# Feasibility, Design, and Deployment Requirements of TCR for Bistatic SAR radiometric Calibration
## 6 Discussion  
After discussing the requirements of SAR radiometric calibration for calibration targets in principle, the feasibility of the TCR for bistatic SAR radiometric calibration at the X-band was investigated. Both electromagnetic simulations and SAR imaging simulation results demonstrated that the TCR can be used as a bistatic SAR calibration target for relatively small bistatic angles (less than 6\({}^{\circ}\)) if the boresight for different bistatic angles is not considered. Moreover, the bistatic angle can be extended to 20\({}^{\circ}\) by taking the change of the TCR boresight into account, which has solved the problem that the TCR is not suitable for a large bistatic angle [11]. However, the lower peak RCS will affect the SCR as the bistatic angle becomes larger. In order to design an appropriate TCR as the bistatic calibration target, the interleg dimension must be large enough to ensure the target stable  
Figure 19: The simulation results for 20\({}^{\circ}\) bistatic angle with the consideration of the TCR optimum boresight. (**a**) The point target image after upsampling 48 times; (**b**) the azimuth profile based on the results of the electromagnetic simulation. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Bistatic Angle** & **IRW (m)** & **Range PSLR** & **Range ISLR** & **Azimuth PSLR** & **Azimuth ISLR** & **Energy** \\ \hline
2\({}^{\circ}\) & 2.6041 & \(-\)13.3613 & \(-\)10.2023 & \(-\)13.2855 & \(-\)10.3628 & 49.8637 \\
6\({}^{\circ}\) & 2.0313 & \(-\)13.3295 & \(-\)10.2343 & \(-\)13.29544 & \(-\)10.2122 & 47.5656 \\
20\({}^{\circ}\) & 1.7187 & \(-\)13.3458 & \(-\)10.3923 & \(-\)13.29661 & \(-\)10.1582 & 44.3411 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Image quality analysis parameters of the TCR. Figure 18: The simulation results for 6\({}^{\circ}\) bistatic angle with the consideration of the TCR optimum boresight. (**a**) The point target image after upsampling 48 times; (**b**) the azimuth profile based on the results of the electromagnetic simulation. RCS characteristics within the optical region. Furthermore, since the SCR is closely related to the radiometric calibration accuracy, if there is to be no more than a 0.5-dB measurement error, the SCR must be at least 25 dB. Otherwise, the target deployment sites should be chosen carefully to consider the clutter characteristics. To further prove the feasibility, the quality of the bistatic SAR images were compared with typical bistatic angles as the TCR boresight is considered or not. However, if the bistatic angle is larger than 6\({}^{\circ}\), taking the TCR optimum boresight into account can improve the imaging quality and point target energy. The most important considerations when design a suitable TCR for bistatic calibration are the interplate orthogonality, the machining error of length, the pointing deviation, and the thermal deformation. Generally, the simulation results at the X-band imply that if an interplate angle deviation is less than 0.5\({}^{\circ}\), the length error is less than 1 mm, and the pointing deviation is less than 0.5\({}^{\circ}\), the resulting RCS will be within 0.5 dB of the peak value. Under the same accuracy, the requirements regarding those error factors for monostatic and bistatic calibration are different. The main motivation behind analyzing the TCR is that the bistatic RCS characteristics of the TCR are not so clear, and whether it can be used as a bistatic calibration target is questionable. Moreover, compared to active calibration targets and some passive calibration targets, the performance of TCR is that it is simple to manufacture, efficient to work, and has a relatively high bistatic RCS. Although we have analyzed how to design the TCR at the X-band, it can be easily extended to other bands that are similar to the bistatic SAR. In this paper, the feasibility of the TCR in bistatic tandem mode was validated, i.e., the mode where the bistatic angle appears in the azimuth (along track) direction.