However, note that I wrote "good display." Some sailors are justifiably concerned about A.I.S. screen clutter, and while I've yet to see any screen shots illustrating a real problem, it could become one in some areas unless some good work is done on optimizing A.I.S. plotting presentations as well as the associated alarms. While just about every multifunction display and charting program made today will overlay A.I.S. data input from a receiver or transponder, the techniques vary a great deal, and not one seems just right yet. This situation applies to ship bridges, too, and Class B enthusiasts should note that today the A.I.S. plotting on many of those bridges is actually less effective than it is on many yachts. While it's true, as noted earlier, that a Class A transponder must display all targets, often that just means a list of the transmitting ships' names, ranges, bearings, C.P.A., and T.C.P.A. (these two terms, calculated Closest Point of Approach and the Time of that event, are carried over from radar target plotting) rather than a graphic display of their position.
Sometimes when I've called ships to ask if they could see my Class B transponder, it's taken a few minutes for someone on the bridge team to look it up. In other words, don't presume that a transponder-equipped ship will necessarily see your transponder-equipped boat, not because they've turned you off, as the legend purports, but because they don't yet have A.I.S. overlay on their primary plotter and radar screens. But this, too, shall pass.
The I.M.O. is working on mandates for such A.I.S. integration along with guidelines on how to do it well. And some of those good ideas are already being borrowed by the recreational marine-electronics developers. A Garmin update last summer, for instance, started treating targets as either Active-a bold icon with name, heading line, and speed-or Inactive-a small, simple, nondistracting icon. A user can activate a target manually or let it happen automatically when its C.P.A. comes within a user-set threshold, which also turns the icon red and graphically marks the C.P.A. spot. This plotting scheme works quite well, minimizes clutter, and will, one hopes, become common, but Garmin still needs to add several parameters so users can avoid unnecessary and irritating A.I.S. audible alarms. You don't need to be buzzed, for instance, just because you putter close to a docked vessel with its transponder on.
For a good look at how nuanced A.I.S. alarms can and should be, check out the Vesper Marine AISWatchMate (www.vespermarine.com), an unusual low-power A.I.S. plotting display designed especially for bluewater sailors. The original $500 model takes NMEA 0183 output from any receiver or transponder, while the new $700 RX model includes its own receiver. Even if its 5-inch grayscale screen isn't something you want aboard, the AISWatchMate may teach you some tricks you'll want to pass along to whoever develops the plotter or software you do use to monitor A.I.S. data.
Though A.I.S. target plotting and alarming is variable and dynamic in ways that new users may find daunting, the transponder hardware element is fortunately much more stable. In fact, I doubt that there's another niche in marine electronics where basic features and performance is so similar and reliable from model to model. That makes sense both because these devices are built and tested to very strict rules and also because most of them contain core circuit boards made by a specialty company in the United Kingdom, SRT. The important differences between Class B transponders, then, are extra features.
For instance, the high-end Simrad AI50 includes a 4-inch color plotting screen with a global basemap, the ability to record voyages and targets to an S.D. card, and a SimNet/NMEA 2000 interface (in addition to the standard 0183 interface). A stand-alone A.I.S. screen is useful if you want your main navigation and plotting screen more zoomed in or turned off altogether when out to sea or in port. (Safety aside, A.I.S. can be fun for boat watching, and all Class B transponders can be switched to Silent Mode if you don't want to be watched yourself.) NMEA 2000 is an easier, more robust protocol for sharing target data around a boat and also for getting optional heading data to the transponder (so your bow plots correctly even when you're stopped or being set by current). Simrad also uses SimNet to connect the AI50 with certain of its VHF radios, so you can place a direct D.S.C. call to a target without having to key in its Maritime Mobile Service Identity number. Just being able to call target vessels by name is often cited by A.I.S. users as a major benefit; making the target's radio ring distinctively is even better.
Icom has just introduced the MA-500TR Class B transponder, which has its own monochrome plotting screen and that same automated target-calling feature if interfaced with certain Icom radios (via NMEA 0183).
The new Garmin AIS 600 Class B, like the Raymarine AIS 500, features NMEA 2000 interfacing and a built-in antenna splitter so that you can use a single stick for A.I.S. tx/rx and regular VHF (which is tricky electronically). The FA-50 from Furuno, which built many of the Class A transponders in use, features the only Ethernet data output. But mind you, all these extra features add cost, and in my experience, none of these higher-end transponders transmit or receive A.I.S. any better than the more conventional models from ACR, West Marine, Shine Micro, Comar, Digital Yacht, and the like.