Airborne Collision Avoidance System (ACAS) is an ICAO (International Civil Aviation Organization) standard specified in ICAO Annex 10 Vol IV which provides pilots with a system independent of air traffic control to detect the presence of other aircraft which may present a threat of collision. Where the risk of collision is imminent, the system provides an indication of a manoeuvre that will reduce the risk of collision.
As of 2006, the only implementation that meets the ACAS II standards set by ICAO is Version 7.0 of TCAS II (Traffic Alert and 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 Standards Order (TSO) C74c, contained errors which affect both air traffic control radar beacon system (ATCRBS) "radar" and Traffic Alert and 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 the "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.
AIS-P 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.