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Within the framework of a project comprising part of the Spanish Space Program related to the JASON-1 CNES (Centre National d'Etudes Spatiales)/NASA (National Aeronautics and Space Administration) mission, a campaign was conducted on June 9-17, 2003, in the Absolute Calibration Site of the island of Ibiza. The objective was to determine the local marine geoid slope under the ascending (187) and descending (248) Jason-1 ground tracks, in order to allow a better extrapolation of the open-ocean altimetric data with on-shore tide gauge locations, and thereby improve the overall precision of the calibration process. For this we have used a catamaran with two GPS antennas onboard, following the Corsica/Senetosa design (Bonnefond et al., 2003a). Five GPS reference stations were deployed in order to reduce the distance between the areas covered by the catamaran and the fixed GPS receiver used in the kinematic process. The geodetic activities (e.g., GPS, leveling) have enabled to build a very accurate (few mm) network in a reference frame compatible with the satellite altimetry missions (ITRF2000). The GPS kinematic data were processed using two different softwares allowing to check the consistency of the solutions. If the standard deviation of the differences (3.3 cm) is close to the kinematic process precision they exhibits some large values (up to 14 cm). These large discrepancies have been reduced using a weighting based on the crossover differences. Inasmuch the distances between the tide gauges and the areas covered by the GPS catamaran we have used the MOG2D ocean model (Carrère and Lyard, 2003) to correct the sea surface from tides: in the farest areas it shows an improvement by a factor of two on the crossover differences. Finally, we also present preliminary results on Jason-1 altimeter calibration using the derived marine geoid: from this analysis the altimeter bias is estimated to be 120 ±5 mm. The quality of this first result validate the whole GPS-based marine geoid processing, for which the accuracy is estimated to be better than 3 cm rms at crossovers.

Data from the altimeter onboard the European Remole Sensing Satellite (ERS 1) was used to study the circulation of the Alboran Sea between-6° to 0° and 35"N to 38"N. This region is dominated by two anticyclonic gyres which am quasi-stationary in space but at times collapse over periods as short as nine days. Maps of sea level residual, between Feb. 1992 and Dec. 1993, were created from the 13RS 1 data with }]igh correlation between sea surface temperature and residual sea level in the eastern part of the Alboran Sea. The spatially coherent signal was then extracted using the technique of complex empirical orthogonal analysis with the first 3 modes statistically significant. The variability of the first CEOF mode appears to be associated with the incoming jet through the straits of Gibraltar with three time periods of high amplitude during the 92-93 time frame. The second and third modes appear to be associated with both the variability of the western and eastern gyre with westward propagation in the third mode evident in the area of the eastern anticyclonic gym and consistent with previous model results. The results indicate that combining sea surface temperature data and sea level data from altimetry hold promise for understanding the circulation of the Western Mediterranean. t , * , periodicities associated with the appearance and disappearance of the western and eastern anticyclonic gyres in the Alboran Sea. If successful, the use of ERS 1 and TOPEX/Poseidon altimetric data can be applied to the entire Western Meditemanean. Specifically the sea surface temperatu~ will be used to qualitatively validate the altimeter data and the appearance and disappearance of the gyres. Once the altimetic data are validated qualitatively, a set of complex empirical orthogonal functions (CEOF) will be derived to separate the variability into spatially coherent modes and examine the spatial and temporal scales.

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Abstract-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 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 aI., 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 cam paign 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.