Note on the use of an altimeter mean sea-surface for mesoscale variability studies

This report discusses whether, by the year 1995, altimeter mean surfaces such as those generated by Geosat, ERS-1 [35 days] and Topex-Poseidon will be accurate enough for extracting the variable ocean signal. The mean oceanic signal is estimated from altimetry using an inverse method. Because of the large number of altimeter measurements, the original one-step method is transformed to a two-step method, the iterative inverse method. The space-time distribution of the altimeter data is only needed to calculate the formal error on the altimeter mean surface. Using a year of satellite altimeter measurements, the mean rms errors are 10 cm for Geosat, 8 cm for ERS- 1 and 13 cm for Topex-Poseidon. The along-track minima are respectively 8, 7 and 5 cm rms. Errors can be reduced to 4 cm rms minimum to 6 cm rms maximum by combining two years of Geosat and ERS-1 data. These results are preliminary but indicate that the altimeter missions of the 1990s should provide the necessary accuracy.