Definition
A paired system used in air traffic control where ground-based radar sends out radio pulses to detect aircraft, and an aircraft-mounted transponder receives those pulses and sends back a coded reply that identifies the aircraft and reports its altitude. Together they allow controllers to see, identify, and track aircraft on radar displays.
Plain English
Radar is the equipment on the ground that finds aircraft using radio waves. A transponder is the small unit in the aircraft that hears the radar and answers back with a code so the controller knows which aircraft it is and how high it is flying.
Context Anchor
You see this term when studying instrument flying equipment, air traffic control surveillance, traffic displays, and weather avoidance systems.
Derivation
Radar comes from 'Radio Detection And Ranging' -- it detects objects and measures their distance using radio waves. Transponder is a blend of 'transmitter' and 'responder' -- a device that responds by transmitting when it is interrogated. Knowing this makes the system easy to picture: the ground radar interrogates, the airborne transponder responds.
Why Pilots Care
These systems allow controllers to maintain safe separation between aircraft and provide navigation assistance in instrument conditions.
Intuition Check
Do not think of radar as a camera or a transponder as a constant tracker. Radar detects by radio energy, and a transponder replies when it is asked by a compatible signal.
Example Sentence 1
The chapter on radar and transponders explains how ATC identifies each aircraft on the controller's display.
Example Sentence 2
Proper operation of radar and transponders is required when flying in controlled airspace under instrument flight rules.