Analysis of GLORI Airbone GNSS-R Data for Soil Moisture Estimation

This study aims to map surface soil moisture (SSM) using airborne measurements from the GLORI instrument, which utilizes GNSS-R technology. Measurements were obtained in July 2021 at the Urgell site in Spain. In situ measurements of soil moisture, roughness, and vegetation cover were collected concurrently with the flight measurements. The study analyzes the reflectivity behavior of copolarization (Γ
RR
) and cross-polarization (Γ
RL
) as a function of incidence angle, normalizing the reflectivity with respect to the incidence angle. The sensitivity of reflectivities to surface soil moisture is then examined. An empirical model, incorporating soil moisture and the normalized difference vegetation index (NDVI), based on the tau-omega model, is used to invert GNSS-R reflectivity (Γ_RL) and estimate soil moisture. The model is calibrated and validated using a threefold cross-validation approach. Finally, SSM mapping at a 100 m resolution is proposed using data from the studied site and three flights.