Enhancing 3 km soil moisture retrieval: data fusion of SMAP brightness temperature with GNSS-R reflectivity from Spire and CYGNSS

This study investigates the fusion of GNSS-R reflectivity from Spire Global Inc. and the CYGNSS constellation to enhance SMAP soil moisture retrieval at 3 km resolution. Bias corrections were applied to align Spire BDS and Galileo to GPS and to reconcile differences with CYGNSS, yielding a consistent +2.41 dB offset and overall correlation of 0.81, with land cover-dependent variability. After fusion and spatial completion using the previously observed behavior interpolation method, the combined reflectivity was used to downscale SMAP brightness temperature (TB) from 9 km to 3 km. Analysis showed that TB decreased with increasing soil moisture but was influenced by vegetation and surface conditions, whereas surface emissivity exhibited a stronger relationship to soil moisture ( (Formula presented) (Formula presented) ). Validation against Sentinel/SMAP 3 km products, SMAP 9 km products and in-situ measurements from 85 ground stations demonstrated improved performance of the Spire+CYGNSS/SMAP retrievals, with higher correlations (up to (Formula presented) (Formula presented) ) and lower root-mean square error (as low as 0.038 cm³ cm⁻³) relative to SMAP alone, particularly over sparsely vegetated areas. The fused product retains SMAP’s 2–3 day revisit cycle while enhancing spatial resolution, underscoring the complementarity of multi-mission GNSS-R for high-resolution hydrological applications.