Coastal monitoring of sea state using airborne GNSS reflectometry

Abstract

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 antenna linked to two independent front-end receivers, one for the Right-hand and one for the Left-hand circular polarized signals. The antenna was mounted on a gyrocopter and tilted ~43° with respect to the zenith direction. A software receiver is used to process the direct and reflected signals from the Right-hand polarization channel. The reflected signals are tracked aided by the signal path difference between the direct and reflected link retrieved from a specular reflection model. The resulting in-phase and quadrature components (at 50Hz) of the reflected re-tracked signals are analyzed in the spectral domain every minute to obtain the power, surface reflectivity, and relative Doppler shift. The sensitivity of the reflected signal with respect to the sea state is determined by correlating the estimates: Doppler shift standard deviation, and the reflectivity residual (Rres) (observed minus modeled reflectivity) with the significant wave height (SWH) and wind speed (WS). 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 that at calm sea (SWH: 0.26m and WS: 2.9m/s), low elevation satellites (E<10°) present a Doppler spread of 0.02Hz, and high elevation events (E>30°) the Doppler spread is 1.29Hz. At a higher sea state (SWH: 0.55m and WS: 6.5m/s), Doppler spread increases to 0.71Hz and 4.90Hz for low and high elevation events, respectively. Besides, there is a relationship between the reflected signal estimates and the ancillary sea state parameters depending on the GNSS satellites' elevation. At low elevation events, Rres and WS present an anticorrelation coefficient of -0.73. On the other hand, Doppler spread and WS show a correlation coefficient of 0.94. At high elevations events, the correlation decreases (-0.12 and 0.60, respectively).