How it works
A reference antenna picks up the direct signal from an FM broadcast tower, DVB-T station, or 4G cell. A surveillance antenna picks up reflections from aircraft. Cross-correlate the two with enough computing power and time-difference-of-arrival gives you target range and direction.
Why it's hard
The reflected signal is millions of times weaker than the direct one. The transmitter is uncooperative — its waveform was designed for radio, not radar. You need clean reference geometry, beefy signal processing, and patience. But 2010s GPU computing finally made it practical.
Real systems
Lockheed Silent Sentry, Selex AULOS, BAE Aurora, Czech VERA-NG. NATO members operate passive radar quietly in the Baltic and Black Sea. China and Russia have publicly fielded their own systems.
Why it matters
Anti-radiation missiles home on a radar's emissions. A passive radar emits nothing — there's no signal to home on. For a defender, this is a survivable backup when conventional radars are jammed or destroyed.