The aircraft-based air traffic control interferometric antenna

An interferometric array of the air traffic control interrogation link is reviewed. To eliminate the ambiguity in angular coordinate readings, the base switching and the request signal encoding have been used to remove the influence of parasite lobes, providing for the angular resolution of the ATC equipment comparable to that of airborne radar phased array antennas. The feasibility of placing the interferometric array outside the phased array antenna aperture of airborne radars considerably reduces intercoupling between them, while control of phase shifters offers a synchronous electric beam scanning for both phased arrays. The drawback to this design of interferometric phased array is that at considerable angles several false request zones occur (if only the P/sub 2 /check pulse is used) generating random replies which, however, cannot be received by a correlation interferometer and do not affect the operation of the ATC interrogator. This drawback is remedied if two check pulses are utilized and an additional reception array is used as a transceiver generating a difference pattern in the transmission mode.