GRAVITY MODEL IMPROVEMENT USING GEOS-3 (GEM-9 AND GEM-10)

To be most effective the spaceborne altimeter missions of Geos 3 (50cm accuracy) and the future SEasat (10cm accuracy) require precise knowledge of the radial position of the spacecraft. Though errors in previous gravity models have produced large uncertainties in the orbtial position of Geos 3, significant improvement has been obtained with the new geopotential solutions, Goddard Earth Model (GEM) 9 and 10. The solution for GEM 9 was derived by combining laser data from Geos 3, Lageos, and Starlette: S band measurements on Landsat 1; and data from 26 other satellites used in previus solutions. GEM 10 is a combination solution contianing a global set of surface gravity anomalies along with the data in GEM 9. Radial errors of Geos 3 for 5-day arcs have been reduced from c5m to 1m on the basis of orbital intercomparisons, station navigations, and analyses emloying crossover points from passes of altimetry. The use of highly accurate laser data in a constrained least squares solution has permitted GEM 9 to be a larger field than previous derived satellite models, GEM 9 having harmonics complete to 20 x 20 with selectedh igher-degree terms. The satellite data set has c840 000 observations, of which 200 000 are laser ranges taken on 9 satellites equipped with retroreflectors. GEM 10 is complete to 22 x 22 with selected higher-degree terms out to degree and order 30 amounting to a total of 592 coefficients. Comparisons with surface gravity and altimeter data indicate a substantial improvement in GEM 9 over previous satellite solutions; GEM 9 is in even closer agreement with surface data than the previously published GEM 6 solution which contained surface gravity.-Authors