Atmospheric effects resolved in coherent airborne GNSS reflectometry

Abstract

The advent of the Global Satellite Navigation Systems (GNSS) allowed the emergence of multiple satellites, airborne and terrestrial systems for remote sensing, and Earth observation. GNSS signals, designed for positioning and navigation, can be used to obtain characteristics of the Earth's surface once they get reflected. Recent studies have made use of GNSS Reflectometry as a tool for atmospheric sounding with promising results, broadening the spectrum for the use of the reflected GNSS signals. This study examines the occurrence of high-precision phase delay information for airborne reflectometry under the condition of various sea states in coastal waters. We fuse GNSS and ancillary data to resolve the tropospheric residual from the signal path change over the observed period. The experiment consisted of four flights performed with a gyrocopter in July 2019 along the coast between Calais and Boulogne-Sur-Mer, France. The processing comprises the integration of aircraft trajectory, GNSS satellites orbits, and geoid model for direct and reflected signal path difference modeling. Path predictions are used for GNSS-R data processing by means of a model-aided software receiver. The resulting reflected signal is passed through a retracking module to obtain the corrected Doppler shift and residual phase observable comparable with the tropospheric residual retrieved from ray-tracing modeling assuming a standard atmosphere. Initial results have shown promising performance at calm sea and grazing angles. Satellites with low elevations (E < 10°) reveal coherent observations that allow resolving tropospheric effects from GNSS-R airborne data.