RADIAL ORBIT ERROR REDUCTION AND MEAN SEA-SURFACE COMPUTATION FROM THE GEOSAT ALTIMETER DATA

Using the altimeter data from the Geosat Exact Repeat Mission, we have produced yearly averaged mean profiles and a global mean sea surface The racial en-or of each 6-day orbital arc computed with the GEM-T2 ge opotential was first estimated by calculating the amplitude and phase of the nine dominant frequencies of the difference between the altimet ric profiles and the mean sea surface obtained when adding the permane nt sea surface topography (computed from the Levitus' Climatological A tlas) to the GEM-T2 geold. We show that this operation is little affec ted by the choice of the geoid or by its formal error. The resulting c orrection has been subtracted from each individual arc. Yearly mean pr ofiles were then obtained by averaging the corrected altimetric data o f each repeat cycle on a yearly basis. Their noise level is 1 to 2 cm and their resolution is 20 km, but the differences of the altimetric h eights at crossovers of ascending and descending tracks are still 30 c m rms. The latter can be reduced to 7 cm rms by a crossover analysis. In addition to the mean values, standard deviations were computed at e ach point of the repeat cycle. This ''yearly along-track variability'' is of the order of 10 cm rms and is dominated by the ocean mesoscale variability. A global yearly mean sea surface has been derived by bili near interpolation. Its resolution ranges approximately from 160 km to 80 km, depending on the latitude. It is shown to be much less noisy t han those deduced from GEOS 3 and Seasat data.