A major new computation of a terrestrial gravitational field model has been performed by the Geodynamics Branch of Goddard Space Flight Center (GSFC). In the development of this new model, designated Goddard Earth Model GEM-T1, the design decisions of the past have been reassessed in light of the present state of the art in satellite geodesy. With GEM-T1 a level of internal consistency has been achieved which is superior to any earlier Goddard Earth Model. For the first time a simultaneous solution has been made for spherical harmonic parameters of both invariant and tidal parts of the gravitational field. The solution of this satellite model to degree 36 is a major factor accounting for its improved accuracy. The addition of more precise and previously unused laser data and the introduction of consistent models were also accomplished with GEM-T1.

Another major factor allowing the creation of this model was the redesign and vectorization of our main software tools (GEODYN II and SOLVE) for the GSFC Cyber 205 computer. In particular, the high-speed advantage (50:1), gained with the new SOLVE program, made possible an optimization of the weighting and parameter estimation scheme used in previous GEM models resulting in significant improvement in GEM-T1. The solution for the GEM-T1 model made use of the latest International Association of Geodesy reference constants, including the J2000 reference System. It provided a simultaneous solution for (1) a gravity model in spherical harmonics complete to degree and order 36; (2) a subset of 66 ocean tidal coefficients for the long-wavelength components of 12 major tides. This adjustment was made in the presence of 550 other fixed ocean tidal terms representing 32 major and minor tides and the Wahr frequency dependent solid earth tidal model; and (3) 5-day averaged Earth rotation and polar motion parameters for the 1980 period onward. GEM-T1 was derived exclusively from satellite tracking data acquired on 17 different satellites whose inclinations ranged from 15¿ to polar.

In all, almost 800,000 observations were used, half of which were from third generation (<5 cm) laser systems. A calibration of the model accuracies has been performed showing GEM-T1 to be a significant improvement over earlier GSFC ''satellite-only'' models based purely on tracking data for both orbital and geoidal modeling applications. For the longest wavelength portion of the geoid (to 8¿8), GEM-T1 is a major advancement over all GEM models, even those containing altimetry and surface gravimetry. The radial accuracy for the anticipated TOPEX/POSEIDON orbit was estimated using the covariances of the GEM-T1 model. The radial errors were found to be at the 25-cm rms levels as compared to 65 cm found using GEM-L2. This simulation evaluated only errors arising from geopotential sources. GEM-L2 was the best available model forTOPEX prior to the work described herein. A major step toward reaching the accuracy of gravity modeling necessary for the TOPEX/POSEIDON mission has been achieved. ¿ American Geophysical Union 1988