How does a passive radar system detect targets without transmitting its own signal?

A passive radar system detects targets without transmitting its own signal by using existing radio frequency transmissions (called illuminators of opportunity) as the radar illumination source. The system uses two or more receive channels: a reference channel that receives the direct signal from the transmitter, and one or more surveillance channels that receive the same signal after it has been reflected from targets. By correlating the reference and surveillance signals, the system measures the time difference (bistatic range) and frequency difference (bistatic Doppler) between the direct and reflected paths, determining target position and velocity. Common illuminators include FM radio broadcasts (88-108 MHz), digital television (DVB-T, 470-790 MHz), cellular base stations (700 MHz-2.6 GHz), WiFi, and DAB radio. The primary advantages of passive radar are that it emits no RF energy (making it inherently covert and immune to anti-radiation missiles), it requires no allocated spectrum, and it can be deployed with relatively simple, low-cost receive-only hardware. The primary limitations are that detection performance depends on the power and location of available illuminators, spatial resolution is typically poorer than active radar, and the signal processing is computationally intensive.