Combination of Ers-1 and Topex Altimetry For Precise Geoid and Gravity Recovery In the Mediterranean Sea

Abstract

SUMMARY From the separate processing and crossover analysis of repeat ERS-1 and 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 coverage of both missions that the resolution of ERS-1 data is superior to that of the TOPEX data, since the distance across track in the TOPEX mission is about four times the corresponding distance in the ERS-1 mission for the 35-day repeat. Therefore, taking into account the advantages and drawbacks of both missions, a common adjustment of both altimeter data sources was carried out in order to improve the prediction accuracy of gravity anomalies and geoid heights in the Mediterranean Sea. The computed sea-surface heights (SSHs) have been used in the following two trials. The first one was the recovery of sea gravity anomalies by an inversion of the adjusted altimeter data. The prediction of gravity anomalies has been performed in three individual test subareas of the Mediterranean Sea, located in the western, central and eastern parts, respectively. The method used was the flexible least-squares collocation (LSC) procedure. In order to assess the quality of the recovered gravity anomalies an external comparison was made with observed (control) gravity values. The standard deviation (sd) of the differences was found to vary from 13.4 mGal in the eastern test subarea to 6.5 mGal in the western subarea. In the central test subarea the sd of the corresponding differences reached a level of 5.8 mGal. The second test run comprised comparisons between the adjusted SSHs and the corresponding gravimetric geoidal heights in order to obtain a rough estimation of the sea-surface topography (SST) in the Mediterranean Sea. The gravimetric geoidal heights have been derived from sea gravity data in the western and central test subareas using the fast leastsquares collocation (FLSC), the 2-D planar fast fourier transform (PFFT) and the 2-D spherical FFT (SFFT) methods. In the eastern test area the LSC approach was used due to the irregular distribution of the gravity data. The computed gravimetric geoid heights have been compared with the corresponding SSHs in the three aforementioned test subareas. The sd of the differences was found to vary from 55 cm in the eastern test subarea to 7cm in the central subarea. In the western test subarea the sd of the corresponding differences reached a level of 12 cm.