Local Sea Level Observations Using Reflected GNSS Signals

Abstract:

Sea level rise due to global warming is predicted to have a large impact on human society, especially for populations living in coastal regions and on islands. It is therefore of great importance to monitor the sea level and to increase the understanding of the local hydrodynamic and meteorological responses to a global sea level rise.

The focus of this thesis is to estimate the local sea level using Global Navigation Satellite System (GNSS) signals reflected off the sea surface. These signals were recorded in two different ways using a GNSS tide gauge at the Onsala Space Observatory, consisting of standard geodetic-type commercially off-the-shelf GNSS equipment. First, the phase-delay of the reflected GNSS signals were recorded directly with a receiver connected to a nadir-looking antenna. Together with the phase-delay of the direct signals, recorded with a receiver connected to a zenith-looking antenna, standard geodetic analysis provided GNSS sea level observations. Second, the Signal-to-Noise Ratio (SNR) recorded with the receiver connected to the zenith-looking antenna, provided an indirect measurement of the reflected GNSS signals, as the reflected signals interfered with the direct GNSS signals and affected the recorded observables. From analysis of the multipath oscillations, an additional type of sea level observation was possible. Furthermore, the SNR-analysis method allowed other GNSS stations, located close to the ocean, in different parts of the world to become GNSS tide gauges.

The GNSS-derived sea level from the GNSS tide gauge at the observatory was compared with independent observations of sea level from co-located traditional tide gauges, showing a high level of agreement with correlation coefficients of 0.89-0.99. The sea level results from the phase-delay analysis performed better with respect to the traditional sea level records than the results from the SNR-analysis. As an example, the Root-Mean-Square (RMS) differences from 1 month of observations between the GNSS-derived sea level (using frequency band L1) and the sea level from the co-located tide gauge were 3.2-3.5 cm and 4.0-4.7 cm for the phase-delay analysis and the SNR-analysis, respectively.

Sea level results applying the SNR-analysis for data of 5 different GNSS stations around the world were compared to independent co-located traditional tide gauge records. The results showed RMS differences on the order of 6.2 cm for stations with low tidal ranges (up to 165 cm) and 43 cm for stations with high tidal ranges (up to 772 cm). In this case, an extended SNR-analysis approach was applied, modelling a time dependent sea level.