Exploiting GPS as a New Oceanographic Remote Sensing Tool

Michael Armatys, Dallas Masters, Attila Komjathy, Penina Axelrad and James L. Garrison

Abstract:	It has been demonstrated that ocean-reflected GPS signals can be used to infer sea state conditions. By measuring the cross-correlation of the reflected GPS signal with the receiver generated pseudorandom noise (PRN) code for a particular satellite, parameters such as wind speed and wind direction can be derived from the cross-correlation waveform shape. A specialized, software-configurable GPS receiver is flown on air craft and high altitude balloons to measure the cross-correlation power versus delay with respect to a specular reflection point on the ocean surface. The resulting waveform is similar to the well understood altimetric response, and the GPS reflected signal can be modeled using standard bistatic radar techniques. We discuss current processing techniques to retrieve wind speed from correlation waveform shapes by comparing measured waveforms to those predicted by a theoretical model. The modeled waveform is a function of surface slope variances driven by wind speed and satellite/specular point/receiver geometry. One retrieval technique directly matches waveform shapes to retrieve wind speed only. These wind speed estimates are compared with ground truth observation from buoys to determine accuracy of the GPS retrieval. Initial results indicate the GPS-derived wind speeds are accurate to within 2 - 3 m/s of the ground-truth data for a high signal-to-noise receiver design. A more recent retrieval technique matches measured GPS waveforms to modeled waveforms by iteratively solving for the set of surface-slope variances that minimize the measurement residual in a least-squares sense. A method has been developed to relate these estimated variances to an absolute wind speed and direction. In addition to estimating the wind speed to within 2 m/s of truth, the wind direction may also be estimated to within 20 degrees of truth. With further refinement of the retrieval processes, it is envisioned that these GPS remotely sensed ocean wind parameters may become standard operational products for marine and meteorological models.
Published in:	Proceedings of the 2000 National Technical Meeting of The Institute of Navigation January 26 - 28, 2000 Pacific Hotel Disneyland Anaheim, CA
Pages:	339 - 347
Cite this article:	Armatys, Michael, Masters, Dallas, Komjathy, Attila, Axelrad, Penina, Garrison, James L., "Exploiting GPS as a New Oceanographic Remote Sensing Tool," Proceedings of the 2000 National Technical Meeting of The Institute of Navigation, Anaheim, CA, January 2000, pp. 339-347.
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