Definition
An underwater detection system that listens for sound waves emitted by other vessels or objects without transmitting any signal of its own. Because it only receives sound, it does not reveal the listener's position.
Plain English
A listening-only system used underwater to find other ships or submarines by the sounds they make, without giving away the listener's location.
Context Anchor
Seen in maritime patrol, military aviation, and helicopter operations that involve locating or tracking objects under water.
Derivation
Sonar comes from Sound Navigation And Ranging. Passive means it only receives -- it takes no action of its own to produce the signal, in contrast to active sonar which transmits a pulse and listens for the echo.
Why Pilots Care
Enables detection of submarines or underwater objects while remaining undetected, preserving the aircraft's tactical advantage.
Analogy
It is like standing quietly in a room and listening for footsteps, instead of calling out and waiting for someone to answer.
Intuition Check
Passive does not mean the system is doing nothing. Here, passive means it is detecting sound without sending out its own signal.
Example Sentence 1
The patrol aircraft dropped a field of sonobuoys and used passive sonar to track the submarine without alerting it.
Example Sentence 2
Using passive sonar, the crew monitored underwater sounds without revealing their own position.