A digital tracker of aircraft represented by a binary coded signal particularly suited for a midair collision avoidance system such as SECANT, an acronym of a system for separation control of aircraft by non-synchronous techniques. The tracker located on an aircraft station receives pulses, representing both interrogation responses, termed ''''hits,'''' from other aircraft in the environment to ''''probe'''' interrogation signals transmitted from the station aircraft, and interrogation responses, termed ''''fruit,'''' from other aircraft responding to interrogation (probe) signals from aircraft other than the aircraft station. The tracker provides an output comprising the average ''''target'''' range of aircraft within a predetermined critical range of the station aircraft, as well as the target''s range rate, the rate at which the range is changing, and the time (''''tau'''') to possible impact with the target.