GNSS Interferometric Reflectometry measurements of sea levels and wave height at E. B. Scripps Pier, California

Sepulveda, Ignacio; Cao, Bing; Haase, Jennifer S.; Hudson, Phoebe ORCID: https://orcid.org/0000-0003-3503-5601; Murphy, Michael J.. 2020 GNSS Interferometric Reflectometry measurements of sea levels and wave height at E. B. Scripps Pier, California. [Poster] In: AGU Fall Meeting 2020, Online, 1-17 Dec 2020.

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Abstract/Summary

Modeling the transfer of energy from the open ocean to the coast (e.g. waves, storm surge) is challenging but of critical importance for assessing the impacts of climate change and sea level rise. In particular, the sparsity of wave and tide gauges makes it difficult to calibrate these types of models. GNSS Interferometric Reflectometry (GNSS-IR), although it has lower accuracy, constitutes an attractive alternative for sensing nearshore surface properties because of its low deployment costs. We evaluate the accuracy of GNSS-IR measuring long-period sea level variations and short-period wave heights by running a 4-month experiment at the E. B. Scripps Pier, in La Jolla, California. We use a standard GNSS geodetic right hand circularly polarized choke ring antenna to capture the GPS, GLONASS and Galileo constellations, allowing more measurements than GPS alone. For low satellite elevation angles, the captured signals result from interference between the direct signal and the coherent reflection from the sea surface, while above a certain elevation angle threshold, there is interference with an incoherent reflection depending on wave height. We use the coherent reflection interference to measure sea level variations, presenting RMS differences of 18 cm with respect to a conventional tide gauge record. The differences are of high-frequency, so long-period variations and tidal constituents are measured with acceptable accuracy as compare to traditional instruments. The incoherent reflection at large elevation angles is employed to determine short-period wave heights, which we demonstrate for the first time using a standard antenna. Adopting the Rayleigh criterion, we relate the significant wave height (Hs) to the elevation angle threshold in which the interfering reflection changes from coherent to incoherent. We propose a criterion to determine the elevation angle threshold that retrieves Hs with low dispersion. The GNSS-derived Hs have RMS differences of 14 cm with respect to a conventional wave gauge. Hence, a calibration period, for example with a movable buoy, combined with the elevation angle threshold criterion can be employed to retrieve Hs. Our results demonstrate that GNSS-IR is a valid alternative for sensing sea state when traditional instruments are nonexistent.

Item Type:	Publication - Conference Item (Poster)
Date made live:	16 Apr 2021 07:18 +0 (UTC)
URI:	https://nora.nerc.ac.uk/id/eprint/530069

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