A passive radar (passive coherent location, PCL) transmits nothing at all itself. Instead it uses existing third-party transmitters — FM broadcast, DAB/DVB-T transmitters, mobile phone masts — as "illumination". The system receives two things: the direct signal from the broadcast transmitter (the reference) and the reflection of that signal off a target. By comparing the two (travel time difference, Doppler shift, direction) the processor calculates the target's position and velocity.
Why is this militarily interesting?
- Invisible and not jammable in the classic way: there is no emission of its own to detect, locate or attack with an anti-radiation missile. The radar cannot be "taken out" by finding it.
- Low frequencies: broadcast transmitters operate at VHF/UHF — wavelengths against which stealth shaping works relatively poorly.
- Cheap and mobile: the "transmitter" is already there and someone else pays for it.
There are limitations too: performance depends on transmitters you do not control yourself (coverage, power, waveform), the accuracy is coarser than that of your own active radar, and the signal processing is heavy. Passive radar is therefore mainly a complementary sensor: a silent early warner that tells active radars where to look (briefly).
Passive radar ≠ ESM. An ESM system listens to the target's own emissions (its radar, its radio). A passive radar detects reflections of someone else's transmitters off the target — the target itself need not transmit anything at all.