Coastal Sea State Measurements Using GPS Reflected Signals from an Airborne Platform

Estel Cardellach, Josep Ma. Aparicio, David Pino, Josep Torrobella and Antonio Rius

Abstract:	Recent studies about applications of the reflected GPS signals for oceanic altimetry have showed that the observables are contaminated by a scatterometric effect, i.e. additional delay induced by the multipath during the reflection at a rough surface. This paper presents a preliminary exercise to try to separate the altimetric from the scatterometric components. To this end, an alternative technique for the sea roughness retrieval is presented. The results, far from being conclusive, show possibilities of success of such method. In Earth Sciences, monostatic radar altimeters and scatterometers, placed on board aircrafts and satellites, use the electromagnetic waves reflected off the sea to map the sea surface properties. The PARIS concept was conceived in 1993 [1] to use the coded signals transmitted from navigation satellites, such as the USA Global Positioning System (GPS) constellation, to perform sea surface altimetry. Recent GPS reflection (GPSR) experiments have assessed a precision of 2-centimetre for static low altitude altimetry (~480 m height) [2], and 5- centimetre precision for 150 seconds observations from an aircraft flying at 3000 meters height [3]. The scatterometric applications of the GPS reflected signals have been demonstrated, as well. The current budget for the precision of the GPSR wind observations is set to ~2 m/s [4], [5]. In [6], we presented the analysis of airborne-GPSR data gathered over a coastal sea area where the surface topography presents features at the 50 cm level over horizontal scales of 30 Km. The altimetric model included a scatterometric term, to account for the extra delay produced by the multipath of the 'GPS glitter'. The work demonstrated the strong correlation between the altimetric and the scatterometric components of the delay, turning into a severe difficulty to separate both terms. The separation would be achievable by using external data of the sea roughness (not always available) or, as already suggested in that text, by extracting the sea state information from independent analysis of the proper GPSR signal. The current paper focuses on the second option, presenting a preliminary exercise thought as a first step to demonstrate the capability of the own GPSR data to break the ambiguity between the scatterometric and the altimetric residuals. Moreover, a new simple technique to retrieve the surface roughness is also presented, which uses the sole peak of the correlation function. For that end, we • summarise the technique implemented in [6] (Section 3), • present a new scatterometric technique to obtain the sea surface roughness from the peak of the correlation function (Section 4), • to finally compare the results and expose the limitations of the concept (Section 5).
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	2262 - 2265
Cite this article:	Cardellach, Estel, Aparicio, Josep Ma., Pino, David, Torrobella, Josep, Rius, Antonio, "Coastal Sea State Measurements Using GPS Reflected Signals from an Airborne Platform," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 2262-2265.
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