The development and analysis of geopotential coefficient models to spherical harmonic degree 360

The GEM-T2 potential coefficient model (incomplete to degree 50) has been combined, in a least squares sense, with 30 arc min mean anomalies, to obtain an adjusted set of coefficients and gravity anomalies. The adjusted anomalies were then harmonically analyzed to yield a set of potential coefficients to degree 360. The 30 arc min mean anomalies were estimated from terrestrial gravity data, from altimeter-derived anomalies, and from 1°×1° terrestrial anomalies where such data were available. For areas devoid of gravity information, the anomalies were computed in two ways: (1) from the GEM-T2 coefficients and (2) from the GEM-T2 coefficients to degree 36 plus coefficients implied by a topographic/isostatic model. These “fill-in” anomalies led to two potential coefficient models: OSU89A and OSU89B. The new models were checked in several ways including satellite orbit residual analysis, Geosat undulation comparisons, and Global Positioning System (GPS)/leveling undulation differences. The orbit fits (carried out by NASA) showed improvement over GEM-T2. After correction for sea surface topography, orbit error, and permanent tidal effects, the geoid undulations from the OSU89B model have an RMS discrepancy with the Geosat-implied undulation of ±59 cm over a complete 17-day exact repeat cycle. The comparisons with GPS information indicate the accuracy of the computation of a relative undulation is of the order of 3–4 ppm of the distance between stations. The new models represent a substantial improvement over previous high-degree expansions.