To achieve the orbit determination requirement of the EOS ICESat, which is 5 cm and 20 cm in radial and horizontal components, respectively, using the GPS tracking system, it is required to use the precise GPS ephemeris. Three different levels of GPS orbit errors were generated through a simulation, and the effect of these GPS orbit errors on ICESat POD was investigated when the GPS ephemeris is fixed in the process of ICESat POD. Two different simulation cases were investigated. First, GPS orbit errors only case was studied to isolate the GPS orbit error effect from the effect of other errors on ICESat POD. It was found that as the GPS orbit accuracy improved, the resulting ICESat orbit accuracy improved quadratically. The effect of radial, transverse, and normal components of the GPS orbit errors on the ICESat orbit was considered individually. It was found that the transverse GPS orbit errors contributed the most to the ICESat orbit errors. Second, combined errors case was investigated to assess the effect of the GPS orbit errors in the presence other errors. The effect of the GPS orbit errors on the gravity tuning was evaluated. Significant improvement in the radial and the horizontal orbit accuracies were demonstrated from the tuned gravity field using betterdetermined GPS ephemeris.