## 4 Results and Discussion
### Indoor Experimental Results of High-Resolution SAR Imaging  
A series of indoor SAR imaging experiments with different qualification factors are performed to further verify the effectiveness of the proposed novel space-borne high-resolution SAR system, using non-uniform hybrid sampling technology. The experimental scene is shown in Figure 8, and the satellite model is used as the imaging target. The azimuth size of the satellite is 1.5 m (including the satellite body and two solar wings), the distance between the front surface of the satellite body and that of the solar wings is about 20 cm. The transmitted and receiving antennas of the imaging system are separated and move at a constant speed along the orbit, which is laid along the azimuth direction. The experimental parameters are listed in Tables 5 and 6. The SAR imaging result using Nyquist real-time sampling by oscilloscopes (Tektronix, DPO71254C) is shown in Figure 9a. The satellite body and the front surface of the two solar wings can be clearly distinguished from the enlarged figure. The imaging results using the proposed non-uniform hybrid sampling technology with qualification factors 2, 6/5 and 2/3 are displayed in Figure 9b-d, respectively. The actual sampling rate is 2.5 GSPS, so the corresponding upper limit of non-uniform hybrid sampling pulse interval is 0.66 s, 1.09 s and 1.97 s, respectively. The imaging result in Figure 9b shows that the three front surfaces are clearly distinguished, and the imaging quality is consistent with that in Figure 9a. Similar to the simulation results, the imaging results in Figure 9c,d are obviously defocused in the azimuth direction, the front surface of the satellite body cannot be clearly distinguished, and the front surfaces of the two solar wings also become blurred. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Quality Parameters** & \(Q\) = 2/3 (\(t_{\text{hs}}\) = 50 us) & \(Q\) = 6/5 (\(t_{\text{hs}}\) = 28 us) & \(Q\) = 2 (\(t_{\text{hs}}\) = 15 us) & Nyquist Sampling \\ \hline IRW (R) & 7.00 cm & 7.10 cm & 7.10 cm & 7.50 cm \\ PSLR (R) & \(-9.1137\) dB & \(-14.8800\) dB & \(-15.7200\) dB & \(-15.4531\) dB \\ ISLR (R) & \(-13.7754\) dB & \(-15.2739\) dB & \(-15.8848\) dB & \(-16.2579\) dB \\ IRW (A) & 15.00 cm & 9.00 cm & 7.80 cm & 7.50 cm \\ PSLR (A) & \(-25.3909\) dB & \(-20.0400\) dB & \(-17.2000\) dB & \(-13.9207\) dB \\ ISLR (A) & \(-30.0375\) dB & \(-19.9563\) dB & \(-18.8613\) dB & \(-18.0941\) dB \\ \hline \hline \end{tabular}
\end{table} 
Table 4: The analysis of imaging quality for single point target with different factors. Figure 6: Imaging scene of point targets with the interval of 15 cm. Figure 8: Imaging scene of the satellite target. Figure 7: The SAR simulated results of point array targets with different Q factors: (**a**) Nyquist real-time sampling; (**b**) \(Q=2\), \(t_{\rm hs}=15\) us; (**c**) \(Q=6/5\), \(t_{\rm hs}=28\) us; (**d**) \(Q=2/3\), \(t_{\rm hs}=50\) us. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Quantification Factor** & \
\begin{tabular}{c} **Range of Hybrid Sampling** \\ **Interval (us)** \\ \end{tabular} &
\begin{tabular}{c} **Hybrid Sampling** \\ **Interval (us)** \\ \end{tabular} \\ \hline \(Q=2/3\) & \(2.20<t_{\text{hs}}<1.98\) & \(t_{\text{hs}}=1.97\) \\ \(Q=6/5\) & \(2.20<t_{\text{hs}}<1.10\) & \(t_{\text{hs}}=1.09\) \\ \(Q=2\) & \(2.20<t_{\text{hs}}<0.66\) & \(t_{\text{hs}}=0.66\) \\ \hline \hline \end{tabular}
\end{table} 
Table 6: The interval between two adjacent hybrid samplings in experiment.