Abstract: A device is provided for interrogating airborne SSR transponders at an airport, in which in certain regions which are made non-active, P1 pulses are transmitted followed, with the correct timing, by P2 pulses which have the same or a larger amplitude than the P1 pulses. As a consequence, SSR transponders in these regions are blocked or muted, and this blocking or muting is repeated, whereas in one or a few selected regions, normal interrogating signals are transmitted. Spreading of transmitted pulses to higher levels above ground is countered by keeping the energy of the pulses low and the vertical gain of transmitter antennas small.

Abstract: The invention provides a Secondary Surveillance Radar (SSR) to be used on airports, in which the airport is divided in a considerable number (preferably 100 to 250) divisional regions, each of said regions having at least one and preferably at least two transmitters (1,2 if FIG. 1; 1,3; 2,4; 3,5; 4,6 in FIG. 4) and at least two receivers (1,2,3 in FIG. 1; 1,2,3; 2,3,4; 3,4,5; 4,5,6 in FIG. 4) to determine the location of interrogated transponder by means of multilateration, in which a transmitter may have the same location as a receiver and the transmitters and receivers are connected to a central processor.