High-resolution soil moisture and freeze–thaw records toward the third pole using GNSS-R reconstructed observations during 2018–2022

Spaceborne Global Navigation Satellite System-Reflectometry (GNSS-R) observations are effective for extensive monitoring of soil moisture (SM) and freeze–thaw (F/T) state. However, there is a lack of long-term, well-defined accuracy, high spatiotemporal resolution SM and F/T datasets for alpine regions. To address these issues, this study first generates five years of daily continuous high spatial resolution GNSS-R F/T and SM datasets for Qinghai-Tibet Plateau based on a proposed multivariate time-fitted observation reconstruction method. Specifically, this study generates spatio-temporally seamless observations based on Cyclone GNSS (CYGNSS) observations for SM and F/T retrieval. The RMSE and R of the SM retrieval are 0.063 \({\text{cm}}^{3}\text{/}{\text{cm}}^{3}\) ${\text{cm}}^3{\text{/cm}}^3$ and 0.52, respectively, which are consistent with the SM retrieval accuracy performed pre-reconstruction. Similarly, the F/T retrieval accuracy was 84.1%, which was also comparable to the F/T retrieval accuracy performed pre-reconstruction. In-situ station evaluation demonstrated that the RMSE and R of the SM retrieval and the F/T retrieval accuracy were 0.063 \({\text{cm}}^{3}\text{/}{\text{cm}}^{3}\) ${\text{cm}}^3{\text{/cm}}^3$ , 0.70, and 85.2%, respectively. This is consistent with the performance of the SM and F/T results from original observations. Notably, the temporal resolution of the CYGNSS reconstructed observations was improved by 278% over the original observations at 9 km. Therefore, we argue that the method proposed in this study addresses the problem of mutual constraints on spatial and temporal resolution in CYGNSS SM and F/T retrievals. Furthermore, this study developed the first high-accuracy and high spatiotemporal resolution SM and F/T dataset for the Qinghai-Tibet Plateau region in the GNSS-R domain.