How do GLAS data improve upon past ice sheet altimetry data?
Seasat data collected in 1978 provided scientists with 20 km elevation grids that resolved only the largest surface undulations. Geosat (1985-86) was designed to track better over variable surfaces, with 10 km gridded elevations plotted over the ice sheets. Initial ERS-1 satellite altimetry data sets (with 35-day repeat cycles), collected from April 1994 to March 1995, provided less dense coverage than Geosat, but better spatial coverage of the polar ice sheets - between 81.5°N and 81.5°S. ERS-1 and ERS-2 data (with 168-day repeat cycles) were eventually combined with previous Geosat data to create a more dense gridding of elevation data for the ice sheets.
The laser altimeter on the GLAS instrument will improve upon this progression by measuring height from the spacecraft to the ice sheet with an intrinsic precision of better than 10 cm with a 60 m surface spot size, thereby providing the capability to measure subdecadal changes in ice sheet thickness of only a few tens of centimeters. Mass balance models will be greatly improved as well as topographic detail of the ice sheets. The accuracy of height determinations over land is yet to be determined, but will be assessed using ground slope and roughness.