Although the terms ACAS and TCAS are often used interchangeably, they are not synonymous:
is the term for airborne collision avoidance systems generally.
is also the term used by ICAO to mean ACAS II - a collision avoidance device that meets the technical performance parameters specified in ICAO Annex 10,
is the term used in the FAA Technical Standard Orders (TSO) in relation to the approval of such equipment.
TCAS II version 7.0
is the only system which is compliant with ICAO ACAS II technical standards.
TCAS II version 7.1
will, from 1 January 2017, be the only system which is compliant with ICAO ACAS II technical standards.