Coastal monitoring of sea state using airborne GNSS reflected signals

Kurzfassung

Global Navigation Satellite Systems signals have shown excellent performance in applications such as geodesy and geophysics, timing, defense, meteorology, and many other areas. Most of these applications rely on the signals that directly reach the receivers. However, there is a portion of them that bounce off the Earth's surface before they reach the receiver from which properties of the reflecting surface can be derived by using the GNSS Reflectometry (GNSS-R) technique. The focus of this study is to investigate the possibility of using GNSS-R from low altitude airborne measurements to retrieve the phase, power, and Doppler shift of the reflected signal, and determine their sensitivity to sea state (surface roughness). The experiment consisted of multiple flights at an altitude of ~780m (a.m.s.l) along Opal Coast, between the cities of Calais and Boulogne-sur-Mer, France, from the 12th to the 19th of July 2019. The direct and reflected signals were received by a dual-polarized (Right-Handed and Left-Handed Circular Polarizations) antenna mounted on a gyrocopter. A software receiver is used to process the direct and reflected signals. The reflected signals are tracked and re-tracked aided by the signal path difference between the direct link and the reflected link retrieved from a specular reflection model. The resulting in-phase and quadrature components (at 50Hz rate) of the reflected re-tracked signals are analyzed in the spectral domain every minute to obtain the power estimates, surface reflectivity, and relative Doppler shift. The sensitivity of the reflected signal parameters and the sea state is determined by the correlation between the reflectivity response with wind speed and significant wave height (SWH). The latter two were obtained from the atmospheric, land and oceanic climate model, ERA5, provided by the European Centre for Medium-Range Weather Forecasts (ECMWF). Initial results have shown an impact in the reflectivity residual (observed minus modeled reflectivity) and the Doppler shift distribution of the reflected signal with respect to wind speed and SWH, depending on the GNSS satellites' elevation. At low elevation events (E<10°), reflectivity residual and wind speed present a Pearson correlation coefficient of -0.73 and the Doppler shift spread shows a correlation of 0.94. At high elevations events (E>30°), the correlation decreases (-0.12 and 0.60, respectively). The impact of receiver position accuracy, antenna gain pattern, or tropospheric/ionospheric disturbance on the retrievals is currently studied.