Definition
An airborne radio transponder equipped with the Mode C function, which automatically reports the aircraft's pressure altitude to air traffic control radar in addition to its assigned four-digit identification code. When interrogated by a secondary surveillance radar, it replies with both the squawk code and altitude data, allowing controllers to see the aircraft's position and altitude on their radar display.
Plain English
A radio device on the aircraft that answers radar pings from air traffic control and tells them both who you are and how high you are flying.
Context Anchor
Seen in departure procedures, airspace requirements, and air traffic control instructions when altitude reporting is required for radar monitoring and separation.
Derivation
Transponder combines 'transmitter' and 'responder' — a device that responds by transmitting when interrogated. 'Mode C' is simply the third operating mode defined for these systems (Modes A, C, and S), with Mode C being the one that adds altitude reporting to the basic identification reply.
Why Pilots Care
It lets controllers see your altitude on their radar display so they can maintain safe vertical separation between aircraft.
Intuition Check
Mode C is not a navigation mode or a radio communication setting. Here, it means the transponder can automatically send altitude information to air traffic control equipment.
Example Sentence 1
Before departure, the controller asked us to confirm our transponder with Mode C was operating so they could verify our altitude on radar.
Example Sentence 2
Before entering Class B airspace the pilot verified the transponder with Mode C was squawking the assigned code.