Definition
A rotating ground-based antenna that transmits interrogation signals to aircraft transponders and receives the coded replies, allowing an air traffic control radar system to determine the aircraft's identity, range, and bearing. It is part of the secondary surveillance radar (SSR) system and typically rotates in unison with, or is mounted atop, the primary radar antenna.
Plain English
The spinning antenna on the ground that sends a question to your transponder and listens for its answer, so controllers can see who you are and where you are on their screens.
Context Anchor
Seen in radar navigation and transponder discussions, especially when explaining how air traffic control radar identifies an aircraft.
Derivation
The word 'beacon' comes from the Old English 'beacn,' meaning a sign or signal. In radar terms, a 'beacon' is a device that responds to an interrogation with a coded signal, so the 'beacon antenna' is the antenna that handles those interrogations and replies.
Why Pilots Care
It enables reliable radar identification, altitude reporting, and traffic separation in instrument conditions.
Intuition Check
Do not read “beacon” here as an airport rotating light. Here, “beacon” means the aircraft’s electronic radar reply system.
Example Sentence 1
As the beacon antenna swept past, the controller saw the aircraft's data tag refresh with current altitude and squawk.
Example Sentence 2
During preflight the pilot confirmed the beacon antenna was securely mounted and free of damage before the IFR departure.