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

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Abstract
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.