In the image above from radartutorial.eu, we can see that the sequence of bits are c1 a1 c2 a2 c4 a4 during the reply of ATC transponder mode A.
Why are the bits not transmitted like c1 c2 c4 then a1 a2 a4. Why this mixing up or interleaving?
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Mode 1 transponders only had six bits: A1, A2, A4, B1, B2 and B4, which clearly wasn’t sufficient to identify specific aircraft. Each day, all pilots would be given a common code to squawk when returning so the air defense folks would know they were friendly, whereas a foe would only have a 1/26 chance of guessing correctly.
Mode 3/A upped this to twelve bits, adding C1, C2, C4, D1, D2 and D4. However, the C bits were cleverly interleaved with the A bits, and the D bits with the B bits, at twice the transmission rate. The result was that (for instance) Mode 1 code “12” and Mode 3/A code “1200” looked the same, at least to the primitive equipment of the era. This was specifically intended so that Mode 1 aircraft could work with Mode 3/A radar sites and vice versa, allowing a gradual upgrade. Where the new equipment was in use by everyone, individual (“discrete”) codes could be assigned to each aircraft.
This is also why, to this day, xx00 codes are mostly “non-discrete”, i.e. they have a general meaning rather than identifying a specific aircraft.
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$\begingroup$ A transponder response is not directional, at least so far as I know. How did they prevent the enemy from eavesdropping on responses to know what to squawk? $\endgroup$ May 30, 2021 at 2:42
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$\begingroup$ @randomhead It wasn’t perfect, but it was arguably better than nothing. That’s sometimes the best you get in the middle of a war. $\endgroup$– StephenSMay 30, 2021 at 2:53
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$\begingroup$ Probably the enemies couldn't know what to squwak because of range $\endgroup$ May 30, 2021 at 15:16