Calibration of radar altimeters and validation of orbit determination in the Corsica-Capraia area

Recent events have shown the importance of monitoring the absolute calibration to make certain a correct interpretation of altimeter data in terms of secular rise of the mean sea level or in terms of seasonal or interannual variations. The accuracy of an absolute local calibration, as made permanently in Harvest or in the past in Lampedusa is only a few centimeters and this fact makes the direct detection of a possible drift of an altimeter at the level of few mm/year over a few years difficult. The problem also is to monitor the absolute altimeter calibration at as low a cost possible. For doing that a probationary experiment is presently carried out between Grasse (France), Corsica (France) and Capraia (Italy) to show that with a fundamental geodetic station as Grasse, with some tide gauges well located in a homogeneous system, it is possible to monitor properly the absolute altimeter calibration in using short-arc techniques. A Mobile Laser Station (FTLRS) has been used with the Grasse laser station as well as GPS and DORIS to allow a control of the calibration with a reliable error budget. In the future, the idea is to extend the principle of this experiment in using tide gauges under the repetitive tracks of satellite in other regions around the world covered by laser tracking, or if necessary to use transportable tracking systems like the FTLRS. Cooperative actions will be carried out with different countries (e.g. at Ibiza, Tahiti, ...). As there are systematic errors at each site, one may hope that in averaging the data, a better accuracy will be obtained.