Definition
Ground-based radar used by air traffic control to detect, locate, and track aircraft. It works by transmitting radio energy and either measuring the reflected return from the aircraft (primary radar) or interrogating the aircraft's transponder for a coded reply that includes identity and altitude (secondary radar).
Plain English
The radar systems on the ground that controllers use to see where aircraft are. Some types just bounce a signal off the aircraft; others ask the aircraft's transponder to send back its identity and altitude.
Context Anchor
Seen in Automatic Dependent Surveillance–Broadcast (ADS-B) discussions when comparing ADS-B position reports with traditional radar used by air traffic control.
Derivation
Radar comes from 'RAdio Detection And Ranging.' It detects objects by sending out radio waves and timing how long the echo takes to return. 'Air-traffic radar' simply applies that technology to tracking aircraft.
Why Pilots Care
Most ATC services -- separation, traffic advisories, vectors -- depend on the controller seeing the aircraft on radar. Knowing whether you are in radar coverage affects what services you can expect and how you communicate with ATC.
Intuition Check
Do not think of air-traffic radar as a radar screen in the airplane. Here it means radar used by air traffic control to monitor aircraft from the ground.
Example Sentence 1
Once clear of the mountains, the controller picked us up on air-traffic radar and issued a direct routing.
Example Sentence 2
In areas without radar coverage, pilots must use other methods because air-traffic radar cannot provide continuous tracking.