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Airborne collision avoidance system
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<p>An airborne collision avoidance system (ACAS, usually pronounced as <i>ay-kas</i>) operates independently of ground-based equipment and <a href="/facts/Air_traffic_control/G2cQebVb">air traffic control</a> 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 <a href="/facts/Convention_on_International_Civil_Aviation/yKSXno0S">Convention on International Civil Aviation</a>. Much of the technology being applied to both military and general aviation today has been undergoing development by <a href="/facts/NASA/TIMCV0yV">NASA</a> and other partners since the 1980s. </p><p>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 <i>en route</i> separation between aircraft (5 nautical miles (9.3 km) horizontal and 1,000 feet (300 m) vertical). </p>