Passive Radar - Principle

Principle

In a conventional radar system, the time of transmission of the pulse and the transmitted waveform are 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 from 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 objects location, heading and speed

These are described in greater detail in the sections below.