# Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results
Besides, the working frequency of the satellite digital TV signal is typically from 10 to 13 GHz, providing a centimeter-level wavelength and a high-resolution displacement estimation capability. Furthermore, although different standards have been adopted (for example, ISDB-S and ISDB-S3 in Japan, DVB-S and DVB-S2 in Europe, and ABS-S in China), satellite digital TV signal can be received globally, giving the potential of using PB-GB-SAR at different geographical locations for different applications. Although the basic principle of PB-GB-SAR seems simple and the properties, such as working frequency, resolution, and ambiguity function, of the geostationary satellite digital TV signal seem promising (as indicated in [37; 38; 39; 40; 41]), the real implementation of a PB-GB-SAR system for imaging and displacement estimation purposes has several problems. For example, for most PBR implementations, two channels (i.e., reference channel and surveillance channel) are inherently required to receive the illuminating signal and the target reflections. The coherence between the two channels is important to increase the signal to noise ratio (SNR) by using a long integration time or adding multiple short-term measurements together. Because commercial-off-the-shelf (COTS) hardware used for the reception of satellite digital TV signal is easily available on the market, the first problem we faced is how to have two coherent (frequency and phase synchronized) channels based on COTS antennas, low noise blocks (LNBs), and amplifiers, without building a radar system from scratch to reduce the system cost. Furthermore, since PB-GB-SAR is a bistatic system, its geometry should be considered for the following imaging and interferometric processes. Although we have established a simplified geometry in the current study, PB-GB-SAR imaging of a local area still needs further studies. Another issue related to the bistatic geometry is the different concept of Line-of-Sight (LoS) displacement with respect to conventional monostatic GB-SAR systems. The geometry-related issues are thus the second problem we faced. To solve these problems, we propose several methodologies. Firstly, we propose a synchronization method for multiple COTS LNBs with different local oscillators. The synchronization can help to increase the integration time of the reference and surveillance channels, which is important to get a sufficient SNR for target imaging. It has been observed that, under the proper conditions, the frequency/phase coherence is stable over a long period. Secondly, we modify the time-domain back-projection algorithm (BPA) and range-migration algorithm (RMA) using a simplified geometry and plane wave approximation to focus PB-GB-SAR images. Compared to BPA which is simple and easy to implement, RMA is more time-saving to image a local scene with a fine grid size and can obtain a comparable performance. At last, the formula of displacement estimation of the target imaged by PB-GB-SAR is derived. All the proposed methodologies are then validated by field experiments. The designed PB-GB-SAR system can provide a focused SAR image of different targets (such as fence, light pole, small house, and car) within a 100-meter range and the displacement of the target can be estimated with submillimeter accuracy. The remainder of this paper is organized as follows. In Section 2, the designed PB-GB-SAR system, link budget analysis, and LNB synchronization method are presented. The signal model and two imaging methods for PB-GB-SAR are presented in Section 3. In Section 4, experiment results of imaging and displacement estimation are shown. In Section 5, results are discussed and perspectives for future work are given. Finally, Section 6 concludes this paper.