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Partial answer: Is the backwards compatibility seamless and simultaneous? Yes. Able to reply to both mode S and mode 3 interrogators in real time? Yes. Or a toggle switch between modes, in which when mode S is selected mode 3 interrogations will be ignored? No - a Mode S transponder will respond to interrogations by a "pure" mode 3 interrogator. ...


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I think the problem is that you see a Mode 3/A (identity) reply, but you try to interpret it as Mode C (altitude reply). In Mode C, altitude is Gillham encoded, using all pulses except for D1 and X. This gives a range of -1200 to 126700 ft in 100 ft increments. If you see a D1 pulse, you are not looking at a Mode C reply. In Mode A, a 12 bit identity (...


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