[en] Soil moisture is an essential climate variable that needs to be accurately understood and monitored. At large scales, its mapping primarily relies on satellite microwave remote sensing missions. NASA’s Soil Moisture Active Passive (SMAP) provides global soil moisture retrievals using a radiometer with a spatial resolution of approximately 40 km and a revisit time of 2-3 days. To improve the spatiotemporal resolution of SMAP and provide daily retrievals at both 3 and 9 km, we fused SMAP’s enhanced 9-km product with spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) observations. GNSS-R uses L-band signals of opportunity to infer properties of the reflecting surface, which are sensitive to variations in biogeophysical parameters. The proposed algorithm combines data from Cyclone GNSS (CYGNSS) and Spire Near-Nadir GNSS-R missions and SMAP, applying a gap-filling approach. Our results for Australia in 2021, compared to 21 in-situ sites, demonstrate that the fused soil moisture product maintains the accuracy of SMAP, with median unbiased root-mean-square error (ubRMSE) of 0.054, 0.056, and 0.056 cm3cm-3 for SMAP-only, fused 9-km and fused 3-km, respectively. This approach enhances the spatiotemporal resolution to 3 km and provides daily estimates.
