How Passive Radar Works

Passive radar is radar that works by listening passively. It doesn't transmit anything; it detects signals that already exist in the environment. By listening to how broadcasts like FM radio and digital TV bounce off objects, it's possible to determine their positions and velocities. The result is a radar system with no transmitter, no expensive hardware, and no need for a broadcast license, unlike traditional, or "monostatic" radar. Radar's General Principles All radar relies on two core physical phenomena: the Doppler effect and signal delay . Doppler Effect and Doppler Shift When a source of waves and an observer are moving relative to each other, the observed frequency changes. An ambulance siren sounds higher-pitched as it approaches and lower-pitched as it drives away. This is the Doppler effect. Radar exploits the same principle with radio waves. When a radio signal bounces off a moving object (like an aircraft), the reflected signal's frequency shifts slightly: Object moving toward the receiver → frequency increases (positive Doppler shift aka blueshift) Object moving away → frequency decreases (negative Doppler shift aka redshift) The Doppler Effect ( Source ) The size of this shift is proportional to the object's radial velocity — how fast it's moving toward or away from the receiver. This lets radar measure an object's speed. Delay The second measurement is simpler: time. A radio signal travels at the speed of light. If a reflected signal arrives later than the dir…