Offshore Absolute Calibration of Space-Borne Radar Altimeters

Abstract

Absolute calibration of sea level measurements collected from space-borne radar altimeters is usually performed with respect to collocated sea level in situ records from tide gauges or GPS buoys (Ménard et al. 1994; Haines et al. 1996; Bonnefond et al. 2003; Haines et al. 2003; Schum et al. 2003; Watson et al. 2003; Watson et al. 2004). Such a method allows regular and long-term control of altimetric systems with independent records. However, this approach is based on a single, geographically dependent point. In order to obtain more significant and accurate bias and drift estimates, there is a strong interest in multiplying the number of calibration opportunities. This article describes a method, called the “offshore method” that was developed to extend the single-point approach to a wider regional scale. The principle is to compare altimeter and tide gauge sea level data not only at the point of closest approach of an overflying pass, but also at distant points along adjacent satellite passes. However, connecting sea level satellite measurements with more distant in situ data requires a more accurate determination of the geoid and mean ocean dynamic topography slopes, and also of the ocean dynamical changes. In this demonstration experiment, 10 years of averaged TOPEX/Poseidon mean sea level profiles are used to precisely determine the geoid and the mean ocean circulation slope. The Mog2d barotropic ocean model (Carerre et Lyard 2003) is used to improve our estimate of the ocean dynamics term. The method is first validated with Jason-1 data, off Corsica, where the dedicated calibration site of Senetosa provides independent reference data. The method is then applied to TOPEX/Poseidon on its new orbit and to Geosat Follow On. The results demonstrate that it is feasible to make altimeter calibrations a few tens to hundreds of kilometers away from a dedicated site, as long as accurate mean sea level altimeter profiles can be used to ensure the connection with reference tide gauges.

Acknowledgments

This work, supported by CNES, was conducted by the CNES SALP project and the NOVELTIS Company in collaboration with LEGOS, OCA, ENSIETA, and IGN. TOPEX/Poseidon and Jason-1 GDR data were provided by AVISO and GFO GDR data by NOAA/NESDIS. Our special thanks to C. Gaillemin who has taken care of the instrument maintenance at Senetosa since 1996. Thanks also to J. Lillibridge (NOAA) for bringing us his expertise in the GFO data and to Météo-France for providing the atmospheric fields needed to run the Mog2d model. Rosemary Morrow (LEGOS) took time to read an early version of this article and to provide useful corrections complementary to the pertinent comments of the two reviewers.