PRECISION ORBIT DETERMINATION FOR TOPEX POSEIDON/

The TOPEX/POSEIDON mission objective requires that the radial position of the spacecraft be determined with an accuracy better than 13 cm RM S (root mean square). This stringent requirement is an order of magnit ude below the accuracy achieved for any altimeter mission prior to the definition of the TOPEX/POSEIDON mission. To satisfy this objective, the TOPEX Precision Orbit Determination (POD) Team was established as a joint effort between the NASA Goddard Space Plight Center and the Un iversity of Texas at Austin, with collaboration from the University of Colorado and the Jet Propulsion Laboratory. During the prelaunch deve lopment and the postlaunch verification phases, the POD team improved, calibrated, and validated the precision orbit determination computer software systems. The accomplishments include (I) increased accuracy o f the gravity and surface force models and (2) improved performance of both the laser ranging and Doppler tracking systems. The result of th ese efforts led to orbit accuracies for TOPEX/POSEIDON which are signi ficantly better than the original mission requirement. Tests based on data fits, covariance analysis, and orbit comparisons indicate that th e radial component of the TOPEX/POSEIDON spacecraft is determined, rel ative to the Earth's mass center, with an RMS error in the range of 3 to 4 cm RMS. This orbit accuracy, together with the near continuous du al-frequency altimetry from this mission, provides the means to determ ine the ocean's dynamic topography with an unprecedented accuracy.