[aprssig] Low-level packet info

[scott at opentrac.org]

When I started working on my OpenTracker project, I discovered that there's
little reliable documentation on the low-level details of 1200 baud packet
radio.  Even the AX.25 spec (V2 anyway) was flawed, to the point that if you
followed its instructions on bit stuffing, it'd never work.

I documented a lot of this stuff in my source code as I figured it out, and
I finally got around to writing it up in a more accessible form.  I've
posted the document at:

http://n1vg.net/packet/index.php

If anyone has any corrections or additions, let me know.  Also, if you've
got known-good example code for the FCS calculation, send me a link - there
are plenty of incorrect code samples on the web.  (Thanks to John Hansen,
W2FS, for providing the correct example that I based my code on.)

And on a completely unrelated note, the OpenTracker now supports __GLL
sentences, for those of you with ancient GPS receivers...

Scott
N1VG