COMBINING MULTIPLE ALTIMETER MISSIONS

Viewing altimeter data only at che points where separate altimeter mis sions' ground tracks cross provides a method to observe long time peri od sea surface height (SSH) variations and avoids many of the problems inherent in combining separate altimeter data sets through an indepen dently determined geoid. TOPEX/POSEIDON (T/P) data over the time perio d from January 1, 1993, to December 31, 1995, form a mean SSH that is used as a reference by other altimeter data sets. A least squares anal ysis of the mean T/P SSH determines the portion of the Geographically Correlated Orbit Error (GCOE) that may be observed through crossover d ifferences and removes this portion of the GCOE. The analysis removes errors of 0.86 cm RMS at 1 cycle per orbit revolution (cpr) and indica tes negligible errors at higher frequencies. After the GCOE removal, t he accuracy of the T/P reference mean is better than 1 cm RMS as measu red by crossover differences. The GCOE contained in the Geosat-Exact R epeat Mission (ERM) and ERS 1 data with orbit solutions using the Join t Gravity Model (JGM) 3 is evaluated through an adjustment to the T/P reference mean surface. The Geosat-ERM data indicate a bias of about 2 8 cm averaged over the globe, and the ERS 1 bias is 44 cm. The T/P dat a used here is not corrected for the oscillator drift correction error so that the actual bias is less by about 13 cm. Both the Geosat-ERM a nd ERS 1 GCOE are mainly 1 cpr. GCOE estimates at frequencies above 1 cpr indicate little actual orbit error but are more correlated to inst rument correction errors (particularly water vapor). Simultaneous T/P and ERS 1 SSH anomalies to the T/P mean indicate good correlation.