Links for the APRS specification

APRS has a spec, but it is understood to be incomplete. The official spec document called "APRS101.PDF" and is available from many places. BUT THAT DOCUMENT IS OBSOLETE and there are many tweaks that have been made to the actual practice of APRS since that document was created. Many of these tweaks are found in sprawling text documents created by Bob Bruninga that he called "addenda" to the spec.

Ignore the addenda at your peril. I just got bitten myself by reading the spec document for area objects, only to find that none of the clients I tried to use actually did what the spec said --- but in fact they do exactly what the 1.1 "addendum" says as a correction to the spec.

APRS spec is a good repository on github that has not only the APRS 1.01 spec PDF but also links to the 1.1 addenda and "1.2 proposal". The site also contains DRAFT versions of a comprehensive "1.2" specification document, but that's not finished.