Definition
The ground-based equipment in a Secondary Surveillance Radar (SSR) system that transmits coded interrogation signals to aircraft transponders and receives the coded reply signals sent back. The replies are decoded to display the aircraft's identity (squawk code) and, when Mode C or Mode S is used, altitude information on the controller's radar scope.
Plain English
It is the ground unit that sends out a question signal to the aircraft and listens for the answer that the aircraft's transponder sends back. That answer tells the controller which aircraft is which and how high it is flying.
Context Anchor
Seen in instrument flying discussions of radar services, transponders, and how air traffic control identifies aircraft on a radar display.
Derivation
A 'beacon' is a signal source meant to be detected. In this system the ground equipment sends an interrogation and the aircraft's transponder acts as the beacon by replying — so the ground unit is described as a beacon transmitter/receiver because it both sends the prompt and receives the beacon reply.
Why Pilots Care
It allows ATC to positively identify the aircraft and receive its altitude, enabling safe separation from other traffic in controlled airspace.
Intuition Check
Do not think of this as a radar screen in the aircraft. It is the aircraft’s reply unit: it listens for a radar request and sends back a coded answer.
Example Sentence 1
The radar beacon transmitter/receiver at the ATC facility interrogates the aircraft's transponder and displays the returned code and altitude on the controller's scope.
Example Sentence 2
Before entering Class B airspace the pilot confirmed the radar beacon transmitter/receiver was set to altitude mode.