6.1 INTRODUCTION

As outlined in Chapter 1, passive bistatic radars ( PBRs) are a subset of bistatic radars that exploit nonradar transmitters of opportunity as their sources of radar illumination. They are often designed for military or civil air surveillance. When more than one transmitter is simultaneously exploited, the configuration becomes multistatic. In this case, measurements from transmit-receive pairs with overlapping coverage can be combined to locate a target usually via multilateration, for example, by determining the intersection of isorange contours generated by each pair. One transmitter operating with multiple receivers is also multistatic and can be used for multilateration.

The most common transmitters exploited by PBRs are terrestrial broadcast transmitters used by TV and FM stations. Other potential transmitters include satellite TV broadcasts, HF radio broadcasts, and even cell phone communication networks. Communications and navigation satellite transmitters have also been considered. These transmitters can be either cooperative (allied or friendly) or noncooperative (hostile or neutral), depending on the scenario of interest to the PBR. In either case, the transmitter has no knowledge that it is being exploited. The PBR is just another receiver in its net. This feature is one of the operational benefits of a PBR.

These exploited transmitters are characterized by broad, fixed, transmit antenna coverage and CW operation, using either analog or digital modulation dedicated to information transmission. These characteristics distinguish the PBR from hitchhikers, which exploit monostatic...