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This paper considers the system engineering of a secondary surveillance radar, comprising a ground interrogator and airborne transponders, for use in an air traffic control system. It deals in general terms with the choice of frequencies, considers how closely it should be associated with the primary radars now used for the control of air traffic and deals with other system problems as follows: the elimination of interrogations by the sidelobe of the ground radar radiation, the design of ground and airborne aerials, transponder saturation and methods of countdown, the difficulty of airborne units being Â¿capturedÂ¿ by one ground station to the exclusion of others, second-time-round signals, unlocked responses due to remote ground stations, and the form of coding which may be used, with particular emphasis on the air-to-ground channel. The discussion is kept intentionally at the systems level so that the general considerations can be appreciated unhampered, so far as possible, by technical detail. It has nevertheless been thought necessary at some places to point the argument with sufficient detail to give some further insight into the problems involved.