The Automatic Identification System is a tremendous advance in collision avoidance — and who doesn't want plenty of that? But just how deeply a cruising sailor should get involved with the technology is confusing.
Are there enough A.I.S.-broadcasting ships and boats passing through your cruising area to even bother purchasing the technology? If you do sometimes cross paths with vessels that automatically transmit their vital data out over the two dedicated A.I.S. VHF channels, is it good enough just to install a receiver? Or should you step up to a Class B A.I.S. transponder that will also put your boat's name, position, course, speed, and so forth onto those shared airwaves? And if you do adopt either form of A.I.S., how easy will it be to view the output target data with your plotter or PC charting program?
There are many ifs, ands, and buts involved in the answers to these questions, and the transponder decision has become even more clouded by a misunderstanding that's grown nearly to urban-legend status.
If A.I.S. basics are unfamiliar, please read "The ABCs of A.I.S." (see below), because I'm going to charge right into the myth that there's no point in carrying your own Class B transponder because ships just ignore those signals. You can find the legend expressed in many places, but this line from an online sailing forum sums it up well: "My recollection is that A.I.S. Class A transceivers fitted to commercial vessels have a big red IGNORE CLASS B button to declutter their displays and concentrate on avoiding vessels that will do more than smudge their paint in a collision."
There's no such button; in fact, Class A transponders are required by International Maritime Organization regulations to show all targets on their included displays. The cynical presumption this myth grew upon-that big-ship crews don't care about small boats-is largely bogus, too. And besides, if the act of purposely ignoring an A.I.S. target causes a collision, it's likely to be documented by other A.I.S. users within range, and it's likely to get some officer in charge in serious trouble.
The misunderstandings go the other way, too. I've seen sailors get so excited about how well A.I.S. plotting works that they jump to the conclusions that just a receiver costing a few hundred dollars can take the place of a radar for minding dangerous traffic and that a Class B transponder-costing from about $600 to $1,500, depending on optional features-makes a radar reflector obsolete, too. Neither notion is true. While all international ships over 300 gross tons now carry Class A transponders, as required by the I.M.O., and many other commercial vessels carry them under individual national mandates, there are still many sizable boats that you'd really like to find in the system-like the fast ferries of Long Island Sound-but aren't. I'd argue that many such vessels should have voluntarily incurred the approximate $4,000 cost of a Class A transponder, like most megayachts have, but that issue should become mute in about a year when the U.S. Coast Guard enacts a proposed ruling that will require Class A or Class B transponders on some 15,000 U.S. ferries, tugs, fishing vessels, and passenger boats, depending on their size and speed.
Add all those commercial vessels-some of the busiest and scariest along our coasts-to the recreational boats that are rapidly adding Class B transponders since the F.C.C. belatedly made them legal in late 2008, and the "Do I need it?" calculation will change significantly. Still, radar will remain the preferred collision-avoidance tool, especially in areas like the foggy coast of Maine, at least until the swarming lobster boats are transmitting A.I.S., which will happen eventually, I believe.
Ultimately, though-if power, space, and financial budgets permit-it's wiser to think of A.I.S. and radar not as an either/or decision but rather as two very useful and complementary technologies. If you see them working together on a good display, you'll soon learn that A.I.S. is better than such a radar feature as MARPA-Mini Automatic Radar Plotting Aid-at clarifying target dangers and that, like radar chart overlay, it's another valuable tool for helping a navigator distinguish the many knowns from the unknowns.