# Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results
## 2 System Overview  
### PB-GB-SAR Concept  
A conventional GB-SAR system is shown in Figure 1, where the transmitter and receiver are both moved along a rail to generate a synthetic aperture. As a dedicated transmitting unit is used, the system is expensive. The purpose of this study is to replace the transmitter with an existing non-cooperative illuminator (e.g., a TV satellite) to develop a PB-GB-SAR system with only receivers. PB-GB-SAR can not only reduce the system cost but also improve the system flexibility, as illustrated in Figure 2, based on which the three-dimensional (3D) displacement vector of the target (e.g., a dam) can be obtained by coherently processing all the received signals. Moreover, low-cost and low-weight receivers can be deployed on different platforms at separated locations to observe the spatial diversity of the target scattering properties. In this study, we used the Communication Satellite (N-SAT-110) on the geostationary orbit of the 110 degrees east longitude (CS 110\({}^{\circ}\) E) for digital TV transmission (with the ISDB-S standard via right-hand and left-hand circular polarizations) as a non-cooperative illuminator. Limited by the hardware, only the right-hand circularly polarized transmitted signal is received. The frequency of the CS digital TV signal is within the Ku band, from approximately 12.27 to 12.75 GHz with 12 channels and a bandwidth of about 474.5 MHz (each channel has a bandwidth of 34.5 MHz), which can provide a maximum range resolution of 0.32 m. The spatial coverage of the CS digital TV signal is shown on the left of Figure 3 with different equivalent isotropically radiated powers (EIRPs) and the spectrum of the real-sampled CS digital TV signal is shown on the right of Figure 3, where 12 channels can be clearly observed. The imaging geometry of the PB-GB-SAR system using satellite digital TV signal is shown in the left subfigure of Figure 4, where, similar to most PBR implementations, two channels (i.e., reference and surveillance channels) are used to receive the illuminating signal and the target reflections. The reference antenna is at a fixed position and its looking angle is fine-tuned to the satellite. In such a case, the \(y\) direction can be defined by the line from the TV satellite to the reference antenna. In order to achieve a high azimuth resolution, the surveillance antenna is moved along a linear rail. Therefore, the \(x\) direction (the wavefront of the satellite digital TV signal) can be defined as perpendicular to the \(y\) direction on the plane determined by the antenna moving direction and the \(y\) direction. In practical implementations, the reference antenna can be moved together with the surveillance antenna, which  
Figure 1: Conventional ground-based synthetic aperture radar (GB-SAR) system: (**left**) the imaging geometry and (**right**) a real stepped-frequency continuous waveform (SFCW)-based GB-SAR system used for landslide monitoring [4]. Figure 3: Communication satellite (N-SAT-110) digital TV signal: (**left**) spatial coverage with different EIRPs [42] and (**right**) spectrum of the real-sampled data. Figure 2: Passive bistatic (PB)-GB-SAR with multiple receivers for 3D displacement vector estimation. Note the difference between the left and right subfigures with different numbers of reference and surveillance antennas. More configurations can be used for specific applications. can help to simplify the imaging process. The aim is to make the antenna moving direction parallel with the \(x\) axis to more easily determine the antenna position, which will be detailed in Section 3 and is shown in the right subfigure of Figure 4.  
To prove the concept with a low-cost set-up, we would like to use the COTS hardware that is easily obtainable on the market to build a PB-GB-SAR system. For the reception of satellite digital TV signal, several COTS components can be used, such as parabolic antenna, LNB, and amplifier (a.k.a. \"booster\" on the COTS market).