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Why from the SELCAL code pool, the letters "I", " N", "O" are omitted?

Thanks in advance.

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From the ASRI SELCAL Users Guide:

The four tones required for a single SELCAL code assignment are selected from a total of sixteen tones available in the ground station SELCAL installation. For convenient reference, the sixteen tones are designated by letters of the alphabet (“A” through “S”; with “I”, “N”, and “O” omitted). Tones “P”, “Q”, “R”, and “S” were the four tones that were added to the exiting 12 tones in 1985.

Although this specification states that N is not used, it does not say why. However, there may be a clue in that the first implementation used 12 letters, A through M (omitting I). When four more codes were added in 1985, they presumably wanted to omit O for the same reason they omitted I (confusion with digits 0 and 1). If they continued in the same sequence then the "new" codes would be N, P, Q, R. Knowing that they were going to have to skip O, it may have been seen as simpler at the time to introduce the four new codes in contiguous sequence: P, Q, R, S.

The linked copy of the document above does not mention SELCAL32, which has been approved for implementation in November 2022. SELCAL32 uses a set of 32 codes, 16 from the previous SELCAL plus T through Z and 1 through 9. Again note that 0 is omitted. More information can be found at the ASRI SELCAL Registrar Services page.

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  • $\begingroup$ Still we cant find the answer $\endgroup$ Feb 13, 2023 at 16:55

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