## 3 Results and Analysis
### Ground-Truth Experiment Data for Validation
In the GNSS-R measurements, the major axis of the first Fresnel zone (the region surrounding the specular point from, which power is reflected with a phase change across the surface constrained to \(\pi\) radians, see Figure 1), for satellites in our geometrical condition (high elevation angle and a height of tripod of 1.5 m) was around 1 m. The TDR portable system was moved around to cover this area. Five measurements are performed with a cross scheme as shown in Figure 11 and the results are the averages of the five (black circles). In the following, four campaigns are discussed in detail as shown in Table 3.  
Figure 11: Tektronix Metallic Cable Tester 1502 for time-domain reflectometry (TDR) measurements. Figure 10: Precipitations in Grugliasco and Agliano during January to March 2016. The traces of satellites in the sky during the experiment are plotted using different colors as shown in the skylplot of Figure 12. It shows the positions of satellites in terms of elevation and azimuth. The elevation was scaled by the concentric rings nested within one another. The outside ring was 0\({}^{\circ}\) and the middle of the plot was a 90\({}^{\circ}\) elevation. The azimuth is the direction angle with respect to the North (0\({}^{\circ}\)) measured clockwise. The sky plot for experiments in Grugliasco and Agliano was determined before each measurement to choose a proper system position and obtain the input variable (\(\delta\)). Especially, for static measurements, when the receiver is only a few meters high above the ground, the knowledge of the positions of the satellites and the first Fresnel zone coverage is sometimes crucial for analyzing the obtained data and provide the range of the data for comparing the results with other kinds of measurements. Here we also added the information of the position of the receiver and the direction that the bar was pointing to, which greatly helped with the GNSS-R retrieval analysis. \begin{table}
\begin{tabular}{c c c c} \hline
**Date** & **Soil Condition** & **Location** & **Soil Type** \\ \hline
27 January 2016 & Dry condition & Grugliasco & Loamy sand \\
5 February 2016 & Dry condition & Agliano & Silty clay loam soil \\
3 March 2016 & Wet condition & Grugliasco & Loamy sand \\
7 March 2016 & Wet condition & Agliano & Silty clay loam soil \\ \hline \end{tabular}
\end{table} 
Table 3: Summary of the experimental campaign. Figure 12: _Cont_. More than that, the georeferencing specular points on the x-y plane with Fresnel zones that relatively corresponded to each satellite, and the antenna footprint obtained at a height of 1.45 m (see Figure 9) was depicted by a big green circle, indicating the signal coverage of the antenna. The receiver projection was at the origin of coordinate and it was represented by a red point. The corresponding ellipse Fresnel zones surrounded the receiver projection represented by different color ellipses. The x-axis represents the distance in meter in West-East direction and the y-axis represents the distance in South-North direction. The antenna footprint will change accordingly with the azimuth and elevation angles, which makes it useful for planning a measurement aiming at receiving reflection signals from certain PRNs. In this measurement, this information is quite useful for the GNSS-R measurement and also indicating the location of the TDR instrument probe to precisely evaluate the permittivity as shown in Figure 13.  
Figure 12: The skyplot with bar and equipment position, specular points mapped on the x–y plane with Fresnel zones and antenna footprint, (**a**) 27 January 2016, (**b**) 5 February 2016, (**c**) 3 March 2016, and (**d**) 7 March 2016. Figure 13: The results of TDR and GNSS-R soil moisture (SM) retrieval, with time series, (**a**) 27 January 2016, (**b**) 5 February 2016, (**c**) 3 March 2016, and (**d**) 7 March 2016. The obtained GNSS-R and TDR results with time series are shown in Figure 13. A good correlation between the GNSS-R and the TDR measurements with a certain PRN in each case can be observed.