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The Automatic Identification System

Whether you want to join the Automatic Identification System or just listen in, your gear choices are multiplying.
Boating Safety

A.I.S. receivers are unregulated and thus vary more, even in raw performance. For instance, I think that you should avoid the older, extra-inexpensive models, which only listen to one A.I.S. channel at a time. They worked OK when all A.I.S. was Class A, but they cause especially jumpy Class B plotting because B only transmits dynamic data every 30 seconds on alternate channels. Besides, there's a new generation of small, inexpensive true dual receivers that are popping up everywhere. Digital Yacht has one entirely built into what looks like a GPS mushroom with a stubby VHF on top; another is built into a 5-inch C-Map plotter.

And perhaps the hottest A.I.S. product of the season is Standard Horizon's GX2100, which combines a full-feature DSC VHF with an A.I.S. receiver, including a small plotting screen, automated target calls, and even backup navigation functions, for about $400. We'll no doubt see A.I.S. receivers built into more plotters, radios, and who knows what else.

So what should you do about A.I.S.? Many sailors are already using receivers or transponders (my preference) and loving them. But if you wait, A.I.S. will become more widespread, easier to use, and less expensive. You can't lose or win, depending on how you look at it!

To keep up with the latest developments, stop by my blog (

The ABCs of A.I.S.

The International Maritime Organization created the Automatic Identfication System back in the 1990s, and it's proven itself exceptionally robust and useful. It was based in part on the digital-selective-calling technology already developed to automate some marine-radio functions. In fact, a new A.I.S. transponder has to be programmed with the same unique Maritime Mobile Service Identity number that, one hopes, is already programmed into a boat's D.S.C.-enabled VHF. What's more, some integrated systems can easily "direct dial" VHF calls to A.I.S. targets using D.S.C. As an aside, M.M.S.I. numbers for vessels intending to head overseas should be obtained along with an F.C.C. ship's license, but boats staying within U.S. waters can use free ones available from BoatU.S., SeaTow, and other organizations.

All A.I.S. transponders use two redundant dedicated VHF channels, 87B and 88B, to broadcast standardized digital messages at carefully organized intervals. Class A was the first style of transponder developed and is the type mandated on SOLAS ships (though any boat can install one). A Class A unit broadcasts at 12 watts, with a range of 20 to 60 miles, depending on antenna height, and it transmits what's called a dynamic data message-a vessel's lat/long, heading, speed, and other fast-changing values, plus its M.M.S.I.-every 2 to10 seconds when under way, depending on speed and rate of turn. The longer static data message-vessel name, dimensions, type, destination, E.T.A., and the like-goes out every six minutes, which explains why you'll often see a target plotted well before it gets a name. But such data thrift, along with the underlying scheme of time-slot sharing, is why hundreds of nearby vessels can all receive each other's data without interference.

Class B transponders, meant for smaller vessels, were always part of the I.M.O.'s plan, but the specification wasn't completed until 2006, and the finished products weren't available in the United States until late 2008. A Class B transmits at 2 watts, with a range of 5 to 12 miles. Its dynamic data message goes out every 30 seconds if your vessel is going over 2 knots, and the every-6-minute static data isn't as detailed. Class B transponders have dual receivers to get all Class A and Class B transmissions, as do Class A transponders. Devices that only receive A.I.S. signals are sometimes called Class C, but that's a misnomer as they're not regulated, while the transponders are built and tested to very tight specs.

The success of A.I.S. is hard to overstate. Glitches have been relatively minor, while the extended-use possibilities are major. Some buoys already exist that identify themselves using A.I.S., including a few that also transmit local weather phenomenon, such as currents in hairy passes, using especially designed A.I.S. messages. Lifeboat rescue beacons (called SARTs) using A.I.S. instead of electronic radar reflectors are just becoming available, and A.I.S.-enabled crew-overboard devices are being studied. The U.S. Coast Guard is building a system to monitor all A.I.S.-equipped traffic within 200 miles of the coast, and though the goal is primarily homeland security, the safety implications are significant. Hobbyists have also set up shore listening stations, many of which are tied in with various websites. In fact, one of the best ways to learn about A.I.S. is via ( Choose "English Channel" in the upper left "Go to region" box, then contemplate what a difference A.I.S. would make if you were crossing that busy waterway.

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The U.S. Coast Guard is asking all boat owners and operators to help reduce fatalities, injuries, property damage, and associated healthcare costs related to recreational boating accidents by taking personal responsibility for their own safety and the safety of their passengers. Essential steps include: wearing a life jacket at all times and requiring passengers to do the same; never boating under the influence (BUI); successfully completing a boating safety course; and getting a Vessel Safety Check (VSC) annually from local U.S. Coast Guard Auxiliary, United States Power Squadrons(r), or your state boating agency's Vessel Examiners. The U.S. Coast Guard reminds all boaters to "Boat Responsibly!" For more tips on boating safety, visit

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