# Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results
## 4 Experiment Results  
In this section, experiment results are presented to assess the performance of the developed PB-GB-SAR system and proposed methods.  
At the beginning, a simple target, i.e., a metallic plate at near range, as shown in Figure 14, is imaged by PB-GB-SAR. The synthetic aperture length is 1.2 m with a step of 5 mm, the integration time is 100 us, and the processing time is 1 us. Based on Equations (25) and (32), SAR images of the metallic plate obtained by BPA and RMA are shown in Figure 15. It can be seen that the target can be well focused, while strong artifacts are generated in the range direction. Although these two methods can get similar results, the computational complexity of RMA is smaller than BPA (the desired computing accuracy of type-I and type-II NUFFTs is set to be 10\({}^{-12}\)). RMA requires only \(\sim\)0.79 s (measured by the TIC and TOC instruction in MATLAB on a Core i5, 2.5 GHz, and 8 GB RAM PC), while, although the grid size has been reduced 8-fold (2 times in the \(x\) direction and 4 times in the \(y\) direction), BPA still needs \(\sim\)8.54 s to get the SAR image, indicating the efficiency of RMA in such a case.  
Figure 14: PB-GB-SAR imaging of a metallic plate: The plate angle is tuned to make its reflection maximal and the antenna moving direction is adjusted to be parallel with the wavefront of the TV signal.  
Figure 15: Normalized focused SAR images of the metallic plate obtained by (**left**) BPA and (**right**) RMA.  
To assess the displacement estimation capability of the PB-GB-SAR system, the metallic plate is moved along a positioner from 1 mm to 15 mm with a step of 1 mm. The accumulated displacements estimated using the BPA and RMA-focused SAR images are shown in Figure 16. Since the angle of the metallic plate is close to zero, i.e., \(\theta\approx 0\), as shown by the imaging result, its displacement along the LoS direction is about \(\delta r=\lambda\Delta\sigma_{\text{D-}In}(x,y)/4\pi\), as given in Equation (35). It can be observed that both methods can accurately estimate the displacement: the root mean square errors (RMSEs) are 0.264 mm and 0.268 mm for BPA and RMA, respectively.  
To show the performance of the PB-GB-SAR system for imaging natural and man-made targets at short and middle range, a further experiment was conducted, as shown in Figure 17. Using the same 100 \(\upmu\)s integration time, the processing time is increased to 4 \(\upmu\)s due to the larger detection distance. To focus SAR images, BPA is used instead of RMA, as a lot of zero-padded samples in the azimuth direction make RMA significantly time-consuming, while BPA can use a larger grid size to reduce the computational complexity. The resulting image is shown in Figure 18, where the fence, light pole, tree, and small house can be imaged, as clearly shown in the right subfigure. Furthermore, the image in the left subfigure shows two cars located at approximately 60 m and 65 m in the parking lot, as indicated by the rectangle. By overlaying the SAR image with the aerial picture provided by Google maps, it can be seen that the obtained SAR image matches well with the real scene, as shown in Figure 19.  
Figure 16: Displacement estimation of the metallic plate sledded on the rail.  
Figure 17: PB-GB-SAR imaging of natural and man-made targets: The green solid line indicates a fence, the dotted rectangle indicates a light pole, the yellow ellipse includes a tree and a small house, and the red dotted line indicates some bicycles.