Ibiza Absolute Calibration Experiment: Survey and Preliminary Results

Abstract

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 from June 9–17, 2003, on 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 the building of a very accurate (few mm) network in a reference frame compatible with the satellite altimetry missions (ITRF 2000). The GPS kinematic data were processed using two different software programmes, allowing checking of the consistency of the solutions. If the standard deviation of the differences (3.3 cm) is close to the kinematic process precision, they exhibit some large values (up to 14 cm). These large discrepancies have been reduced using a weighting based on the crossover differences. Inasmuch as the distances between the tide gauges and the areas covered by the GPS catamaran were becoming large, we have used the MOG2D ocean model (Carrère and Lyard 2003) to correct the sea surface from tides. In the farthest areas, the crossover differences show an improvement by a factor of two. 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 validates the whole GPS-based marine geoid processing, for which the accuracy is estimated to be better than 3 cm rms at crossovers.

Keywords:

• altimeter calibration
• geoid
• GPS buoy
• kinematic GPS

Acknowledgments

The IBIZA 2003 experiment has been developed within the framework of the Spanish National R and D Project in Space Research (Ministerio de Ciencia y Tecnologia), ref: ESP2001-4534-PE. Special thanks to the Estado Mayor de la Armada (Ministerio de Defensa), to ROA and to Deva Captain. Enrique Rodriguez de Santiago and its crew. Special thanks for the French support. My best thanks to Marta Sastre, Miriam Moysset, and Anna Serrano from the ICC and Nieves Lantada from the UPC for the support for the Ibiza campaign. GPS (SANB) and tide gauge data from Ibiza and San Antonio were obtained also within the framework of the ESEAS-RI EU project under contract EVR1-CT-2002-40025.

Notes

*Data lacking on 11/06 from 08h09 to end of day.

*Height relative to the water line.