# Passive Bistatic Ground-Based Synthetic Aperture Radar: Concept, System, and Experiment Results  
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1 Graduate School of Environmental Studies, Tohoku University, 980-8579 Sendai, Japan 1  
12 FEMTO-ST, Time & Frequency Department, 25000 Besancon, France 2  
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to generate a synthetic aperture. With respect to the transmitting signal, frequency-modulated continuous waveform (FMCW) or stepped-frequency continuous waveform (SFCW) is commonly used by GB-SAR [1]. To get a high range resolution, the frequency bandwidth of GB-SAR is typically several-hundred megahertz. The working frequency is normally within the Ku band (e.g., 17 GHz) to get a high-resolution displacement measurement. FMCW or SFCW transmitter can be achieved with an acceptable hardware requirement at the present day and GB-SAR/MIMO radar has been successfully used for various applications. However, here is a question: to further reduce its cost, improve its system flexibility, and obtain different scattering properties of the target, can we design a GB-SAR system without a dedicated transmitter, while, at the same time, its capabilities of continuous illumination, short revisiting period, high spatial resolution, and high accuracy in displacement estimation are not sacrificed? In this study, we try to answer this question by describing a passive bistatic GB-SAR (PB-GB-SAR) system, whose practical imaging and displacement estimation performance has been validated by field experiments. The core idea of PB-GB-SAR is to exploit an existing illuminator of opportunity (IO) to replace the GB-SAR transmitter. Actually, using the non-cooperative IO signal (e.g., communication signal, broadcasting signal, or navigation signal) for passive bistatic radar (PBR) applications has been thoroughly demonstrated in the last decades [22; 23]. Different techniques have been developed for PBR, such as moving target range-Doppler mapping and tracking, inverse SAR imaging, ground/maritime moving target indication, and passive bistatic SAR imaging [24; 25; 26; 27; 28; 29; 30; 31; 32]. Compared to conventional active radar, several advantages can be achieved by PBR, such as no need for frequency allocation, smaller vulnerability, higher flexibility, and lower cost. Therefore, PBR has been viewed as an effective complementary technique to active radar and has been extensively developed for various applications, such as harbor protection, traffic density monitoring, through the wall moving target detection, and coherent change detection [33; 34; 35; 36]. For PBR, the commonly used IOs can be summarized as [23] (a) terrestrial or satellite communication system, such as 4G long term evolution (LTE), Wireless Fidelity (WiFi), and Globalstar signal; (b) terrestrial or satellite broadcast system, such as FM radio, digital audio broadcast (DAB), and digital television signal; (c) global navigation signal system, such as GPS, GLONASS, Galileo, and BeiDou; and (d) other terrestrial or spaceborne radar systems used for remote sensing applications, such as TerraSAR-X. Among these IO signals, a suitable one should be selected for GB-SAR applications. To satisfy the requirements of wide bandwidth, continuous illumination, and high-accuracy displacement estimation, continuous digital TV signal broadcast from a geostationary satellite is used in this study. Specifically, Japan communication satellite (CS) digital TV signal is used for PB-GB-SAR. Compared to other existing commercial signals (such as FM, DAB, and GNSS), satellite digital TV signal has a wider bandwidth, and multiple TV channels from different transponders can be combined to get a submeter range resolution. Compared to the PBR systems using the signal transmitted from other radar systems, its geostationary orbit provides the capability to continuously monitor a nationwide region.