Abstract: An aircraft collision-avoidance system utilizing a known-location navigational ground station to enable a primary aircraft to determine its own location and employing a known-location air surveillance radar including an interrogator to determine the location, velocity, course and altitude of a potentially conflicting other aircraft. The technique utilizes the time between the inception of an interrogating pulse and the reception of a transponder response from the target aircraft to compute the position and altitude of said target aircraft, and employs storage of repeated timed samplings of the data and subsequent replacement of the data as a series of dots on a display to establish a line showing the relative motion of the target aircraft with respect to the primary aircraft. An alarm device is provided to direct aircrew attention to potential conflicts predicted by a data processor carried by the primary aircraft and forming part of the system.