# Adaptive 3D Imaging for Moving Targets Based on a SIMO InISAR Imaging System in 0.2 THz Band
## 5 Experiments
### Target Tracking and Imaging  
In the experiments, the targets were initially located in the left terminal of the sliding rail and move to the right terminal with a constant velocity of 0.052 m/s, with its  
Figure 8: The painted plane model used in the experiments. Figure 7: The experiment set-up in laboratory environment. coordinates varying from (-1.0 m, 4.5 m, 0) to (1.0 m, 4.5 m, 0). In consideration of both integration gain and tracking efficiency, the step-tracking period was set as 0.1 s, which means a number of 25 echoes are processed in each period. Consequently, the total tracking duration was about 38.5 s and 385 step-tracking periods were experienced, corresponding to 385 tracking records. Moreover, the actual positions of the targets during movement could be obtained from the parameters of sliding rail, which provided a reference for the evaluation of tracking accuracy. The parameters of the system and targets in the experiments are listed in Table 3.  
Figure 9 presents the tracking results of the plane model. Both the tracking result of azimuth angle and its comparison to the estimated true angle curve are shown in the figure. It's seen from the curves that the tracked angular curve matches well with the real one, revealing a high stability of the tracking method. The estimated angular root mean square error (ARMSE) was 4.18 mrad, which satisfies the requirement of accurate tracking for imaging. In post-processing, the recorded echoes from the moving targets during tracking were processed with the imaging method introduced in the above section. Figure 10 displays the ISAR imaging results of the plane model. Figure 10 presents the image produced by the virtual sum signal. The imaging time window is in the middle of the whole tracking duration. A number of 4096 echoes were processed, corresponding to an integration time of 16.384 s. The ISAR image is well-focused, which clearly shows the target shape and size information. Next, the ISAR imaging for each receiving channel was performed using the same data set. Figure 10b-d show the ISAR images of the three channels (Rx1, Rx2, and Rx4) after the combined motion compensation processing. It can be seen that all of the three channels can image the moving targets with high quality. The interferometric imaging results are shown in Figure 11. Figure 11a-d display the 3D view, \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Symbol** & **Value** \\ \hline Carrier frequency & \(f_{c}\) & 0.2 THz \\ Bandwidth & \(B\) & 15 GHz \\ Chirp period & \(T_{p}\) & 4 ms \\ IF sampling frequency & \(f_{s}\) & 1.024 MHz \\ Baseline length & \(L_{x},L_{z}\) & 5 mm \\ Target size & — & 25 cm \(\times\) 20 cm \\ Target range & — & 4.5 m \\ Target velocity & \((V_{x},V_{y},V_{z})\) & (0.052, 0, 0) m/s \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Parameters of the system and targets in the experiments. Figure 9: Azimuth tracking results of the plane model. plane, side view on the YOZ plane, and bird's eye view on the XOY plane, respectively. In order to guarantee the visual quality, only the pixels whose power level is higher than \(-10\) dB are screened and applied interferometry. From the 3D images, the target shape and structure can be clearly identified, which are very similar to the real model. Furthermore, the dynamic 3D imaging in different tracking periods was attempted. Another two imaging windows were selected from the whole observation, corresponding to the two periods during which the targets are moving in the left part and right part of the sliding rail. The window length was still set as 16.384 s and partially overlapped with the middle imaging window. Figure 12 displays the imaging results in the left window. The ISAR image acquired by the virtual sum signal and the 3D image are shown in Figure 12a,b, respectively. The corresponding ISAR and 3D imaging results in the right window are presented separately in Figure 13a,b.