A new gravitational model for the Earth from satellite tracking data: GEM‐T1

Abstract

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 level 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 for TOPEX 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.

Dynamic sea surface topography, gravity, and improved orbit accuracies from the direct evaluation of Seasat altimeter data

“…PGS-S4 is a gravitational model that utilized SEASAT altimetry to help reduce geoid errors arising in terms above degree and order 6. More recently, Tai (1988) has incorporated the latest GSFC gravitational model GEM-Ti (Marsh et al, 1987). Recently, Tapley et al(1988) and Engelis and Knudsen (1989) have published an analysis of the SEASAT dynamic topography which parallel, somewhat, the activity described in this document.…”

“…If several separate satellite orbits are simultaneously evaluated over a wide range of inclinations, mean motions, and eccentricities, further adjustments can be performed on the global conservative force models. The Goddard Earth Models, as well as other satellite derived geopotential and tidal models, are developed in this way (Marsh et al, 1988).…”

“…The satellite accelerations (R) are calculated from an evaluation of numerous forcing effects (Marsh, et al, 1988) The minimization of b r is done within a least squares adjustment. This adjustment is complicated by imperfect models, algorithms and unknown systematic sources of error.…”

“…This solution is an outgrowth of the work being performed at GSFC to develop advanced gravitational models in preparation for the TOPEX Mission. GEM-Ti (Marsh, et al, 1987(Marsh, et al, , 1988 is the first of a new series of GSFC models that will be produced for TOPEX. GEM-Ti has now been extended through the inclusion of surface gravimetry and SEASAT altimetry to estimate along with the other parameters in the solution, a model of the QSST that is complete to degree and order 10.…”

An Improved Model for the Earth’s Gravity Field

“…The gravity field solution will represent the first complete reiteration of the historical tracking data used to define the NASA Goddard Space Flight Center (GSFC) Earth Model series. The GSFC GEM-T1 [Marsh et al, 1988] and the University of Texas (UT) TEG-I fields, described in this paper, are preliminary versions of the Topex gravity field solution.…”