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Abstract:
A refined gravity field model, Goddard Earth Model GEM-L2, has been derived using the Lageos orbital data yielding better baseline measurements for the analysis of tectonic plate motion. This field also contributes to an improved understanding of long wavelength features, such as the sea slope across broad ocean basins, through its significant improvement of the long wavelength geoid (through degree and order 4). The geoid for these terms has an accuracy estimated at ± 8 cm in GEM-L2. GEM-L2, as in all recent Goddard Earth Models, relies heavily on the precise near-Earth satellite laser ranging data, in this case provided by NASA's Crustal Dynamics Program. Two and a half years of Lageos laser data acquired from over 20 well-distributed stations were combined with the existing data from the best satellite-derived model, GEM-9, to develop the new Lageos model. Testing shows that the Lageos gravity field error at long wavelengths is less than half that for GEM-9. Independent tests using well determined longitude accelerations of 24-hour satellites have verified the improved accuracy of the new model. A comparison of global laser "base" stations from independent data sets of alternating 15 day data segments over the 2 1/2 years of Lageos show total inter-station positioning to ± 1.8 cm when using this new field. The same comparison using the 1979 versus the 1980 Lageos data yields ± 5.2 cm; this difference in agreement reflects the change in data distribution and other systematic errors along with the tectonic motion which has occurred between these chronologically distinguishable data sets. Five day average polar motion values with a precision of 10 cm and change in length of day values accurate to better than .5 msec have been derived in the solution. The adjustment of these parameters are necessary to achieve the accurate stations and geopotential results in GEM-L2.
