2.2.2. Sentinel-1

The Sentinel-1 (S1) satellites were launched by the European Space Agency (ESA) in the frame of Europe's Copernicus program, including Sentinel-1A (S1A) and Sentinel-1B (S1B). The orbital period of S1A is 98.6 min, the revisit period is 12 days, and the combined AB satellite is 6 days. The interferometric wide (IW) imaging model with a spatial resolution of 5 m × 20 m provides a more accurate *σ*◦ and better meets the needs of this study [50].

We use the ground range detection (GRD) products of S1 in IW acquisition mode with the VV and VH polarizations from Google Earth Engine (GEE). Compared to the VH polarization of S1, the VV polarization has greater potential for SM retrieval [40,65–67]. Some researchers have used both VV and VH in SM retrieval studies and characterized the effect of vegetation by the ratio of VV and VH [41,68]. However, in the study, only VV single polarized data is acquired before February 2017. Therefore, the optical vegetation index NDVI is used in this study for characterizing vegetation. The backscattered images of VV polarization in the completely thawed season (months July, and August) and the completely frozen season (January to February) from 2016 to 2019, in a total of 89 acquisitions, are employed for the SM retrieval algorithm development and validation.
