NEW ERROR CALIBRATION TESTS FOR GRAVITY MODELS USING SUBSET SOLUTIONS AND INDEPENDENT DATA - APPLIED TO GEM-T3

Orbit error projections based on the error covariance estimates of Goddard Earth Model (GEM)-T3 have been shown to be reliable through their projection on observation residuals within independent data sets. Special geopotential solutions were developed based upon the same data set and weighting used in the GEM-T3 gravity model, but with a significant satellite data set eliminated from the solution. These subset gravity models are then used to compute the observation residuals within orbital solutions for the omitted satellite and the results are compared to their predicted values based on the error covariance of these models. To ensure meaningful results, the tests were designed so that the observation residuals are dominated by geopotential modeling errors. This yields a reliable test of the error estimates of the subset solutions and hence tests the data weighting used in the construction of these models (GEM-T3 and subset solutions alike). The error estimates for GEM-T3 are based upon an optimal data weighting method and have been obtained in a separate calibration process. The test results shown here indicate that the GEM-T3 error estimates for the gravity parameters are calibrated and that the predicted orbit errors correspond well with actual orbit accuracies. Test results of the complete GEM-T3 model with totally independent high precision DORIS Doppler tracking data acquired on the French SPOT-2 satellite confirms these conclusions.