Geosat and ERS-1 Datum Offsets Relative to Topex/Poseidon Estimated Simultaneously with Geopotential Corrections from Multi-Satellite Crossover Altimetry

Abstract

Crossover residuals, averaged over a sufficient long time period carry the signature of geopotentially based orbit errors. In addition they may exhibit geographical pattern which are inconsistent with orbit dynamics. Tracking system offsets and different orientations, for example, map to the so called “forbidden” harmonics: the degree 1 and the degree 2, order 1 coefficients. Long-term averaged dual satellite crossover residuals among Geosat, ERS-1 and Topex/Poseidon, all with JGM3 based orbits, have been used to solve simultaneously for i) corrections to the gravity field harmonics and ii) parameters which account for a coordinate frame offset relative to Topex/Poseidon. The parameters are estimated by a least squares adjustment. The paper investigates in particular the obstacle of large correlation between the shift parameters and the low degree order one harmonic coefficients. Results of the inversion indicate significant shifts between the coordinate frames of the altimeter missions involved. They must be taken into account, if secular sea level changes are to be investigated.