## 3 Methods
### Calculating Azimuth and Elevation Angles for Automatic CRs and Conducting In Situ Measurements  
The ground experiment made use of 15 automatic CRs, as depicted in Figure 4, which were developed by CASPLOS_CCSI and employed as the ground control points. Each of these 15 sets of trihedral CRs had an inner leg length of 1 m. They exhibited radar cross section errors of less than 0.16 dB in the C-band. The console was situated beneath the CR and the azimuth and elevation angle adjustment errors were less than \(\pm 0.1^{\circ}\). In this experiment, the iRTK5 device, as illustrated in Figure 5, was employed to measure the latitude/longitude position and elevation information of the phase center of the CR, indicated by the red circle in Figure 4. The iRTK5 is equipped with a professional GNSS antenna, ensuring stable operation even in complex environments. It maintains a data measurement error of less than 9 mm for dynamic data acquisition and 2.5 mm for static data acquisition, adhering to D-level surveying and mapping standards [20]. This level of precision effectively guarantees the positional accuracy of the CRs and the accuracy of the subsequent angle calculations and other processes. During the transit of SAR satellites, it is crucial to align the normal angle of the CR with the radar incident beam direction [21] to maximize the backscattered echoes and ensure the point target is clearly visible in the image. To achieve this, the satellite's orbital parameters are employed to calculate its azimuth and elevation angles during transit [22]. Based on the spatial geometric relationship between the satellite and the ground, we calculated the azimuth angle \(\varphi_{\mathrm{s}}\) (defined as the angle between the CR's direction towards the satellite S and due north, indicated by the red arc in Figure 6a) and the elevation angle \(\theta_{\mathrm{s}}\) (defined as the angle between the CR's direction towards the satellite S and the horizon plane, marked by the red arc in Figure 6b) of the satellite. Then, we determined the azimuth angle \(\varphi_{CR}\) and elevation angle \(\theta_{CR}\) of the CR (the vertexes of the CR, O, A, B, and C are shown in Figure 4) based on our previous work in [23]. For instance, when the satellite is in the  
Figure 3: Experimental method flow. descending orbit and right-looking, the geometric relationship between the CR's azimuth angle \(\varphi_{CR}\) and the satellite azimuth angle \(\varphi_{S}\) is shown in Figure 6a, and the geometric relationship between the CR's elevation angle \(\theta_{CR}\) and the satellite elevation angle \(\theta_{s}\) is shown in Figure 6b. Notably, the angle \(\theta_{1}\) between the normal direction and the bottom surface of the trihedral corner reflector is \(35.264^{\circ}\). Figure 4: Automatic trihedral CR (the letters on the 4 vertices of the corner reflector correspond to the vertex positions in Figure 6). Figure 5: RTK instrument used for the location of CR in in situ measurements. Figure 6: Schematic for the calculation of CR’s azimuth and elevation angles: (**a**) azimuth angle calculation and (**b**) elevation angle calculation. Geometric Correction of C-SAR Images Based on RPC Model and Theoretical Row-Column Coordinate Calculation for Ground CRs  
The RPC model was employed for the correction of C-SAR images. This model is versatile and adaptable to various sensor types, eliminating the need for complex knowledge about imaging processes and sensor system characteristics. This simplifies and expedites photogrammetric processing, making it user-friendly even when dealing with new sensor parameters [24]. Presently, geometric correction parameters based on RPC models are provided for GF-3, C-SAR/01 and C-SAR/02 [25]. Therefore, in this study, the RPC model was utilized to establish a correlation between the geodesic coordinates of the ground point (\(P\), \(L\), \(H\)) and the corresponding image point coordinates (\(Y\), \(X\)) using Equation (1). \[ \begin{cases}Y=\frac{N_{L}(P,L,H)}{D_{L}(P,L,H)}\\ X=\frac{N_{S}(P,L,H)}{D_{S}(P,L,H)}\end{cases}