Over the years, air traffic has continued to increase. The developments of modern air traffic control systems have made it possible to cope with this increase, whilst maintaining the necessary levels of safety. Despite technical advances in ATC systems, there are cases when the separation provision fails due to a human or technical error. Any separation provision failures may result in an increased risk of a midair collision. Therefore, an airborne collision avoidance system, acting as a last resort, has been considered from the 1950s. In 1955, Dr John S. Morrel proposed the use of the slant range between aircraft divided by the rate of closure or range rate for collision avoidance algorithms (i.e. time rather than distance, to the closest point of approach). Today’s airborne collision avoidance system is based on this concept.
During the late 1960s and early 1970s, several manufacturers developed prototype aircraft collision avoidance systems. Although these systems functioned properly during staged aircraft encounter testing, it was concluded that in normal airline operations, these systems would generate a high rate of unnecessary alarms in dense terminal areas. This problem would have undermined the credibility of the system with the flight crews.
In 1978, the collision between a light aircraft and an airliner over San Diego, California led the US Federal Aviation Administration to initiate, three years later, the development of TCAS (Traffic alert and Collision Avoidance System). In 1986 the collision between an airliner and a light aircraft over Cerritos, California resulted in a US Congressional mandate that required some categories of US and foreign aircraft to be equipped with TCAS II for flight operations in US airspace.
In parallel to the development of TCAS equipment, ICAO (International Civil Aviation Organization) has developed, from the beginning of the 1980s, standards for ACAS (Airborne Collision Avoidance Systems).