Airborne collision avoidance system

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The U.S. Air Force's F-16D Ground Collision Avoidance Technology (GCAT) aircraft. F-16D ACAT Aircraft.jpg
The U.S. Air Force's F-16D Ground Collision Avoidance Technology (GCAT) aircraft.

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. [1] 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. [2]


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). [3]


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). [4] :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.[ citation needed ] In 2009, a new version, TSO C74d [5] was defined with tighter technical requirements.[ citation needed ]

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.[ citation needed ] It awaits member country submission to the ICAO as a requested approval.[ citation needed ]

Other collision avoidance systems

Modern aircraft can use several types of collision avoidance systems to prevent unintentional contact with other aircraft, obstacles, or the ground.

Aircraft collision avoidance

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. [6]

Terrain collision avoidance

See also

Related Research Articles

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In aviation, a controlled flight into terrain is an accident in which an airworthy aircraft, fully under pilot control, is unintentionally flown into the ground, a mountain, a body of water or an obstacle. In a typical CFIT scenario, the crew is unaware of the impending disaster until it is too late. The term was coined by engineers at Boeing in the late 1970s.

<span class="mw-page-title-main">Ground proximity warning system</span> Alert system meant to prevent pilots from flying or taxiing into obstacles

A ground proximity warning system (GPWS) is a system designed to alert pilots if their aircraft is in immediate danger of flying into the ground or an obstacle. The United States Federal Aviation Administration (FAA) defines GPWS as a type of terrain awareness warning system (TAWS). More advanced systems, introduced in 1996, are known as enhanced ground proximity warning systems (EGPWS), a modern type of TAWS.

<span class="mw-page-title-main">Traffic collision avoidance system</span> Aircraft collision avoidance system

A traffic collision avoidance system, also known as a traffic alert and collision avoidance system, is an aircraft collision avoidance system designed to reduce the incidence of mid-air collision (MAC) between aircraft. It monitors the airspace around an aircraft for other aircraft equipped with a corresponding active transponder, independent of air traffic control, and warns pilots of the presence of other transponder-equipped aircraft which may present a threat of MAC. It is a type of airborne collision avoidance system mandated by the International Civil Aviation Organization to be fitted to all aircraft with a maximum take-off mass (MTOM) of over 5,700 kg (12,600 lb) or authorized to carry more than 19 passengers. CFR 14, Ch I, part 135 requires that TCAS I be installed for aircraft with 10-30 passengers and TCAS II for aircraft with more than 30 passengers. ACAS/TCAS is based on secondary surveillance radar (SSR) transponder signals, but operates independently of ground-based equipment to provide advice to the pilot on potentially conflicting aircraft.

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<span class="mw-page-title-main">Secondary surveillance radar</span> Radar system used in air traffic control

Secondary surveillance radar (SSR) is a radar system used in air traffic control (ATC), that unlike primary radar systems that measure the bearing and distance of targets using the detected reflections of radio signals, relies on targets equipped with a radar transponder, that reply to each interrogation signal by transmitting encoded data such as an identity code, the aircraft's altitude and further information depending on its chosen mode. SSR is based on the military identification friend or foe (IFF) technology originally developed during World War II, therefore the two systems are still compatible. Monopulse secondary surveillance radar (MSSR), Mode S, TCAS and ADS-B are similar modern methods of secondary surveillance.

The air traffic control radar beacon system (ATCRBS) is a system used in air traffic control (ATC) to enhance surveillance radar monitoring and separation of air traffic. It consists of a rotating ground antenna and transponders in aircraft. The ground antenna sweeps a narrow vertical beam of microwaves around the airspace. When the beam strikes an aircraft, the transponder transmits a return signal back giving information such as altitude and the Squawk Code, a four digit code assigned to each aircraft that enters a region. Information about this aircraft is then entered into the system and subsequently added to the controller's screen to display this information when queried. This information can include flight number designation and altitude of the aircraft. ATCRBS assists air traffic control (ATC) surveillance radars by acquiring information about the aircraft being monitored, and providing this information to the radar controllers. The controllers can use the information to identify radar returns from aircraft and to distinguish those returns from ground clutter.

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<span class="mw-page-title-main">Airport surveillance radar</span> Radar system

An airport surveillance radar (ASR) is a radar system used at airports to detect and display the presence and position of aircraft in the terminal area, the airspace around airports. It is the main air traffic control system for the airspace around airports. At large airports it typically controls traffic within a radius of 60 miles (96 km) of the airport below an elevation of 25,000 feet. The sophisticated systems at large airports consist of two different radar systems, the primary and secondary surveillance radar. The primary radar typically consists of a large rotating parabolic antenna dish that sweeps a vertical fan-shaped beam of microwaves around the airspace surrounding the airport. It detects the position and range of aircraft by microwaves reflected back to the antenna from the aircraft's surface. The secondary surveillance radar consists of a second rotating antenna, often mounted on the primary antenna, which interrogates the transponders of aircraft, which transmits a radio signal back containing the aircraft's identification, barometric altitude, and an emergency status code, which is displayed on the radar screen next to the return from the primary radar.

<span class="mw-page-title-main">FLARM</span> Traffic avoidance system in light aircraft and gliders

FLARM is a proprietary electronic system used to selectively alert pilots to potential collisions between aircraft. It is not formally an implementation of ADS-B, as it is optimized for the specific needs of light aircraft, not for long-range communication or ATC interaction. FLARM is a portmanteau of "flight" and "alarm". The installation of all physical FLARM devices is approved as a “Standard Change”, and the PowerFLARM Core specifically as a “Minor Change” by the European Aviation Safety Agency.; and in addition the Minor Change also approves the PowerFLARM Core for its IFR and at night.

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<span class="mw-page-title-main">Transponder (aeronautics)</span> Airborne radio transponder

A transponder is an electronic device that produces a response when it receives a radio-frequency interrogation. Aircraft have transponders to assist in identifying them on air traffic control radar. Collision avoidance systems have been developed to use transponder transmissions as a means of detecting aircraft at risk of colliding with each other.

Automatic Independent Surveillance – Privacy (AIS-P) is a data packet protocol for the TailLight system of aircraft Traffic Collision Avoidance System (TCAS), wherein a single Mode S 64 microsecond message is transmitted by an aircraft ATCRBS or Mode S transponder, and received by aircraft and Air Traffic Control on the ground. This is an augmentation to aircraft transponders, which report aircraft position and velocity in such a way as to minimize interference with any other avionics system, maximize the possible number of participating aircraft, while not relying on any equipment on the ground, and protecting aircraft from potential attack. AIS-P and ADS-B are competing protocols for aircraft based surveillance of traffic, a replacement technology for Mode S radar and TCAS.

A portable collision avoidance system (PCAS) is an aircraft collision avoidance system similar in function to traffic collision avoidance system (TCAS). TCAS is the industry standard for commercial collision avoidance systems but PCAS is gaining recognition as an effective means of collision avoidance for general aviation and is in use the world over by independent pilots in personally owned or rented light aircraft as well as by flight schools and flying clubs. Its main competitor is FLARM.

<span class="mw-page-title-main">Terrain awareness and warning system</span> Technological equipment to prevent pilots from flying into obstacles

In aviation, a terrain awareness and warning system (TAWS) is generally an on-board system aimed at preventing unintentional impacts with the ground, termed "controlled flight into terrain" accidents, or CFIT. The specific systems currently in use are the ground proximity warning system (GPWS) and the enhanced ground proximity warning system (EGPWS). The U.S. Federal Aviation Administration (FAA) introduced the generic term TAWS to encompass all terrain-avoidance systems that meet the relevant FAA standards, which include GPWS, EGPWS and any future system that might replace them.

The aviation transponder interrogation modes are the standard formats of pulsed sequences from an interrogating Secondary Surveillance Radar (SSR) or similar Automatic Dependent Surveillance-Broadcast (ADS-B) system. The reply format is usually referred to as a "code" from a transponder, which is used to determine detailed information from a suitably equipped aircraft.

L-3 SmartDeck - is a fully integrated cockpit system originally developed by L-3 Avionics Systems. and acquired in 2010 by Esterline CMC Electronics through an exclusive licensing agreement.

<span class="mw-page-title-main">Automatic Dependent Surveillance–Broadcast</span> Aircraft surveillance technology

Automatic Dependent Surveillance–Broadcast (ADS-B) is a surveillance technology and form of Electronic Conspicuity in which an aircraft determines its position via satellite navigation or other sensors and periodically broadcasts it, enabling it to be tracked. The information can be received by air traffic control ground stations as a replacement for secondary surveillance radar, as no interrogation signal is needed from the ground. It can also be transmitted and received point-to-point by other aircraft to provide situational awareness and allow self-separation. ADS-B is "automatic" in that it requires no pilot or external input. It is "dependent" in that it depends on data from the aircraft's navigation system.

Traffic information service – broadcast (TIS–B) is an aviation information service that allows pilots to see aircraft that are not emitting ADS-B data but have a basic transponder.

Airport surveillance and broadcast systems are a set of runway-safety tools that display aircraft on and near an airport.


  1. "EUROCONTROL - ACAS II ICAO Provisions". Archived from the original on 2010-04-21. Retrieved 2010-04-18.
  2. "NASA-Pioneered Automatic Ground-Collision Avoidance System Operational". NASA website. Retrieved 8 Oct 2014.
  3. [Hoekstra, J.M. (2002). Free flight with airborne separation assurance. Report No. NLR-TP-2002-170. National Aerospace Laboratory NLR.]
  4. 1 2 Airborne Collision Avoidance System (ACAS) guide (PDF). Eurocontrol. March 2022.
  5. "TSO C74d Air Traffic Control Radar Beacon System (ATCRBS) Airborne Equipment" (PDF). Federal Aviation Administration.
  6. "Electronic conspicuity devices". UK CAA. Retrieved 13 September 2022.
  7. Jedick, Rocky (14 December 2014). "Ground Collision Avoidance System". Go Flight Medicine. Retrieved 16 Dec 2014.