## 4 Experimental Results
### Simulation Results of Sliding-Spotlight GEO-LEO BiSAR  
The simulation parameters have been listed in Table 1. The raw data of nine point targets of sliding-spotlight GEO-LEO BiSAR are generated. The positions of the nine simulated targets are shown in Figure 6. The point targets are evenly distributed in the 8 km \(\times\) 8 km imaging scene with 4 km distance between adjacent targets both in x and y directions. The imaging performance of the proposed method is compared with the RMOSE method in [24]. In order to better illustrate the performance of the competing methods, the'stop-and-go' assumption error is taken into consideration in the RMOSE method as simulated in Section 2.3. Moreover, the two-step preprocessing scheme [56] is implemented for RMOSE method to remove the azimuth aliasing in sliding-spotlight mode. The imaging results of the nine point targets processed by the proposed method and RMOSE are illustrated in Figures 7 and 8, respectively. It can be observed that all nine targets are defocused by RMOSE in Figure 8, especially in az  
Figure 5: Flowchart of the proposed imaging algorithm for GEO-LEO BiSAR in sliding-spotlight mode. Figure 6: Positions of nine targets. reason for the performance degradation of RMOSE is the range model error shown in Figure 3. The range error increases with longer synthetic aperture time and introduces significant phase error during the imaging procedure in sliding-spotlight GEO-LEO BiSAR. For the targets away from the reference point, the imaging results are further deteriorated due to the errors in 2-D Stolt mapping. On the other hand, the proposed method achieves satisfactory imaging performance for all nine targets as shown in Figure 7.  
The measured metrics of the imaging results of the proposed method and RMOSE are given in Tables 2 and 3, respectively. The theoretical values of peak sidelobe ratio (PSLR) and integrated sidelobe ratio (ISLR) are \(-13.26\) dB and \(-10.4\) dB, respectively. The 3 dB impulse response width (IRW) is \(0.68\) m in the azimuth direction and \(1.01\) m in the range direction, which are calculated based on the system parameters and bistatic configuration. It can be seen that the imaging performance metrics of the proposed method are close to the theoretical values. However, for RMOSE, the PSLR and ISLR in azimuth direction are greatly degraded with widening of azimuth IRW. As analyzed above, the two-dimensional linear spatial variation of the echo data is removed using two-dimensional Stolt interpolation. The space-invariant higher-order residual phase error is compensated for by using the higher-order phase error of the scene center. Therefore, the main error source that limits the imaging swath width of the algorithm is the residual spatial variant higher-order phase error. When the spatial variant higher-order phase term reaches \(\pi/4\), it will affect the focusing effect and introduce focusing degradation. Therefore, the imaging swath of the method in both range and azimuth will be limited. In addition, the focusing effect becomes worse as the target is further away from the scene center. The residual higher-order phase error of the simulated GEO-LEO sliding-spotlight SAR in Section 4.1 is given in Figure 9, where the simulation  
Figure 7: Imaging results of the nine point targets by the proposed method based on BiCoT-ESRM. parameters and system configuration are given in Table 1. From Figure 9, it can be seen that when the image width is within 10 km \(\times\) 10 km, the residual higher-order phase is less than \(\pi/4\), and the image quality is not affected.