Interpolating SMAP Soil Moisture to 3 km Using CYGNSS and Spire Reflectivity Observations: Regional Case Study

This study utilizes GNSS-reflectometry (GNSS-R) reflectivity data from Spire Global Inc. and CYGNSS constellations to interpolate SMAP brightness temperature to enhance its resolution from 9-km to 3-km. Previously Behavior Interpolation Method (POBI) is firstly employed to fill spatial gaps of CYGNSS and Spire data gridded at 3-km, which are then used to downscale the SMAP 9-km brightness temperature to achieve a fine resolution. Soil moisture is derived from the enhanced brightness temperature using the tau-omega model. The POBI-processed CYGNSS and Spire datasets are comparable with a mean difference of 0.27 dB across the Southern Continental United States and central America. Our results demonstrate that the 3-km soil moisture maps generated by combining SMAP and Spire/CYGNSS data offer an improved spatial resolution over the standalone SMAP 9 km product and significantly increased spatial coverage compared to CYGNSS measurements.