A Study on Detection of Sea Level Variation Using GPS Signal Reflected by Sea Surface

T. Yoshihara, S. Saito, N. Fujii, and T. Sakai

Abstract:	Multi-path is a major error source of ranging in Global Positioning System (GPS) and it is important to mitigate or remove its effects from GPS measurements. However, if it is possible to exactly extract a multi-path component from observed GPS signals, it can provide information of reflective point such as its location. Since a multi-path signal affects code correlation curve, its monitoring will make it possible to estimate and extract a multi-path component from the received GPS signals. As a simple case of multi-path, the authors focused on a reflected signal from the sea surface and tried to estimate the sea level and its variation from the code correlation curve measurements. Signal Quality Monitoring (SQM) receiver, which have been developed by Electronic Navigation Research Institute (ENRI), Japan, can observe correlation curves of C/A code for In-phase and Quadrature-phase (I, Q) signals using 127 correlators in real time. In the above mentioned aspect, the authors carried out an experiment to observe multi-path signal reflected by the smooth sea surface at a site with an altitude of 758.3 meters. Using time series of the code correlation curve of in-phase tracking signal for a certain GPS satellite with elevation angles from 2.4 to 3.2 degrees, time series of the path difference between direct-path and multi-path was estimated. From this result, it is confirmed that the estimated path differences were increasing and decreasing as GPS satellite was rising and setting during about 3 minutes, respectively. A relative height was successfully estimated by applying the least square method to time series of the estimated path difference for each satellite with RMS of the path difference residual of about 5 meters except a large bias error. A main factor of this large bias was thought as tropospheric delay difference between direct-path and multi-path, which was very sensitive to determination of the sea level height. In comparison among 10 events during 9 hours, the estimated relative heights were consistent each other and an averaged relative height was calculated with RMS of 4.2 meters. In this study, several-meter precision to estimate the sea level from the code correlation curve was realized, however, the precision is not still enough to observe ocean tide, which varies with a typical range of about 1.5 meters in this region. Therefore, it is necessary to improve analysis method to estimate time series of more accurate path difference from the code correlation curve and to develop suitable tropospheric delay difference model. Otherwise, it is also suggested that it is possible to estimate tropospheric delay difference between the observation site and the sea surface with a given sea level from the other observations.
Published in:	Proceedings of the 2006 National Technical Meeting of The Institute of Navigation January 18 - 20, 2006 Hyatt Regency Hotel Monterey, CA
Pages:	217 - 223
Cite this article:	Yoshihara, T., Saito, S., Fujii, N., Sakai, T., "A Study on Detection of Sea Level Variation Using GPS Signal Reflected by Sea Surface," Proceedings of the 2006 National Technical Meeting of The Institute of Navigation, Monterey, CA, January 2006, pp. 217-223.
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