An Improved Model of the Earth’s Gravity Field: GEM-T3
An improved model of the Earth’s gravitational field has been developed from a combination of conventional satellite tracking, satellite altimeter and surface gravimetric data (GEM-T3). This combination model represents a significant improvement in the modeling of the gravity field at half-wavelengths of 300 km and longer. GEM-T3 is complete to degree and order 50. This model gives improved performance for the computation of satellite orbital effects as well as a superior representation of the geoid from that achieved in any previous Goddard Earth Model. The GEM-T3 model uses altimeter data directly to define the orbits, geoid and dynamic height fields. Altimeter data acquired, during the GEOS-3 (1975–76), SEASAT (1978) and GEOSAT (1986–87) Missions were used to compute GEM-T3. In order to accommodate the non-gravitational signal mapped by these altimeters, spherical harmonic models of the dynamic height of the ocean surface were recovered for each mission simultaneously with the gravitational field. Herein, each of these dynamic height fields are referenced to a common geoidal model and are tied into the Conventional Terrestrial Reference System established by Satellite Laser Ranging (SLR). The tracking data utilized in the solution includes more than 1300 arcs of data encompassing 31 different satellites. The observational data base is highly dependent on SLR, but also includes TRANET Doppler, optical, S-Band average range-rate and satellite-to-satellite tracking acquired between ATS-6 and GEOS-3. The tracking data are largely the same as used to develop GEM-T2 with certain important improvements in data treatment and expanded laser coverage. The GEM-T3 model has undergone extensive error calibration. The method employed an optimal data weighting technique which insures reliable estimates of the model’s uncertainty. This method relies on statistical testing using a subset solution technique. The subset solution testing is based on the premise that the expected mean squares deviation of a subset gravity solution from the overall solution is predicted by the solution covariances. Data weights are iteratively adjusted until this condition is satisfied.
KeywordsSatellite Laser Range Satellite Altimetry Altimeter Data Dynamic Height Spherical Harmonic Model
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