## 3 Results and Analysis
### Ground-Truth Experiment Data for Validation  
Several ground-truth experiments were done in various test sites. Two sets of data particularly with two different typical soil compositions and soil moisture content (dry terrain and a wet terrain) were considered. Two ground-based campaigns with a controlled environment are shown in Figure 9. The first site is located in Grugliasco, Torino (45\({}^{\circ}\)03\({}^{\prime}\)58.5\({}^{\prime}\)N, 7\({}^{\circ}\)35\({}^{\prime}\)33.8\({}^{\prime}\)E), in the Dipartimento Inter-ateneo di Scienze Progetto e Politiche del Territorio (DIST) of Polito. In this place, a wide field of known characteristics (mainly 50% of sand) was available. The second site was located in Agliano (44\({}^{\circ}\)47\({}^{\prime}\)29.1\({}^{\prime}\)N, 8\({}^{\circ}\)15\({}^{\prime}\)19.8\({}^{\prime}\)E), where it is an area of smooth hills mainly devoted to wine production. In this second case, the composition of the soil is 50% silt and 37% clay. The details of the soil compositions for the two sites are reported in Tables 1 and 2. During the experiment, GNSS-R equipment and time-domain reflectometry (TDR) setup were used to make measurements before and after rain in bare fields. They were intentionally chosen due to their different terrain composition. The measurements in the dry condition were done after a long drought, and the wet condition was determined after several rainfalls. The timeline of the rainfall and the measurements are shown in Figure 10. The TDR measurement can provide a high spatial resolution between 1.0 and 2.0 cm for SMC between 10% and 40% [60] and reliable permittivity profiles that would be used in the GNSS-R performance analysis [61; 62]. Figure 8: Variable importance sensitivity to different types of soils, Grugliasco (**a**) and Agliano (**b**), when _estimators_ = 2000, \(n\) = 5000, _col_ - _sample_ - _tree_ = 0.5 and 0.6. Figure 9: Two ground-based setups in Grugliasco (**left** panel) and Agliano (**right** panel). The GNSS-R system consists of two commercial frontends connected to two antennas and PCs for data acquisition. It was performed in bistatic GNSS-R configuration, which has one right-hand (RH) antenna pointing to the sky for receiving the direct signal and another left-hand (LH) antenna pointed downwards to receive the reflected signals in Figure 9. The antennas used in the experimental were two commercial antennas produced by ANTCOM Corp. that were able to receive the GNSS signals in L1 and L2 bands. Two commercial front-end SiGe GN3S v2 USB RF developed by the Colorado Center for Astrodynamics Research were used [63]. The front-end connected to the two antennas and a cable was used to transfer the sampled data to a PC. The antennas were mounted on a plastic tripod. The board was installed on the end of the bar that was kept horizontal at a height of 1.45 in both places (Grugliasco and Agliano). The acquisition of GPS data was performed by using N-Grab GNSS data grabber that was developed by the NavSAS group of Polito di Torino [64]. Then the raw data were post-processed for obtaining the SNR of each satellite. The reflected signals mainly contained the LH signal, and this measurement was done in the condition regardless of the surface roughness and incoherent components. The values of the permittivity are obtained from local measurements based on time-domain reflectometry (TDR) technique [62]. A rod sensor (length 15 cm) Tektronix Metallic Cable Tester 1502 manufactured by Tektronix Inc., Beaverton, OR, USA, was used in the measurements (Figure 11). Then the value of permittivity was obtained from the travel time of the TDR probe. In this measurement, the position of the TDR probe was tilted to 30\({}^{\circ}\) with respect to the surface, thus, only around 7 cm of the surface were taken into account in the TDR measurements. This was done in order to satisfy the TDR results with those obtained with GNSS-R that sense only the first few centimeters of the surface (2-5 cm).