Analysis of SMAP-R Polarimetric Information for Improved Sea Surface Wind Retrieval

Spaceborne Global Navigation Satellite System reflectometry (GNSS-R) sea surface wind speed retrieval commonly relies on the normalized bistatic radar cross section (NBRCS), whose response depends on observation geometry, link conditions, signal quality, and sea state. Under low-wind, high-swell conditions, local wind speed alone cannot fully explain variations in reflected signals. We use SMAP-R hybrid compact polarimetric observations and ERA5 wind and wave data to assess whether polarimetric features improve sea surface wind speed retrieval. A dataset of 2,067,971 observations is divided by complete months into training, validation, calibration, and test sets. With total significant wave height included in every baseline, paired non-polarimetric and polarimetric configurations are evaluated on identical samples using a geophysical model function with minimum-variance fusion (GMF/MV), extreme gradient boosting (XGBoost), and a fully connected network trained by back-propagation (BP-FCN). Adding the degree of polarization and normalized third Stokes component reduces the overall root mean square error (RMSE) by 7.83%, 5.49%, and 5.31%, respectively, with the polarimetric BP-FCN configuration achieving the lowest RMSE of 1.666 m s−1. Wind speed-stratified analysis shows that the relative error reduction from polarimetric information varies non-monotonically across wind speed intervals. Across all signal-to-noise ratio (SNR) intervals represented in this study, all three methods achieve positive polarimetric gains, with paired month-block bootstrap 95% intervals lying above zero, indicating benefits across different SNR conditions. Under low-wind, high-swell conditions, RMSE decreases by 10.58%, 11.26%, and 9.82%, respectively, exceeding the overall relative gains. Error decomposition indicates that these improvements mainly reflect reduced systematic overestimation. Comparisons against ASCAT scatterometer and NDBC buoy winds using all collocated samples also show positive polarimetric gains of 4.83–5.35% and 4.80–5.78%, respectively. These results demonstrate that polarimetric observations can further improve GNSS-R wind speed retrieval accuracy beyond the information already included in the baselines, with more pronounced gains under the low-wind, high-swell conditions sampled in this study, providing useful complementary information for sea surface wind speed retrieval under complex sea states.