## 2 Materials and Methods
### Determination of Site Suitability Criteria and Constraints
#### 2.1.2 Triangular Trihedral Corner Reflector (CR) Installation Requirements  
The CR is made of aluminum, which is more expensive than steel; however, the structure's weight is much lighter than iron or steel and is notable for its low density, corrosion resistance, and tolerance to oxidization. The use of corner reflectors as coherent targets is considered an advantage due to the conceptually simple and economic construction. Also, CRs do not require a source of power or any electronic components that could affect or decay their performance over time. Nevertheless, periodic maintenance is needed since they are exposed to weather conditions (e.g., snow, rain, high temperatures, strong winds, etc. ), fauna, vandalism, or theft during long-term measurements or site occupations [14]. Additionally, CRs, due to their large size, have a significant weight (especially C-band compliant CRs); thus, they need to be strongly mounted and anchored to the ground. The mount should enable orientation flexibility in elevation and azimuth, and it is usually fixed to a partially buried concrete base of sufficient dimension to ensure stability in typical weather conditions. Various types of CR exist, ranging from a single flat plate to dihedrals (2 orthogonal plates), trihedral (3 orthogonal plates) and more exotic configurations [15]. A permanently deployed CR intended for use with various sensors at different wavelengths should be sufficient size to satisfy minimum RCS requirements at a longer wavelength. According to [PERSON] and [PERSON] [16], the magnitude of RCS is influenced by the size and the shape of the corner reflector; thus, some of the major components being used are flat plate, dihedral and trihedral, while the latter is categorized by the following four types: square, cubic, circular and triangular trihedral. Although the triangular trihedral CR has the lowest RCS of all the mentioned reflector shapes, it is the most commonly used due to its structural rigidity, installation simplicity, requirement of less material to manufacture and it is cheaper [17]. A triangular trihedral side length of 1.5 m is generally suitable for C-, L-, and X-Bands. For optimal performance, it must be re-aligned each time a different sensor makes an acquisition [18]. This can be avoided by aligning the CR in the average direction of the sensors and using a triangular trihedral whose RCS is less affected by misalignments. Fortunately, SAR satellites tend to be in sun-synchronous polar orbits with very similar ground headings and azimuth alignment requirements. In order to be useful for both the ascending and descending passes of a satellite, two CRs are necessary with roughly opposite azimuth orientations. The RCS of a CR is proportional to the fourth power of its physical size and inversely proportional to the square of the wavelength, \(\lambda\). Typical SAR sensors operate at X-Band (\(\lambda\) = 3.1 cm, TerraSAR-X, COSMO-SkyMed, TanDEM-X), C-band (\(\lambda\) = 5.6 cm, Sentinel-1A/B, RADARSAT-2, RCM) and L-band (\(\lambda\) = 23.5 cm, ALOS-2/PALSAR-2, SAOCOM-1, NISAR-L) [19]. The RCS of a CR is then smaller at L-Band than at X-Band; nonetheless, this can be compensated for by increasing the size of the corner reflector [20]. Based on the above, the CR type used in CyCLOPS was decided to be the triangular trihedral with an inner length dimension of 1.5 m to support all current SAR missions. The RCS changes depend on its azimuth orientation and elevation angles; hence, when deployed, the CR should be oriented in such a way that the azimuth and elevation of the boresight vector (originated from the intersection of the three triangular plates) is directed in the SAR satellite's line of sight [16]. A triangular trihedral CR can be considered as an object whose RCS is determined by its open face. The maximum RCS direction is determined as normal to the CR's open face.