Definition
A ground-based secondary surveillance radar system that interrogates aircraft transponders and displays the resulting coded replies — including aircraft identity (Mode A code) and pressure altitude (Mode C) — on the controller's radar scope. ATCRBS supplements primary radar by relying on an active reply from the aircraft rather than a reflected radio signal, producing a stronger, more reliable target with identifying information attached.
Plain English
A radar system on the ground that sends a question to the aircraft's transponder, and the transponder answers back with the aircraft's assigned code and altitude. The controller then sees the aircraft on the screen along with its identity and altitude.
Context Anchor
You encounter ATCRBS when ATC assigns you a transponder code, identifies your aircraft on radar, or uses your reported altitude while providing radar service.
Derivation
The name describes the method: a radar that listens for a 'beacon' reply from the aircraft rather than a passive echo. The word 'beacon' comes from the Old English 'beacen', meaning a signal or sign — fitting, since the transponder's reply acts as the aircraft's signal back to the ground station.
Why Pilots Care
It gives controllers precise aircraft identification and altitude, allowing safer separation and fewer voice calls for position reports.
Intuition Check
ATCRBS is not the radar screen itself. It is the question-and-answer system between ATC equipment and your aircraft transponder that adds identity and altitude information to the radar display.
Example Sentence 1
When the controller assigned squawk 4271, the transponder began replying to ATCRBS interrogations and the aircraft's data tag appeared on the controller's scope.
Example Sentence 2
ATCRBS replies let the controller track the plane accurately even when it is not visible on primary radar.