Sea Surface Height Determination in the Mediterranean Sea by Local Adjustment of GEOSAT Altimeter Data

Abstract

The pre-processing and the crossover adjustment of GEOSAT altimeter data are carried out in order to obtain a precise approximation of the gravity field in the Mediterranean sea. Altimeter data from the U.S. National Oceanographic Data Center (NODC) have been used covering a time period of about one year. With the rejection of outliers and sub-satellite points whose standard deviation (s.d.) exceeds ±0.09 m, the s.d. of the crossover differences was ±3.5 m. After applying a fit and crossover adjustment model, the s.d. of the crossover differences decreased to ±0.16 m. In order to test the efficiency of the sea surface heights performed, a number of predictions was carried out using as data: (a) adjusted satellite altimetry and (b) sea-gravimetry. Least squares collocation was then used to predict subsets of the data types (a) and (b) from subsets of the same data types. The results given here are in terms of s.d. of observed minus predicted values. Using adjusted altimeter data for gravity prediction a s.d. of ±9.36 mgal was found. When gravity values spaced 10′ apart were used to predict, (1) gravity values in a grid with the same mesh width shifted 5′ and (2) altimeter observations, a s.d. of ±5.08 mgal and ±0.18m, respectively, were found. A combination of gravity and altimeter data made the first s.d. of ±5.08 mgal decrease to ±4.34 mgal showing the impact of combining the two data types. In all the above prediction computations the use of topographic-isostatic effects has been taken into account.