# Feasibility Study of Passive Bistatic Radar Based on Phased Array Radar Signals
## 1 Introduction  
A passive bistatic radar (PBR) system performs target detection and localization by exploiting noncooperative illuminators of opportunity. PBR offers many advantages over conventional monostatic radar systems, including lower cost, harder to detect, higher immunity to electronic countermeasures, no requirement for frequency allocation, and the ability to counter stealth targets due to the bistatic configuration of PBR systems. Therefore, many kinds of civil illuminators have been exploited by PBR systems, such as FM radio [1], Integrated Services Digital Broadcasting-Terrestrial (ISDB-T) [2], Digital Video Broadcasting-Terrestrial (DVB-T) [3; 4; 5], Long-Term Evolution (LTE) [6], Wireless Fidelity (Wi-Fi) [7], and Global Navigation Satellite System (GNSS) [8]. However, those communications signals are not designed for use in radar, which may cause ambiguities due to the signal structure [9], and the maximum detectable range may be small due to the limited transmitting power. Compared with civil illuminators, a dedicated radar transmitter usually transmits signals with a more ideal ambiguity function and higher transmitting power. German passive radar system Klein Heidelberg [10] was the first PBR system based on existing hostile radar, and in recent years, some PBR systems which utilize existing radar as their transmitter have been proven to perform target detection successfully [11; 12; 13]. Furthermore, phased array radar (PAR) has the advantages of strongertransmitting power and higher reliability. However, the complexity of PAR signals poses unique challenges to the signal processing of a PBR system, so PBR systems based on PAR signals still remains a broad area of research. The exploited illuminator of our PBR system is a high-power air surveillance PAR, which is defined as cooperative but non-dedicated. It is cooperative in the sense that the information about signal waveform is available but is considered non-dedicated as its operations is solely for its own monostatic radar purposes and that no changes are applied to enhance PBR capabilities. The PAR adopts agile radar parameters to improve the anti-jamming ability. Liner frequency modulated (LFM) waveform is used as a transmitting signal. The PAR transmits a group of signals with the same carrier frequency (CF) and bandwidth (BW) for seconds; the CF and BW change randomly among different groups, while the pulse width (PW) and pulse repetition interval (PRI) are different among different pulses in each pulse group. The agility of the waveform parameters of transmitting signal brings two problems to the signal processing of our PBR system. The first problem is the time and frequency synchronization of the system, and the second problem is the coherent integration for moving target detection and weak target detection. For the first problem, we use a reference channel to receive the direct wave signal and to estimate the parameters of the transmitted signal in real time to achieve time and frequency synchronization of the system. The parameters we need to estimate include BW, PW, CF, and time of arrival (TOA). To solve the problem of LFM signal detection and parameter estimation, many literatures have proposed many algorithms for different applications, such as Wigner-Hough transform [14], Lv's distribution [15], and Ensemble Empirical Mode Decomposition-Fractional Fourier Transform (EEMD-FRFT) [16]. [PERSON] proposed a method based on generalized Radon Fourier transform for the parameter estimation of direct wave signal [17], but it requires a lot of searching and calculation, which cannot be realized in real time. Since the PAR transmits a deterministic signal and the parameter template library of the signal waveform has been established in advance, we propose a direct wave parameter estimation method based on template matching to realize the time and frequency synchronization of the PBR system.