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

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 similar to 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. Plain Language Summary Signals from Global Navigation Satellite Systems (GNSS), such as the Global Positioning System (GPS), that reflect off the ocean surface, when compared to signals that arrive directly at the antenna, reveal variations in signal strength that can be used to determine the elevation of the ocean surface. When the satellite elevation angle is too high above the horizon, the method to measure sea level no longer works. However, observing the elevation angle where this transition occurs allows us to measure the height of the waves that are preventing a strong reflected signal from being recorded. We develop criteria for obtaining the closest agreement between the GNSS-IR observations and the tide and wave gauge instruments on the Ellen Browning Scripps Memorial Pier in southern California over a 1 year period. Because it simultaneously measures changes in sea level and wave height, the method could be useful for measuring coastal hazards such as storm surge and flooding from hurricanes.

Automated summary

Global Navigation Satellite System Interferometric Reflectometry (GNSS-IR) is used to measure water level and significant wave height by analyzing the interference pattern in signal-to-noise ratio. A method was developed to simultaneously measure sea levels and waves using standard geodetic antennas, achieving improved retrieval rate and reduced errors. This technology could be useful for monitoring coastal hazards in under-sampled regions.