Can ships TURN OFF Class B AIS targets? The answer is NO!

by Ben Ellison on 11 Apr 2010 It has become a frequently repeated marine myth that ships, faced with a lot of clutter on their AIS screens, can TURN OFF the signals from Class B AIS targets - thus ridding themselves of the worry of potential collisions with vessels which may only scrape their paintwork.

Ben Ellison, of www.panbo.com, arguably the premier electronics blog for cruisers, and a member of the Cruisers Network Online, here addresses the issue, in his own inimitable style:

Recently, in a (to be unnamed in Sail-World) sailing magazine an anonymous writer spread AIS misinformation, reinforcing a myth that threatens to curtail adoption of Class B transponders.

After first establishing himself as a sailor who uses an AIS receiver, he writes knowingly (but ignorantly) about specific AIS requirements around the world. And then comes the doozy: 'My recollection is that AIS Class A transceivers fitted to commercial vessels have a big red IGNORE CLASS B Transceivers button to declutter their displays and concentrate on avoiding vessels that will do more than smudge their paint in a collision'. That's unadulterated BS on all counts -- the big red button DOES NOT EXIST, the watch standers who don't care about small vessels are rare, and this blowhard has probably never seen a ship's bridge to 'recollect' -- but the myth continues to spread...

Yesterday I came across an even worse instance, because the source was a industry tech support person! Cruiser Glenn Tuttle recently moved up from an AIS receiver to a transponder, and was disappointed when several cruise ships he called in shipping lanes couldn't seem to see his vessel target a couple of miles away. (I don't know what happened, but wonder if in fact they saw his dynamic target data but not the static data with his name, size, etc. -- a known issue with older Class A transponders.) At any rate, Glenn researched the problem, as he reported on the Cruisers Network Online (which he happens to moderate):

I called Electronics Unlimited in Ft Lauderdale, Florida, where I purchased the unit and antenna. They just referred me to the tech support at Simrad. I finally was able to speak to a Simrad technician who told me 'It is normal for Class A units not to be able to communicate with Class B units.' However, he continued to say 'my Class B unit should be able to be seen by all Class B units'.

That misinformation makes me want to scream! Class A and Class B AIS are carefully designed to work together; they are an integrated System. The Class B transponders I've tried have been visible to almost every ship I asked (though sometimes not the static data). Yet Glenn understandably goes on to say, 'Had I known this, I would have just replaced my old passive unit with another passive receive only unit and saved a bundle, or maybe bit the bullet and bought a Class A unit.'

My hope is that Glenn will soon have some positive Class B experiences, like being called by name while crossing a ship channel on a snotty night. My entry on that incident -- which included the theory that this ships-ignoring-Class-B fear has become a 'meme' -- drew a lively response. I'd like to add a few points:

* First of all, let's start describing this issue as 'ignoring' Class B, because 'filtering' is a vaguer term and hence confusing. For instance, a ship captain says he regularly filters Class B -- just meaning he chooses not to show Class B target name text on his plotter -- but a listener assumes that he's ignoring Class B...and the myth spreads further.

* In fact, I now understand that Coastal Explorer's option to not 'Show Class B' -- once called out here -- does not mean they are ignored. Class B targets pop up again if CPA/TCPA alarm thresholds are crossed. And I believe that Fugawi removed the 'Ignore Class B' option from its software completely.

In that same thread, incidentally, a knowledgeable commenter claimed that new ship radars and ECDIS were mandated to offer the ability to ignore Class B, but it turned out that the rules being formulated -- explained here -- are quite the opposite, allowing targets of various sorts to be filtered but demanding that dangerous ones be shown.

* When you think about it, can you imagine that the IMO would allow anyone to ignore its carefully designed anti-collision system?

Hell, no. As the good Kennebec Captain pointed out in yet another Panbo discussion on this subject, Rule 5 of the International Rules of the Road states that 'Every vessel shall at all times maintain a proper lookout by sight and hearing as well as by all available means appropriate in the prevailing circumstances and conditions so as to make a full appraisal of the situation and of the risk of collision' and those means certainly include AIS.

* To make the point stronger, can you imagine a Captain before a judge and jury trying to explain that he set his equipment to ignore Class B vessels, one of which he inadvertently ran over, because they cluttered his screen?

I think the truth here is pretty obvious. The big question is why does the myth grow that ships can and do ignore Class B transponders.

Read on that discussion and more on other aspects of electronics by going to the www.panbo.com website

To visit the Cruisers Network Online, click http://groups.yahoo.com/group/Cruisers_Network_Online/!here
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Response from Dennis P. Hogan at Simrad:

There are a couple of issues regarding AIS Class B data showing up on Class A vessels.

1) The majority of Class A vessels will have had the AIS fitted before the Static data message was defined and will therefore not decode the Class B static data.

What typically happens is that people with class B AIS ask a Class A vessel if they can see them ' Can you see ship XYZ' the answer is most likely no since the Class A cannot decode the name of the class B vessel.
The vessel will show up as a target showing position and the MMSI will be available. So if the Class B vessel had asked can you see vessel 0012345678 (mmsi number) then they will get a positive answer.

2) Another issue is likely to be down to the Class A vessel's method of displaying the AIS data either on Radar, Ecdis or ECS. These will have filters on to de-clutter the screen and I believe there are IMO rules on how to filter the Class B information on screen on Class A vessels. The target will be shown on screen as a target but any data will not be shown e.g. mmsi and any static data (even if the AIS can decode it). The data is likely to be available but the Class A operator will have to scroll over and/or select the class B target to get to the information.

So in summary all class A and Class B targets positions will show up on both Class A and Class B AIS devices. However the Static Data from Class B may not be decoded by class A devices, in addition to this the display may be filtering any class B information on screen.

This can lead to people incorrectly believing that the Class B AIS do not appear on Class A devices.

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