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

I was recently sailing from Hawaii to San Francisco, and encountered a
freighter that eventually passed about one mile north of us. We were both
heading east. The strange thing is that their position as reported by their
AIS transmitter showed them passing about one mile to the south of us! I am
using my own AIS program, so I assumed at the time that there was a bug in
my code. I captured the raw NMEA data (an option in my program), and after
looking at it, and running it through some commercial programs, it seems
that my code was OK and the ship was reporting its position incorrectly.

I've seen bad data show up in the "static and voyage related data" messages,
but until now haven't seen bad latitude / longitude data. The reference
position offset fields only allow for up to 63 meters of beam offset, so
that couldn't account for the position error I was seeing. 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 guess that this underscores the need for a visual or radar confirmation
during a close AIS encounter! Obviously the calculated CPA was quite
different from the actual.

FYI, here is the ship data my program saved:

Time: 8/4/06 3:01:52 PM
Name: LADY MADONNA
Callsign: 3EKW8
Latitude: 40.728833 deg
Longitude: -152.034833 deg
SOG: 12.2 kt
COG: 86.0 deg
Destination: CEDROS_MEXICO
Ship Type: Cargo ship
Ship Status: Under Way using engine
MMSI: 352730000

Any thoughts?

Thanks,
Paul