Definition
Ground-based or airborne radio transmitters that send out a signal used by air traffic control radar or aircraft equipment to identify, track, or locate an aircraft or a fixed point. In the radar/transponder context, beacon transmitters refer to the components in the secondary surveillance radar system and the aircraft transponder that emit the coded reply signals controllers use to identify and track each aircraft.
Plain English
Radio devices that send out a signal so that controllers or aircraft systems can find and identify something — usually an aircraft or a navigation point.
Context Anchor
Seen in instrument flying discussions of radar, transponders, and how air traffic control identifies aircraft.
Derivation
Beacon comes from the Old English beacen, meaning a sign or signal — originally a fire lit on a hill to warn or guide. A transmitter is something that sends. Together: a device that sends a guiding signal. The original idea of a hilltop fire guiding travellers maps cleanly onto a radio device guiding controllers to your aircraft.
Why Pilots Care
They enable air traffic control to identify specific aircraft and receive altitude and other data even when primary radar returns are weak or absent.
Intuition Check
Do not read “beacon transmitter” as an airport rotating light or an emergency locator beacon. In this context, it means a ground radio transmitter that asks aircraft transponders for a reply.
Example Sentence 1
The aircraft's beacon transmitter replied to the ground radar's interrogation, allowing the controller to see the flight's identity and altitude on the scope.
Example Sentence 2
Secondary radar depends on beacon transmitters to supply detailed flight data beyond basic position returns.