RWR

device that warns a pilot against tracking by radar or an enemy missile

Typically fitted to military aircraft, radar warning receivers (RWR) detect the radio emissions of radar systems, whether ground-based or on-board other aircraft. The system is often capable of classifying the source of the radar by type (Pulsed or continuous wave radar, for example) and strength. It may be further capable of using this data to identify the type of threat the detected radar poses, e.g. an incoming radar-guided surface-to-air missile (SAM).
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