the commercial launch of the Silent Sentry system, that exploited FM radio and analogue television transmitters. Typical illuminators Passive radar systems have been developed that exploit the following sources of illumination: Analog television signals FM radio signals Cellular phone base stations Digital audio broadcasting Digital video broadcasting Terrestrial High-definition television transmitters in North America GPS satellites (GPS reflectometry). Satellite signals have generally been found more difficult for passive radar use, either because the powers are too low or because the orbits of the satellites are such that illumination is too infrequent. However, there have been significant developments in the area over the past years. The possible exception to this is the exploitation of satellite-based radar and satellite radio systems. In 2011, researchers Barott and Butka from Embry-Riddle Aeronautical University announced results claiming success using XM Radio to detect aircraft with a low-cost ground station. https://ieeexplore.ieee.org/stamp/stamp.jsp?arnumber=6096159 Principle In a conventional radar system, the time of transmission of the pulse and the transmitted waveform is exactly known. This allows the object range to be easily calculated and for a matched filter to be used to achieve an optimal signal-to-noise ratio in the receiver. A passive radar does not have this information directly and hence must use a dedicated receiver channel (known as the "reference channel") to monitor each transmitter being exploited, and dynamically sample the transmitted waveform. A passive radar typically employs the following processing steps: Reception of the direct signal from the transmitter(s) and the surveillance region on dedicated low-noise, linear, digital receivers Digital beamforming to determine the direction of arrival of signals and spatial rejection of strong in-band interference Adaptive filtering to cancel any unwanted direct signal returns in the surveillance channel(s) Transmitter-specific signal conditioning Cross-correlation of the reference channel with the surveillance channels to determine object bistatic range and Doppler Detection using constant false alarm rate (CFAR) scheme Association and tracking of object returns in range/Doppler space, known as "line tracking" Association and fusion of line tracks from each transmitter to form the final estimate of an object's location, heading and speed. These are described in greater detail in the sections below. Processing Receiver system A passive radar system must detect very small target returns in the presence of very strong, continuous interference. This contrasts with a conventional radar, which listens for echoes during the periods of silence in between each pulse transmission. As a result, the receiver must