# Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results
## 5 Discussions
### Improvement Directions  
Although the performance of BPA and RMA based PB-GB-SAR imaging has been validated by experiments, they are not suitable to image a large observation area with fine grid size because of the high computational cost. Since the focus of this paper is on short-range and middle range targets, the computing burden of BPA or RMA is acceptable and thus other fast algorithms are not introduced. If a large scene is to be measured, some approximations can be used to derive faster imaging methods. For example, similar to the methods in [13; 56], based on the far-field approximation and the condition that the synthetic aperture length is comparable to the range resolution, 2D FFT can be used to get the focused SAR image of the target. Besides, some limitations of the developed PB-GB-SAR system are low SNR (and thus small coverage), narrow observation angle, long data acquisition period, and simple imaging geometry. The current implementation of PB-GB-SAR only uses COTS hardware to prove the concept. Dedicated hardware components, such as surveillance antenna with a wider beamwidth and high-quality low noise amplifiers should be used to improve the SNR and broaden the imaging angle. Since a digital oscilloscope is used to sample the data, which has a limited memory depth, an integration time longer than 100 us impossible by a single measurement. Besides, because of the data transmission from the oscilloscope to the PC, the data acquisition time should be further reduced. Therefore, more proper integrated digital data sampling platform should be used to increase the integration gain of matched filtering and increase the data acquisition rate. As a simplified imaging geometry is adopted in the current experiment, the potentials of PB-GB-SAR have not been fully exploited. Although it is a bistatic system, the obtained scattering properties of the imaging scene by the current PB-GB-SAR system is similar to the case of the conventional monostatic GB-SAR system. Therefore, more advanced  
Figure 20: **(Left)** SAR image of the metallic plate obtained by RMA after frequency gap filling and (right) comparison between the results with and without gaps filling in the \(y\) direction. geometries should be adopted to get different bistatic and monostatic SAR images of the observation scene. Moreover, the system should not be limited on the ground; the low-cost and light receivers can be mounted on different platforms, such as an airship [57], to measure different displacement components of the targets. Actually, the LNB synchronization circuit can provide three coherent channels for measurement, as shown in Figure 8, while only two of them are used in the current setup. More synchronized receivers can be employed to generate a high-resolution digital elevation model (DEM) of the observation scene by setting a proper spatial baseline. Moreover, since both the right-hand and left-hand circular polarizations have been employed by the satellite digital TV broadcasting in Japan, a PB-GB-SAR system that can measure different polarizations should be employed to have a better understanding of the imaging scene, like that in GB-SAR [19; 20; 21]. Furthermore, in the derivation of the displacement estimation formula in Equation (35), the assumption of \"the target only has the displacement along the LoS direction of the surveillance antenna\" has been used. If this assumption is not satisfied, only the two-way displacement can be estimated, given by  
\[D=\delta y+\delta r=\delta y(1+\cos\theta)+\delta x\sin\theta \tag{42}\]  
In such a case, with a single surveillance antenna, the target displacement along the \(x\) and \(y\) directions cannot be derived. To solve this problem, as indicated by Figure 21, two surveillance antennas can be used to get the 2D displacement vector of the target as  
\[\left\{