CONSTRAINTS ON THE REPETITIVITY OF THE ORBIT OF AN ALTIMETRIC SATELLITE - ESTIMATION OF THE CROSS-TRACK SLOPE

The effect of a poorly constrained repetitivity of the orbit of an alt imetric satellite is analyzed. From existing data, 35% of the marine g eoid slopes are found to exceed 1.5 cm km-1. This may be due either to short-distance-scale features (seamounts, fracture, and subduction zo nes) or to the large-scale geoid (in the Indian and North Atlantic oce ans). A geoid cross-track slope (CTS) can be calculated locally from t he tracks inside the repetitivity band. Assuming that the various meas urement errors and ocean variability signals are decorrelated, it has a precision of 0.2-2 cm km-1, depending on the orbit cycle (which cons trains the number of repeat passes per year) and on the width of the b and (from 1 to 10 km in these calculations). This can be used as a cor rection but increases the noise level by at least 50%. Alternatively, the CTS can be derived from a mean sea surface. This adequately correc ts for the large-scale signals but, with present mean sea surfaces, it is inadequate for the short-distance-scale features. Future high-dens ity altimetric samplings such as that of the ERS-1 176-day orbit shoul d improve this precision to about +/-0.1-0.2 cm km-1. Above continenta l ice, larger than 0.3% along-track slopes were encountered for more t han 10% of the time above an altitude of 500 m. These slopes result mo stly from undulations of the ice topography. Over one year, a median h eight profile inside the repetitivity band can be derived at 8-16-cm p recision, depending on the number of tracks used and assuming that the measurement noise is 50 cm. From one year to the next, a CTS correcti on needs to be applied to compare the yearly median height profiles. T he latter can be estimated at a 13-130 cm km-1 precision, depending on the repetitivity (from 0.5 to 2.5 km at a latitude of 70-degrees) and the number of tracks. In each case, the precision is comparable with the expected signals (e.g., mesoscale variability of the ocean dynamic topography or climatic variation of the snow accumulation rate). Thes e signals can, however, be recovered by space-time analysis of the dat a. A more elaborate analysis of the covariances of these corrections i s thus required.