COMBINATION OF ERS-1 AND TOPEX ALTIMETRY FOR PRECISE GEOID AND GRAVITY RECOVERY IN THE MEDITERRANEAN-SEA

From the separate processing and crossover analysis of repeat ERS-1 an d TOPEX altimeter data in the Mediterranean basin it was assessed that the TOPEX data are characterized by higher accuracy than that of ERS- 1 and other previous missions' data. It is also known from the coverag e of both missions that the resolution of ERS-1 data is superior to th at of the TOPEX data, since the distance across track in the TOPEX mis sion is about four times the corresponding distance in the ERS-1 missi on for the 35-day repeat. Therefore, taking into account the advantage s and drawbacks of both missions, a common adjustment of both altimete r data sources was carried out in order to improve the prediction accu racy of gravity anomalies and geoid heights in the Mediterranean Sea. The computed sea-surface heights (SSHs) have been used in the followin g two trials. The first one was the recovery of sea gravity anomalies by an inversion of the adjusted altimeter data. The prediction of grav ity anomalies has been performed in three individual test subareas of the Mediterranean Sea, located in the western, central and eastern par ts, respectively. The method used was the flexible least-squares collo cation (LSC) procedure. In order to assess the quality of the recovere d gravity anomalies an external comparison was made with observed (con trol) gravity values. The standard deviation (sd) of the differences w as found to vary from 13.4 mGal in the eastern test subarea to 6.5 mGa l in the western subarea. In the central test subarea the sd of the co rresponding differences reached a level of 5.8 mGal. The second rest r un comprised comparisons between the adjusted SSHs and the correspondi ng gravimetric geoidal heights in order to obtain a rough estimation o f the sea-surface topography (SST) in the Mediterranean Sea. The gravi metric geoidal heights have been derived from sea gravity data in the western and central test subareas using the fast least-squares colloca tion (FLSC), the 2-D planar fast fourier transform (PFFT) and the 2-D spherical FFT (SFFT) methods. In the eastern test area the LSC approac h was used due to the irregular distribution of the gravity data. The computed gravimetric geoid heights have been compared with the corresp onding SSHs in the three aforementioned test subareas. The sd of the d ifferences was found to vary from 55 cm in the eastern test subarea to 7 cm in the central subarea. In the western test subarea the sd of th e corresponding differences reached a level of 12 cm.