Aircraft collision avoidance system

Electronic equipment in an aircraft that gives the pilot relative position information concerning nearby aircraft; some versions also display resolution advisories of maneuvers to avoid a collision. The system operates independently of ground-based air-traffic control equipment, but makes use of transponders carried by most aircraft to reply to air-traffic control surveillance.

To achieve surveillance of aircraft carrying Air-Traffic Control Radar Beacon System (ATCRBS) transponders (see illustration), TCAS transmits interrogations once per second using 0.8-microsecond pulses at 1030 MHz, in a format similar to those of the ground radars. The transponders reply with a block of 0.45-μs pulses at 1090 MHz, their sequence announcing the aircraft's barometric altitude quantized to the nearest 100 ft (30.5 m). TCAS measures the round-trip time from its interrogation to the received reply to determine the slant range, and decodes the altitude information contained in the reply sequence.

Relation of the Traffic Alert and Collision Avoidance System (TCAS) to the existing air-traffic control system. TCAS interrogations elicit replies from transponders of nearby aircraft, enabling TCAS to show the pilot their relative position on a traffic display, and when appropriate, to issue a resolution advisory. Here, TCAS instructs the pilot to descend to avoid a threat aircraft 300 ft above.

The collision avoidance logic must distinguish a genuine collision threat from routine safe passages. The relative range rate is derived from successive range reports. Likewise, an altitude rate is estimated from the other aircraft's altitude, and also for the system's aircraft. An estimate of the time of closest approach (τ) can be calculated from the equation below.

Each nearby aircraft is evaluated once per second, and is deemed a threat if the range is already small or if τ is small, and if the relative altitude is predicted to be small. When a threat is declared, the effects of potential climb and descent maneuvers are estimated. The maneuver sense that gives the greater separation is chosen, except that a vertical crossing is not selected if the noncrossing sense gives adequate separation. An advisory is selected (such as limit climb; do not climb; descend) that is predicted to prevent a collision while minimizing the displacement from the aircraft's flight path.

An important part of TCAS is its traffic display, enabling crews to locate nearby traffic even when weather hinders visual sighting. At present, the format in use depicts nearby aircraft in plan view, centered about the TCAS aircraft (see illustration).