OBSERVATIONS OF GEOGRAPHICALLY CORRELATED ORBIT ERRORS FOR TOPEX POSEIDON USING THE GLOBAL POSITIONING SYSTEM

We have compared Global Positioning System (GPS)-based dynamic and red uced-dynamic TOPEX/Poseidon orbits over three 10-day repeat cycles of the ground-track. The results suggest that the prelaunch joint gravity model (JGM-1) introduces geographically correlated errors (GCEs) whic h have a strong meridional dependence. The global distribution and mag nitude of these GCEs are consistent with a prelaunch covariance analys is, with estimated and predicted global rms error statistics of 2.3 an d 2.4 cm rms, respectively. Repeating the analysis with the post-launc h joint gravity model (JGM-2) suggests that a portion of the meridiona l dependence observed in JGM-1 still remains, with a global rms error of 1.2 cm.