An airborne collision avoidance system (ACAS, usually pronounced as ay-kas) operates independently of ground-based equipment and air traffic control in warning pilots of the presence of other aircraft that may present a threat of collision. If the risk of collision is imminent, the system recommends a maneuver that will reduce the risk of collision. ACAS standards and recommended practices are mainly defined in annex 10, volume IV, of the Convention on International Civil Aviation. Much of the technology being applied to both military and general aviation today has been undergoing development by NASA and other partners since the 1980s.
A distinction is increasingly being made between ACAS and ASAS (airborne separation assurance system). ACAS is being used to describe short-range systems intended to prevent actual metal-on-metal collisions. In contrast, ASAS is being used to describe longer-range systems used to maintain standard en route separation between aircraft (5 nautical miles (9.3 km) horizontal and 1,000 feet (300 m) vertical).
As of 2022, the only implementations that meets the ACAS II standards set by ICAO are Versions 7.0 and 7.1 of TCAS II (Traffic Collision Avoidance System) produced by Garmin, Rockwell Collins, Honeywell and ACSS (Aviation Communication & Surveillance Systems; an L-3 Communications and Thales Avionics company).: 14, 16
As of 1973, the United States Federal Aviation Administration (FAA) standard for transponder minimal operational performance, Technical Standard Order (TSO) C74c, contained errors which caused compatibility problems with air traffic control radar beacon system (ATCRBS) radar and Traffic Collision Avoidance System (TCAS) abilities to detect aircraft transponders. First called "The Terra Problem", there have since been individual FAA Airworthiness Directives issued against various transponder manufacturers in an attempt to repair the operational deficiencies, to enable newer radars and TCAS systems to operate. Unfortunately, the defect[clarification needed] is in the TSO, and the individual corrective actions to transponders have led to significant differences in the logical behavior of transponders by make and mark, as proven by an FAA study of in-situ transponders. In 2009, a new version, TSO C74d was defined with tighter technical requirements.
AIS-P (ACAS)[clarification needed] is a modification which both corrects the transponder deficiencies (the transponder will respond to all varieties of radar and TCAS), then adds an Automatic Independent Surveillance with Privacy augmentation. The AIS-P protocol does not suffer from the saturation issue in high density traffic, does not interfere with the Air Traffic Control (ATC) radar system or TCAS, and conforms to the internationally approved Mode S data packet standard. It awaits member country submission to the ICAO as a requested approval.
Modern aircraft can use several types of collision avoidance systems to prevent unintentional contact with other aircraft, obstacles, or the ground.
Some of the systems are designed to avoid collisions with other aircraft and UAVs. They are referred to as "electronic conspicuity" by the UK CAA.