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Here Comes Class B Page 2

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Norris' work also inspired the diagram below, which illustrates the effect of Class B's slow dynamic data rate. The Class A rate varies with vessel speed, reaching 30 times per minute for a ship going more than 14 knots, which exceeds the normal radar sweep rate of 24 rpm. But the maximum any Class B will transmit its position, speed, heading, etc. is twice per minute, which is going to take some getting used to. I haven't seen the results yet but am picturing a sort of jumping-bean effect, which may be especially confusing if the Class B vessel is maneuvering at speed.


The diagram shows how a vessel going 30 knots for one minute would look as a target in radar, Class A AIS, and Class B AIS.

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. I'm guessing that a graphical solution will be invented eventually, but in the meantime prepare yourself for jumping bean AIS targets, which 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 these single-channel receivers will go out of fashion, receiver sensitivity will become more critical as weaker Class B messages abound. That's why it may be useful to know which Class B units use SRT's PCB, as they should all perform similarly, and why I'm keeping an eye on the American company Shine Micro, which has a sterling performance reputation based on its dual channel receivers, and its own Class B design.

I'll add here that, on top of the predictable growing pains, a few boaters 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 onboard tool to avoid close contact with SOLAS vessels."

But I'll give the last word to another nav guru, Milt Baker, because he and his wife Judy recently cruised their Nordhavn 47 Bluewater (like the Books and Chart company they founded) more than 8,500 miles using a Nobeltec dual-channel AIS receiver. Milt says, "AIS is a giant step forward that will save thousands of lives and millions of dollars in damage in the decades ahead." Bluewater is headed transatlantic this summer, equipped with a new Furuno Class A transponder, which is an increasingly more affordable option for any vessel. But most of us will wait for Class B, which may even be here by the time you read this.


Should I get an MMSI for my DSC radios and AIS from the FCC or BoatUS?—J. E., via e-mail


If your boat will stay in U.S. waters, by all means get a free MMSI (Maritime Mobile Service Identity) at BoatUS or Sea Tow, both of which make the initial online or paper process as well as later updates easy. If, on the other hand, you’re headed off to foreign ports or your yacht is more than 20 meters (65'6"), or she has an SSB radio, you’re required to get a Radio Station License, which includes an MMSI from the FCC. Be forewarned that the agency’s Universal Licensing System is notoriously complex and the resulting nontransferable ticket costs $160 for ten years. Thus the real question arises when you fit in the first category but might one day evolve into the second.

That’s because a boat should have the same MMSI for every compatible device, they are not easy to change, and the FCC will not transfer an MMSI you have already obtained. That situation bothers many knowledgeable mariners, including the National GMDSS Implementation Task Force, a largely volunteer group chartered by the U.S. Coast Guard to assist in implementing the Global Maritime Distress and Safety System. Its early 2006 petition asking the FCC to recognize existing MMSI numbers reveals frustration, suggesting that many boaters are tempted to ignore the licensing regulation “since it is well known that the rule is not enforced and appears to serve no practical purpose.” The FCC is expected to treat Class B AIS like VHF, i.e. no license will be needed if you’re in category one, but has not yet issued its ruling.

Got a marine electronics question? Write to Electronics Q&A, Power & Motoryacht, 261 Madison Ave., 6th Fl., New York, NY 10016. Fax: (212) 915-4328. e-mail PMYElectronics For fastest response, visit the PMY Electronics forum. No phone calls, please.

This article originally appeared in the March 2007 issue of Power & Motoryacht magazine.