Airborne collision avoidance system

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 initiates 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 ( horizontal and vertical).
As of 2009, 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 three manufacturers: Rockwell Collins, Honeywell and ACSS (Aviation Communication & Surveillance Systems; an L-3 Communications and Thales Avionics company).
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 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) 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:
Large, sophisticated aircraft may use several or all of these systems, while a small general aviation aircraft might have only PCAS or benefit from the OCAS system, or no collision-avoidance system at all (other than the pilot's situational awareness)