# Feasibility, Design, and Deployment Requirements of TCR for Bistatic SAR radiometric Calibration
## 3 Bistatic RCS Characteristics
### FEKO Electromagnetic Simulation  
FEKO is a simulation software for three-dimensional (3D) structural electromagnetic field analysis, and offers a variety of core algorithms such as the Method of Moments (MOM), Physical Optics (PO), Multilevel Fast Multipole Method (MLFMM), Finite Element Method (FEM), Uniform Theory of Diffraction (UTD), and several of the above algorithms are used together. In addition, theoretical analysis or simulation computation has been used as a standard for RCS analysis and evaluation of the calibration target. Paper [26] has concluded that FEKO's RCS is more reliable and accurate than the results of the normal anechoic chamber. So, we give the results of FEKO electromagnetic simulation. Combining the MOM with the MLFMA to calculate the RCS of TCR under different bistatic angles has the advantages of shortening the calculation time and having a high accuracy. The bistatic RCS observation geometry of TCR is shown in Figure 6, where \(\theta\) is the elevation angle, \(\varphi\) is the azimuth angle, and \(\beta\) is the bistatic angle. We get the bistatic RCS azimuth pattern at different bistatic angles with the elevation angle fixed at 54.74\({}^{\circ}\) and the bistatic angle illustrated in the azimuth. As shown in Figure 7, the bistatic RCS azimuth patterns of the TCR were simulated using the FEKO electromagnetic simulations. It can be seen that the main lobe of the bistatic RCS pattern became highly rippled and dramatically lower with the bistatic angle increasing. Furthermore, the peak RCS and beamwidth varied greatly for different bistatic angles, so even in the case of a small bistatic angle, the monostatic and bistatic RCS characteristics are different. However, the peak RCS for different  
Figure 5: The bistatic RCS azimuth pattern of the TCR based on the monostatic bistatic equivalence theorem. Figure 6: The bistatic RCS observation geometry of the TCR. bistatic angles corresponded to different azimuth angles, which was the same as the results obtained by using the monostatic bistatic equivalence theorem. By comparing the results of the FEKO electromagnetic simulations (see Figure 7) and those of the monostatic bistatic equivalence theorem (see Figure 5), it was found that the peak RCS and the beamwidth of the two results were different for the same bistatic angle. The TCR is specifically designed to be a backscatter, and the coherent addition of the waves work only in that direction. In most of the other directions, i.e., the bistatic condition, there is very little scattering, and in all of the other directions, the scatter is less than in the backscatter [24]. It has a narrow backscattering lobe, with the beamwidth of the lobe proportional to the wavelength divided by a characteristic length of the trihedral, so the TCR would be the worst case for the monostatic bistatic equivalence theorem. Therefore, it is not suitable to use the equivalence theorem to compute the bistatic RCS of TCR. For these reasons, the computation of the bistatic RCS characteristics of the TCR in this paper were based on the FEKO electromagnetic simulations. As shown in Figure 7, when the bistatic angle is larger than \(6^{\circ}\), the ripples are too large and dramatic to align the peak RCS pointing of the TCR with the SAR antenna's boresight. Therefore, the RCS becomes unpredictable, which would degrade the SAR image quality and induce radiometric calibration errors. For bistatic angles of less than \(6^{\circ}\), the bistatic RCS patterns of TCR are wide, with a beamwidth up to \(30^{\circ}\), so it satisfies the beamwidth requirement of the calibration target when the bistatic SAR is working in tandem mode. On the other hand, the peak RCS is more than 15 dB, and also satisfies the SCR requirements that were discussed in Section 2. As is widely known, the TCR boresight of the maximum RCS is \(\theta=54.74^{\circ}\), \(\varphi=45^{\circ}\) for monostatic SAR. We got the bistatic RCS azimuth patterns (Figure 7) with the elevation angle fixed at \(54.74^{\circ}\).