AIS Oddities

[steve77]

Had two unusual things on AIS the past few days. The first was on the trip from Capri to Napoli. LOTS of passenger ferries on this route, and I was watching one northbound and one southbound that were passing a few miles east of me. They passed within half a mile of one another, and when the targets separated the northbound ferry appeared to be moving sideways. I checked details on the AIS and his heading was 105, but course was 010. The display on the chart plotter showed him moving sideways at almost 30 knots!

The second was a little harder for me to explain. We were north of Napoli and my wife had the helm while I napped. When I woke up, she told me that a 78 foot fishing vessel was on AIS about a quarter of a mile off the port beam but she couldn't find it. We looked, but there was nothing there. The course, SOG, etc. all showed this to be a vessel underway, but we couldn't see it. I turned on the radar and found what I think was the vessel, about three miles off to starboard. I watched it for a while and it appeared to track the course and speed of the phony AIS target, so I believe that was it.

This second one is puzzling to me. How can one's AIS transmitted data show a position so far off of actual?

Cheers!

Steve

[chris mac]

We just finished a charter in the gulf islands. Lots of ferries there, I made sure to keep track of any close to us. At one point I showed a ferry passing in front of us about a little over a,mile away. A second ais target showed in his wake. No info was listed except speed of slightly less than the ferry and somewhere around 70 feet(can't remember exact length)
We never saw it, as it crossed or after we passed the ferries wake.
So it was either a ghost or underwater????
Or electronics sometimes mess up and we have to remember to look up and trust our eyes.

[Paul Elliott]

It's not unheard of for a vessel's heading data to be wrong, which will show the boat pointing in some random direction.

As for the wrong position being reported, I used to see this from time to time, with perhaps a one or two-mile error. The reason (or one reason) was that in some old chartplotter systems the GPS output was "adjusted" to compensate for chart datum differences, and this adjusted position was then fed to the AIS transponder. This was obviously wrong, and technical notices were sent out to inform shipowners that they should correct this problem. I think this has been largely fixed, but perhaps not in all cases?

[steve77]

Yes, the incorrect heading is easy to explain. I could do that on our boat by unmounting the electronic compass and turning it 90 degrees. I Just thought it odd that it happened while the vessel was underway.

The explanation you give for the second oddity sounds reasonable. Thanks for that!

Cheers!

Steve