A GEOPOTENTIAL MODEL FROM SATELLITE TRACKING, ALTIMETER, AND SURFACE GRAVITY-DATA - GEM-T3

An improved model of Earth's gravitational field, GEM-T3, has been dev eloped from a combination of satellite tracking, satellite altimeter, and surface gravimetric data. GEM-T3 provides a significant improvemen t in the modeling of the gravity field at half wavelengths of 400 km a nd longer. This model, complete to degree and order 50, yields more ac curate satellite orbits and an improved geoid representation than prev ious Goddard Earth Models. GEM-T3 uses altimeter data from GEOS 3 (197 5-1976), Seasat (1978) and Geosat (1986-1987). Tracking information us ed in the solution includes more than 1300 arcs of data encompassing 3 1 different satellites. The recovery of the long-wavelength components of the solution relies mostly on highly precise satellite laser rangi ng (SLR) data, but also includes TRANET Doppler, optical, and satellit e-to-satellite tracking acquired between the ATS 6 and GEOS 3 satellit es. The main advances over GEM-T2 (beyond the inclusion of altimeter a nd surface gravity information which is essential For the resolution o f the shorter wavelength geoid) are some improved tracking data analys is approaches and additional SLR data. Although the use of altimeter d ata his greatly enhanced the modeling of the ocean geoid between 65-de grees-N and 60-degrees-S latitudes in GEM-T3, the lack of accurate det ailed surface gravimetry leaves poor geoid resolution over many contin ental regions of great tectonic interest (e.g., Himalayas, Andes). Est imates of polar motion, tracking station coordinates, and long-wavelen gth ocean tidal terms were also made (accounting for 6330 parameters), (GFM-T3 has undergone error calibration using a technique based on su bset solutions to produce reliable error estimates. The calibration is based on the condition that the expected mean square deviation of a s ubset gravity solution from the full set values is predicted by the so lutions' error covariances. Data weights are iteratively adjusted unti l this condition for the error calibration is satisfied. In addition, gravity field tests were performed on strong satellite data sets withh eld from the solution (thereby ensuring their independence). In these tests, the performance of the subset models on the withheld observatio ns is compared to error projections based on their calibrated error co variances. These results demonstrate that orbit accuracy projections a re reliable for new satellites which were not included in GEM-T3.