AIS orientation on plotter

[Guitarrich]

I've been cruising in Greece since April 2010 (this time out). My fabulous suite of electronics includes a chartplotter which overlays AIS symbols. My wife is particularly enamoured of this function as she always used to fret most about large vessels when she was on watch alone.

In recent days I have had problems. Four seperate vessels; a cargo ship and three different ferries have had faulty compass information. Now the little AIS symbol is orentated according to the (transmitting) vessel's electronic compass, although the collision avoidance stats are worked out on COG/SOG, (thank goodness).

So, most recent example - crossing the windy steep seas of the Cyclades - I kept a keen eye on the plotter, and all other means in accordance with rule 5 - honestly!

I have to admit that, especially with the genoa deployed that taking a good visual of the entire horizon doesn't necessarily happen every single minute. I do get used to watching a screen full of little grey triangles, and like it when none of them are pointing at me!

Now I try to be dilligent, and I don't think this last ferry got ALL that close, but the fact is his "Heading" information and his real "COG" were SO disparate that we COULD have come a cropper, say in poor vis and if the radar was not in use.

After a little digging, I've learned that the IEC Spec: "IEC 62388 Ed.1 Maritime navigation and radio-communication equipment and systems" in annex J states that the symbol shall be alligned according to HEADING information, or COG if HDG is unavailable.

Now I might well have agreed with this part of the policy, had I not seen, on four occasions in the last few weeks vessels with a problematic compass. I fear this may cause an accident in the future, and would personally like to see the IEC spec changed, so that the orientation of the symbol accords with REAL LIFE, ie COG, and not Heading.

I agree, in an ideal world, if every ship was perfectly maintained then heading would be better. But the shipping world is far from ideal.

Or, how about this; if the HDG and COG are more than say 15 degrees apart, then use COG?

If you care to look at the attached jpegs, one shows the details the ship was transmitting, (COG and Heading more than 60 degrees apart!), the other, with vectors turned on, shows the vessel as it appeared on the chartplotter. It looked a lot more innocent BEFORE I turned on the vectors BTW!

Sorry for such a dull post, but thanks for reading!

Rich

 

[fireball]

you're not the only one to have noticed these sorts of anomalies ...
In the solent it is usually the Wightlink that are able to travel in completely different direction to their heading (error or not I don't know) ... however, the easy way to resolve it in our plotter is to turn on their vectpr - as you have done ... then check out the vector line rather than the icon ...

If you're really lucky the programmers of your CP may give you an option to use COG over a certain speed .. which would eliminate most of the problems - but I wouldn't hold your breath ...

 

[Bilgediver]

you're not the only one to have noticed these sorts of anomalies ...
In the solent it is usually the Wightlink that are able to travel in completely different direction to their heading (error or not I don't know)

Are Whitelink double ended so forwards is backwards


The watch officer maybe is too busy trying to demolish yachts to switch the AIS to a reciprocal???

 

[duncan]

not dull - rather the opposite

whilst there will be times when there are significant differences between the HDG and COG I tend to agree that this should trigger some additional response (flashing icon for example)

having to retain vector lines all the time can make for a very confused screen!

 

[deep denial]

surely in tidal waters the heading is a particularly useless piece of information. I had no idea that anything other than cog was used.

 

[noelex]

I have not got AIS yet but your suggestion sounds very sensible to me.
Don't forget to keep looking under the Genoa I am sailing the same area

 

[Leighb]

If the transmitting vessel is a tug the heading could genuinely be very different from the COG

 

[Guitarrich]

AIS - good consensus

Thanks all for your positive comments on this. I too have noticed the IOW ferry going "backwards" but didn't attach too much importance to it at the time - yes, no doubt OOW sleepy!

A tug would quite possibly be pointing a different direction to travel, but this would probably be obvious from other aspects/context etc. It's also perfectly true that a vessel in a big tideway may be pointing a strange way. Overall though, at it seems there is consensus on this it would be better if COG were used either instead of HDG or where they are very different values.

I'm afraid that the makers of my plotter simply quoted the "official specs" at me and that was that. The days have passed when manufacturers had the luxuary to go out of their way to do the RIGHT thing, rather than the fastest, most convenient thing. However, the net result is that many more of us can afford nice kit!

Thanks all,

Rich

 

[DJE]

Many ferries have drectional thrusters like these and are quite capable of going sideways even in non-tidal waters.

 

[jerrytug]

Guitarrich that`s dead interesting,thanks for posting it. Always assumed it was COG not ship`s head as well. Cheers. I can vouch that the Pompey-Fishbourne boats can do 10 ahead or astern and 5 sideways BTW! Triple VT units,one at the front,two at the other end.

 

[Bilgediver]

In recent days I have had problems. Four seperate vessels; a cargo ship and three different ferries have had faulty compass information. Now the little AIS symbol is orentated according to the (transmitting) vessel's electronic compass, although the collision avoidance stats are worked out on COG/SOG, (thank goodness).

The AIS unit on a ship has to be told certain information. It is not unusual to see the AIS transmitting incorrect information for one reason or the other and I certainly wouldn t rely on the icon for direction expecially regarding a ferry.

The AIS is programed with some informtion when installed such as the ships details and heading details however from time to time the heading seems to sneek in as a reciprocal. In the case of a double ended ferry the watch officer has to tell the AIS at every departure which is the sharp end and which is the blunt end. Sometimes they forget and the AIS thinks the ship is going backwards.

The watch officer also has to input cargo details and other details such as under sail,under power and at anchor. It is possible to see anchored ships moving at speed I believe.

It seems that it is not obvious to those on board if any of these entries are incorrect. You just have to visit an AIS site to see examples of these errors.
Just have to learn to live with them till the sytem of on board indication is improved.For now just accept that ferries often go fall speed astern between ports.

 

[bonny]

I have encountered a similar situation in the Thames estuary, a Cargo vessel apparently dragging it's anchor at 12Kts and heading straight for me (from astern). He was anchored off Margate when I passed by. He continued to drag anchor until he dissapeared from my AIS range all the way up the Thames.

 

[timbartlett]

Sorry for such a dull post, but thanks for reading!

Not a dull post at all! Interesting stuff.
Just as the advent of radar forced a rethink about collision avoidance, and led to the introduction of new concepts such as CPA, maybe AIS is about to lead us into another rethink.

... the collision avoidance stats are worked out on COG/SOG, (thank goodness)...

Why "thank goodness"?

Thinking about the way radar plotting and ARPA/MARPA work, I would suggest that collision-avoidance data is based on the change of bearing with range, and that sea-stabilzed (based on CTW/STW) ARPA is infinitely preferable to ground-stabilized (COG/SOG).

... like it when none of them are pointing at me!

I agree with you if you are stationary or if the ship is directly ahead or astern but in anything other than those three specific situations, I always feel quite relieved when I see a ship heading straight at me, because it means that I have crossed safely ahead. i.e. if he's pointing straight at you are now, then by the time he gets to where you are now, you will have moved on

 

[supra30]

ARPA/MARPA bobbins!

Why "thank goodness"?

Thinking about the way radar plotting and ARPA/MARPA work, I would suggest that collision-avoidance data is based on the change of bearing with range, and that sea-stabilzed (based on CTW/STW) ARPA is infinitely preferable to ground-stabilized (COG/SOG).

Why on earth would you suggest that?

ARPA from an unstable platform like a small yacht in any significant sea normally has pretty poor performance, even with the best fast heading sensors available.

AIS CPA/TCPA calculations on the other hand give solid results. It's not even particularly necessary for any AIS software to apply a weighted moving average approach to the yacht's COG/SOG data from the GPS as all GPS receivers are doing that already!

Regards,

Mike

 

[timbartlett]

Why on earth would you suggest that?

ARPA from an unstable platform like a small yacht in any significant sea normally has pretty poor performance, even with the best fast heading sensors available.

AIS CPA/TCPA calculations on the other hand give solid results. It's not even particularly necessary for any AIS software to apply a weighted moving average approach to the yacht's COG/SOG data from the GPS as all GPS receivers are doing that already!

Regards,

Mike

Sorry, I think we are misunderstanding each other.

I was not suggesting that MARPA is superior to AIS: I was suggesting that for collision avoidance, a vessel's track (CoG) is irrelevant: it is the course (through the water) that matters.

As I understand it, the reason AIS CPAs are so good is because they are based on rock solid position data from both vessel's GPSs: and are not messed about by the lousy speed and heading data that makes such a mockery of small craft ARPA.

I've never seen proper ARPA on a yacht, but on ships it is pretty good, because they have better sensors and data that changes less quickly. And in that context, Ground Stabilised ARPA is capable of giving information that is very precise, but very, very wrong. Sea-stabilised ARPA, on the other hand, is very much better, because it tells you what the vessels are doing, not what the tide is doing to them.

One classic example was the Wakhuna collision, in which the Nedloyd Vespucci was criticised for having their ARPA in ground stabilised mode -- not least because it led them to believe that the Wakhuna was doing three knots up-channel after the collision -- when if she was doing three knots anywhere, it was three knots downwards!

 

[supra30]

Sorry, I think we are misunderstanding each other.

I was not suggesting that MARPA is superior to AIS: I was suggesting that for collision avoidance, a vessel's track (CoG) is irrelevant: it is the course (through the water) that matters.

Sorry Tim, I think you're wrong there.

Assuming both vessels are in the same current (which you'd expect them to be in plenty of time whenever there's a real and imminent risk of collision) then having position and COG/SOG data as the basis of the calculation makes no odds at all from using 'through the water' data.

As I understand it, the reason AIS CPAs are so good is because they are based on rock solid position and SOG/COG data from both vessel's GPSs: and are not messed about by the lousy speed and heading data that makes such a mockery of small craft ARPA.

Absolutely!

Regards,

Mike

 

[timbartlett]

Assuming both vessels are in the same current (which you'd expect them to be in plenty of time whenever there's a real and imminent risk of collision) then having position and COG/SOG data as the basis of the calculation makes no odds at all from using 'through the water' data.

I see where you're coming from, Mike, and I agree that if two vessels are going to collide anyway, then it makes very little difference whether the skippers concerned refer to Track or course (any more than it matters whether they were using true or magnetic) though I'm not completely convinced that an erratic CoG/SoG caused by a small boat's ability to yaw and speedup/slow down is any better than an erratic course and speed, no matter how precisely you measure it !

The point that intrigues me is the effect that using CoG/SoG might have on skippers' efforts to avoid collision, just as the increasing use of radar affected the way people dealt with potential collisions in the 60s/70s/80s -- and was blamed for causing a few that might never have happened without it.

For instance, in my sketch below, A and B are both meant to be power driven vessels making about 6knots in a 3knot tidal stream. Looking at their courses, A sees B on his starboard bow, so A is the give way vessel. But if the skipper of A were to look only at their tracks, then he would see that he was to starboard of B's track, and might therefore believe that B was the give way vessel. Or he might just be confused, and come to the conclusion that his AIS was "wrong".

 

[supra30]

Cross current, low speed situations

AIS and RADAR can give three kinds of 'pictures' of the situation, depending upon what inputs they have available, the software in question and the setup the operator has configured.

'Traditional RADAR' style views are, I suspect, the most useful. A heading up display with bearing and range info, plus CPA/TCPA data is the simplest for assessing a collision risk. In my opinion, the best and most useful AIS displays for yotties have this capability, and they are excellent for the 'frogger' problem when crossing shipping lanes or similar high density interactions with commercial vessels. The fact that this sort of presentation allows target vectors to be extended for all 'at risk' targets to their CPAs gives instant at-a-glance info for resolving course conflicts.

As you say though, these sorts of displays may not provide a true stand-on/give-way vessel indication in a few specific (and rather rare) circumstances.

'Ground stabilised' views, I'm not keen on. That's the traditional AIS on a plotter, or RADAR overlay approach. They don't tend to provide clear info on target risk levels (because it's difficult to plot course lines and associated CPAs clearly without clutter). They are also no better at definitively resolving your stand-on vessel question either.

Sea stabilised views do provide a resolution to the stand-on vessel question... but I don't think they are available in many AIS sets or software at present - at least as far as I am aware. There are no reasons why these views couldn't be generated though, as long as CTW/Heading and STW info are available at the receiving station. No doubt expensive ECDIS systems will provide this sort of thing in the future, if only to help resolve the spurious heading data that stations sometimes provide!

For yachties though, it's irrelevant.

When you're small and doing 5 knots and the targets are big and are doing 25, we are never really stand-on vessel. The tactic is always the same, hold course while you can, then keep clear and shape to pass astern of the target of greatest concern. To deal with that, I'll take a course-up RADAR style AIS plot as my no. 1 helper every time.

Regards,

Mike