Definition 1 of 2
Definition
A function of certain ATC automated radar systems that alerts the controller to an observed Mode C transponder-equipped aircraft (or non-transponder-equipped aircraft, if its altitude is known) that is predicted to enter, or is currently within, the airspace surrounding an aircraft to which ATC is providing service, and whose path appears to be in conflict.
Plain English
An automatic warning that flashes up on a controller's radar screen when another aircraft -- one whose altitude the system can see -- is getting close to an aircraft the controller is working, in a way that could cause a conflict.
Context Anchor
Encountered in ATC radar services, traffic advisories, and discussions of how controllers detect possible traffic conflicts.
Derivation
Mode C' refers to the transponder mode that reports an aircraft's pressure altitude back to ATC radar. 'Intruder' here means an aircraft entering the protected airspace around the one being worked. Knowing 'Mode C' is the altitude-reporting mode explains why the alert works: the system needs altitude data to predict a real conflict, not just lateral proximity.
Why Pilots Care
Prompts immediate visual search and possible altitude or heading change to maintain separation from the other aircraft.
Grounding Statement
The key idea is that ATC automation can warn the controller when an unknown altitude-reporting aircraft may become a traffic conflict.
Intuition Check
“Intruder” does not mean hostile or illegal aircraft here. It means another radar target, often simply unidentified, that may be close enough to matter for traffic separation.
Example Sentence 1
The controller's Mode C Intruder Alert activated, prompting an immediate traffic call to the inbound Cessna.
Example Sentence 2
The pilot scanned outside after receiving a Mode C Intruder Alert on the traffic display.