Wiktionary
n. A receive-only radar that uses ambient radio signals for search, tracking, surveillance, identification, guidance and mapping.
Wikipedia
Passive radar systems (also referred to as passive coherent location and passive covert radar) encompass a class of radar systems that detect and track objects by processing reflections from non-cooperative sources of illumination in the environment, such as commercial broadcast and communications signals. It is a specific case of bistatic radar, the latter also including the exploitation of cooperative and non-cooperative radar transmitters.
Usage examples of "passive radar".
She could tell from her passive radar receiver that at least one of the aircraft over the test area was reacting.
The passive radar detector indicated that the boat's own radars were not operating.
The WLR-I and WLR-II systems aboard the Valley Mistress were passive radar-detection systems--they did not require the use of radar to pick up an enemy presence.
The Air Force plane had also been sending out millimeter-wave microwave signals in an effort to raise the ROC's active-passive radar reflector.
The Burya missiles used passive radar homing devices to zero in on the early-warning radars, and once they had computed the radar's exact position, they could not miss.
One watched the hyper detection gear, the other the passive radar scans.