Sea level monitoring and sea state estimate using a single geodetic receiver

Abstract

GNSS-Reflectometry (GNSS-R) altimetry has demonstrated a strong potential for sea level monitoring. Interference Pattern Technique (IPT) based on the analysis of the Signal-to-Noise Ratio (SNR) estimated by a GNSS receiver, presents the main advantage of being applicable everywhere by using a single geodetic antenna and receiver, transforming them to real tide gauges. Classical SNR analysis method used to estimate the variations of the reflecting surface height h(t) has a limited domain of validity due to its variation rate dhdtt assumed to be negligible. We present here a significant advance in this altimetric methodology using GNSS multipath to conjointly estimate h(t) and dhdtt over areas characterized by high amplitudes of tides and presence of waves. It drastically enhances the temporal and spatial monitoring of tides and waves. Inversion approach is based on a Least Square Method (LSM), combining simultaneous measurements from different GNSS constellations (GPS, GLONASS). Our method is validated with SNR data acquired on an offshore site of 60-meter height, in conditions were assumptions of the classical SNR analysis method are not valid (i.e. with a semi-diurnal tide amplitude of ~ 4 m, vertical velocity of the sea surface due to tide reaching 0.2 mm/s, and presence of waves with amplitude up to few meters). Linear correlation between the estimates with our method and tide gauges records are better than 0.97, whereas it only equals 0.82 with the classical method over the whole 3 months of acquisition. Our dynamic SNR method allows a very good estimate of the main tide periods and permits to detect swell and waves with realistic amplitudes and periods, which is not the case with tide gauges (located in protected areas) or classical SNR analysis method.