Ibiza Absolute Calibration Experiment: Survey and Preliminary Results

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. Introduction Description of the Ibiza 2003 campaign The Jason-1 project has set up a CalVal Plan organizing calibration and validation activities during the validation and operational phase of Jason-1. Amongst the studies concerning the verification of the altimetric system components, some of them are devoted to the altimeter sea-level measurement assessment by comparing space-derived measurements to in situ ones. The Jason-1 project has already equipped two main NASA and CNES devoted calibration sites: at the Senetosa Cape in Corsica for the French side (Bonnefond et al., 2003b), and on the Harvest Oil platform off the Californian coast for the USA side (Haines et al., 2003). Other sites are already equipped, or in installation phase for in situ measurements mainly using tide gauges, in order to help the verification of altimeter range measurements. This is the case for Bass Strait, Tasmania (Australia) (Watson et al.,2003), Gavdos Island (Greece) (Pavlis et al.,2002), Lake Eire (USA) (Shum et al.,2003) and Ibiza/ Cape of Begur (Spain). A Spanish (with French support) JASON-1 calibration campaign, IBIZA 2003, was carried out in June 9-17, 2003 around Ibiza in the NW Mediterranean Sea (Figure 1a). It has required a deep involvement and logistics in the field campaign, in the processing of the collected GPS/tide gauges data and in the analysis of results. The main objective of the campaign was to map the instantaneous sea level/local geoid gradient in three areas around Ibiza: at the crossing point of ascending and descending JASON-1 tracks located to the north of the island, and along these tracks to the SE and SW of the Island (Figure 1b). The campaign was based on the experience gained from three previous pilot experiments at Begur Cape (NE Spain). They were carried out in March 1999 and July 2000 for TOPEX/POSEIDON and August 2002 for JASON-1, as part of the JASON-1 CalVal Team. The Spanish 1999 campaign using a GPS buoy (Born et. al. 1994; Kruizinga, 1997) included the first direct absolute altimeter calibration of TOPEX Altimeter Site B in the western Mediterranean (Martinez-Benjamin et al. 2000). Single point calibrations were performed (Martinez-Garcia et al., 2002). GPS buoys presents potential application to deep-sea studies (Colombo et al., 2000). A major component of the campaign was a newly designed, calibrated GPS catamaran built up at ICC, following the original design from Senetosa experiences (Bonnefond et al., 2003a). It was towed by the Spanish Navy Patrol Deva, with a crew of 17 persons on board plus researchers from ROA, UPC and