Exploring Spire GNSS Reflections for Global Soil Moisture Monitoring

Spaceborne GNSS reflectometry (GNSS-R) has emerged as an effective tool for observing soil moisture content at relatively high temporal and spatial resolutions. This study presents an initial evaluation of using reflection signals captured by Spire Global's GNSS-R satellites for global soil moisture monitoring, based on the sample data on Nov 1, 2023. The land surface is covered by ∼16.7% when gridding the good-quality reflection samples into a map with a resolution of 36 km. Moreover, the Spire reflectivity observations of an example track over the region without dense vegetation show a good agreement with the SMAP soil moisture measurements, with a correlation coefficient of 0.71. The Spire soil moisture retrievals have a root mean square difference (RMSD) of 0.066 m³/m³, compared to the SMAP observations. More accurate Spire soil moisture measurements are expected after correcting the impact of vegetation, surface roughness, and signal incidence angles.