## 2 Materials and Methods  
### Field Campaign at the Valencia Anchor Station  
The Valencia Anchor Station validation site is located on the Utiel-Requena plateau, 80 km west of the city of Valencia, in Spain, and the SOMOSTA experiment took place at the Valencia Anchor Station MELBEX (Mediterranean L-Band characterization Experiment) site, within the El Renegado vineyard area with coordinates (39\({}^{\circ}\)31'17.98\" N, 1\({}^{\circ}\)17'29.29\" W) and an altitude of 800 m. During this experimental campaign, the ESA GNSS-R Oceanpal [21] antenna was installed on the same tower as the ESA ELBARA-II passive microwave radiometer, the Oceanpal 11 m high from the ground, and both measuring instruments having a similar field of view. The Oceanpal Radio Frequency Unit (ORFU) and Data Management Unit (DMU) was installed at a small protected hut at the bottom of the tower.  
Figure 1 shows the experimental set-up composed of the ELBARA-II L-band radiometer tower, also holding the Oceanpal GNSS-R antennas that are shown in detail in Figure 1b.  
According to Figure 1b, the GNSS-R instrument has an up-looking antenna for receiving direct signals in RHCP (Right-Hand Circular Polarization) from GNSS satellites, and a down-looking antenna for receiving LHCP (Left-Hand Circular Polarization) and RHCP-reflected signals from the soil surface. The boresight direction of both antennas was pointing 20\({}^{\circ}\) away from the tower to reduce tower impact and for a larger observation range. The direction of the Oceanpal antenna is about 285\({}^{\circ}\) towards northwest (360\({}^{\circ}\) is the north). All three antennas have the same gain pattern, as shown in Figure 2a. Figure 2b combines the observation geometry from both instruments and the GNSS-R antenna pattern overlay on Google Earth. The yellow ellipses correspond to ELBARA-II footprints for incidence angles 30\({}^{\circ}\), 35\({}^{\circ}\), 40\({}^{\circ}\), 45\({}^{\circ}\), 50\({}^{\circ}\), 55\({}^{\circ}\), the azimuth was fixed to 305\({}^{\circ}\), and the blue ellipses correspond to projections of Oceanpal gain patterns. The upper layer shows the projection of the Oceanpal down-looking antenna's gain over the soil surface, together with the ELBARA-II footprint, the bottom layer is a photo taken by a camera drone overlaid on the Google Earth base map. The visible area of Oceanpal is within 20 m from the tower, and the azimuth range for observation is 265\({}^{\circ}\) to 345\({}^{\circ}\).  
Two soil moisture ThetaProbe sensors [22], type ML2x from the company Delta-T Devices, UK, were placed at the bottom of the tower. As displayed in Figure 3, the ThetaProbe 1 is close to a vine stump, and the ThetaProbe 2 is in the middle of two rows, both of them measuring soil moisture for top 5 cm. The averages of the measurements performed with these two probes were assumed to be representative of the soil moisture conditions in the MELBEX-III field site, and thus considered as the soil moisture reference in this study. A Davis Vantage Pro2 Plus weather station [23] was installed on the tower, measuring rainfall, temperature, humidity, and solar radiation.  
Figure 1: (a) ELBARA-II L-band radiometer tower also holding the Oceanpal GNSS-R antennas over a vineyard field at El Renegado MELBEX site at the Valencia Anchor Station; (b) Detail of the GNSS-R deployment indicating the essential elements.  
Figure 2: (**a**) Oceanpal antenna gain pattern; (**b**) projection of Oceanpal gain pattern and ELBARA-II footprint overlay on Google Earth. The upper layer shows the projection of the Oceanpal down-looking antenna’s gain over the soil surface, together with the ELBARA footprint, and the bottom layer is a photo taken by a camera drone overlay on the Google Earth base map; (**c**) The incidence angle is the complementary angle of elevation, as shown in the graph.