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Transponder? >> this is an old-fashioned transponder. they have electronic readouts today. you put the numbers in, four numbers. air traffic control sees on the radar scope an identification of those numbers with the altitude of the airplane. to turn a transponder off you turn it off. air traffic control loses that information. >> if you believe the transponder was turned off how is it that we know the altitude of this particular plane? >> well, i understand that they have cross refrned fr -- re...