INCIDENCE ANGLE NORMALIZATION OF SPACEBORNE GNSS-R SURFACE REFLECTIVITY FOR SOIL MOISTURE RETRIEVAL

Large-scale near-surface soil moisture can be retrieved from Global Navigation Satellite System Reflectometry (GNSS-R) surface reflectivity observations, which are dependent on the signal incidence angle and therefore need to be normalized. Using 4 years of Cyclone GNSS (CYGNSS) data, in this study we propose a new method for this normalization, accounting for the spatially varying effects of coherent and incoherent scattering. The method is based on a linear regression between the gridded incidence angle and surface reflectivity. We applied the normalized surface reflectivity observations in our soil moisture retrieval algorithm and found a median unbiased root-mean-square error (ubRMSE) of 0.0504 cm(3)cm(-3) using the Soil Moisture Active Passive (SMAP) as the reference, an improved result compared to other incidence angle correction methods described in the literature.