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enter image description hereIf I understand correctly, originally DME and the equivalent distance measurement aspect of TACAN had only the "X" channels where the pulse pairs are 12 μs apart, and later the "Y" channels were introduced (to TACAN only?) with 30 μs separation. When did this happen, in the late 60s/70s era? So airborne TACAN equipment from before that would not have any X/Y selection possibility, right? Picture of the TACAN panel in the XB-70, which doesn't seem to have any X/Y switch as I see in newer aircraft.

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Civil DME and TACAN share the same frequencies and the same 252 channels.

TACAN is standardized in MIL-STD-291.

  • 1956: MIL-STD-291
  • 1958: MIL-STD-291A
  • 1967: MIL-STD-291B
  • 1998: MIL-STD-291C

1958

I have not been able to find the two first versions, and cannot know directly whether these channels existed in 1958, but some documentation is available from NATO archives, e.g. in this description dated May 1959 only 126 channels are mentioned.

TACAN panel from this era, for ARN21:

TACAN ARN21 control panel

ARN21 control panel, source

1967

MIL-STD-291B from 1967 mentions 126 X channels and 126 Y channels, and also that 126 channels taken from X and Y modes are allocated for Air/Air communication.

So the change occurred between 1959 and 1967.

In the meantime

Looking at NATO archives it appears a problem with the number of channels in Europe arose around 1964. A study was conducted.

The decision to double the number of channels was taken after this study, in March 1967:

enter image description here

The new standard with 252 channels was published in December 1967.

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