Radar Altimeter Calibration of the Envisat Satellite: An Autonomous System of High-Precision for Instantaneous Sea Surface Height Determination

T. Schueler

Abstract:	The European Space Agency’s Environmental Remote Sensing Satellite “ENVISAT” was successfully launched in March 2001 and carries - among a number of other sensors - a radar altimeter capable of measuring the instantaneous sea surface height at unprecedented precision of a few centimetres that can be used to study the marine gravity field, the ocean’s currents and tides, and many other features of the sea. However, the altimeter exhibits a systematic offset that cannot be calibrated in the laboratory in advance. For this reason, an extensive field campaign needs to be carried out during the so-called “commissioning phase”. Precise centimetre-level orbit determination using the French DORIS system together with accurate values for the instantaneous sea surface height allow to compute the distance between satellite and surface that can be compared with the radar altimeter measurements yielding the instrumental bias. Several methods can be thought of to accomplish this task: Tide gauges, for instance, constitute an important part of ESA’s calibration activities, and continuous GPS-measurements on a permanently operated oil-platform in the Mediterranean Sea near the Spanish coast are collected. With a commissioning phase as short as 6 to 9 months it is, however, not possible to derive the altimeter’s systematic error with sufficient precision from these few sites alone. A method to improve the situation is the use of carrier phase GPS-positioning onboard of buoys which serve as costeffective sensor platforms. This paper describes the measurement system developed at the University FAF Munich, and points out its advantages in comparison to conventional GPS-buoy calibration approaches from the designer’s point of view. The data processing chain will be depicted and results of the sea surface height determination over short baselines using a commercial GPS processing package as well as a GPS carrier phase adjustment software developed at the University are presented and, finally, conclusions for future work are drawn. All in all, it can be stated that the high-sea buoy approach is a significant and innovative contribution to the radar altimeter calibration campaign that carries some additional potential for future applications of this and similar kind.
Published in:	Proceedings of the 2003 National Technical Meeting of The Institute of Navigation January 22 - 24, 2003 Disneyland Paradise Pier Hotel Anaheim, CA
Pages:	397 - 406
Cite this article:	Schueler, T., "Radar Altimeter Calibration of the Envisat Satellite: An Autonomous System of High-Precision for Instantaneous Sea Surface Height Determination," Proceedings of the 2003 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2003, pp. 397-406.
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