[aprssig] APRS Top-level parser

[Robert Bruninga]

While discussing the basic APRS parsing technique this weekend,
I realized that it did not exist on paper anywhere other than in
code.  So I thought it would be a good idea to put it in pseudo
English and in a document so everyone could have a common sheet
of music.

APRS only has 4 basic packet types.  It is not what we do with
POSITIONS, STATUS, MESSAGEs and QUERY packets that is bothering
me, it is what we do with the other packets that fall out the
bottom of the parser (as none-of-the-above) that have been
inconsistently implemented and therefor gives different users,
different perspectives on what they see on the APRS network.

APRS was designed to give the user a VIEW of everything in ham
radio going on around him that is making itself known via AX.25.
So how we handle the non-APRS packets was always fundamentally
included in APRS.  This document explains:

http://www.ew.usna.edu/~bruninga/aprs/APRSparser.txt

Basically, it describes how every station is supposed to get a
"vicinity position" until his real APRS posit is received.  This
at least gets him into the system while we wait for a posit.

If I have not done an adequate job of capturing the
top-level-parser in the above document, let me know how I can
improve it.  Also, it would be nice to collect a summary of how
various APRS clones did or did not implement these techniques.

Bob, WB4APR