THE USE OF REFERENCE SURFACES TO DETERMINE REPEAT-ORBIT VARIABILITY IN SATELLITE ALTIMETRY

Two alternative techniques for estimating the variability of the radia l orbit error for collinear tracks are investigated using Geosat altim eter data. The first uses sinusoidal fitting to ocean height differenc es around an orbit, and the second uses relatively flat areas of land (in the Simpson Desert, Australia, and the Antarctic Plateau). Using a non-ocean surface requires knowledge of the local surface slope, and we obtain this through the fitting of a plane to the set of repeat hei ght measurements. The difference in the relative-orbit-error estimates from the two techniques is 12 cm root-mean-square (RMS), from which w e conclude that relative orbit error can be reduced to less than 9 cm using ocean fitting, and to between 9 and 12 cm using land fitting. Th e Antarctic plateau could not be used as a reference as the orbit erro r appeared correlated with the cross-track displacement of repeat trac ks, preventing the determination of the local surface slope. The land analysis was also Limited by lack of waveform data and Geosat off-poin ting; current altimeter missions (e.g., ERS-1 and Topex/Poseidon) shou ld be able to achieve higher accuracies.