On Fri, 2014-04-25 at 17:22 -0400, Andy W. wrote:
GitHub shows this fork network:
Right now I'm using bistromath/gr-ais commit
(master HEAD, IIRC), but I'm not getting *any* !AIVDM output
to the terminal window.
Well, I’ve identified one problem.
The “unstuff” operation is being applied before the correlator for the
start flag of 01111110 (0x7e). The start flag, end flag, and preamble
are not subject to bit stuffing per ITU-R M.1371-4 Annex 2 Sections
184.108.40.206 through 220.127.116.11.
I suspect this causes a problem with proper detection of the start of
the message, as the unstuff, as I understand it, will change the start
flag from 01111110 to 0111110m, where m is the first bit of the message
Putting the unstuff operation after the correlators doesn’t fix
everything though. All potential message decodes still fail the CRC
Bypassing the CRC check and inspecting the output !AIVDMs with gpsdecode
verifies they are junk.