# mmSight: A Robust Millimeter-Wave Near-Field SAR Imaging Algorithm
## 4 Experiment and Evaluation
### Experimental Result
#### 4.3.1 Single Target Imaging  
(1) _Fully-Sampled Single Target_. In order to verify the feasibility of the mmSight, a single target is imaged with Fully-Sampled, the parameters are the same as those set in Section 4.2, the distance from the target plane to the array plane (\(z_{0}\)) is 110 mm, and the number of horizontal sampling points and vertical sampling points are both 41, so that the virtual aperture size is 80 mm \(\times\) 80 mm, which can cover the target of size 60 mm \(\times\) 60 mm. The optical image of the circle target is shown in Figure 6a, and the imaging results are shown in Figure 6c. It can be seen that the algorithm has high imaging quality for the circle target and accurate contour restoration. The star target optical image is shown in Figure 6b, and the imaging results are shown in Figure 6d. The star target is more complex than the circle target, and the algorithm, although the general outline is accurately restored, for the detailed part, the restoration is insufficient. In particular, the part connecting the periphery and the center is not well resolved in this part because its width is smaller than the spatial sampling interval (2 mm). In addition, there is a certain amount of spatial noise accompanying it.  
(2) _Sparsely-Sampled Single Target_. Sometimes, to quickly verify the feasibility of the algorithm, it is necessary to image in the case of Sparsely-Sampled, but this produces ghost image, and the general research mostly uses the MIMO approach to avoid ghost image by converting Sparsely-Sampled under SISO to Fully-Sampled under MIMO, but MIMO increases the complexity of the system, so it is more necessary to study the avoidance of ghost image in the case of Sparsely-Sampled under SISO. Therefore, in this paper, based on the Fully-Sampled single-target imaging experiments, the rail parameters are modified and the corresponding Sparsely-Sampled experiments are performed. Figure 7 shows the horizontal and vertical directions are both Sparsely-Sampled (horizontal sampling interval and vertical sampling interval are 4 mm, which does not satisfy the spatial sampling criterion, the number of horizontal and vertical sampling points is 21, and the virtual aperture size is 80 mm \(\times\) 80 mm, which can cover the target), where the mmSight (Figure 7a)  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Number of Steps at Horizontal Scan & scenario-dependent \\ Number of Steps at Vertical Scan & scenario-dependent \\ Horizontal Step Size (mm) & 2 \\ Vertical Step Size (mm) & 2 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Rail parameters.  
gets the imaging effect close to fully-sampled and does not produce ghost image, while the Back Projection (Figure 7b) and Matched Filter (Figure 7c) imaging algorithms produce ghost image in the horizontal and vertical directions. Figure 8 shows the case of Sparsely-Sampled in the vertical direction only (horizontal sampling interval is 2 mm, vertical sampling interval is 4 mm, the number of horizontal sampling points is 41, the number of vertical sampling points is 21, the virtual aperture size is 80 mm \(\times\) 80 mm, which can cover the target), where the mmSight (Figure 8a) obtained the imaging effect of nearly Fully-Sampled, which did not produce ghost image, and the Back Projection (Figure 8b), and Matched Filter (Figure 8c) imaging algorithms produced ghost image only in the vertical direction. In the case of Sparsely-Sampled, the mmSight is based on the Analytic Fourier Transform, compared to the Back Projection, Matched Filter, which effectively avoids the generation of ghost image.