aircraft system based on secondary surveillance radar (SSR) (3.1.121)  transponder signals which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders

NOTE 1 The terms “ACAS” and “TCAS” (traffic alert and collision avoidance system) are often used interchangeably however there is a difference between the two terms: ACAS is typically used when referring to the technical standard, concept or implementation; TCAS is typically used when referring to implementation of the technical standards and concept.

NOTE 2 ACAS sXu is a derivative of ACAS and provides a capability for smaller “uncrewed aircraft system (UAS)” (3.1.137) (which are defined in the relevant MOPS (minimum operational performance standard[s]) as UAS with a wingspan, or equivalent, of 25 feet or less). ACAS sXu is intended for vehicles that are not secondary “surveillance radar (SSR) transponder” (3.1.121) transponder- or “automatic dependent surveillance broadcast (ADS-B)” (3.1.19)- equipped.

[SOURCE: International Civil Aviation Organization (ICAO) Annex 11 to the Convention on International Civil Aviation, Air Traffic Services, 15th edition, July 2018] [4]