## 2 Materials and Methods
### Visibility Analysis  
Beginning from the most favourable locations, if at least one of the aforementioned constraints is not satisfied, the assessment for the optimal site continues to the less convenient location and so on. At first glance, when looking at the orthomosaic, it is noticeable that there is a metallic boundary fence between the two most ideal locations, 9 m away from the southern deeper blue cell and 14 m away from the northern. Hence, the two darker blue coloured cells are excluded from the selection. Subsequently, the next option--the southern cell for the ascending pass--is selected for the analysis. Simultaneously, the following option for the descending pass site is one of the three light blue coloured cells. Due to the proximity to the GNSS station, the suitable location is chosen to be the northwest light blue cell. Performing exploratory 3D analysis in ArcGIS Pro software, one of those locations is chosen to be the final. As mentioned, the distance between the two CRs must exceed 200 m, so an elevation profile was carried out (see Figure 14) to check this constraint. Finally, a visibility analysis was performed for the two CR sites, one for each pass, based on the average incidence angle as described in Table 12.  
Figure 13: The candidate locations inside the AOI boundaries. (**a**) CR candidate sites for the ascending pass; (**b**) CR candidate sites for the descending direction. Concerning the relative orbits of each pass, and assuming that all SAR sensors have a line of sight that is perpendicular to the satellite platform's travel direction [22], the CR1 was oriented at a 260\({}^{\circ}\) azimuth angle, whereas the CR2 has a 100\({}^{\circ}\) azimuth angle. As described in Section 2.1.2, to succeed the maximum RCS, for a given incidence angle of \(\sim\)40\({}^{\circ}\), the CR for ascending pass should have a baseplate elevation angle of \(\sim\)15\({}^{\circ}\). Figure 15 illustrates the viewsheds in a 3D model, while in Table 13 the viewshed parameters of the CR are described. Figure 14: The elevation profile of the two CRs in ALEV location. The distance between them is 230 m. CR1 is referred to the site of the ascending pass and CR2 to the descending. Figure 15: Viewshed perspective of the candidate sites with sigma 0 values overlayed on the terrain. Red lines demonstrate the open face orientation angle of 45\({}^{\circ}\). The lower red line illustrates the baseplate line of sight when CR is tilted at a specific angle. The upper red line points at the line of sight at an elevation angle of 35.26\({}^{\circ}\). (**a**) viewshed perspective of the CR for the ascending pass; (**b**) 3D viewshed of the CR for the descending direction. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Location** & **Absolute Orbit** & **Direction and Relative Orbit** & **Average Incidence Angle** & **Average Sigma 0 (dB)** \\ \hline Corner Reflector 1 & 23,286 & Ascending (160) & 39.7\({}^{\circ}\) & \(-\)10.65 \\ \hline Corner Reflector 2 & 23,293 & Descending (167) & 39.22\({}^{\circ}\) & \(-\)11.53 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: The average incidence angle and the sigma 0 value for the two locations. The data were acquired using Sentinel-1 SLC products from August to September 2020. Determination of Candidate Locations for GNSS  
An exploratory 3D analysis was performed in GIS environment to check the visibility of the GNSS antenna as depicted in Figure 16, and the distance between the GNSS antenna and the 2 CRs (see Figures 17 and 18). The closest obstruction was found at 70 m (>30 m) to the South of the GNSS antenna. Nonetheless, this was the most suitable site because of the lack of nearby trees. Furthermore, the aforementioned obstruction was calculated to be 5 degrees above the antenna reference point (ARP) horizon. Therefore, signals can reach the GNSS antenna uninterrupted or unreflected, increasing the likelihood of a reliable position estimate [54]. Figure 16: The View Dome visibility analysis of the GNSS antenna illustrates its visible volumetric spaces in 3D.