Sea-Ice Thickness Retrieval Using Integrated Grazing-Angle and Near-Nadir GNSS-R Reflectivity From the Spire Constellation

Abstract

Accurate estimation of sea-ice thickness (SIT) is essential for understanding polar climate processes and supporting operational monitoring. This study demonstrates, for the first time, the use of grazing-angle global navigation satellite system reflectometry (GNSS-R) reflectivity from the Spire CubeSat Constellation to retrieve SIT, alongside sea-ice salinity and density. We extend previous work on near-nadir GNSS-R by integrating both near-nadir and grazing-angle observations using a three-layer ice reflectivity model and inversion framework. Weekly SIT products are generated and validated against the CryoSat-2/SMOS (CS2SMOS) reference dataset. The combined retrieval reduces the unbiased root-mean-square error (ubRMSE) to 0.540 m (0.376 m within the 0.5–1 m SIT range), compared with individual full-Arctic product ubRMSEs of 0.609–0.743 m. Within the 0.5–1 m SIT range, where individual Soil Moisture and Ocean Salinity (SMOS) and CryoSat-2 retrievals are less reliable, the combined retrieval achieves lower ubRMSE than every individual GNSS-R product, although a slight positive bias remains. Retrieved salinity and density fields, derived as by-products of the inversion, show spatial patterns consistent with known geophysical behavior. While some seasonal biases remain, especially underestimation during early and late ice growth periods, this study demonstrates the potential of Spire GNSS-R as a complementary observation source for sea-ice remote sensing, offering high revisit frequency and moderate spatial resolution that may contribute to future fused SIT records as the technique matures.