There are two advanced airborne collision avoidance systems: TCAS II and ACAS II.

The Traffic Alert and Collision Avoidance System (TCAS II) operates software
level 6.04.
The Airborne Collision Avoidance System (ACAS II) operates software level 7
(Change 7) and is mandatory in Europe.
The software difference for the European ACAS II is consistent with the RVSM requirement of 1,000 ft separation for all aircraft flying between FL290 and FL410. The European ACAS II is designed principally to eliminate nuisance warnings when an aircraft is flying within RVSM airspace.

TCAS II operates as an airborne secondary surveillance radar that interrogates the ATC transponders of nearby aircraft. Range, relative bearing and altitude of the nearby aircraft are measured by the TCAS II system. The system provides both aural and visual warnings in the event TCAS II predicts a near miss or potential collision. TCAS II depicts aircraft with operating Mode C or Mode S transponders on one or more instrument panel displays.

TCAS II installation is mandated in United States airspace for aircraft with more than 30 passenger seats. TCAS II with Change 7 satisfies the requirements imposed by the International Civil Aviation Organisation (ICAO) called Airborne Collision Avoidance System (ACASII). ACAS has been mandated in Europe since January 2000.

TCAS-commanded avoidance manouvers are in the vertical plane only, so Resolution Advisory (RA) commands appear on the two vertical speed instruments or the vertical speed display fields of integrated electronic flight display systems.

TCAS:

- Enhances a crew's situational awareness of nearby traffic
- Enhances safety by detecting and displaying potential collision threats
- Computes avoidance manouvers with aural and visual commands
- Coordinates complementary commands in encounters with other TCAS II-equipped aircraft.

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