Abstract: A protected aircraft having a discrete address beacon transponder includes a ground base air traffic control radar beacon system (ATCRBS) interrogator, a collocated discrete address beacon system (DABS) transponder and a directional antenna on the protected aircraft. The protected aircraft synchronizes a local clock with the pulse repetition frequency (PRF) and scan rate of the ATCRBS interrogating his field of interest from which information the protected aircraft can calculate the azimuth angle of responding intruder aircraft with respect to the ground station. By means of his directional antenna the protected aircraft also determines the azimuth of the intruder with respect to itself. Interrogation by the protected aircraft of a collocated ground DABS transponder or a DME or other suitable means of measuring range provides a data base which together with the aforementioned azimuth angles permits the protected aircraft to calculate instantaneous position of the intruder relative to his own.

Abstract: An integrated airport terminal area surveillance system, including a plurality of spaced apart interrogating and receiving stations and central equipment for controlling the stations operates in search and track modes by transmitting interrogation messages and receiving resulting replies. In the search mode the stations interrogate by directional beams all aircraft operating within the entire terminal area and enrolling the position and identification of replying aircraft into a system memory. In the track mode those aircraft enrolled in the system memory are individually interrogated. A system map is divided into memory cells corresponding to predetermined geographical cells. The means by which an aircraft is interrogated is determined by the geographical cell in which the aircraft is located.