Altimetric calibration experiences in the Western Mediterranean

Since many years, space borne radar altimeters have brought a powerful contribution in monitoring the dynamic sea surface topography, and in understanding better the ocean circulation and its impact on the earth system. Today, altimetric satellites are observing the whole oceans, measuring the sea surface height with a rms precision of 3-4 cm at 1 Hz sampling, as demonstrated by TOPEX/POSEIDON, launched in 1992, by Jason-1, launched in 2001 and by ENVISAT, launched in 2002. Such a high level error budget was achieved thanks to the tremendous improvements which have been obtained in radar performances as well as in precise orbit determination. Indeed, applications of altimetry in oceanography and geodesy requires very precise measurements of the satellite-sea level range, along with appropriate environmental corrections, but also an accurate knowledge of the satellite position with respect to the Earth reference. One campaign has also been made in June 2003 at the Ibiza island area (Martinez-Benjamin et al., 2003). The marine geoid has been used to relate the coastal tide gauge data from Ibiza and San Antonio harbours to off-shore altimetric data. A technical Spanish contribution to the calibration experience has been the design of GPS buoys and GPS catamaran taking in account the University of Colorado at Boulder and Senetosa/Capraia designs. We present a synthesis of the sea level results results obtained from the altimeter calibration campaign at Ibiza island on June 2003 using the direct measurements from GPS buoys and the derived marine geoid. The main objective of the marine campaign was to check the value of Ibiza Island as a permanent calibration site in the western Mediterranean Sea, to complement the Corsica site in the network of altimeter calibration sites.