How Passive Radar Works

Passive radar detects objects using existing radio broadcasts (FM, digital TV) instead of a dedicated transmitter. It works by comparing a direct signal from a broadcast tower with echoes reflected off objects, extracting Doppler shift (speed) and time delay (distance). Because the transmitter and receiver are in different locations (bistatic geometry), delay maps to an ellipse rather than a circle; intersecting ellipses from multiple transmitters pinpoints an object's location. Key advantages include no transmitter hardware, no broadcast license, low cost using off-the-shelf SDR hardware, and inherent covertness. Drawbacks include dependence on third-party signals, lower precision, complex signal processing (direct signal can be 60–80 dB stronger than echoes), and limited range resolution.