The Digital NOTAM concept includes the automatic generation of the ICAO NOTAM from the digital AIXM (features and Event) data. As explained in the General Principles, the use of both upper and lower case characters is allowed in Digital NOTAM encoding. However, as long as the NOTAM messages need then to be transmitted on the ICAO Aeronautical Fixed Service (AFS) networks, certain limitations are imposed on the character set. While lower case text can be easily converted into upper case, special characters and language specific characters are more difficult for automatic conversions. Taking into consideration both the Annex 10 to the ICAO Convention (section 4.1.2.1) and the need for lower case the following characters are allowed in the text parts of the Digital NOTAM code:

Letters: ABCDEFGHIJKLMNOPQRSTUVWXYZ abcdefghijklmnopqrstuvwxyz
Figures: 1 2 3 4 5 6 7 8 9 0
Other signs:(hyphen)

- - (hyphen)
- ? (question mark)
- : (colon)
- ( (open bracket)
- ) (close bracket)
- . (full stop, period, or decimal point)
- , (comma)
- ’ (apostrophe)
- = (double hyphen or equal sign)
- / (oblique)
- + (plus sign)
- < (carriage return)
- ≡ (line feed)
- (space)

Restrictions on the character set are already embedded in the AIXM schema for elements such as names, identifiers, etc. For AIXM elements of type annotation, instructions, etc. that are allowed by the AIXM Schema to use any Unicode character it is important that Digital NOTAM applications limit the character set that can be used to the one mentioned above.

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