Observing Sea Level Using Reflected Global Navigation Satellite System Signals

Licentiate thesis, 2011

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. Presented in this thesis is a technique to measure local sea level using Global Navigation Satellite System (GNSS) signals. This GNSS-based tide gauge acquires both the directly received GNSS signals and the GNSS signals that are reflected off the sea surface, using standard geodetic GNSS receivers. With the directly received signals the installation measures land-surface height changes, whereas the reflected signals are used to measure sea-surface height changes. Both measurements are done with respect to the Earth's centre of mass. By combining these observations it is possible to estimate the local sea level, which is directly related to the volume of the ocean. Several GNSS-based tide gauge campaigns have been carried out at the Onsala Space Observatory (OSO) on the west coast of Sweden. Today the installation is still in place and continues to record GNSS data with a sampling frequency of 1 Hz. In this thesis, data from the campaigns are analysed in a post-processing mode with an in-house developed software. Sea-level estimates are produced with a temporal resolution from 5 s to 20 min and compared to independent sea-level observations from stilling well gauges located approximately 18 km south and 33 km north of OSO. The results for three months of GNSS-derived sea level show an agreement, with respect to the stilling well gauge sea level, with typical root-mean-square differences of better than 6 cm and correlation coefficients of higher than 0.95. Additionally, using an ocean-tide analysis of three months of sea-level observations from the GNSS-based tide gauge, it was possible to determine several tidal components, i.e., M2, S2, N2, O2, and M4. The amplitudes and phases show reasonable agreement with the ones derived from one year of stilling well gauge sea-level data.