On Mon, 28 Aug 2006 00:19:10 -0700, "Paul" wrote:

Has anyone seen a ship position as reported by AIS being off by over one
nautical mile?

Since the
"position report" message should get it's position directly from a GPS unit,
I can't understand how an error of this magnitude could occur. I have not
seen any similar errors with other ships. I will be digging deeper into the
captured data to look at the "position accuracy" and "time stamp" fields to
see if perhaps the ship was in some sort of dead reckoning mode or was
reporting other position-fixing errors.

I discovered a problem over a year ago with Furuno GPS units interfaced into
AIS systems. I reported it to the US Coast Guard & they published some info
he

http://www.uscg.mil/hq/gm/moa/docs/1-05.pdf

More info on the fix he

http://www.uscg.mil/hq/gm/moa/docs/furunosafety.pdf

So, next time you chat to them, ask them if they have a Furuno GP80 or GP90
GPS, and if they do, let them know there is a firmware upgrade. I found the
problem initially when installing an AIS base station & seeing about 10% of
the vessels transmitting positions on land when I could see them out the
window. The bust was about 250 metres in my case, and the offset to WGS-84 on
the paper charts that everyone cheated off was 250 metres. I went onboard a
couple of the vessels to check out their systems & needless to say, the
Captains were pretty shocked when I plugged my notebook into their AIS pilot
port & showed them their boat 200m up the shoreline.

Dave