HYDRO 2016 Paper 8A2

Filtering methods to extract the tide height from Global Navigation Satellite Systems (GNSS) signals for hydrographic applications

Faisal Alsaaq, Ahmed El-Mowafy, Michael Kuhn, Paul Kennedy


Hydrographic surveys have traditionally relied on the availability of tide information for the reduction of sounding observations to a common datum where the datum derived from a certain phase of the tide. In most cases, tide information is obtained from tide gauge observations and/or tide predictions over space and time using local, regional or global tide models. While the latter often provides a rather crude approximation, the former relies on tide gauge stations that are spatially restricted and often have sparse and limited distribution. A more recent method that is increasingly being used is Global Navigation Satellite System (GNSS) positioning which can be utilised to monitor height variations of a vessel or buoy, thus providing information on sea level variations during the time of a hydrographic survey. In this study, a different type of filtering methods to extract the tide signal from GNSS height observation has been applied and compared. The GNSS heights were estimated by using data collected by a Fugro Starpack GNSS receiver installed on a floating pontoon at Hillarys Boat Harbor, Western Australia. The receiver recorded continuously both (GPS and Glonass) signals for 30 days. Sea level heights determined by a traditional tide gauge at Hillarys were used as a reference to evaluate the effectiveness of using GNSS height in extracting the tide signal. A high degree of agreements of the tide information obtained by GNSS and the tide gauge was found in the time domain, with correlation coefficients of up to 0.98. Different low pass filters (Moving averaging, Savitzky-Golay, Gaussian, Zero-Phase and 1-D digital filters) were implemented to eliminate high-frequency components due to waves and the dynamic draft. Amplitudes and phases of the four major of tidal harmonic constituents (M2, S2, K2, and O1) were determined by spectral analysis and compared to model predictions. The standard deviation of the residuals between the GNSS heights and tide gauge signals was less than 5 cm. This level of positioning is proportional with International Hydrographic Organization (IHO) Special Order of hydrographic surveys.

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