Corsica: an experiment for long-term altimeter calibration and sea level monitoring
Pascal Bonnefond(1) , Pierre Exertier(1) , Olivier Laurain(1) , Yves Ménard(2) , François Boldo(3) , Eric Jeansou(4) , and Gwenaele Jan(4)
avenue Nicolas Copernic,
(2) CNES, 18 avenue Edouard Belin, 31055 Toulouse, France
(3) IGN-CNES, 18 avenue Edouard Belin, 31055 Toulouse, France
(4) NOVELTIS, Parc Technologique du Canal, 2 avenue de l'Europe, 31520 Ramonville, France
The Corsica site, which includes Ajaccio-Aspretto site, Senetosa Cape site, and Capraia (Italy) in the western Mediterranean area has been chosen to permit the absolute calibration of radar altimeters. Thanks to the French Transportable Laser Ranging System (FTLRS) for accurate orbit determination, and to various geodetic measurements of the local sea level and mean sea level, the objective is to measure the altimeter biases and their drifts.
The expected outputs of this on site verification experiment are dedicated obviously to the determination of the calibration bias of TOPEX/Poseidon and Jason-1 but has been extended to other radar altimeters (ERS, EnviSat, GFO). On the other hand, it is also an opportunity to contribute to the orbit tracking of oceanographic and geodetic satellites and to the analysis of the different error sources, which affect altimetry. In the field of positioning, we expect to contribute also to the decorrelation between the possible vertical displacements of our site (Earth crust) and the Mediterranean mean sea level.
The double geodetic site in Corsica (Aspretto, near Ajaccio and Senetosa Cape 40 km south under the Jason-T/P ground track N° 85) has been used to calibrate the TOPEX/Poseidon altimeters from 1998, and the Jason-1 ones since the beginning of the mission. Permanent and semi-permanent geodetic equipments are used to monitor these calibrations.
Concerning the Aspretto site, a permanent GPS station and an automatic tide gauge have been installed since 1999 (close to an EnviSat ground track). Two dedicated tracking campaigns of the French Transportable Laser Ranging System have been realized in 2002 and 2005.
At Senetosa cape, permanent geodetic installations have been installed since 1998 and different campaigns have been conducted in view of Jason-1 mission. Four tide gauges are installed at the Senetosa Cape and linked to ITRF using GPS and leveling. In parallel, since 2000, a GPS buoy is deployed during overflights at Senetosa (10 km off-shore). Besides, two GPS campaigns (1998 and 1999) have been performed to measure the marine geoid slope from the coast to 20 km off Senetosa cape - in this area the geoid slope can reach 6 cm/km. Moreover, since 2003, a permanent GPS has been installed to monitor possible vertical displacements of our site. In addition, using a local weather station, we derived the wet tropospheric path delay from GPS measurements which are compared to the Jason Microwave Radiometer ones at the overflight times.
The extension of this calibration experiment includes now Capraia island where a dedicated GPS campaign has been realized in 2004 to measure the geoid slopes under Jason-1, EnviSat and GFO tracks around the island. In a near future (October 2005), a similar campaign will be realized for the Envisat ground track close to Ajaccio.
Our semi-permanent experiment is planned to last over several years in order to detect any drift in the space borne instruments.