## 4 Experimental Results
### Equivalent Experiment with GF-3 Transmitter and Stationary Receiver  
To further test the performance of the proposed method, a preliminary equivalent bistatic SAR experiment is conducted, which mainly aims at simulating the echo characteristics of sliding-spotlight GEO-LEO BiSAR. It should be noted that other aspects of GEO-LEO BiSAR, including ionospheric interference and illuminating power, etc., are not considered in the equivalent experiment. In the equivalent experiment, China's C-band GF-3 LEO-SAR satellite was utilized as the transmitter, which works in sliding-spotlight mode. A stationary receiver was mounted on the top of a mountain, with an additional pair of antennae and corresponding channels for direct signal synchronization. The system and platform parameters of the bistatic SAR experiment are given in Table 4. The experiment was conducted in October 2020 in Zhoushan City, China. The total data take duration is 6 s in sliding-spotlight mode, corresponding to theoretical 2-D resolution of 0.91 m in azimuth direction and 0.76 m in range direction, respectively. The equivalence of the experiment with GEO-LEO BiSAR regarding echo characteristics is analyzed in three aspects, where the same system parameters are assumed. First, due to the lack of operating GEO-SAR system, a stationary platform is used to approximate the motion characteristics of a GEO-SAR transmitter. The angular speed of a GEO-SAR is generally quite small due to the high-orbit altitude [6]. The relative motion between GEO-SAR and the target scene is slight during several seconds of synthetic aperture time. Therefore, the stationary platform can approximate the motion of GEO-SAR without significant error  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Target Index**} & \multicolumn{3}{c}{**Azimuth**} & \multicolumn{3}{c}{**Range**} \\ \cline{2-7}  & \(\mathbf{\rho_{a}}\) **(m)** & **PSLR (dB)** & **ISLR (dB)** & \(\mathbf{\rho_{r}}\) **(m)** & **PSLR (dB)** & **ISLR (dB)** \\ \hline
1 & 0.91 & \(-\)3.60 & \(-\)1.5 & 1.01 & \(-\)13.24 & \(-\)10.5 \\ \hline
2 & 0.78 & \(-\)5.85 & \(-\)3.5 & 1.01 & \(-\)13.24 & \(-\)10.4 \\ \hline
3 & 0.78 & \(-\  
in cases with low inclination or geostationary orbit. It should be noted that the exchange of transmitting and receiving platforms does not affect the echo characteristics. The azimuth resolution and its spatial variance of GEO-LEO BiSAR and the equivalent experiment are illustrated in Figure 10. The spatial variance is shown in a scene size of 2 km \(\times\) 2 km. It can be observed that similar azimuth resolution performance can be achieved by the equivalent BiSAR experiment compared to GEO-LEO BiSAR. The spatial variance of azimuth resolutions are linear in both \(\times\) and \(\times\) directions with approximately the same magnitude. The azimuth resolution is dependent on the rotation angles of the transmitting and receiving platforms with respect to the target scene during the synthetic aperture [6]. For a GEO-LEO bistatic SAR system, the contribution of GEO transmitter is negligible compared with that of LEO receiver within a few seconds of synthetic aperture. Therefore, the equivalent experiment can provide satisfactory approximation regarding azimuth resolution. Finally, the azimuth phase and its spatial variance of GEO-LEO BiSAR and the equivalent experiment are illustrated in Figure 11, which are deter