Passive radars (PRs) have drawn the attention of the scientific community for many decades, as they offer a number of advantages over conventional active radar systems [14, 9, 6, 10, 8, 16, 2, 7]. A passive radar is intrinsically a bistatic radar, since the transmitter and the receiver are not co-located. Differently from the bistatic radar, PRs exploit illuminators of opportunity (IOs) as electromagnetic sources to illuminate targets of interest. The use of non-cooperative IOs imposes the need to use two receiving channels so as to perform the matched filter at the receiver. One channel gathers the reference signal, namely the transmitted signal, so as to have a copy of the transmitted signal at the receiver, while the other channel gathers the echoes from all the targets in the surveillance area, namely the received signal. A pictorial representation of how a PR works is depicted in Figure 5.1, where at the receiver location two antennas define both the reference channel and the surveillance channel. The bistatic geometry offers an enhanced resilience against electronic counter measure (ECM) with respect to active radars and
a solution against stealth technology, which is designed primarily to defeat monostatic radars.