System Errors?

A few days ago I was piloting a container ship up the Fraser River (west coast Canada near Vancouver) when I observed on my AIS display a tug heading my way on a reciprocal course at about 10 knots. The problem was that the vessel was not there! Not on radar, visual nor in the VTS system.

Later that day I called the company who own and operate the tug and learned that the tug had been in Allied Shipyard(north vancouver) for the last week. Allied Shipyard is 11 NM as the crow flies from where I observed the AIS target.

So there where two errors:

• Error in position of 11 NM
• Error in course and speed. Actual co&sp Zero at shipyard,,,AIS indicated course made good about 250 deg true..Speed about 10 Kts.

The operators of the tug had no explanation for this anomaly and I have never heard of errors of this magnitude before. If anyone has any theories on this one, I'd love to hear them.

If errors such as this can occur then I have to question the value and credibility of the system.

Feedback

I read with interest the above reported problem (from a Pilot taking a container ship up the Fraser River and who saw a tug on a reciprocal course on his AIS, when the tug was actually in a dockyard about 11 miles away). I would humbly suggest the following possible explanations:

1. The AIS display that the Pilot was using may not have been properly interpreting the "invalid position"  or "invalid course and speed" bits in the AIS transmissions from the Tug. If these bits are not interpreted properly by the receiving AIS display then the vessel concerned can "appear" miles away from where she actually is. Transmissions from Tugs in particular seem to be affected by this on a regular basis and my guess is that it is because of the nature of their duties, as their GPS antennas can become shielded due to the close proximity of high sided vessels, dock walls and other obstructions etc.
    
2. Not particularly likely, but many GPS receivers have a simulator mode and it might just have been that a technician was using simulator mode when servicing equipment on the Tug.
   Not sure about this, as I would hope that the AIS on the Tug would alarm if this was the case.
    
3. Not particularly likely,  but the AIS specification does include provision for re-transmission of "pseudo targets" by a suitably equipped VTS base station.