**2. Methods and Datasets**

If the land surface is relatively flat and smooth, the roughness of the scattering region is lower than the scale of the wavelength of the incoming GNSS signal; then the scattering mechanism is different from the diffuse scattering general occurring over the ocean surface. The land-coherent scattering only comes from the first Fresnel zone around the specular point instead of the whole glistening zone. The image theory and Friis transmission equation are used to explain this coherent forward scattering process, and the geophysical characteristics of the reflection surface are indicated by reflectivity [31]. As the surface roughness increases, the contribution of the incoherent component dramatically increases [32]; when the surface roughness approaches the signal wavelength scale, the conditions on the sea surface will recur. Next, we first introduce the scattering model, then present our coherence detection estimators, dataset, and GNSS-R soil moisture retrieval algorithm.
