Passive secondary surveillance radar using signals of remote ssr and

Abstract

The range from which transponder replies can be received at a virtual secondary surveillance radar of the type described in U.S. Pat. No. 4,115,771 is extended by replacing the omnidirectional antenna previously utilized to receive replies from transponders within the range of the SSR with a stationary array of a plurality of directive antennas arranged in a circle and having radiation patterns pointing in different directions from the center of and covering a respective sector of the circle, and sequentially switching on the antenna of the array covering the sector in which the rotating beam of the associated SSR is then pointing. In an arrangement in which the SSR and PSSR are so widely separated that in certain pointing directions the SSR beam overlies more than one sector of the antenna array, the antennas are sequentially switched on at calculated switching times determined from the pointing direction of the SSR beam and the times it takes in the geometrical arrangement between the SSR beam and the radiation patterns of the antennas of the array, for a given SSR interrogating pulse to propagate, from launch, along the beam until it reaches the line separating the first encountered sector from the next adjacent sector, between second and third sectors, and so on, such that a high gain directive antenna is always pointing in the proper direction to receive reply signals from a transponder intercepted by the scanning beam of the SSR.