# High-Resolution and Wide-Swath 3D Imaging for Urban Areas Based on Distributed Spaceborne SAR
## 3 Experiments and Results
### The Results of Range Ambiguity Resolution and HRWS 3D Imaging  
The SAR image with range ambiguity is shown in Figure 9. It can be seen that the far and near scenes overlap obviously. Meanwhile, the road and the buildings overlap, seriously affecting the subsequent 3D reconstruction process. In SAR imaging, the \"far range\" and \"near range\" refer to the two ends of the range direction in the SAR image. The far range corresponds to the region in the SAR image that is farthest away from the radar sensor or the imaging platform. It represents the maximum distance at which the radar signal has traveled and interacted with the target. The far range is typically depicted at the top or the right side of the SAR image. On the other hand, the near range represents the region in the SAR image that is closest to the radar sensor or the imaging platform. It corresponds to the minimum distance at which the radar signal has interacted with the target. The near range is usually shown at the bottom or the left side of the SAR image. The near and far ranges in SAR imaging are determined by the radar system's range resolution, which is the ability to distinguish between two targets located at different distances. The range resolution is determined by the pulse length of the radar signal, and it affects the level of detail and clarity in the SAR image. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Center frequency & \(f_{c}\) & 10 GHz \\ Bandwidth & \(B_{ov}\) & 500 MHz \\ Channel number & \(N\) & 14 \\ Baseline interval & \(b\) & 0.2 m \\ The horizontal inclination of & \(\beta\) & 0 deg \\ the baseline & \(\beta\) & 0 deg \\ Flight height & \(H\) & 3.5 km \\ Central incidence angle & \(\theta_{c}\) & 35 deg \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main parameters of the system. Figure 9: The SAR image of the study area with range ambiguity. As described in Section 2.3.1, the range ambiguity is mainly caused by the high PRF of the radar-transmitted pulse. In the process of spaceborne SAR, the echo signal of its transmitted pulse is generally received by the receiving radar after N-pulse repetition cycles are delayed. Considering the impact of the curvature of the earth, the time dispersion of useful echo signals in the observation zone may exceed the interval between pulses. This will cause the echo signals of the previously transmitted pulse or several transmitted pulses to overlap with the echo signals of the current transmitted pulse in time and result in interference with the echo signals of the current transmitted pulse, thus causing ambiguity. Reflected in the SAR image, the far and near target scenes are mixed, and the azimuth defocus will occur due to the different modulation frequencies of different range units. The method proposed in this paper is used to separate the far and near target scenes so that the received echo points to the real position and range ambiguity resolution is achieved. Four typical areas of ABCD are shown in Figure 9, and the corresponding results are illustrated in Figures 10-13 in turn, where (a) shows the image with range ambiguity, and the far and near scenes overlap; (b) and (c) show the near and far scene after range ambiguity, respectively; (d) and (e) show the HRWS 3D imaging results of (b) and (c), respectively. It can be seen from Figures 10a, 11a, 12a and 13a that there is significant azimuth defocusing, so azimuth refocusing is necessary. Then, a detailed analysis is performed on area B. Taking channel 1 and channel 2 as examples for analysis, the interferometric phase before range ambiguity resolution is shown in Figure 14a. The range ambiguity of the ambiguous image is calculated, and the weight of the areas with low ambiguity is increased. The flat ground with low ambiguity can be adopted as the registration point. After coarse image registration, according to the proposed method, the DOA of the target scene corresponding to each range moment is estimated by the echo of each range unit.