# mmSight: A Robust Millimeter-Wave Near-Field SAR Imaging Algorithm
## 4 Experiment and Evaluation
### Experimental Result
#### 4.3.3 Multiple Targets Imaging  
(1) _Same-Distance Multiple Targets_. In real scenarios, there are usually multiple targets, and making a distinction between multiple targets and imaging them separately and accurately is more complex than in a single-target scenario. In addition, more targets also require larger apertures, longer sampling times, and higher requirements for the stability of the system, making it necessary to conduct experiments related to multiple targets. Most of the parameter settings in this experiment are consistent with the first group in Section 4.3.1, but the distance (\(z_{0}\)) from the target plane to the array plane is 87 mm, the number of horizontal sampling points is 81 and the number of vertical sampling points is 41, making the virtual aperture size 160 mm \(\times\) 80 mm, which can cover two targets. The multi-targets scenario, imaging results are shown in Figure 10. From the results, it  
Figure 8: Vertical direction sparsely-sampled only: (**a**) mmSight; (**b**) Back Projection; (**c**) Matched Filter. Figure 9: Hidden target: (**a**) outside; (**b**) inside; (**c**) imaging result. can be seen that different targets are distinguished in the same distance plane. However, because the distance between the two planes (87 mm) was shortened than Section 4.3.1 Experiment one (110 mm), according to the literature [11] Equation (13), the interval of the spatial sampling criterion also became smaller, so the original sampling interval of 2 mm, does not meet the new sampling criterion requirements for the Back Projection (Figure 10b), Matched Filter (Figure 10c), results of them appeared unwanted spatial noise, while the Analytic Fourier Transform (Figure 10d) and the mmSight (Figure 10e) are unaffected and still maintain high-quality imaging results. (2) _Different-Distance Multiple Targets_. For multiple targets in the same distance plane, only a large aperture needs to be set, but for targets in different distance planes, the algorithm often focuses on a single distance plane only, resulting in poor imaging of targets in other distance planes. In this regard, the literature [16,17] proposed the maximum projection approach to integrate targets from multiple planes, but this requires imaging for each plane and finally projection, which has more steps, while the mmSight in this paper can take into account multiple planes at once, which greatly reduces the workload compared to the projection method. Most of the parameters set in this experiment are consistent with the first group of Section 4.3.3. The distance from the plane where the star target is located to the array plane is 108 mm, while the distance from the plane where the circle target is located to the array plane is 135 mm, and the difference between the planes where the two targets are located is 27 mm, as shown in Figure 11a. The final results are shown on the right side of Figure 11. The mmSight (Figure 11e) can take into account the targets of other secondary distance planes, and the result, which is close to that of the dual targets of the same distance plane, is one of its advantages over the Analytic Fourier  
Figure 10: Same-Distance multiple targets: (**a**) real scenario; (**b**) Back Projection; (**c**) Matched Filter; (**d**) Analytic Fourier Transform; (**e**) mmSight. Transform (Figure 11d). In addition, since the inter-plane distance is 108 mm, which is farther than the 87 mm distance of the previous experiment, Back Projection (Figure 11b) and Matched Filter (Figure 11c), do not appear ghost image, but the residual spatial noise of them is still more. In addition, we made a comparison with the work in literature [31; 32], as shown in Figures 12-15. Figure 12, the primary target is the metal sheet, and the secondary target is the bracket. From the results, it can be seen that mmSight images the bracket better than the other algorithms, but the projection is not as good as Figure 11e because of the large distance between the two targets, which exceeds the 27 mm set in the previous experiment.