C21B-01 INVITED 08:00h
Overview of the ICESat Mission and Results
NASA's Ice, Cloud, and Land Elevation Satellite (ICESat), launched in January, 2003, has been measuring surface elevations of ice and land, vertical distributions of clouds and aerosols, vegetation-canopy heights, and other features with unprecedented accuracy and sensitivity. The ICESat mission, which was designed to operate continuously for 3 to 5 years, has so far acquired science data during five periods of laser operation ranging from 33 to 54 days each. The primary purpose of ICESat has been to acquire 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 improve estimates of the present and future contributions to global sea level rise. ICEsat's atmospheric measurements are providing fundamentally new information on the precise vertical structure of clouds and aerosols. In particular, cloud heights are important for understanding radiation balance and their effects on climate change. Other applications include mapping of polar sea-ice freeboard and thickness, high-resolution mapping of ocean eddies, glacier topography, and lake and river levels. ICESat has a 1064 nm laser channel for near-surface altimetry with a designed range precision of 10 cm that is actually 2 cm on-orbit. Vertical distributions of clouds and aerosols are obtained with 75 m resolution from both the 1064 nm channel and the more sensitive 532 nm channel. The laser footprints are about 70 m spaced at 170 m along-track. The accuracy of the satellite-orbital heights is about 3 cm. The star-tracking attitude-determination system should enable footprints to be located to 6 m horizontally when attitude calibration is completed. The spacecraft attitude is controlled to point the laser beam to within 100 m (35 m goal) of reference surface tracks at high latitudes and to point off-nadir up to 5 degrees to targets of interest. The remaining laser lifetime will be used for approximately 33-day periods at 3 to 6 month-intervals to optimize the science return. The first ICESat was intended to be followed by successive missions to measure changes over 15 years, and has clearly proven the unique capability of laser measurements to meet multi-disciplinary science objectives. An example of continuing requirements is: "Continued observations with satellite altimeters, including . the laser altimeter on ICESat . should be continued for at least 15 years . to establish the climate sensitivities of the ice mass balance and decadal-scale trends" (Climate Change 2001, IPCC, 2001).
C21B-02 08:15h
Laser Pointing Determination for ICESat Altimetry
Since its launch aboard the Ice, Cloud, and land Elevation Satellite (ICESat) in January 2003, the Geoscience Laser Altimeter System (GLAS) has produced high-quality elevation profiles for the Greenland and Antarctic ice sheets, as well as for other land and ocean surfaces, during five operational periods. The computation of individual elevations begins with a scalar range, inferred from the time required for a single pulse to travel from a reference point in the GLAS instrument to the surface, and back. This value is then combined with the laser-pointing direction, determined from ground-based processing of data collected by the uniquely designed, on-board Stellar Reference System (SRS). The resulting range vector, together with the geocentric position vector of the GLAS reference point, defines the position and the elevation of the laser spot on the surface. Consequently, while the accuracy of a GLAS-derived elevation depends on the knowledge of the instrument position, as with radar altimeters, it also depends directly on the knowledge of the laser-pointing direction. The position of the GLAS reference point is obtained through ground-based precision orbit determination (POD), using the navigation data collected by the on-board BlackJack GPS receiver. Current assessment of these results suggests that the critical radial component has an accuracy of 2-3 cm, well within the 5-cm requirement. Faced with a number of challenges in the SRS after launch, significant effort has been expended to calibrate the laser-pointing determination. Through a series of refinements in the precision attitude determination (PAD), the resulting pointing solutions have steadily improved the accuracy of the computed elevations, as determined through a variety of calibration/validation techniques. This presentation will highlight the improvements observed for elevation data obtained during the first two operations periods, February-March and September-November 2003.
C21B-03 08:30h
Geoscience Laser Altimeter System (GLAS) on the ICESat Mission: Science Measurement Performance since Launch
The Geoscience Laser Altimeter System is a space lidar and the primary instrument on NASA's ICESat mision. Since launch in January 2003 GLAS has produced about 544 million measurements of the Earth's surface and atmosphere. It has made global measurements of the Earth's icesheets, land topography and atmosphere with unprecedented vertical resolution and accuracy. GLAS was first activated for science measurements in February 2003. Since then its operation and performance has confirmed many pre-launch expectations and exceed a few of the most optimistic expectations in vertical resolution and sensitivity. However GLAS also suffered an unexpected failure with its first laser, and the GLAS measurements have yielded some surprises in other areas. This talk will give a post-launch assessment of the science measurement performance of the GLAS instrument, and compare the measurement environment and its science measurements to pre-launch expectations. It also will address some of what has been learned from the GLAS design, operations and measurements which may benefit future space lidar.
C21B-04 INVITED 08:45h
Global and Polar Cloud Cover from the Geoscience Laser Altimeter System, Observations and Implications
The Geoscience Laser Altimeter System (GLAS) on board the Ice, Cloud and Land Elevation Satellite provides space-borne laser observations of atmospheric layers. The GLAS laser profiling permits unambiguous detection of clouds with high sensitivity and direct height profiling. Climatologies of clouds in the high latitudes in particular are compiled to much greater accuracy than passive satellite observations where clouds and snow surfaces appear similar at both infrared and visible wavelengths. Since its launch in early 2003, GLAS has obtained a substantial sample of cloud observations by space borne lidar. In addition to the detection of the coverage and height distribution of clouds, the GLAS data algorithms also derive the optical thickness and scattering cross sections for transmissive clouds. A substantial fraction of clouds are sufficiently transmissive that a laser pulse from surface scattering is obtained. The accurate results for cloud coverage and distribution obtained from GLAS may be contrasted with previous climatologies and other satellite observations. Overall GLAS has detected a 70 percent average global cloud cover with 45 percent of the total being single layered. For many parameters there is good agreement with MODIS and other passive cloud retrievals but in some cases, especially night time and polar clouds, there are significant discrepancies. Polar clouds and aerosol are an important factor for the accuracy of the altitude measurements of ice sheet elevation change. Multiple scattering through even the thinnest clouds introduces an effective pulse spreading and altitude bias. Pulse spreading up to 10 of meters is observed in data. The GLAS mission was designed so that cloud clearing and corrections would permit centimeter accuracies for surface elevation change. Results showing the adequacy the cloud detection and clearing for surface measurements are presented.
http://glo.gsfc.nasa.gov/
C21B-05 INVITED 09:00h
ICESat Observations of Inland Surface Water Stage, Slope, and Extent: a new Method for Hydrologic Monitoring
River discharge and changes in lake, reservoir and wetland water storage are critical terms in the global surface water balance, yet they are poorly observed globally and the prospects for adequate observations from in-situ networks are poor (Alsdorf et al., 2003). The NASA-sponsored Surface Water Working Group has established a framework for advancing satellite observations of river discharge and water storage changes which focuses on obtaining measurements of water surface height (stage), slope, and extent. Satellite laser altimetry provides a method to obtain these inland water parameters and contribute to global water balance monitoring. Since its launch in January, 2003, the Ice, Cloud, and land Elevation Satellite (ICESat), a NASA Earth Observing System mission, has achieved over 540 million laser pulse observations of ice sheet, ocean surface, land topography, and inland water elevations and cloud and aerosol height distributions. The ICESat mission has demonstrated the following laser altimeter capabilities relevant to observations of inland water: (1) elevation precision of 2 to 3 cm, suitable for detecting river surface slopes along long river reaches or between multiple crossings of a channel, (2) decimeter single pulse absolute elevation accuracy for flat surfaces and clear atmosphere, suitable for detection of stage changes, (3) detection of water surface elevations beneath vegetation canopies, suitable for measuring water stage in flooded forests, (4) precise spacecraft pointing so as to position the laser profile on the Earth's surface with an accuracy of 50 m (1 sigma), enabling small water bodies to be targeted and re-observed through time, (5) adequate signal levels from quasi-specular water surfaces up to 5° off-nadir, enabling access to any location on the Earth's surface from the ICESat repeat orbit, and (6) day and night operation with successful laser ranging to the Earth's surface through thin to moderate cloud cover, enabling more frequent measurements than can be achieved by passive optical sensors. Here we illustrate these capabilities by showing ICESat observations through time for selected river and lake locations and discuss their implications for estimating discharge and storage changes.
C21B-06 09:15h
ICESat Validation of SRTM C-Band Digital Elevation Models
Understanding the quality of the DEM data sets is crucial to their use in land process studies and in detection of change obtained from comparison of DEMs acquired at different times. Elevation data from the Geoscience Laser Altimeter System (GLAS) on board the Ice Cloud and Land Elevation Satellite (ICESat) provides a globally distributed data set that can be used to independently estimate the accuracy of available Digital Elevation Models (DEMs), such as the ones produced by the Shuttle Radar Topography Mission (SRTM). Calibration and validation activities indicate that the accuracy of the best ICESat data is currently approximately 30 m horizontal and 50 cm vertical (1 sigma for 1 degree laser beam incidence angle on the Earth's surface), with further improvements expected from ongoing calibration efforts. The current accuracy is of sufficient quality for assessment of the SRTM DEMs. Our efforts have concentrated on documenting elevation differences between ICESat and SRTM C-band elevations for various regions of the world with variable topography and vegetation cover. ICESat data enables the estimation of radar phase center elevation biases in vegetated regions with respect to the top of the canopy and ground. Errors in the SRTM elevations data are quantified based on the distribution of the ICESat to SRTM elevation differences, and their relationship with local relief and land-cover characteristics. Correlations between ICESat to SRTM elevation differences and waveform extent, a surrogate for canopy height, and canopy closure are also examined. Regional analyses, previously conducted in the Western US, Alaska and the central Amazon Basin, have been expanded and now include data from all the continents (except Antarctica which lacks SRTM coverage). Based on the results of this work, the ICESat data product for each laser footprint location will include the elevation from the SRTM 90 m resolution product where available, facilitating global analyses using these complimentary elevation measurement systems.
C21B-07 09:30h
Estimating forest canopy height from Geoscience Laser Altimeter System waveforms and Shuttle Radar Topography Mission digital elevation models
Although the exchange of carbon between the atmosphere and forested ecosystems is a vital component of the global carbon cycle, it is a portion of the cycle that is not well-characterized or well-understood. Lidar remote sensing has a unique capability for estimating forest canopy height, which has a direct and increasingly well understood relationship to aboveground carbon storage. While the Geoscience Laser Altimeter System (GLAS) on the Ice, Cloud and land Elevation Satellite has collected an unparalleled data set of lidar waveforms over terrestrial targets, processing of GLAS data to create reliable estimates of forest height is complicated by elements of sensor design related to its primary mission-the topographic mapping of the ice sheets of Greenland and Antarctica. In this research we demonstrate a straightforward method utilizing Shuttle Radar Topography Mission data to correct for the broadening of collected waveforms due to sloped ground surfaces- a major complication to using the GLAS data for land surface applications. Using corrected waveforms, we then demonstrate the potential applicability of GLAS data to both carbon cycle and fire management questions. To date GLAS has transmitted over 540 million globally-distributed laser pulses. This data set is an important and unique source of information on global forest canopy height and aboveground biomass.
C21B-08 INVITED 09:45h
High-resolution Antarctic topography from the ICESat mission near $86\deg$S
At the southern limit of ICESat's coverage, the satellite ground-tracks converge. Between $86^o$S and $85^o$S the density of topographic data allows the generation of topographic maps with a resolution of 300-500 meters. At this resolution, fine-scale features of mountain and outlet glaciers as well as aeolean and depositional ice-sheet surface features become visible, in addition to the large-scale ice sheet morphology. In the Transantarctic mountains, dramatic flow-striping is revealed on the Reedy, Scott and Amundsen glaciers. Through the Horilick mountains a unique regime of glacier-flow appears that is mid-way between ice-sheet flow and valley glacier flow. On the East Antarctic plateau, our data reveal the height distribution and orientation of megadune fields. In the relatively featureless parts of the ice sheet where no mountains or nunataks are present, these data also allow the extraction of two-dimensional Fourier spectra of the ice-sheet surface. Analysis of these spectra demonstrates variations in the topographic texture of the ice-sheet that reflect variations in ice- thickness and flow conditions.