Constant Offset - Problem & Advice

Problem encountered:

I observed a constant off-set of AIS position on a vessel I recently piloted. The error remained constant throughout the port transit. Whilst on-board, I managed to identify where the problem was originating. The vessel was equipped with two GPS receivers but they were not providing the same position. Checking with the chart one of the units was providing an accurate position but the other was giving the same erroneous position as the AIS unit. Checking the “own data” function of the AIS confirmed that the position used by this AIS unit was from the erroneous GPS. Unfortunately the Captain spoke very poor English but he indicated that the AIS could not be re-programmed to use the correct GPS unit. I was very puzzled by all this because I always understood that the AIS units had their own integrated GPS receiver which were independent of the ships main system. I also cannot understand why the position was not only inaccurate but so consistently adrift.
I can only think of two reasons:

• The GPS had an error in its internal clock
• It was a GLONASS rather than GPS unit.

On the same screen there is also a vessel alongside the jetty which is also not in the correct position. This small offset is common and I understand that this may be caused by the fact that most AIS units have an internal GPS which is a basic low grade unit rather than a DGPS unit and it is therefore prone to normal GPS signal error. Would this be a correct assumption?

Technical response:

There are several reasons why a GPS may be giving an erroneous error. The most common problem is on older equipment, where it has been set to a datum of other than WGS84. On newer GPS equipment the messages between the GPS and AIS would make the AIS alarm if the GPS was not set to WGS84. I suspect that it wasn't a Glonass problem as the error remained constant. Because Glonass does not have a full constellation of satellites, errors can occur but the magnitude of these errors will change fairly significantly over, say, an hour.

If there are two GNSS (eg GPS) on board one should be designated the primary and the other the secondary. The primary should normally be used unless there is a fault. On some AIS equipment selection of the primary or secondary external GNSS is possible. Most only have provision for one external GNSS. If this is faulty the AIS should be set to the internal GNSS. The Master should have set the AIS to its internal GNSS. Unfortunately, most users have not been properly trained in AIS.

Obviously, it is good practice to use the same positional information for the navigation of the ship and to broadcast via AIS, whenever possible.

In US waters the Coast Guard would take aggressive action against a vessel with such gross inaccuracies in its broadcast position.

With regards to the vessel alongside, varying offsets of up to about 15 metres can be observed when DGPS is not being used. If there is a constant small offset it could be due to an incorrect datum being set in the external GNSS or, perhaps more probably, that the GNSS antenna coordinates have not been set up correctly within the AIS.