Definition
The portion of the Air Traffic Control Radar Beacon System (ATCRBS) ground equipment that translates the coded reply signals received from an aircraft's transponder into the discrete code and altitude information displayed on the controller's radar scope.
Plain English
The ground-based equipment that reads the signal coming back from your transponder and turns it into the code and altitude the controller sees on their radar screen.
Context Anchor
Seen in radar, transponder, and air traffic control surveillance discussions.
Derivation
From 'de-' (reverse) and 'code.' A decoder reverses the coding process — taking the encoded transponder reply and turning it back into readable information for the controller.
Why Pilots Care
Enables controllers to see your assigned code and altitude, allowing safe aircraft separation and traffic advisories.
Intuition Check
A decoder is not a person solving a secret message here. It is electronic equipment that translates coded aircraft replies into information air traffic control equipment can display.
Example Sentence 1
After the controller reported losing the aircraft's Mode C readout, the technicians checked the decoder at the radar facility.
Example Sentence 2
Decoder data showed the Mode C altitude next to the transponder reply.