The accuracy of a Goddard TOPEX gravity model as seen by independent resonant data

Abstract

A recent satellite geopotential (Goddard Earth Model T1; Marsh, 1986, EOS 67, 909) has been derived from almost 800,000 precise tracking observations (laser ranges, optical positions and doppler range rates) to 17 spacecraft. This model was designed principally to minimize radial orbit errors on the forthcoming altimeter bearing ocean TOPography Experimental satellite TOPEX. The largest geopotential component of this error (over weekly data arcs) is estimated to arise from a 13th order resonance with a period of 3 days on the 127 revs per 10 day TOPEX orbit. This orbit is not near one of the 17 in the GEM T1 solution. To assess the reliability of the resonances predicted by the new model, 240 geopotential constraints derived from independent (mostly deep) resonant orbits have been examined (orders 2 and 9 through 15). Comparisons of these data with Gem T1 are generally superior to comparisons with all previous models from satellite data only. However, the error calibration results are considerably more variable than for past Goddard models. Using a single constant to scale the covariance matrix of Gem T1, these comparisons show that the new solution has probably improved the geopotential in the higher orders by about 25% overall to degree 36 from the best previous model Gem L2 (Lerch et al., 1982, Geophys. Res. Lett. 9, 1263). If this scale is valid for the significant m daily effects of low orders as well (and there is some evidence for it using 24-h satellite accelerations), TOPEX radial errors using Gem T1 (for weekly tracking arcs) will be (at best) in the neighborhood of 43 cm (r.m.s.).