Collision Avoidance Systems Information

Collision avoidance systems are designed to prevent or reduce the severity of vehicle collisions by providing aural and visual warnings to operators or by proactively executing defensive maneuvers such as braking when a danger of collision is imminent. Collision avoidance systems are used in aerospace, automotive, rail, and marine applications.

Aircraft collision avoidance systems (ACAS) include Traffic Alert and Collision Avoidance Systems (TCAS), Portable Collision Avoidance Systems (PCAS), Ground Proximity Warning Systems (GPWS), Terrain Awareness and Warning Systems (TAWS), and Obstacle Collision Avoidance Systems (OCAS). Radar, laser position sensors and radio frequency transponder interrogation are some of the technologies used in these systems. Collision avoidance systems are also known as: pre-crash systems, forward collision warning systems, or collision mitigating systems.


Crash avoidance features in automobiles include:

  • Forward collision warning
  • Autobrake
  • Lane departure warning
  • Lane departure prevention
  • Adaptive headlights
  • Blind spot detection

Volvo XC60 forward collision avoidance system



There are three types of aircraft collision avoidance systems:

  • ACAS I gives traffic advisories but does not recommend any maneuvers.
  • ACAS II gives traffic advisories and resolution advisories in the vertical sense (direction).
  • ACAS III gives traffic advisories and resolution advisories in vertical and horizontal directions.

The benefits of using ACAS are expressed in terms of the risk ratio. The most important factor affecting the performance of ACAS if the response of pilots to resolution advisories (RA's), when using these, the risk of collision for a specific aircraft can be reduced by a factor greater than three. Another aircraft collision avoidance system is FLARM. FLARM works by determining position and altitude with a highly sensitive GPS receiver. Based on speed, acceleration, heading, track, turn radius, wind, altitude, vertical speed, configured aircraft type, and other parameters, a very precise projected flight path can be calculated. The flight path is encoded and sent over an encrypted radio channel to all nearby aircraft at least once per second. The FLARM device receives the same encoded flight path from all surrounding aircraft. The latest version includes collision risk models for sailplanes, motor gliders, piston airplanes, turbine airplanes, helicopters, UAVs, paragliders, skydivers, and balloons.

New technology is currently being developed by the Federal Aviation Administration (FAA) called ACAS X. This new technology will eventually replace TCAS II (ACAS II). Two key differences between TCAS II and the current concept for ACAS X are the collision avoidance logic and the sources of surveillance data. Anticipated benefits of ACAS X are:

  • Reduction of unnecessary advisories
  • Adaptability to future operational concepts
  • Extending collision avoidance to other classes of aircraft
  • Use of future surveillance
  • Environment safety improvement
  • It will use the same hardware (antennas, processors, and displays) as the current TCAS II system


IIHS - Crash avoidance features by make and model


SKYbrary - ACAS X


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