GRAVITY MODEL DEVELOPMENT FOR TOPEX POSEIDON - JOINT GRAVITY MODEL-1 AND MODEL-2/

The TOPEX/POSEIDON (TIP) prelaunch Joint Gravity Model-1 (JGM-1) and t he postlaunch JGM-2 Earth gravitational models have been developed to support precision orbit determination for T/P. Each of these models is complete to degree 70 in spherical harmonics and was computed from a combination of satellite tracking data, satellite altimetry, and surfa ce gravimetry. While improved orbit determination accuracies for T/P h ave driven the improvements in the models, the models are general in a pplication and also provide an improved geoid for oceanographic comput ations. The postlaunch model, JGM-2, which includes T/P satellite lase r ranging (SLR) and Doppler orbitography and radiopositioning integrat ed by satellite (DORIS) tracking data, introduces radial orbit errors for TIP that are only 2 cm RMS with the commission errors of the marin e geoid for terms to degree 70 being +/- 25 cm. Errors in modeling the nonconservative forces acting on T/P increase the total radial errors to only 3-4 cm RMS, a result much better than premission goals. While the orbit accuracy goal for T/P has been far surpassed, geoid errors still prevent the absolute determination of the ocean dynamic topograp hy for wavelengths shorter than about 2500 km., Only a dedicated gravi tational field satellite mission will likely provide the necessary imp rovement in the geoid.