Spatialization and analysis of the GNSS-R measurements around of the Cordouan lighthouse

As the Cordouan lighthouse [N45°35'11"; W1°10'24"] is constructed in the Bay of Biscay, GNSS reflectometry-monitored time series of sea level are characterized by important tidal variations for this site. However, these GNSS-R measurements are impacted by submerged sandy banks that appear at low tide and spoil the estimates of pure sea level. We propose the spatialization of the GNSS reflection points to identify the areas with sandy or rocky parts around the lighthouse. For this purpose, the surrounding of the receiving antenna is divided into juxtaposed geographical cells forming a map filled by water heights estimates according to the position of the reflection points. Water heights are determined using the method of periodogram of 1-second Signal-to-Noise-Ratio (SNR) data on a sliding elevation window of several degrees. In particular, correction of the atmospheric delay effects enables to reduce the dispersion of the water heights versus low elevations by a factor around 2. High variability (more than one meter) and higher means of water heights estimated over months in the eastern part are very consistent with the presence of the shoals, while long-term means and dispersions in cells of plain ocean are much smaller. These results are highly emphasized by the distinction between low and high tides.