Definition
The coded radio signals an aircraft's transponder transmits back to ground-based secondary surveillance radar after being interrogated. Each reply contains the assigned four-digit code (and, in Mode C or Mode S, pressure altitude and other data), allowing controllers to identify and track the aircraft on their radar display.
Plain English
When ground radar 'pings' your aircraft, your transponder answers with a short coded signal. That answer is the reply, and it tells the controller who and where you are.
Context Anchor
Seen in radar, air traffic control, and transponder discussions when explaining how a controller’s radar display receives aircraft information.
Derivation
Reply' comes from the Latin 'replicare', meaning 'to fold back' or 'answer.' The transponder is literally folding the radar's question back as an answer, which is why the flashing light on the unit is called the reply light.
Why Pilots Care
Transponder replies make the aircraft visible on ATC radar displays and to collision-avoidance systems, enabling controllers and other pilots to maintain safe separation.
Intuition Check
A transponder reply is not a voice reply from the pilot. It is an automatic electronic answer sent by the aircraft’s equipment.
Example Sentence 1
The controller noted that transponder replies from the aircraft showed an altitude 300 feet below the assigned level.
Example Sentence 2
Mode C transponder replies provide pressure altitude to ATC even when the pilot is not speaking on the radio.