THE JOINT GRAVITY MODEL-3

An improved Earth geopotential model, complete to spherical harmonic d egree and order 70, has been determined by combining the Joint Gravity Model 1 (JGM 1) geopotential coefficients, and their associated error covariance, with new information from SLR, DORIS, and GPS tracking of TOPEX/Poseidon, laser tracking of LAGEOS 1, LAGEOS 2, and Stella, and additional DORIS tracking of SPOT 2. The resulting field, JGM 3, whic h has been adopted for the TOPEX/Poseidon altimeter data rerelease, yi elds improved orbit accuracies as demonstrated by better fits to withh eld tracking data and substantially reduced geographically correlated orbit error. Methods for analyzing the performance of the gravity fiel d using high-precision tracking station positioning were applied. Geod etic results, including station coordinates and Earth orientation para meters, are significantly improved with the JGM 3 model. Sea surface t opography solutions from TOPEX/Poseidon altimetry indicate that the oc ean geoid has been improved. Subset solutions performed by withholding either the GPS data or the SLR/DORIS data were computed to demonstrat e the effect of these particular data sets on the gravity model used f or TOPEX/Poseidon orbit determination.