Rongor Posted January 24, 2023 Posted January 24, 2023 Hey there. I know that as per MIL-STD-291 the interrogating and reply frequency pairs are cut down to 1025-1150 MHz in mode Y, instead of using the full range of 962-1213 MHz in mode X. Also when switching to A/A while a channel is set to mode X, we essentially get the Y pairings. The only difference in A/A between X and Y seems to be the pulse spacing of 36 μs for mode Y instead of 12 μs in mode X. So what is the deal here? What are the benefits, what is the idea behind these different pulse spacings? Why does Y use the longer spacing? In A/G, mode Y also has the longer spacing, yet its only 30 μs there. Why?
peanuts0441 Posted April 6, 2023 Posted April 6, 2023 Using different spacing doubles the amount of available channels from 126 to 252 Around the 1960s there became an issue with the amount available channels, so a new standard was introduced to double the amount of available channels Since in mode Y the pulses are spaced 36μs for the interrogation pulse and 30μs for the reply pulse, this can be differentiated from the 12μs pulse spacing of the X channels, thus doubling the amount of available "bandwitdh" while staying within the same frequency range Also in A/G, mode Y has a 30μs pulse spacing to differentiate ground transmittors from the 36μs pulse spacing coming from aircraft on the same frequency
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