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

8:30 PM Sat 10 Apr 2010 GMT

'Shipping lanes can get pretty busy - ORBVOMM AIS data, showing ships at sea graphically represented in the Indian ocean' .

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):

Can they turn off your AIS Class B signal? - Jeff Robbins Click Here to view large photo

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 here

by Ben Ellison