Class B AIS is coming, growing pains included

“All AIS is good; and Class B is going to give people a better picture of what’s going on out there, no question about it, but my problem with it is that update rate of just once every 30 seconds,” said former tanker captain and current Penobscot Bay pilot Capt. Skip Strong. And he’s just one of several AIS experts who say that Class B transponders, the just-about-to-arrive third phase of the AIS marine safety revolution, will take some getting used to.
To review, phase one was the mandated installation of Class A AIS transponders on almost all large commercial vessels, letting them “see” each other with more precision and detail than the most sophisticated radar (which they neatly complement). Phase two was the arrival during the last two years of AIS receivers that, along with compatible charting programs and plotters, let smaller boats “see” the big guys too. Now even some little recreational-level plotters can deliver a wealth of critical information about nearby – possibly too nearby – ships, and both Raymarine and Furuno just introduced sophisticated AIS receivers.
Phase three will consist of low-cost, easy-to-install Class B Automatic Identification System transponders designed to allow smaller vessels to actually participate in, not just monitor, the system. The U.S. Coast Guard recently granted its first Class B approval, and the Federal Communications Commission may follow suit any day now. (Phase three units are already being deployed in Europe.) Companies awaiting approval, or planning to offer transponders soon, include Furuno, ACR, Shine Micro, Comar and several others. And once Class B is permissible in U.S. waters, the Coast Guard hopes to extend the AIS carriage requirements to include some 17,000 additional commercial vessels, with B-grade transponders as acceptable equipment.
Those 17,000 new U.S. coastal AIS targets will consist of every self-propelled commercial vessel 65 feet and over, plus tugs over 26 feet and 600 hp, and passenger boats carrying more than 50 passengers, according to Jose Arroyo, the Coast Guard’s AIS regulatory project officer. Arroyo often gives presentations on AIS and reports that even some offshore fishermen, who were previously displeased about the prospect of constantly broadcasting their position, are now looking at Class B gear, expected to cost about $1,000, complete with GPS, as a valuable and reasonably priced safety tool. But Arroyo, like Strong, anticipates some “growing pains” with these purposely low-powered (2 watts) and slow-talking transponders.
Creating the Class B standard was delayed as the authorities struggled to ensure that the Class A system, which is working well, will never be disrupted by congestion caused by smaller B-talking boats. In fact, the B standard, just finalized in early 2006, uses a communications protocol and message slots slightly different from Class A. Simulations indicate overall system integrity, but the true test won’t happen until a lot of real Class B transponders are in use. In the meantime, several short-term glitches are apparent, and worth knowing about.
For one thing, existing AIS target plotters may not recognize all Class B data, particularly the “static” variety – boat name, type, dimensions, etc. – which is now slotted differently than anticipated in the original standard. In other words, your plotter, not to mention those on ship bridges, may show a Class B target moving across the screen – i.e. the “dynamic” data – but not its name, etc., until the plotter’s software is updated to understand the final B specifications. Class B, by the way, has much more limited user-entered static data – omitting fields like status (underway, moored, etc.), destination and ETA – but that may be a good thing as such data is often confusingly old or wrong for Class A vessels.
And now is a good time to mention that not all ships plot even Class A AIS targets as well as you might think, a fact that concerns navigation experts like Dr. Andy Norris, who chairs one of the international committees concerned with such matters. While Class A transponders must constantly monitor both AIS frequencies (Class Bs will too), their mandated minimum display is just three lines of text describing the nearest targets, and that’s all some economy-grade bridges have (and some personnel don’t even monitor that). Norris worries that smaller vessels will presume greater Class B-transponder visibility than may actually exist. On the other hand, many ships have superb AIS target displays, even able to “fuse” them with radar targets, and these will be become more common over the next few years.
Norris is concerned about Class B’s slow dynamic data rate. The Class A rate varies with vessel speed, maxing out at 30 times a minute for a ship going over 14 knots, which, for reference, exceeds the normal radar sweep rate of 24 rotations per minute. But the maximum rate at which any Class B will transmit its position, speed, heading, etc. is twice a minute.
That slow rate may produce odd effects. I haven’t seen the results yet, but I am picturing a sort of jumping-bean plot that may be especially confusing if the Class B vessel is maneuvering at speed. Or, as Strong put it, “Plotting a 6-knot sailboat, no big deal, but some knucklehead going 30 knots €¦ you’ll have to understand what Class B’s limitations are.”
Software could do old-fashioned dead reckoning on Class B targets – using their heading and speed to advance their plots – but it will only work well on targets that maintain course, and the idea of it not working well worries many developers. Strong and others predict that a graphical solution will be settled on eventually – say, a DR’d target symbol that fades as it ages – but in the meantime, prepare yourself for jumping-bean AIS plots. The effect will be even worse if you are using one of the inexpensive one-channel-at-a-time AIS receivers that have been popular during phase two. They typically switch back and forth between the two AIS VHF channels, receiving only half the total messages sent, which was good enough for Class A but means a full minute plot lag for Class B targets.
Besides the likelihood that one-channel-at-a-time receivers will go out of fashion aboard unmandated vessels, some skippers – mandated or not – are going to step up to Class A transponders for their superior safety performance. At the moment, any boat can use a Class A unit, which generally costs about $5,000 installed.
I should add here that, on top of the predictable growing pains, a few mariners are truly skeptical about Class B, feeling that the standard is too constrained for effective collision avoidance, and that the authorities are much more interested in using it for Big-Brother-type coastal monitoring anyway. In fact the Coast Guard does see Class B as a possibly valuable element in homeland security, but that’s in addition to its tremendous safety potential. I’ve heard the undeniable enthusiasm in Arroyo’s voice. Norris, too, is an overall believer, writing that in “educated” hands Class B will be “a powerful and affordable on-board tool to avoid close contact with SOLAS vessels.” But let’s give the last words to Skip Strong, “All AIS is good; but if you rely on it too much, you’re screwed!”

After 30 years as a commercial crewman, yacht captain and navigation instructor, Ben Ellison now writes about marine electronics for numerous publications and edits the blog€.

By Professional Mariner Staff