<h2>AAS 99-146</h2>
<h2>Analysis of GPS and Satellite Laser Ranging (SLR) data for ICESAT Precision Orbit Determination                                                  </h2>
<h4>J. Rim, C. Webb, B. Schutz                                                                                                                                                          </h4>
The University of Texas at Austin                                                                                                                                         
<h2> Abstract </h2>
The EOS ICESat (ice, Cloud and Land Elevation Satellite) mission is scheduled for launch on July 2001. The spacecraft features the Geoscience Laser Altimeter (GLAS) to measure ice-sheet topography and associated temporal changes, as well as cloud and atmospheric properties. To accomplish the ICESat science objective, the position of the GLAS instrument in space should be known with an accuracy of 5 cm and 20 cm in radial and horizontal components, respectively. Thi knowledge will be acquired from dta collected by the GPS receiver on ICESat and from the ground-based satellite laser ranging (SLR) data. The predicted radial orbit errors based on recent gravity models (e.g., JGM-3 or EGM-96) are 15-20 cm, which exceed the 5 cm requirement. Gravity model improvement effort (gravity tuning) will be carried out during the Verification Phase (first 90-120 days from launch) using the collected GPS data and the SLR data. This paper summarizes an in-depth simulation study to assess the EOS ICESat POD accuracy based on the GPS and SLR data. Contributions from each data type to POD and gravity tuning are analyzed.