Calibration of Satellite Laser Altimetry Over Precisely Mapped Terrain

Early in 2003, NASA's GLAS laser altimeter aboard ICESat started measuring surface elevations at all latitudes up to 86 degrees latitude, with the prime objective of measuring rates of ice-surface elevation change in Greenland and Antarctica to an accuracy of a few cm/yr. Validation of these measurements and calibration of measurement biases includes comparison of GLAS measurements with detailed topographic maps of narrow stripes (200-300 m wide) of terrain surveyed by NASA's Airborne Topographic Mapper (ATM) - a scanning laser altimeter. Because these segments include a range of surface slopes, it is possible to check overall GLAS-derived elevations at the 10-cm level and to infer pointing to less than an arc second. Validation sites are in Greenland, western US, and Antarctica, including arid regions free from snow cover and vegetation which can be used to check data consistency throughout the ICESat mission and for cross calibration with follow-on missions. The ATM survey data is sufficiently dense to allow calculation of GLAS return waveforms which are then compared to the observed waveforms with differences used to estimate range bias, pointing errors, and timing errors. These comparisons can be made for surfaces with slopes of 15 deg or more to which ranges have a high sensitivity to pointing errors. Here, we present overall comparisons between GLAS and ATM, showing the level of agreement between GLAS and recent ATM surveys of ice surfaces, along with our estimates of instrument biases based on ATM/GLAS comparisons along individual orbit tracks.