The density of air traffic, particularly in the USA, is now X increasing at such a rate that the overloading of the Air Traffic Control system seems inevitable. Fig. 1 shows the expected increase in the size of the US civil air fleet and its operations over an eleven-year span, from 1966 to 1977.

Since January 1st 1968, there have been 58 mid-air collisions in the USA, including 19 this year. The majority of these were under positive Air Traffic Control in the vicinity of airports. Some form of airborne system to prevent collisions seems essential for the safe operation of aircraft in the 1970s.

A considerable amount of discussion and development has been carried out in the USA during the past 14 years, and some prototype equipments are now flying. The purpose of this paper is to describe the technique employed in these systems, and to suggest an original principle as a design basis for a collision avoidance system (CAS).