http://books.google.com/books?id=ueT...%20iff&f=false
By May 1967, several new systems were entering service on the U.S. side that would tip the battle scales in its favor. The first was a system that would have a tremendous impact on air-to-air combat for the rest of the war: the QRC-248 enemy IFF transponder interrogator.
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The most effective mode for the QRC-248 was the active mode, where it could actually interrogate the MiG's IFF (...)