C33B-0343 1340h
Recent Results in ICESat Geolocation Parameter Calibration From Range Residual Analysis
The Ice, Cloud and land Elevation Satellite (ICESat), launched on January 12, 2003, carries the Geoscience Laser Altimeter System (GLAS), which has a primary measurement of short-pulse laser-ranging to the Earth's surface at 1064nm wavelength at a rate of 40 pulses per second. The instrument has collected (at the time of this writing) over 540 million observations of the Earth's surface including precise elevation measurements of the ice sheets, sea ice roughness and thickness, ocean and land surface elevations and surface reflectivity. The accurate geolocation of GLAS's surface returns, the spots from which the laser energy reflects on the Earth's surface, is a critical issue in the scientific application of these data. Pointing, ranging, timing and orbit errors must be compensated to accurately geolocate the laser altimeter surface returns. Towards this end, the laser range observations can be fully exploited in an integrated residual analysis to accurately calibrate these geolocation/instrument parameters. The ICESat laser altimeter data have been simultaneously processed as direct altimetry from ocean sweeps and around-the-world scans along with dynamic crossovers in order to calibrate pointing, ranging and timing. The analysis has resolved complex time varying signal resulting from thermal variations of instrument pointing and boresite shadowing. The calibration methodology and current calibration results are discussed along with future efforts and current geolocation accuracies.
C33B-0344 1340h
Validation of GLAS range measurement over the salar de Uyuni, Bolivia
NASA's Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite (ICESat) is Earth's first polar-orbiting satellite laser altimeter. Its primary purpose is to measure ice sheet elevation change, with a scientific requirement of detecting changes as low as 1.5 cm/year. This goal requires precise calibration and validation of the instrument. One approach for validating the GLAS range measurement involves comparison with a land reference target. Dry salt lakes are ideal for this purpose since they are large, stable, easily surveyed using kinematic GPS, and have albedos similar to that of ice. We selected the largest dry salt lake in the world, the salar de Uyuni in Bolivia, as a reference target for GLAS. In September 2002 we surveyed a 54 x 45 km area of the salar with car-mounted kinematic GPS. The resulting DEM of the surface has an estimated precision of 1 cm (RMS) with a total topographic range of less than 1 meter over the survey area. To date there have been six ICESat passes across our surveyed area, along two intersecting tracks. The first two passes occurred during "Laser 2A" operation (September to November 2003), the second two during "Laser 2B" operation (February to March 2004), and the final two during "Laser 2C" (June to July 2004). We compare GLAS elevation measurements from all six passes with the salar de Uyuni DEM, showing how differing atmospheric and surface conditions affected the performance of the GLAS instrument from pass to pass and providing estimates of the relative and absolute accuracies of the range measurement. As an independent check, we compare GLAS elevation profiles with the TOPEX-derived mean sea-surface for ocean areas along the orbits used in the GPS comparison above.
C33B-0345 1340h
Topography of the Flattest Surface on Earth: Using ICESAT, GPS, and MISR to Measure Salt Surface Morphology on Salar de Uyuni, Bolivia
Salt flats are aptly named: they are composed largely of salt, and are maintained as nearly equipotential surfaces via frequent flooding. The salar de Uyuni, on the Altiplano in southwestern Bolivia, is the largest salt flat on Earth, with an area of 9,800 km$^{2}$. Except for a few bedrock islands, it has less than 40 cm of relief. The upper-most salt unit averages 5 m thick and contains 50 km$^{3}$ of nearly pure halite. It includes most of the salt that was in solution in paleolake Minchin, which attained a maximum area of 60,000 km$^{2}$ and a maximum depth of 150 m, roughly 15 kyr ago. Despite ~10 m of differential isostatic rebound since deposition, the salar surface has been actively maintained as an extraordinarily flat and smooth surface by annual flooding during the rainy season. We have used the strong optical absorption properties of water in the visible band to map spatial variations in water depth during a time when the salar was flooded. As water depth increases, the initially pure white surface appears both darker and bluer. We utilized MISR images taken during the interval from April to November 2001. The red and infra-red bands (672 and 867 nm wavelength) were most useful since the water depth is small and the absorption at those wavelengths is quite strong. Nadir pointed MISR images have 275 m spatial resolution. Optical absorption of water has been used previously for bathymetric mapping, but usually in much deeper water. Our application was particularly well suited to obtaining high accuracy results, since the water clarity is high, the substrate reflectivity is high and quite uniform, and we can measure it directly when the surface water is gone. To aid in our evaluation of water depth variations over the salar surface, we utilized two sources of direct topographic measurements: several ICESAT altimetry tracks cross the area, and a 40 x 50 km GPS grid was surveyed to calibrate ICESAT. A difficulty in using these data types is that both give salt surface elevations relative to the ellipsoid, whereas the water surface will, in the absence of wind or tidal disturbances, follow an equipotential surface. Geoid height is not known to the required accuracy of a few cm in the central Andes. As a result, before comparing optical absorption from MISR to salt surface topography from GPS or ICESAT, we removed the longest wavelengths from both. Our model agrees quite well with the GPS grid and ICESAT tracks (5 cm RMS misfit) and shows a very flat and level, but not completely featureless salt surface. The most prominent topographic feature is a peripheral moat, or depression near the edge of the salar, which is most pronounced in locations near to sources of fresh water input during the rainy season. Another prominent feature is a series of wave-like ridges, with 20-30 cm amplitude, and 4-5 km wavelength. They are mainly found near the western edge of the salar, and have crests and troughs oriented mainly N-S. The process of formation of these features is still enigmatic, but seems related to wave action in the water during wet episodes.
C33B-0346 1340h
Seasonal Variation of Antarctic Sea-Ice Freeboard Heights and Thicknesses from ICESat
The distribution of sea ice affects Earth's radiative balance and atmosphere and ocean circulation. The study of sea-ice thickness distribution and volume variation is an important part of understanding the Earth's climate system. ICESat measures the mean surface elevation of flat surfaces to better than 3 cm over its 70 m laser footprints spaced at 170 m. This provides an important tool to study sea ice. Previous knowledge of Antarctic sea-ice freeboard and thickness is based on very limited information from surface and ship-based measurements. The ICESat orbit has an inclination of 94° and its ground tracks cover all sea ice surrounding Antarctica. Using open water and thin ice as reference sea level, a novel technique has been developed to measure sea-ice freeboard using ICESat measured elevation data. With estimates of snow, brine, and sea-ice density, combined with snow depth data from AMSR-E, sea-ice thickness can be derived from the freeboard. Sea-ice freeboard and thickness were calculated along ICESat ground tracks first and then gridded to 50 x 50 km grid maps. Three periods of Antarctic sea-ice freeboard and thickness data have been studied. Sea-ice freeboard and thickness distributions show clear seasonal variation. During the Antarctic winter (Oct-Nov, 2003), sea ice grows to its seasonal maximum. Thicker sea ice surrounds the Antarctic continent; thinner sea ice is distributed near Princess Ragnhild Coast and the Amundsen and Bellingshausen Seas; the mean thickness of winter sea ice is 2.9 m. During the Antarctic summer (Feb-Mar, 2004), thinner sea ice melts away. Sea ice is mainly distributed in the Weddell Sea near the Antarctic Peninsula and Ross Sea, with a mean thickness of 2.4 m. During the Antarctic fall (May-Jun, 2004), large areas of new, thinner sea ice forms. Thinner sea ice covers large areas of the Weddell and Ross Seas, and the overall mean thickness is 1.9 m. Overall, ICESat measurements provide unprecedented accuracy and spatial and temporal coverage of sea-ice freeboard and thickness and can be used to monitor sea-ice volume, which is an indicator of climate change.
C33B-0347 1340h
Ice Sheet Surface Topography from ICESat Altimetry
Over 150 days of data have been collected by the Geoscience Laser Altimeter System (GLAS) on-board NASA's Ice Cloud and land Elevation Satellite (ICESat). The unprecedented precision and accuracy of this dataset are excellent for creating high quality Digital Elevation Models (DEMs) of the Greenland and Antarctic ice sheets. The second generation of 5km ice sheet DEMs demonstrate improvements over their predecessors due to better attitude calibrations and data processing. Another key to improved ICESat DEMs is a better understanding of GLAS data editing. Complete Antarctica and Greenland 5km DEMs are shown as well as higher resolution models in the high latitude regions. Aircraft Laser Data from the NASA/WFF Airborne Topographic Mapper (ATM) is used as a reference to demonstrate ICESat DEM improvements over radar altimetry DEMs. Several different gridding methods including averaging, spline fitting, and biquadratic interpolation are compared.
C33B-0348 1340h
Balance Fluxes Computed Based on Different Altimeter-derived Topographies Relative to the Observed Ice Flow in the Greenland Ice Sheet
Balance fluxes, representing the ice flow that would correspond to a steady state ice sheet, are computed over the Greenland ice sheet. Input data are ice accumulation rate and altimeter-derived topography. Two topographic datasets derived from Ice, Cloud, and Land Elevation Satellite (ICESat) and European Remote-sensing Satellite (ERS-1&2) in 5km high-resolution grids are used, respectively, for comparison. To decide if the ice sheet is locally in a state of positive or negative mass budget, these balance fluxes must be compared with actual ice fluxes obtained from the observations of ice thickness and surface velocity. This comparison requires the ratio of depth-averaged velocity to surface velocity to convert the measured surface velocities to depth-averaged velocities. In this paper, the values of the ratio, depending on ice rheology, tempearture and other ice flow parameters, are determined from a 3D thermomechanical ice flow model with incorporation of anisotropic rheology and are used for comparing balance fluxes with available Global Positioning System (GPS) measurements (along 2000 metre contour) to evaluate the local ice-sheet state.
C33B-0349 1340h
Ice flow properties derived from ICESat surface topography in polar ice sheets
A major difficulty in numerical modeling of ice sheets is a lack of sufficiently precise information on the flow properties. Analysis of the detailed variations in the ice sheet elevation profiles against bedrock topography provides valuable means of determining ice flow properties. In this study, the power spectral analysis of the detailed ICESat surface undulation data against available bedrock profiles was performed along selected sections near flow-lines over both Antarctica and Greenland. The spectra of the surface undulations are characterized by a minimum damping indicated by a peak value of the power with wave-length of several km, whereas the magnitude of the spectra for the bedrock increases with the wave-length. By using balance velocity and a mean damping factor with wave-length of 20-40 km, the mean effective ice viscosities are estimated and compared with values from laboratory ice deformation tests. The increased departure from inland to coast between topography and laboratory derived viscosities for isotropic polycrystalline ice may indicate systematic changes in the development of crystal anisotropies and the amount of basal sliding.
C33B-0350 1340h
Rapid Ice Flow Related Topography from ICESat Altimetry in NE Greenland
Extending approximately 700 km inland from the coast of Greenland, the `Bronlund' Ice Stream in Northeast Greenland has a clear influence on the topography of the ice sheet from the coast to near the central divide. This onset of flow in this feature is associated with high rates of basal melt; flow in the interior is characterized by high strain rates in distinct shear margins. Fahnestock et al. (2001) investigated the topography and ice flow patterns in the interior based on data from SAR interferometry from Joughin et al. (2001). In this paper, we use ICESat precision laser altimetry data to examine the morphology of the shear margins of this feature. ICESat's high frequency sampling, 172 m along-track, provides consistent and accurate elevation data on the relief associated with the shear margins and surface undulations in the stream itself. The marginal shear zones and the undulating internal topography that define the ice stream are clearly illustrated by the sampling of the ICESat data. Repeat tracks obtained during ICESat's first four operational periods enable the impact of clouds on the elevation data to be minimized. The resulting pattern of ICESat tracks, supported by enhanced MODIS imagery, allows us to revisit the previous study based on interferometry, enabling us to study the dynamically generated topography using a consistently sampled set of detailed elevation profiles over large areas. Narrow marginal troughs are shown to be as much as 10 m deep over distances of a kilometer; this is pronounced topography in ice 3 km thick. The relief and extent of the marginal trough on the southeastern edge of the stream is in contrast to the less well-pronounced margin on the northwest. Significant surface undulations, with horizontal extents of approximately 3 ice thicknesses, are found where ice stream flow speeds reach 50 m/year, and change character downstream as flow speeds increase. Changes in the positions of the marginal troughs may be discernable if the ICESat instrument, or a follow-on mission, repeats the same tracks in a few years with similar accuracy.
http://icesat.gsfc.nasa.gov/
C33B-0351 1340h
An Application of Kriging and Kalman Filtering to Study Surface Characteristics and Height Changes over Antarctica Using ICESat Data
We construct two high resolution DEMs of Antarctica to study height changes and other surficial characteristics by applying a combination of Kalman filtering and kriging techniques to ICESat 33-day repeat data. The first DEM at resolution approximately 5km was constructed by initially taking the mean of Laser2a 33-day data, then refining by applying a kriging/Kalman filter to subsequent repeat-tracks to produced final maps of height changes across four regions in Antarctica. To characterize surface slopes, we take advantage of the geometry of ICESat tracks to create a second DEM at resolution of approximately 1-6km along track, then refine to obtain across/along track slope information along with height changes. Our goal is to be able to construct a DEM as close to true surface height as possible by taking advantage of the very closely-spaced repeated 33-day tracks. Using this DEM and its associated slope information, we should be able to predict accurately surface height at locations close to ICESat footprints, and thus detecting small height changes signals.
C33B-0352 1340h
Multiple Reflections Inside the GLAS Footprint: Estimation of the Altimetry Bias and a Roadmap towards a Correction Scheme
ICESat's primary mission statement is to use laser ranging to estimate the altitude (hence thickness) of the Earth's ice sheets at better than 10 cm per shot, leading to a mapping of the trend at about 1 cm per year accuracy upon temporal averaging. This ambitious goal is dictated by global warming science considerations. Laser altimetry and/or ranging can be done either by detection of the earliest photon returned from a short but intense pulse or by tracking the shift of the maximum of a longer pulse. In altimetry mode, GLAS uses the latter algorithm which makes it vulnerable to spurious shifts caused by multiple reflections in the unresolved terrain. We show that, for GLAS's instrumental characteristics and typical reflection and roughness properties of ice, the resulting negative altimetry bias can be commensurate with the mission's precision goal. The bias will furthermore have a seasonal cycle which may become aliased into the annual mean. Finally, we will argue that, based on our modeling results, GLAS has within its product portfolio much of what is needed to correct the bias.
http://nis-www.lanl.gov/~adavis
C33B-0353 1340h
Cloud and Aerosol Observations by Micro-Pulse Lidars at Arctic and Antarctic Sites During ICESat/GLAS Overpass Measurements
Intended for long-term monitoring of the vertical structure and optical properties of clouds and aerosol in bi-polar regions, we are operating Micro-Pulse Lidars (MPLs) at Ny-Aalesund (79N, 12E), Svalbard in the Arctic and at Syowa Station (69S, 40E), Antarctica. These sites are part of the NASA Micro-Pulse Lidar Network (MPLNET). The Arctic MPL measurement started in 1998, and the Antarctic MPL measurement started in 2001. The Geoscience Laser Altimeter System (GLAS) on board the Ice, Cloud and Land Elevation Satellite (ICESat) was successfully launched in January 2003. The lidar observation in a near-polar orbit with an inclination of 94 degrees provides a global coverage of vertical profiles of clouds and aerosol including both polar regions. Data products include thin cloud and aerosol optical depth. GLAS data validation issues include the sensitivity of cloud detection and optical depth accuracy. The polar MPL measurements include GLAS overpasses in 2003 and 2004. ICESat pointed directly to the Ny-Aalesund and Syowa sites when within five degrees off nadir. In this paper, preliminary results from the Arctic and Antarctic MPL measurements will be shown and discussed as ground truth of cloud and aerosol measurements by ICESat/GLAS.
C33B-0354 1340h
Tools at the National Snow and Ice Data Center (NSIDC) Distributed Active Archive Center (DAAC) for Working With Geoscience Laser Altimeter System (GLAS) Data Products From Ice, Cloud, and Land Elevation Satellite (ICESat)
The Geoscience Laser Altimeter System (GLAS) is the sole instrument on the Ice, Cloud, and land Elevation Satellite (ICESat), launched in January 2003. The main objective of the ICESat mission is to measure ice sheet elevations and changes in elevation through time. Secondary objectives include measurement of cloud and aerosol height profiles, land elevation and vegetation cover, and sea ice thickness. This poster provides an overview of the tools available from the National Snow and Ice Data Center (NSIDC) Distributed Active Archive Center (DAAC) for working with ICESat/GLAS data. It includes descriptions of an IDL visualization tool, an altimetry extractor tool, IDL readers, Fortran 90 code, IDL ellipsoid converter, and subsetting capabilities. For more information about GLAS data products available from NSIDC, see: http://nsidc.org/data/icesat.
http://nsidc.org/data/icesat