Optimizing Simultaneous Water Level and Wave Measurements From Multi-GNSS Interferometric Reflectometry Over 1 Year at an Exposed Coastal Site

Abstract

Global Navigation Satellite System Interferometric Reflectometry (GNSS-IR) measures water level using the interference pattern in signal-to-noise ratio (SNR) from direct and reflected signals off the sea surface, retrieved from standard geodetic antennas. Significant wave height is also measured by determining the satellite elevation angles where reflections become incoherent. We developed an approach for standard geodetic antennas to simultaneously measure sea levels and waves using a criterion for identifying coherent reflections. We tested the method at an exposed coastal environment at the E.B. Scripps Memorial Pier in California. The 1-year test captures a broad range of sea states and benefits from several co-located standard oceanographic sensors. By including GPS, Galileo, and GLONASS observations, the retrieval rate increases by a factor of ∼2 over GPS alone. Uncorrected water levels are estimated with a root-mean-square (RMS) error of 18.2 cm with respect to a conventional tide gauge. We further developed a simplified correction to remove the effect of phenomena altering the SNR oscillatory frequency and phase, which reduces RMS errors to 9.4 cm. We estimate the significant wave height with 15 cm RMS error with respect to a traditional wave gauge. The method, however, requires a short calibration. We find the wave height errors increase abruptly beyond a fixed limit when high waves are present, that may be a result of the particular deployment geometry. With this caveat, the technology could be useful to deploy in under-sampled regions affected by compounded coastal hazards, such as in areas affected by tropical cyclones and flooding.