AIS direction indicators

[[2574]]

i think we are all familiar with the AIS triangles that appear on the AIS or chart plotter screen to indicate the position of an AIS equipped vessel. The triangle indicates the direction of travel of the vessel via the "point" of the triangle pointing in the direction of COG. If one has predicted heading switched on then a longer line indicates extended COG.

However, whilst sailing on Sunday I noticed that the AIS signal arrow of an eastbound passenger ferry indicated a COG which was 180deg opposed to her direction of travel, it indicated a Westbound COG. Now maybe this was because this was a "both ended" ferry and as far as the AIS was concerned it was being driven backwards.

I checked the COG being reported in the AIS details screen for the vessel which was correctly reporting the COG as heading east. Maybe this apparent contradiction is always the case with both ended ferries but it was a bit of a surprise to me and could be a significant factor in poor visibility. One's own radar and AIS arrow would be in conflict or, if one was relying upon AIS only it could be much more serious. One to note - don't absolutely always assume that the indicated direction of travel is correct. I didn't check whether the CPA was also screwed, I should have done but didn't think of it.

 

[prv]

Is the arrow's direction dictated by CoG or by heading? Class A AIS will normally report both.

The CoG is just the movement of the GPS antenna and will be correct regardless of whether the ferry is travelling forwards, backwards, or sideways. But the heading of a double-ended ferry will depend on which end is currently the front, and while this could be automated in several different ways it wouldn't surprise me if there was simply a switch on the bridge. If there is a switch, it *really* wouldn't surprise me if the crew occasionally forgot (or didn't bother) to switch it.

Pete

 

[johnalison]

The arrows are not a good indication of a vessel's movement. On my e7, if there is a vessel that I am interested in I wil always instruct my set to add a vector. This gives an extended line showing the target's course relative to my own boat. this is the same as its charted course when I am stationary, but not otherwise, and this has to be taken into account.

 

[MM5AHO]

I've noted this with Western Ferries (Gourock to Dunoon), which are all doublended types. (and names all "sound" the same).
I suspect that they have a switch to change the heading, as on occasions any of them shows that they're going backwards, but most of the time they're showing correctly.

 

[macd]

It's a common experience with short-hop double-ender ferries in Greece, of which there are many. At a guess, at any one time, 50% of them are going 'backwards'. Makes no difference to collision avoidance. Edit: assuming you've switched on course vectors, as per johnalison.

 

[lpdsn]

Same with the Strangford Narrows ferries. Heading always points at the same 'nominal' bow but COG depends on the actual movement, so sometimes appears they are going astern (well astern-ish as they're usually suffering significant tidal effects).

 

[dunedin]

As well as double ended ferries, seen a few other sizeable boats with AIS heading being substantially differnt from COG. Sometimes seems to be if something is not switched on or functioning correctly on the transmitting vessel, feeding incorrect data to their AIS.
One commercial ship coming up behind us, but showing a heading 90 degrees different, prompted me to radio them to question this - of course they said "no, everything is fine", but 60 seconds after the call the AIS heading was fixed.

 

[pandroid]

One thing I don't understand is why my (Raymarine) plotter keeps giving me alarms for targets that a)are heading in a completely different direction from me and are never going to cross my path and/or b) are stationary. (E.g tied up in a marina). Is this just sh1t software on Raymarine's part?

 

[lpdsn]

One thing I don't understand is why my (Raymarine) plotter keeps giving me alarms for targets that a)are heading in a completely different direction from me and are never going to cross my path and/or b) are stationary. (E.g tied up in a marina). Is this just sh1t software on Raymarine's part?

The i70 has the same irritating feature. As a result I turn off the alarm and only turn it back on when out at sea, if I remember. It works off only two factors I believe, CPA and TCPA. So, say, everything that will be within a mile anytime within the next ten minutes. Takes no account of moored status (which in itself isn't reliable) or SOG.

 

[Uricanejack]

Is the arrow's direction dictated by CoG or by heading? Class A AIS will normally report both.

The CoG is just the movement of the GPS antenna and will be correct regardless of whether the ferry is travelling forwards, backwards, or sideways. But the heading of a double-ended ferry will depend on which end is currently the front, and while this could be automated in several different ways it wouldn't surprise me if there was simply a switch on the bridge. If there is a switch, it *really* wouldn't surprise me if the crew occasionally forgot (or didn't bother) to switch it.

Pete

Probably correct.

Some types of double ended ferries. Have separate control systems for going in each direction. Some of the bigger ones here even separate bridges. On those ships the switching of Control systems, Navigation lights, and AIS. Are all hooked in to a synchronised control transfer system. With separate back up transfer in case of failure.

Some of the smaller double ended ferries, have one small wheelhouse and control consol. There is no need to transfer control, the lights are usually changed over by a manual switch. Some of the older ones predate AIS and initially had a separate switch for AIS to be switched over. This may or may not have later been synchronised with the navigation lights.

The ferries are funny. Often leaving there Nav lights on in the dock.
A small local ferry I routinely travel on. I can usually tell when it gets to the other side, when I see the Nav lights change. Occasionally they forget. I suppose when you do it a dozen or more times a day it gets forgotten occasionally.

 

[Kylora]

The arrows are not a good indication of a vessel's movement. On my e7, if there is a vessel that I am interested in I wil always instruct my set to add a vector. This gives an extended line showing ,the target's course relative to my own boat. this is the same as its charted course when I am stationary, but not otherwise, and this has to be taken into account.

I'm new to chart plotters and ais - can you explain your comments more fully? I have a Raymarine E7 too, and wondered why the other boat 's vector line looks wrong. For example, you can see the line on the chart that a ferry will be following, but her vector line is at an angle to it.

TIA

Ash

 

[dunedin]

I'm new to chart plotters and ais - can you explain your comments more fully? I have a Raymarine E7 too, and wondered why the other boat 's vector line looks wrong. For example, you can see the line on the chart that a ferry will be following, but her vector line is at an angle to it.

TIA

Ash

On the situation we had (also using Raymarine e7) I saw a ship a couple of miles directly astern. The angle of the little AIS ship icon was at 90 degrees to our course so assumed safe to ignore as already across our track astern - and so didn't bother to switch on the optional AIS vector.
A few minutes later I noticed that the course shown by the AIS icon was still 90 degrees to our track - but it had got much closer and was still astern. Hence investigated more closely and spotted that the heading was 90 degrees different from its actual COG. I put the AIS vector on at that point and this accurately calculated that the ship was shaping to pass slightly to port. That's when I VHF called them using the name shown on the AIS to query their AIS heading settings - which rapidly got corrected afterwards

 

[johnalison]

On the situation we had (also using Raymarine e7) I saw a ship a couple of miles directly astern. The angle of the little AIS ship icon was at 90 degrees to our course so assumed safe to ignore as already across our track astern - and so didn't bother to switch on the optional AIS vector.
A few minutes later I noticed that the course shown by the AIS icon was still 90 degrees to our track - but it had got much closer and was still astern. Hence investigated more closely and spotted that the heading was 90 degrees different from its actual COG. I put the AIS vector on at that point and this accurately calculated that the ship was shaping to pass slightly to port. That's when I VHF called them using the name shown on the AIS to query their AIS heading settings - which rapidly got corrected afterwards

You have to imagine that you have two entirely separate displays in the same space. In order to interpret the very useful AIS vector facility, you must ignore the chart completely and imagine you are looking at something like a radar screen with your boat and the AIS symbols only. I think you will find that if you do this it will all become clear. The vector line will show the target's course relative to your boat, not the ground. If it appears to be missing you by, say, a mile, then this is correct. If the target appears to be heading for a charted object, then this is not the case.

 

[dunedin]

You have to imagine that you have two entirely separate displays in the same space. In order to interpret the very useful AIS vector facility, you must ignore the chart completely and imagine you are looking at something like a radar screen with your boat and the AIS symbols only. I think you will find that if you do this it will all become clear. The vector line will show the target's course relative to your boat, not the ground. If it appears to be missing you by, say, a mile, then this is correct. If the target appears to be heading for a charted object, then this is not the case.

The situation I was describing was the AIS icon when the transmitting vessel has incorrect heading data.

Also worth noting that the optional AIS Vector setting works differently in the newer e-Series (where it is a relative bearing) from the older E-series (where it is still absolute heading on the latest available software). We have both types of plotter which showed this difference starkly.

 

[Kylora]

On the situation we had (also using Raymarine e7) I saw a ship a couple of miles directly astern. The angle of the little AIS ship icon was at 90 degrees to our course so assumed safe to ignore as already across our track astern -.......... AIS heading settings - which rapidly got corrected afterwards

I haven't relied too much on the direction of the triangle. I assume that it is set by the Gyro heading info in Class A, but don't know about Class B.

Ash

 

[Kylora]

You have to imagine that you have two entirely separate displays in the same space. In order to interpret the very useful AIS vector facility, you must ignore the chart completely and imagine you are looking at something like a radar screen with your boat and the AIS symbols only. I think you will find that if you do this it will all become clear. The vector line will show the target's course relative to your boat, not the ground. If it appears to be missing you by, say, a mile, then this is correct. If the target appears to be heading for a charted object, then this is not the case.

Thanks for the explanation - that makes sense to me.

What about the info in the little blue box? Is the speed and bearing relative to my vessel too?

I can see that I need to spend more time on study and practice.

TIA

Ash

 

[johnalison]

Thanks for the explanation - that makes sense to me.

What about the info in the little blue box? Is the speed and bearing relative to my vessel too?

I can see that I need to spend more time on study and practice.

TIA

Ash

I don't use little blue boxes much. As far as I remember, the information given is in terms of course and speed, so that would need to be plotted on the chart, which would be much harder, though CPA and time are also given.

 

[Oldfellah]

I have been using the Comar display for my AIS. It takes the signal from my radio which incorporates an AIS receiver and I have found that the readouts always seem spot on. I never rely on the arrows but always open the information box which gives accurate COG and speed. I have found it invaluable when crossing the shipping lanes and by having the information shown on a display separate from my plotter means a very uncluttered look to it. Just what my old eyes need.

 

[[70521]]

If I am that close to anything I can usually see it. AIS is a guide, OK a pretty accurate one, nothing more nothing less.

 

[johnalison]

If I am that close to anything I can usually see it. AIS is a guide, OK a pretty accurate one, nothing more nothing less.

I believe that yachtsmen occasionally managed to cross the Channel or North Sea before the advent of AIS and sometimes came to no harm. However, as an adjunct to radar in restricted visibility it is very useful so long as its ,imitations are recognised. Although this is an infrequent occurrence for me, its chief function is to increase the enjoyment of sailing with my wife, who is more apprehensive of big things than me. If I can see a ship on the screen ten miles away and tell her that under present conditions the ship will be 1.6 miles astern when we meet, that is a great reassurance for both of us, since this is not evident when the ship is first seen over the horizon. It also makes complex situations much easier to sort out, as happened to us when approaching Blankenberg across the entrance to Zeebrugge this year, with a number of ships going in different directions. It was simple to click on a ship and see which port it was heading for and make allowances.

This is just one example, but as I say, it increases the enjoyment, which is why I go sailing.