Precision of ICESat/GLAS Elevation Measurements Over the Continental ice Sheets.

The precision of the Geoscience Laser Altimeter System (GLAS) elevations is investigated using crossing arc residuals formed when ascending and descending passes cross over the same location on the ground. This study uses the 5 contiguous 8-day repeat cycles from February 20th, through March 28th, 2003 of laser number one data and the laser number two data available from Fall 2003. The errors in the GLAS elevations consist of - 1) residual orbit errors, 2) unmodeled attitude error which cause erroneous geolocation of the return, 3) distortion of the received return relative to the return processing model, and 4) errors in the tide and atmospheric delay models. The ICESat mission precision orbit and GLAS attitude supplied by UTCSR are used for this study. Evaluation of crossover residuals within each 8-day repeat cycle for both continental ice sheets shows consistency in the precision of laser 1 throughout all 5 repeats. The distortion of the received return is investigated and crossover residuals are calculated with and without editing the distorted returns showing how well the return processing algorithm accounts for the distortion. Crossover statistics for laser 2 are calculated within each 8-day period and compared with those from laser 1 to show if there are any differences in precision in the measurements between the lasers. The precision as a function of surface slope is presented showing that precision degrades with slope. Higher slopes cause more distortion of the return and increase the effect of the attitude error. Cross-laser crossovers are calculated giving a measure of laser biases over the ice sheets in areas where little surface elevation changes are expected. A comparison is also made with radar altimetry crossovers from ERS-2 showing the expected large improvement in GLAS precision in the higher sloping regions compared with the radar.