by the network used to compare with altimeter data. To compare both datasets, altimeter grids and synthetic climatologies used to compute DHA were spatially and temporally interpolated at the position and time of each in-situ Argo profile by a mapping method based on an optimal interpolation scheme. The analysis was conducted in the entire Mediterranean Sea and different sub-regions of the basin. The second part of this work is devoted to investigate which configuration in terms of spatial sampling of the Argo array in the Mediterranean will properly reproduce the mesoscale dynamics in this basin, which is comprehensively captured by new standards of specific altimeter products for this region. To do that, several Observing System Simulation Experiments (OSSEs) were conducted assuming that altimetry data computed from AVISO specific reanalysis gridded merged product for
Brasseur, P.; Verron, J. A.; Djath, B.; Duran, M.; Gaultier, L.; Gourdeau, L.; Melet, A.; Molines, J. M.; Ubelmann, C.
The upcoming high-resolution SWOT altimetry satellite will provide an unprecedented description of the ocean dynamic topography for studying sub- and meso-scale processes in the ocean. But there is still much uncertainty on the signal that will be observed. There are many scientific questions that are unresolved about the observability of altimetry at vhigh resolution and on the dynamical role of the ocean meso- and submesoscales. In addition, SWOT data will raise specific problems due to the size of the data flows. These issues will probably impact the data assimilation approaches for future scientific or operational oceanography applications. In this work, we propose to use a high-resolution numerical model of the Western Pacific Solomon Sea as a regional laboratory to explore such observability and dynamical issues, as well as new data assimilation challenges raised by SWOT. The Solomon Sea connects subtropical water masses to the equatorial ones through the low latitude western boundary currents and could potentially modulate the tropical Pacific climate. In the South Western Pacific, the Solomon Sea exhibits very intense eddy kinetic energy levels, while relatively little is known about the mesoscale and submesoscale activities in this region. The complex bathymetry of the region, complicated by the presence of narrow straits and numerous islands, raises specific challenges. So far, a Solomon sea model configuration has been set up at 1/36° resolution. Numerical simulations have been performed to explore the meso- and submesoscales dynamics. The numerical solutions which have been validated against available in situ data, show the development of small scale features, eddies, fronts and filaments. Spectral analysis reveals a behavior that is consistent with the SQG theory. There is a clear evidence of energy cascade from the small scales including the submesoscales, although those submesoscales are only partially resolved by the model. In parallel
Walbridge, E. W.
A laser satellite power system (SPS) converts solar power captured by Earth-orbiting satellites into electrical power on the Earth's surface, the satellite-to-ground transmission of power being effected by a laser beam. The laser SPS is an alternative to the microwave SPS. Lasers and how they work are described, as are the types of lasers - electric discharge, direct and indirect solar pumped, free electron, and closed-cycle chemical - that are candidates for application in a laser SPS. The advantages of a laser SPS over the microwave alternative are pointed out. One such advantage is that, for the same power delivered to the utility busbar, land requirements for a laser system are much smaller (by a factor of 21) than those for a microwave system. The four laser SPS concepts that have been presented in the literature are described and commented on. Finally key issues for further laser SPS research are discussed.
Pfeffer, Julia; Seyler, Frédérique; Bonnet, Marie-Paule; Calmant, Stéphane; Frappart, Frédéric; Papa, Fabrice; Paiva, Rodrigo C. D.; Satgé, Frédéric; Silva, Joecila Santos Da
Groundwater plays a fundamental role in rainforest environments, as it is connected with rivers, lakes, and wetlands, and helps to support wildlife habitat during dry periods. Groundwater reservoirs are however excessively difficult to monitor, especially in large and remote areas. Using concepts from groundwater-surface water interactions and ENVISAT altimetry data, we evaluated the topography of the groundwater table during low-water periods in the alluvial plain of the central Amazon. The water levels are monitored using an unprecedented coverage of 491 altimetric stations over surface waters in the central Amazon. The groundwater table maps interpolated at spatial resolutions ranging from 50 to 100 km are consistent with groundwater wells data. They provide evidence of significant spatiotemporal organization at regional scale: heterogeneous flow from the hillslope toward the main rivers is observed, as well as strong memory effects and contrasted hydrological behaviors between the North and the South of the Amazon.
Yunck, Tom P.; Hajj, George A.
The vast illuminating power of the Global Positioning System (GPS), which transformed space geodesy in the 199Os, is now serving to probe the earth's fluid envelope in unique ways. Three distinct techniques have emerged: ground-based sensing of the integrated atmospheric moisture; space-based profiling of atmospheric refractivity, pressure, temperature, moisture, and other properties by active limb sounding; and surface (ocean and ice) altimetry and scatterometry with reflected signals detected from space. Ground-based GPS moisture sensing is already in provisional use for numerical weather prediction. Limb sounding, while less mature, offers a bevy of attractions, including high accuracy, stability, and vertical resolution; all-weather operation; and exceptionally low cost. GPS bistatic radar, r 'reflectometry,' is the least advanced but shows promise for a number of niche applications.
Garvin, J.B.; Williams, R.S.; Frawley, J.J.; Krabill, W.B.
The volumetric evolution of Surtsey has been estimated on the basis of digital elevation models derived from NASA scanning airborne laser altimeter surveys (20 July 1998), as well as digitized 1:5,000-scale topographic maps produced by the National Land Survey of Iceland and by Norrman. Subaerial volumes have been computed from co-registered digital elevation models (DEM's) from 6 July 1968, 11 July 1975, 16 July 1993, and 20 July 1998 (scanning airborne laser altimetry), as well as true surface area (above mean sea level). Our analysis suggests that the subaerial volume of Surtsey has been reduced from nearly 0.100 km3 on 6 July 1968 to 0.075 km3 on 20 July 1998. Linear regression analysis of the temporal evolution of Surtsey's subaerial volume indicates that most of its subaerial surface will be at or below mean sea-level by approximately 2100. This assumes a conservative estimate of continuation of the current pace of marine erosion and mass-wasting on the island, including the indurated core of the conduits of the Surtur I and Surtur II eruptive vents. If the conduits are relatively resistant to marine erosion they will become sea stacks after the rest of the island has become a submarine shoal, and some portions of the island could survive for centuries. The 20 July 1998 scanning laser altimeter surveys further indicate rapid enlargement of erosional canyons in the northeastern portion of the partial tephra ring associated with Surtur I. Continued airborne and eventually spaceborne topographic surveys of Surtsey are planned to refine the inter-annual change of its subaerial volume.
Etala, Paula; Saraceno, Martín; Echevarría, Pablo
Cyclogenesis and long-fetched winds along the southeastern coast of South America may lead to floods in populated areas, as the Buenos Aires Province, with important economic and social impacts. A numerical model (SMARA) has already been implemented in the region to forecast storm surges. The propagation time of the surge in such extensive and shallow area allows the detection of anomalies based on observations from several hours up to the order of a day prior to the event. Here, we investigate the impact and potential benefit of storm surge level data assimilation into the SMARA model, with the objective of improving the forecast. In the experiments, the surface wind stress from an ensemble prediction system drives a storm surge model ensemble, based on the operational 2-D depth-averaged SMARA model. A 4-D Local Ensemble Transform Kalman Filter (4D-LETKF) initializes the ensemble in a 6-h cycle, assimilating the very few tide gauge observations available along the northern coast and satellite altimeter data. The sparse coverage of the altimeters is a challenge to data assimilation; however, the 4D-LETKF evolving covariance of the ensemble perturbations provides realistic cross-track analysis increments. Improvements on the forecast ensemble mean show the potential of an effective use of the sparse satellite altimeter and tidal gauges observations in the data assimilation prototype. Furthermore, the effects of the localization scale and of the observational errors of coastal altimetry and tidal gauges in the data assimilation approach are assessed.
Stober, Manfred; Hepperle, Jorg; Rawiel, Paul
A long-term geodetic project on the Greenland inland ice is performed in order to determine elevations, elevation change, flow velocity, and deformation of the ice surface in the western part of the Greenland ice sheet. There are two main research areas: Swiss Camp (ETH/CU-Camp) which was started in 1991, and ST2, started in 2004. Until 2008 a total of 10 measuring campaigns were carried out at Swiss Camp. The 3D-coordinates of the snow and ice surfaces were measured by ground-based static and kinematical GPS survey. As a result very precise digital elevation models of the research areas are available. The digital terrain models can be used as ground control areas for satellite altimetry. As an example, they were used for validation of ICESat satellite elevation data. Height comparisons along one track show in average a discrepancy of 0.13 m ± 0.06 m. Due to their very high accuracy, the measured areas can also be used as control areas for CryoSat. The next field measurements are planned in summer 2011. The location of the ground measurements will be coordinated with predicted tracks for CryoSat.
Spudis, Paul D.; Reisse, Robert A.; Gillis, Jeffrey J.
Analysis of laser altimetry data from Clementine has confirmed and extended our knowledge of nearly obliterated multiring basins on the moon. These basins were formed during the early bombardment phase of lunar history, have been filled to varying degrees by mare lavas and regional ejecta blankets, and have been degraded by the superposition of large impact craters. The Mendel-Rydberg Basin, a degraded three-ring feature over 600 kilometers in diameter on the lunar western limb, is about 6 kilometers deep from rim to floor, only slightly less deep than the nearby younger and much better preserved Orientale Basin (8 kilometers deep). The South Pole-Aitken Basin, the oldest discernible impact feature on the moon, is revealed as a basin 2500 kilometers in diameter with an average depth of more than 13 kilometers, rim crest to floor. This feature is the largest, deepest impact crater yet discovered in the solar system. Several additional depressions seen in the data may represent previously unmapped ancient impact basins.
Spudis, P D; Gillis, J J; Reisse, R A
Analysis of laser altimetry data from Clementine has confirmed and extended our knowledge of nearly obliterated multiring basins on the moon. These basins were formed during the early bombardment phase of lunar history, have been filled to varying degrees by mare lavas and regional ejecta blankets, and have been degraded by the superposition of large impact craters. The Mendel-Rydberg Basin, a degraded three-ring feature over 600 kilometers in diameter on the lunar western limb, is about 6 kilometers deep from rim to floor, only slightly less deep than the nearby younger and much better preserved Orientale Basin (8 kilometers deep). The South Pole-Aitken Basin, the oldest discernible impact feature on the moon, is revealed as a basin 2500 kilometers in diameter with an average depth of more than 13 kilometers, rim crest to floor. This feature is the largest, deepest impact crater yet discovered in the solar system. Several additional depressions seen in the data may represent previously unmapped ancient impact basins. PMID:17737079
The melt water of the glaciers on earth makes a substantial contribution to global sea-level rise. However, until recently most estimates relied on the extrapolation of measurements of few glaciers only which were upscaled to larger regions. Space-borne laser altimetry data from the ICESat GLAS sensor provides a global data set of elevation changes for the period 2003 - 2009. The accuracy of the altimetry measurements of about ±0.5 m even over rough surfaces along with their small footprint (about 70 m) is making this data very suitable to assess elevation changes not only for the two ice sheets but also for the glaciers. This data set in combination with the recently finalized global glacier inventory (Randolph Glacier Inventory) allowed the first time to determine the glacier volume changes for larger regions. A major challenge with ICESat data is the sparse density of the tracks with increasing horizontal separation with decreasing latitude (from about 10 km up to more than 50 km), and the fact that the repeat tracks can be several hundred metres apart. Plane fitting and the comparison to the available global DEMs (SRTM and GDEM) is required to correct for the repeat track separation. The obtained elevation changes are usually extrapolated to larger glazierized regions based on the glacier hypsometries. Results of two different studies of the local glaciers and ice caps on Greenland using similar data match well for the volume change (-40 km³ vs. -42 km³) with the highest loss in the south-eastern sector and lowest in the northern sector of Greenland. However, they vary between 28 ± 11 and 38 ± 7 Gt a-1 (~10 - 15% mass loss of glaciers on earth) due to different ice-density assumptions. Hence, while the measurements seem to be accurate the major challenge for assessing the glacier mass changes (and hence, their contribution to sea is the conversion from elevation changes to mass changes as snow and ice density and firn compaction have to be estimated or
Barker, M. K.; Mazarico, E.; Smith, D. E.; Zuber, M. T.
The dynamics of planetary bodies can provide valuable, and often unique, information on their interior structure. For instance, surface tidal deformation indicates how a body responds to the gravitational tidal forcing, and can thus give an indication of how the internal structure and temperature varies with depth. In addition, the orientation and spin rate of a planetary body are affected by its interior mass distribution and thermal evolution. In this contribution, we describe recent work to constrain the tidal deformation of the Moon and spin state and orientation of Mercury using altimetric crossovers measured by the Lunar Orbiter Laser Altimeter (LOLA) and MESSENGER Laser Altimeter (MLA). Altimetric crossovers are ideal for detecting the desired small surface changes, as they avoid the problem of aliasing topographic changes due to small-scale, unpredictable and uncorrelated, geologic relief. On the Moon, the tidal surface deformation is small (amplitude ~10 cm), but, using the highest quality LOLA crossovers, Mazarico et al. (2014) made the first measurement of the radial Love number h2 from an orbiting spacecraft. In a follow-up to that work, we are incorporating more crossovers to improve the temporal sampling of the tidal signal, thus enabling analysis of the spatial variation of the tidal amplitude, as might be expected given the thicker and cooler far side crust and the potential presence of a partial melt region below the PKT. Due to tidal torques from the Sun, Mercury experiences longitudinal librations about its 3:2 spin-orbit resonance with an amplitude of ~450 m at the equator. This amplitude is significantly larger than the geolocation uncertainty of the MLA altimetry (~10/100 m in radial/horizontal), and could, thus, be detectable from crossovers alone. However, given the sparse coverage near the equator, where the libration amplitude is largest, it may be necessary to incorporate into the analysis stereo-derived DEMs from the Mercury Dual Imaging
Pistek, Pavel; Johnson, Donald R.
Relatively warm and salty North Atlantic surface waters flow through the Faeroe-Shetland Channel into the higher latitudes of the Nordic Seas, preserving an ice-free winter environment for much of the exterior coast of northern Europe. This flow was monitored along the Norwegian coast using Geosat altimetry on two ascending arcs during the Exact Repeat Mission in 1987-1989. Concurrent undertrack CTD surveys were used to fix a reference surface for the altimeter-derived SSH anomalies, in effect creating time series of alongtrack surface dynamic height topographies. Climatologic CTD casts were then used, with empirical orthogonal function (EOF) analysis, to derive relationships between historical surface dynamic heights and vertical temperature and salinity profiles. Applying these EOF relationships to the altimeter signals, mean transports of volume, heat, and salt were calculated at approximately 2.9 Sverdrups, 8.1 x 10 exp 11 KCal/s and 1.0 x 10 exp 8 Kg/s, respectively. Maximum transports occurred in February/March and minimum in July/August.
Landis, Geoffrey A.; Westerlund, Larry H.
It is possible to use a ground-based laser to beam light to the solar arrays of orbiting satellites, to a level sufficient to provide all or some of the operating power required. Near-term applications of this technology for providing supplemental power to existing satellites are discussed. Two missions with significant commercial pay-off are supplementing solar power for radiation-degraded arrays and providing satellite power during eclipse for satellites with failed batteries.
Henrique Costa, Paulo; Oliveira Pereira, Eric; Maillard, Philippe
Satellite altimetry is becoming a major tool for measuring water levels in rivers and lakes offering accuracies compatible with many hydrological applications, especially in uninhabited regions of difficult access. The Pantanal is considered the largest tropical wetland in the world and the sparsity of in situ gauging station make remote methods of water level measurements an attractive alternative. This article describes how satellites altimetry data from Envisat and Saral was used to determine water level in two small lakes in the Pantanal. By combining the water level with the water surface area extracted from satellite imagery, water volume fluctuations were also estimated for a few periods. The available algorithms (retrackers) that compute a range solution from the raw waveforms do not always produce reliable measurements in small lakes. This is because the return signal gets often "contaminated" by the surrounding land. To try to solve this, we created a "lake" retracker that rejects waveforms that cannot be attributed to "calm water" and convert them to altitude. Elevation data are stored in a database along with the water surface area to compute the volume fluctuations. Satellite water level time series were also produced and compared with the only nearby in situ gauging station. Although the "lake" retracker worked well with calm water, the presence of waves and other factors was such that the standard "ice1" retracker performed better on the overall. We estimate our water level accuracy to be around 75 cm. Although the return time of both satellites is only 35 days, the next few years promise to bring new altimetry satellite missions that will significantly increase this frequency.
Schwatke, Christian; Dettmering, Denise
Satellite altimetry has been designed for sea level monitoring over open ocean areas. However, for some years, this technology has also been used to retrieve water levels from lakes, reservoirs, rivers, wetlands and in general any inland water body. In this contribution, a new approach for the estimation of inland water level time series is presented. The method is the basis for the computation of time series of rivers and lakes available through the web service 'Database for Hydrological Time Series over Inland Water' (DAHITI). It is based on an extended outlier rejection and a Kalman filter approach incorporating cross-calibrated multi-mission altimeter data from Envisat, ERS-2, Jason-1, Jason-2, Topex/Poseidon, and SARAL/AltiKa, including their uncertainties. The new approach yields RMS differences with respect to in situ data between 4 cm and 36 cm for lakes and 8 cm and 114 cm for rivers, respectively. Within this presentation, the new approach will be introduced and examples for water level time series for a variety of lakes and rivers will be shown featuring different characteristics such as shape, lake extent, river width, and data coverage. A comprehensive validation is performed by comparisons with in situ gauge data and results from external inland altimeter databases.
Schwatke, C.; Dettmering, D.; Bosch, W.; Seitz, F.
Satellite altimetry has been designed for sea level monitoring over open ocean areas. However, since some years, this technology is also used for observing inland water levels of lakes and rivers. In this paper, a new approach for the estimation of inland water level time series is described. It is used for the computation of time series available through the web service "Database for Hydrological Time Series over Inland Water" (DAHITI). The method is based on a Kalman filter approach incorporating multi-mission altimeter observations and their uncertainties. As input data, cross-calibrated altimeter data from Envisat, ERS-2, Jason-1, Jason-2, Topex/Poseidon, and SARAL/AltiKa are used. The paper presents water level time series for a variety of lakes and rivers in North and South America featuring different characteristics such as shape, lake extent, river width, and data coverage. A comprehensive validation is performed by comparison with in-situ gauge data and results from external inland altimeter databases. The new approach yields RMS differences with respect to in-situ data between 4 and 38 cm for lakes and 12 and 139 cm for rivers, respectively. For most study cases, more accurate height information than from available other altimeter data bases can be achieved.
Collins, C.; Hermes, J. C.; Reason, C. J. C.
Recently it has been shown that anticyclonic eddies are generated in the Comoros Basin contesting the long-held notion of a single large anticyclonic cell, the Comoros Gyre. Limited knowledge exists about the mesoscale activity within the basin, a potential key source of variability for the Mozambique Channel and subsequently the Agulhas Current. In this paper an automated eddy tracking scheme, applied to satellite altimetry data and a high-resolution model simulation, is used to determine the characteristics of the anticyclonic eddies generated in the Comoros Basin. The generation and characteristics of cyclonic eddies are also investigated. The eddy tracking scheme revealed that anticyclonic eddies are primarily generated west of the tip of Madagascar due to barotropic instabilities whereas cyclonic eddies are mainly generated along the northwest coast of Madagascar as a result of baroclinic instabilities. Anticyclonic eddies, with a mean lifespan of about 3 months, reside in the basin for half their lifespan before propagating into the Mozambique Channel. On the other hand, the majority of cyclonic eddies, with a similar mean lifespan, dissipate within the basin. Initially, the anticyclones, with translation speeds of 6-8 km d-1 and mean radii of 80-100 km, follow the trajectory of the North East Madagascar Current and turn south upon reaching the African coast. The cyclonic eddies tend to be smaller (˜60 km) and have slower translation speeds (2.5-3.5 km d-1) than their anticyclonic counterparts.
Direct estimation of the absolute dynamic topography from satellite altimetry has been confined to the largest scales (basically the basin-scale) owing to the fact that the signal-to-noise ratio is more unfavorable everywhere else. But even for the largest scales, the results are contaminated by the orbit error and geoid uncertainties. Recently a more accurate Earth gravity model (GEM-T1) became available, providing the opportunity to examine the whole question of direct estimation under a more critical limelight. It is found that our knowledge of the Earth's gravity field has indeed improved a great deal. However, it is not yet possible to claim definitively that our knowledge of the ocean circulation has improved through direct estimation. Yet, the improvement in the gravity model has come to the point that it is no longer possible to attribute the discrepancy at the basin scales between altimetric and hydrographic results as mostly due to geoid uncertainties. A substantial part of the difference must be due to other factors; i.e., the orbit error, or the uncertainty of the hydrographically derived dynamic topography.
Fenoglio-Marc, Luciana; Buchhaupt, Christopher; Dinardo, Salvatore; Scharroo, Remko; Benveniste, Jerome; Becker, Matthias
The Delay Doppler/Synthetic Aperture Radar (SAR) altimeter offers a new quality of observational data in comparison to the pulse-limited low resolution mode (LRM) data collected over the past twenty years. Due to the reduced noise in the measurements an improved retrieval of the geophysical signal is expected in SAR. The goal of this study is to characterize these improvements both in open ocean and coastal zone using standard Level 2 and Level 1 data reprocessed with improved algorithms. We have carried out, from CryoSat-2 Level 1a Full Bit Rate (L1a FBR) data, a Delay-Doppler processing and waveform retracking tailored specifically for coastal zone by applying Hamming Window and Zero-Padding, using an extended vertical swath window in order to minimize tracker errors and a dedicated SAMOSA-based coastal retracker (named SAMOSA+). SAMOSA+ accepts mean square slope as free parameter and the epoch's first guess fitting value is decided according to the peak in correlation between 20 consecutive waveforms (in order to mitigate land off-ranging effect). Those products can be extracted from ESA-ESRIN GPOD service (named SARvatore). In order to quantify the improvement with respect to pulse-limited altimetry, we build 20 Hz PLRM (pseudo-LRM) data from CryoSat-1 L1a FBR and retrack them with numerical convolutional Brown-based retracker. Hence, here, PLRM is used as a proxy for real pulse-limited products (LRM), since there is no direct comparison of SAR and LRM possible otherwise. The PLRM data are built and retracked by Technical University of Darmstadt (TUDa). In the open ocean the study consists on the retrieval of short scale geophysical, as the swell signals. The selected areas are the CryoSat-2 Pacific and Atlantic Boxes in which it operated in SAR mode. In the coastal zone of the North Sea the study concentrates on the reduction of land and ships contamination by dedicated procedures including improved retracking. Effects of different options and retracking
Lichten, Stephen M.
The advent of satellite altimetry has greatly improved our ability to observe global ocean circulation. However, the swath of a single, nadir-viewing satellite altimeter is only a few km and the track spacing is several hundred km to resolve the two-dimensional structure of ocean eddies. Our goal is to increase spatial and temporal coverage by monitoring Global Positioning System (GPS) signals reflected from the ocean. A constellation of spacecraft would each carry a GPS receiver capable of recording 8 reflections simultaneously. The reflections are well distributed in azimuth and elevation and can be tracked continuously while the satellite is in view, and another is then acquired, as illustrated below. The diagram depicts a new approach at altimetry measurements where ocean surface reflected GPS signals are simultaneously tracked and processed in a GPS flight receiver in space. The reflected GPS signals from the ocean must be compared precisely with the direct GPS signals in order to infer the characteristics of the ocean from the combined data set. Understanding the features and accuracy of GPS altimetry measurement is crucial to establishing its suitability for oceanography. Preliminary work has enabled us to theoretically model the signal output of the correlator for a variety of system parameters such as wind speed (sea roughness), receiver height, incidence angle, receiver range and Doppler filter bandwidth and antenna gain. Expected signal-to-noise ratio has been estimated from which we have inferred, to a first approximation, the basic receiver gain requirements for a space-based altimeter and the expected range raw error. In 1998, work on a different task led to the extraction of the first reflected GPS signal observed from a spaceborne receiver during the 1995 Space Transportation System-68 (STS-68) Shuttle Radar Laboratory-2 (SRL-2) high resolution synthetic aperture radar mission. Good comparisons with our signal models have been obtained. Having
Babonis, Gregory Scott
There remains much uncertainty in estimating the amount of Antarctic ice mass change, its dynamic component, and its spatial and temporal patterns. This work remedies the limitations of previous studies by generating the first detailed reconstruction of total and dynamic ice thickness and mass changes across Antarctica, from ICESat satellite altimetry observations in 2003-2009 using the Surface Elevation Reconstruction and Change Detection (SERAC) method. Ice sheet thickness changes are calculated with quantified error estimates for each time when ICESat flew over a ground-track crossover region, at approximately 110,000 locations across the Antarctic Ice Sheet. The time series are partitioned into changes due to surficial processes and ice dynamics. The new results markedly improve the spatial and temporal resolution of surface elevation, volume, and mass change rates for the AIS, and can be sampled at annual temporal resolutions. The results indicate a complex spatiotemporal pattern of dynamic mass loss in Antarctica, especially along individual outlet glaciers, and allow for the quantification of the annual contribution of Antarctic ice loss to sea level rise. Over 5000 individual locations exhibit either strong dynamic ice thickness change patterns, accounting for approximately 500 unique spatial clusters that identify regions likely influenced by subglacial hydrology. The spatial distribution and temporal behavior of these regions reveal the complexity and short-time scale variability in the subglacial hydrological system. From the 500 unique spatial clusters, over 370 represent newly identified, and not previously published, potential subglacial water bodies indicating an active subglacial hydrological system over a much larger region than previously observed. These numerous new observations of dynamic changes provide more than simply a larger set of data. Examination of both regional and local scale dynamic change patterns across Antarctica shows newly
Colton, M. T.; Chase, R. R. P.
Theoretical studies on the interaction of a zonal current with a zonal ridge, an isolated bump, and a meridional ridge compare favorably with hydrographic observations within the eastward flowing Antarctic Circumpolar Current where it flows over similar topographic features. However, the existing hydrographic data are insufficient for examining the temporal stability and kinematic behavior of the resulting mesoscale structures. In this study, some of these transient features have been compared with patterns in sea surface variability, derived from collinear satellite altimetric data. When these features occurred near the crossing point of two satellite ground traces, it was possible to characterize their length scales, dynamic height relief, and translational and surface geostrophic velocities.
Brunt, Kelly M.; Neumann, Thomas A.; Amundson, Jason M.; Kavanaugh, Jeffrey L.; Moussavi, Mahsa S.; Walsh, Kaitlin M.; Cook, William B.; Markus, Thorsten
Ice, Cloud, and land Elevation Satellite-2 (ICESat-2) is scheduled to launch in late 2017 and will carry the Advanced Topographic Laser Altimeter System (ATLAS), which is a photon-counting laser altimeter and represents a new approach to satellite determination of surface elevation. Given the new technology of ATLAS, an airborne instrument, the Multiple Altimeter Beam Experimental Lidar (MABEL), was developed to provide data needed for satellite-algorithm development and ICESat-2 error analysis. MABEL was deployed out of Fairbanks, Alaska, in July 2014 to provide a test dataset for algorithm development in summer conditions with water-saturated snow and ice surfaces. Here we compare MABEL lidar data to in situ observations in Southeast Alaska to assess instrument performance in summer conditions and in the presence of glacier surface melt ponds and a wet snowpack. Results indicate the following: (1) based on MABEL and in situ data comparisons, the ATLAS 90 m beam-spacing strategy will provide a valid assessment of across-track slope that is consistent with shallow slopes (< 1°) of an ice-sheet interior over 50 to 150 m length scales; (2) the dense along-track sampling strategy of photon counting systems can provide crevasse detail; and (3) MABEL 532 nm wavelength light may sample both the surface and subsurface of shallow (approximately 2 m deep) supraglacial melt ponds. The data associated with crevasses and melt ponds indicate the potential ICESat-2 will have for the study of mountain and other small glaciers.
Steinbrügge, Gregor; Hussmann, Hauke; Stark, Alexander; Oberst, Jürgen
Measurements of Ganymede's induced magnetic field suggest a salty water layer under the icy crust (Kivelson et al. 2002), in agreement with thermal models based on heat transfer and energy balance equations (e.g., Spohn and Schubert, 2003). Due to the small density contrast between ice-I and liquid water, interior structure models (e.g. Sohl et al. 2003) consistent with Ganymede's moment of inertia and total mass cannot constrain the ice thickness or ocean depth. In order to reduce the ambiguity of the structural models and to constrain the ice thickness, it has been proposed to measure the dynamic response of Ganymede's ice shell to tidal forces exerted by Jupiter characterized by the Love numbers h2 and k2. Similar strategies have been investigated in application to Europa (Wu 2001, Wahr 2006, Hussmann 2011). The body tide Love number h2 depends on the tidal frequency (main tidal cycle is the 7.15 days period of revolution), the internal structure, and the rheology, in particular on the presence of fluid layers, and the thickness and rigidity of an overlaying ice shell. Combined with measurements of the Love number k2, which can be inferred from radio science experiments, and a simultaneous determination of linear combinations of h2 and k2 the obtained data would significantly reduce the ambiguity in structural models (Wahr et al. 2006). A way to determine tidal effects in Ganymede's topography and therefore the h2 value by a spacecraft in orbit is the crossover method: Different orbit tracks will intersect at certain surface locations at different times so that the tidal signal can be extracted from a differential altimetry measurement. The Ganymede Laser Altimeter GALA is one of the instruments selected for the Jupiter Icy Moon Explorer (JUICE). The GALA instrument will perform globally distributed altitude measurements from a low circular orbit. The main challenges for the determination of the tidal amplitude are Ganymede's high surface roughness and low
Bindschadler, Robert A.; Zwally, H. Jay; Major, Judith A.; Brenner, Anita C.
Surface elevation maps of the southern half of the Greenland subcontinent are produced from radar altimeter data acquired by the Seasat satellite. A summary of the processing procedure and examples of return waveform data are given. The elevation data are used to generate a regular grid which is then computer contoured to provide an elevation contour map. Ancillary maps show the statistical quality of the elevation data and various characteristics of the surface. The elevation map is used to define ice flow directions and delineate the major drainage basins. Regular maps of the Jakobshavns Glacier drainage basin and the ice divide in the vicinity of Crete Station are presented. Altimeter derived elevations are compared with elevations measured both by satellite geoceivers and optical surveying.
Safarov, Elnur; Mammadov, Ramiz; Cretaux, Jean-Francois; Arsen, Adalbert; Safarov, Said; Amrahov, Elvin
Sea level fluctuations are among the most outstanding and debated issues of the Caspian Sea. Precipitation, underground water and river input are consistent parts of the inflow of the Caspian Sea water balance. The river input is also considered to be the main driver of the seasonal level changes of the Caspian Sea. Sufficiently large amount of this input is provided by the Volga. Although there is a good network of sea level stations covering the coastline of the sea, these facilities are not capable to reflect the sea level variations over the all surface. Meanwhile, the Caspian Sea is well observed by satellites Jason 1, Jason 2 and ENVISAT. Altimetric data taken from these satellites covers the surface of the sea much better than the data from the in-situ network stations. In this paper we investigate the spatial variability of the sea level that could provide more insight into the influence of river input (especially the Volga river), precipitation and other hydro-meteorological parameters on the Caspian Sea level.The altimetric data was averaged per every 10 square kilometers through all the tracks by means of the pre-prepared program made especially for this work. Also new maps of seasonal spatial variability of amplitude and phase of the annual signal of the Caspian Sea level for each investigated satellite were created by employing ARCGIS software. Moreover, these peaks of sea level amplitude and phase of annual signal results were comparatively analyzed with the corresponding river discharge of the Volga.
Biancamaria, S.; Garambois, P. A.; Calmant, S.; Roux, H.; Paris, A.; Monnier, J.; Santos da Silva, J.
Hydrodynamic laws predict that irregularities in a river bed geometry produce spatial and temporal variations in the water level, hence in its slope. Conversely, observation of these changes is a goal of the SWOT mission with the determination of the discharge as a final objective. In this study, we analyse the relationship between river bed undulations and water surface for an ungauged reach of the Xingu river, a first order tributary of the Amazon river. It is crosscut more than 10 times by a single ENVISAT track over a hundred of km. We have determined time series of water levelsat each of these crossings, called virtual stations (VS), hence slopes of the flow line. Using the discharge series computed by Paiva et al. (2013) between 1998 and 2009, Paris et al. (submitted) determined at each VS a rating curve relating these simulated discharge with the ENVISAT height series. One parameter of these rating curves is the zero-flow depth Z 0 . We show that it is possible to explain the spatial and temporal variations of the water surface slope in terms of hydrodynamical response of the longitudinal changes of the river bed geometry given by the successive values of Z 0 . Our experiment is based on an effective, single thread representation of a braided river, realistic values for the Manning coefficient and river widths picked up on JERS images. This study confirms that simulated flow lines are consistent with water surface elevations (WSE) and slopes gained by satellite altimetry. Hydrodynamical signatures are more visible where the river bed geometry varies significantly, and for reaches with a strong downstream control. Therefore, this study suggests that the longitudinal variations of the slope might be an interesting criteria for the question of river segmentation into elementary reaches for the SWOT mission which will provide continuous measurements of the water surface elevation, the slope and the reach width.
Wilson, Matthew D.; Durand, Michael; Alsdorf, Douglas; Chul-Jung, Hahn; Andreadis, Konstantinos M.; Lee, Hyongki
The Surface Water and Ocean Topography (SWOT) satellite mission, scheduled for launch in 2020 with development commencing in 2015, will provide a step-change improvement in the measurement of terrestrial surface water storage and dynamics. In particular, it will provide the first, routine two-dimensional measurements of water surface elevations, which will allow for the estimation of river and floodplain flows via the water surface slope. In this paper, we characterize the measurements which may be obtained from SWOT and illustrate how they may be used to derive estimates of river discharge. In particular, we show (i) the spatia-temporal sampling scheme of SWOT, (ii) the errors which maybe expected in swath altimetry measurements of the terrestrial surface water, and (iii) the impacts such errors may have on estimates of water surface slope and river discharge, We illustrate this through a "virtual mission" study for a approximately 300 km reach of the central Amazon river, using a hydraulic model to provide water surface elevations according to the SWOT spatia-temporal sampling scheme (orbit with 78 degree inclination, 22 day repeat and 140 km swath width) to which errors were added based on a two-dimension height error spectrum derived from the SWOT design requirements. Water surface elevation measurements for the Amazon mainstem as may be observed by SWOT were thereby obtained. Using these measurements, estimates of river slope and discharge were derived and compared to those which may be obtained without error, and those obtained directly from the hydraulic model. It was found that discharge can be reproduced highly accurately from the water height, without knowledge of the detailed channel bathymetry using a modified Manning's equation, if friction, depth, width and slope are known. Increasing reach length was found to be an effective method to reduce systematic height error in SWOT measurements.
Marks, Karen M.
The July 1995 declassification of the entire Geosat GM satellite altimeter data set enabled a joint Scripps/NOAA effort to compute a new (version 7.2) marine gravity field on a 2-minute grid. This gravity field covers the world's oceans between 72°N and 72°S, and is derived from a combination of ERS-1 and Geosat GM and ERM data. An earlier NOAA Geosat-only gravity field solution was confined to the southern latitudes because the 1992 declassification was limited to GM data south of 30°S. A simple coherence analysis between accurately-navigated ship gravity profiles and comparable gravity profiles obtained from the gravity grids reveals that the Scripps/NOAA gravity field is coherent with ship gravity down to ˜≥ 23-30 km. This slight increase in resolution over the previous NOAA Geosat-only gravity field (short-wavelength resolution of ˜26-30 km) implies that the increased spatial coverage provided by the ERS-I altimeter, when combined with Geosat, improves the solution. Coherence analyses between satellite gravity and ship topography, and ship gravity and ship topography, show that even shorter wavelength gravity anomalies (˜13 km) are present in sea-surface measurements made by ship. Even so, the Scripps/NOAA marine gravity field does an excellent job of resolving most of the short-wavelength gravity anomalies covering the world’s oceans.
It is contended that a spherical harmonic expansion of the difference between the altimeter-derived mean sea surface and the geoid estimate should reveal the large-scale circulation of the ocean surface layer when the low-degree terms are examined. Methods based on this principle are proposed and partially demonstrated over the Pacific Ocean with the aid of the mean sea surface derived from the Seasat altimeter and the Goddard Earth Model 9 earth gravity model. The preliminary results reveal a well-defined clockwise gyre in the North Pacific and a much less well defined counterclockwise gyre in the South Pacific. When the dynamic topography thus obtained is compared with Wyrtki's (1975) dynamic topography derived from hydrographic data, the agreement is found to be within the limit of geoid uncertainties and satellite orbital errors.
Comparisons of gyre-scale acoustic and direct thermal measurements of heat content in the Pacific Ocean, satellite altimeter measurements of sea surface height, and results from a general circulation model show that only about half of the seasonal and year-to-year changes in sea level are attributable to thermal expansion. Interpreting climate change signals from fluctuations in sea level is therefore complicated. The annual cycle of heat flux is 150 +/- 25 watts per square meter (peak-to-peak, corresponding to a 0.2 degreesC vertically averaged temperature cycle); an interannual change of similar magnitude is also detected. Meteorological estimates of surface heat flux, if accurate, require a large seasonal cycle in the advective heat flux. PMID:9721093
Campbell, W.J.; Cheney, R.E.; Marsh, J.G.; Mognard, N.M.
Models for the towing of large tabular icebergs give towing speeds of 0.5 knots to 1.0 knots relative to the ambient near surface current. Recent oceanographic research indicates that the world oceans are not principally composed of large steady-state current systems, like the Gulf Stream, but that most of the ocean momentum is probably involved in intense rings, formed by meanders of the large streams, and in mid-ocean eddies. These rings and eddies have typical dimensions on the order of 200 km with dynamic height anomalies across them of tens-of-centimeters to a meter. They migrate at speeds on the order of a few cm/sec. Current velocities as great as 3 knots have been observed in rings, and currents of 1 knot are common. Thus, the successful towing of icebergs is dependent on the ability to locate, measure, and track ocean rings and eddies. To accomplish this systematically on synoptic scales appears to be possible only by using satelliteborne radar altimeters. Ocean current and eddy structures as observed by the radar altimeters on the GEOS-3 and Seasat-1 satellites are presented and compared. Several satellite programs presently being planned call for flying radar altimeters in polar or near-polar orbits in the mid-1980 time frame. Thus, by the time tows of large icebergs will probably be attempted, it is possible synoptic observations of ocean rings and eddies which can be used to ascertain their location, size, intensity, and translation velocity will be a reality. ?? 1980.
Kern, S.; Khvorostovsky, K.; Skourup, H.; Rinne, E.; Parsakhoo, Z. S.; Djepa, V.; Wadhams, P.; Sandven, S.
One goal of the European Space Agency Climate Change Initiative sea ice Essential Climate Variable project is to provide a quality controlled 20 year long data set of Arctic Ocean winter-time sea ice thickness distribution. An important step to achieve this goal is to assess the accuracy of sea ice thickness retrieval based on satellite radar altimetry. For this purpose a data base is created comprising sea ice freeboard derived from satellite radar altimetry between 1993 and 2012 and collocated observations of snow and sea ice freeboard from Operation Ice Bridge (OIB) and CryoSat Validation Experiment (CryoVEx) air-borne campaigns, of sea ice draft from moored and submarine Upward Looking Sonar (ULS), and of snow depth from OIB campaigns, Advanced Microwave Scanning Radiometer aboard EOS (AMSR-E) and the Warren Climatology (Warren et al., 1999). An inter-comparison of the snow depth data sets stresses the limited usefulness of Warren climatology snow depth for freeboard-to-thickness conversion under current Arctic Ocean conditions reported in other studies. This is confirmed by a comparison of snow freeboard measured during OIB and CryoVEx and snow freeboard computed from radar altimetry. For first-year ice the agreement between OIB and AMSR-E snow depth within 0.02 m suggests AMSR-E snow depth as an appropriate alternative. Different freeboard-to-thickness and freeboard-to-draft conversion approaches are realized. The mean observed ULS sea ice draft agrees with the mean sea ice draft computed from radar altimetry within the uncertainty bounds of the data sets involved. However, none of the realized approaches is able to reproduce the seasonal cycle in sea ice draft observed by moored ULS satisfactorily. A sensitivity analysis of the freeboard-to-thickness conversion suggests: in order to obtain sea ice thickness as accurate as 0.5 m from radar altimetry, besides a freeboard estimate with centimetre accuracy, an ice-type dependent sea ice density is as mandatory