Development of Airborne Collision Avoidance Algorithms Compatible with the National-Airspace System

The Traffic Alert and Collision Avoidance System (TCAS-II) helps pilots avoid midair collisions using vertical escape maneuvers generated by an on-board processor. TCAS-II interrogates the altitude reporting transponders on other nearby aircraft, tracks their location, decides which pose a threat, selects a resolution maneuver against each threat, coordinates with the threat aircraft if possible, and displays the maneuvers to the pilot. To operate compatibly in the highly structured airspace, TCAS-II must minimize unwanted alarms when no true danger exists. A description of the algorithm is presented. Studies of real-world data leading to the selected tradeoffs between maximum protection and lowest alert rate are described. Many features have evolved which minimize the disruptive effects of a resolution advisory, but which provide a high level of protection within the fundamental constraints of the system.