Abstract: A method and apparatus for preventing unauthorized sources from utilizing rcraft-borne I.F.F. for locating same. A bypass switch is inserted in the I.F.F. transponder antenna line which normally sends all incoming signals into a dummy load. Upon receipt of a proper interrogate command, the antenna is switched to the transponder.

Abstract: An improved passive IFF system operating as a supplement to a conventional adar system including a coded variable delay system to produce like time-coded variable phase delayed identifying signal components indicative of cooperating or friendly objects and which overlap signals of other reflecting echo signals from the objects. The IFF system is provided with a retro-directive type antenna. The overlapping cooperating or friendly signal components and the corresponding other echo reflecting object signals are simultaneously detected at the radar system which is provided with a compensating or complementary phase delay means time coded in the same manner as the identifying IFF signal components. The identifying IFF signal components are thus produced in uniform phase and are summed up to provide a signal at the radar system whose magnitude is relatively large as compared to the combined corresponding echo reflecting signals which are in random-phase relationship.