Airborne collision avoidance system

An aircraft-borne system that detects potential collisions with nearby aircraft and provides warnings or maneuver advisories to help prevent them.

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Traffic Collision Avoidance System: An onboard system that interrogates aircraft transponders to detect collision threats and issue traffic advisories and resolution advisories. TCAS is the best-known implementation of airborne collision avoidance, with standardized threat and maneuver logic.
Time to collision: An estimate of the time remaining before two moving objects would collide if their present motion continued. ACAS threat logic uses projected encounter timing, not merely the distance between aircraft.
ACAS X: A family of next-generation airborne collision avoidance systems using optimized threat logic and configurable surveillance inputs. ACAS X is designed to improve on conventional TCAS while supporting new aircraft and surveillance technologies.
Ground-based collision avoidance system: A system that detects aircraft conflict risks using ground-based surveillance and provides warnings or instructions. Unlike ACAS, it depends on infrastructure and surveillance available to air traffic services.
Secondary surveillance radar: A radar system that elicits coded replies from aircraft transponders to determine identity, altitude, and position. ACAS systems use transponder replies to track nearby aircraft without relying on ground radar.
Closest point of approach: The point in a projected encounter at which two moving objects would be nearest to each other. Predicted separation at closest approach helps distinguish threats from harmless crossings.
ACAS Xa: An ACAS X variant intended for commercial aircraft, using active surveillance and optimized collision-avoidance logic. Its design targets airline operations and the limitations of legacy TCAS resolution advisories.
Air traffic conflict alert: A controller-facing warning that aircraft under air traffic control may violate separation standards. Conflict alerts support controllers from the ground, whereas ACAS issues advisories directly to flight crews.
Mode S transponder: An aircraft transponder that provides selective digital replies, including aircraft identity and altitude. Mode S enables addressed surveillance and coordinated advisories between compatible aircraft.
Aircraft transponder: A radio device that replies to interrogation signals with information such as aircraft identity and altitude. Transponders provide the cooperative signals on which conventional ACAS surveillance depends.