## 2 Materials and Methods
### Determination of Site Suitability Criteria and Constraints
#### 2.1.2 Triangular Trihedral Corner Reflector (CR) Installation Requirements
Concordantly, the orientation angle is expressed as follows:  
\[\theta=\frac{\pi}{4}\textit{ or }45^{\lx@math@degree} \tag{1}\]  
Figure 3: (**a**) Multipath graph at ALEV site for 24 h data acquisition, with an elevation cut-off angle of 0 degrees; (**b**) Observation availability of the tracked signals transmitted from each satellite at different elevation angles. The radar wave is normal to the open face when the elevation angle with one of the side plates is as follows (see Figure 4a) [21]:  
\[\psi=tan^{-1}\bigg{(}\frac{1}{\sqrt{2}}\bigg{)}\;or\;35.26^{\lx@math@degree} \tag{2}\]  
Hence, as shown in Figure 4b, for a given incidence angle \(\Phi\), the CR must be tilted at an angle \(\alpha\) from the baseplate to acquire the maximum RCS. The most demanding process in this task was the selection of the appropriate locations that would host the aforementioned equipment and would comply with the literature guidelines. According to the literature [17; 22; 23], numerous parameters should be examined when selecting an installation site for a target to be visible above the background signal level, called clutter. The requirements used for the analysis are presented in Table 1. Besides sigma 0 and the incidence angle, the remaining factors can also be used as criteria for choosing the most suitable GNSS location. The incidence angle is needed for the calculation of the CR's baseplate elevation angle to perform the maximum RCS. As shown in Table 1, the main criteria for selecting the most suitable location for a CR are sigma 0 (\(\sigma_{0}\)), land cover, slope, and aspect. For a CR installation to be considered successful and operational, the sigma 0--the normalized measure for determining the intensity of radar signals reflected by a distributed scatterer in an SAR image--must be less \begin{table}
\begin{tabular}{c c} \hline \hline
**Criteria (Factors)** & **Favourable Values** \\ \hline Backscatter coefficient & \(\leq\)\(-\)10 dB (at C-band) \\ (sigma 0) & Stable solid surface, no dense vegetation, and trees, \\ Land Cover & no metallic structures \\ Slope & \(<\)10\({}^{\circ}\) \\ Land Ownership & Government control area \\ Geological Formation & No marls, clays, sand, silts, or gravel \\ Ground Surface Aspect & Flat, or Oriented toward the direction of the satellite pass \\ Distance between CRs and CORS & Distance between GNSS antenna and CRs \(>\) 30 m \\ Distance between the 2 CR sites \(>\) 200 m \\ Incidence Angle & The radar signal’s average incidence angle with the target \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The requirements used for the suitability analysis. Figure 4: (a) Geometry of a triangular CR. \(\Phi\) represents the azimuth angle and \(\psi\) the elevation angle; (b) Adjustment of a triangular trihedral CR. \(\psi\) represents the elevation angle, \(\Phi\) the off-nadir angle, and \(\alpha\) the CR tilt angle from the base plate. than \(-10\) dB at C-band [22; 24]. Land cover near CRs should be free of dense vegetation and trees, as well as man-made structures, and the surface should be stable and solid. The slope in the installation area should not exceed \(10^{\circ}\), and the ground surface aspect should be oriented as much as possible toward the direction of the satellite pass [23]. These criteria and constraints are using the stage of the MCDA.