AIS: NOT the one-stop-shop anti-collision tool

11:20 PM Sun 18 Apr 2010 GMT

'I called him on the VHF and he didn't answer!' .

Ian Heffernan is a Master Mariner with many years experience as a Mate on British Antarctic Survey ships. He has also done an 18 months stint on the Antarctic research station on Adelaide Island.

While he is also a keen sailor planning his own long range cruise, the advice and opinions he gives here are as a result of his years of professional experience:

Let me start this piece with what seems to be a little known fact among some of the recreational boating community and one that may have marine electronics vendors howling in disagreement, that is, AIS is not an anti-collision tool and should not be used or relied upon for this purpose.

This is not simply my opinion or yet another 'expert' weighing into the argument, this is the advice promulgated by the International Maritime Organisation (IMO) in Resolution A.917(22) 'Guidelines for the onboard operational use of shipbourne automatic identification systems (AIS)'.

In plain English, these are the guys who make the rules, cross them at your peril. Now this is not to say rip out your brand new AIS and return to peering off into the darkness hoping your radar, if indeed you are fortunate enough to have one, will pick up that container ship coming directly for you at 28 knots.

AIS is a fantastic tool but it must be remembered that it is just that, a tool, an aid to situational awareness. Ah that great phrase 'situational awareness', it's the not so new buzz word in collision avoidance. What it basically means is being aware of what's going on around you and for that AIS is a formidable, albeit on occasion unreliable, assistant.

The use of the word unreliable may be of concern to some but before dismissing me completely you would do well to read some of the many reports on the subject. A study in the UK on vessels transiting the Dover straits recorded numerous errors in transmitted AIS data. The good news is that the error rate is dropping; the bad news is that in 2007, 3.5% of vessels were still transmitting errors.

AIS - an aid, but not a one-stop solution - .. .

Another research paper published in 2007 claims 8% of the vessels they studied transmitted erroneous data. I have conducted my own studies recently that again back up these claims. But please don't just take my word for it, ask anybody that has been using AIS for an extended period of time if they have ever noticed any problems, a ship turning up on the horizon but nothing on the AIS, ships going backwards, wrong name, wrong destination, at anchor while steaming?

There are a whole host of possible errors. Where AIS comes unstuck is the fact that for it to operate properly there must be human interaction and where you have human interaction you have the potential for error. Research has shown that 80 to 85% of all recorded maritime accidents are directly due to human error or associated with human error.

Now let us for a minute say that you are going to ignore these human errors, those percentages aren't so bad and hey all you need to do is make sure your AIS is working properly and the bad guys will see you - right? All well and good provided your AIS is actually visible to them, this may not be as straightforward as some would have you believe. The majority of recreational vessels are fitted with class B systems, vessels over 300gt are fitted with class A systems.

These are by no means separate systems and should work together but unfortunately research has shown that this is not necessarily always the case. In certain circumstances AIS-B transmissions may not be detected by AIS-A equipment. The situation most likely is one where the name of a vessel fitted with AIS-B is not displayed but instead the vessels MMSI number is displayed. The report indicates that this problem is more likely with older AIS-A units trying to identify newer AIS-B units.

I have noticed that there seems to be some concern around ships 'turning off' AIS-B reception. What frequently causes this confusion is the difference between how AIS-A and AIS-B works. It is complicated and long winded but in a nutshell AIS-B is a more 'polite' version of AIS-A.

AIS data is broadcast by means of VHF radio, and the VHF frequency is shared by dividing into 2250 time slots repeated every 60 seconds. Class A units use transmissions that are self organised into existing free slots whereas class B units listen to the traffic to determine free slots and decide when to report, hence the more 'polite' approach.

In theory, should all the time slots be used up by class A units the class B units could get squeezed out but there would have to be an extraordinary amount of ships around you for that to happen and in that case I think checking AIS would be fairly low on the priority list of all concerned. I should also mention that some future class B systems might adopt the same approach to transmitting data as the class A systems.

So now we have three reasons why you should not depend too heavily on the AIS: Human error, the possibility of your AIS not being detected by another station, the IMO telling us not to use AIS as an anti-collision device.

Then there's a fourth aspect: Did you know that the master of a vessel has the option to turn off his or her AIS if he/she believes the continual operation of it might compromise the safety or security of his/her ship? It's a point worth remembering with the ever-increasing pirate threats on the world's merchant fleet. Have you ever experienced a problem with GPS? Where do you think AIS gets its positional information?

A fifth question is that of GPS offsets: I've read reports of ships intentionally entering an offset in their GPS set in order to be able to plot position directly onto a chart (you'll find these on many charts, look for the satellite derived positions part). The problem is this offset is then fed to the AIS which transmits an incorrect position for the vessel (or at least a position which does not agree with what your GPS is showing), this can at times be a considerable difference - up to two miles have been reported.

It seems to be that one of the primary uses of AIS is to identify that annoying ship and give it a shout on the VHF just to let them know your there. I will admit that in exceptional circumstances a vessel may have no recourse other than to contact another and arrange a mutually acceptable passing. However this is the exception, most definitely not the rule.

The international regulations for preventing collision at sea were drawn up to cover the majority of situations any of us may ever encounter, so they are best used. There is no excuse for picking up the VHF and attempting to negotiate collision avoidance unless in exceptional circumstances. If it appears that the offending vessel has not seen you then, to paraphrase rule 17 (action by stand-on vessel), get out of the way!

Whether you like it or not you have an obligation to avoid collision by positive action made in ample time and with due regard to the observance of good seamanship, and nowhere in the rules is a VHF mentioned. While ships commonly talk to each other to negotiate their passing, as a judge in a well-known collision case said 'any attempt to use VHF to agree the manner of passing is fraught with the danger of misunderstanding' and yes this still applies in this age of AIS and the readily identified ship.

My advice is simple; instead of wasting your time and distracting the officer on the bridge, who may in any case be difficult to contact, in normal circumstances invest that energy into avoiding collision.

AIS is a great aid to collision avoidance. It is intended to enhance the safety of life at sea by providing additional information to assist situational awareness. Use it to augment the resources you already have whether that be the mark 1 eyeball or an all singing, all dancing, radar fitted with ARPA but please do not be misguided into thinking that AIS is the ultimate quick fix for avoiding collisions at sea.

by Ian Heffernan