## 5 Conclusions  
In this paper, a novel accurate range model, i.e., BiCoT-ESRM, is proposed for GEO-LEO BiSAR. The BiCoT-ESRM takes into consideration the curved trajectory of LEO receiver and'stop-and-go' assumption error in GEO-LEO BiSAR. It can provide high-precision approximation for the range history of GEO-LEO BiSAR within long synthetic aperture. Then, based on BiCoT-ESRM, a 2-D frequency imaging method is put forward. First, the two-step method is modified to deal with the Doppler aliasing caused by beam steering and spatial variance of Doppler centroid. Then, an azimuth trajectory scaling is introduced to resample the platform movement to a uniform linear motion and remove the azimuth variance of motion parameters. Finally, a modified 2-D \(\omega-k\) method is derived for the accurate focusing of sliding-spotlight GEO-LEO BiSAR. The proposed method is tested both in simulated GEO-LEO BiSAR data and real data from an equivalent BiSAR experiment. Imaging results validate the effectiveness of the proposed range model and imaging algorithm. Based on the aforementioned analysis, the BoCoT-ESRM can precisely represent the range history of GEO-LEO BiSAR, which can be applied to most of the imaging modes and configurations. However, the proposed imaging method is designed for sliding-spotlight GEO-LEO BiSAR, which generally cannot be applied to other modes and bistatic configurations, such as  
Figure 14: Comparison of azimuth and range profiles for the selected isolated strong target. (**a**) Azimuth profile of the proposed method. (**b**) Range profile of the proposed method. (**c**) Azimuth profile of the ‘non-stop-and-go’ RMOSE method. (**d**) Range profile of the ‘non-stop-and-go’ RMOSE method.  
Figure 13: Imaging result of the equivalent experiment by ‘non-stop-and-go’ RMOSE method.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Method**} & \multicolumn{3}{c}{**Azimuth**} & \multicolumn{3}{c}{**Range**} \\ \cline{2-7}  & \(\rho_{a}\) **(m)** & **PSLR (dB)** & **ISLR (dB)** & \(\rho_{r}\) **(m)** & **PSLR (dB)** & **ISLR (dB)** \\ \hline proposed method & \(0.99\) & \(-14.97\) & \(-9.68\) & \(0.81\) & \(-9.25\) & \(-7.28\) \\ ‘non-stop-and-go’ RMOSE method & \(1.67\) & \(-13.95\) & \(-8.99\) & \(0.83\) & \(-11.07\) & \(-7.08\) \\ \hline \hline \end{tabular}
\end{table}
Table 5: Comparison of focusing results between proposed and ‘non-stop-and-go’ RMOSE method.  
stripmap or scan SAR. The development of imaging methods for other modes will be our future work.  
[PERSON] put forward the main idea, designed the experiment and wrote the manuscript. [PERSON], [PERSON], [PERSON], [PERSON] ([PERSON]), [PERSON] ([PERSON]), [PERSON] and [PERSON] participated in the experiment. [PERSON] participated in the revision of the manuscript. All authors have read and agreed to the published version of the manuscript.  
This work was supported in part by the National Natural Science Foundation of China under Grant 61901088, Grant 61922023, Grant 61771113, and Grant 61801099, in part by the Postdoctoral Innovation Talent Support Program under Grant BX20180059, and in part by the China Postdoctoral Science Foundation under Grant 2020M683292, 2019M65338 and Grant 2019 TQ0052.  
Not applicable.  
The authors declare no conflict of interest.