AIS-B vs. Radar
Radar is a boon for cruisers in all sorts of ways, but it’s not the perfect solution to dealing with shipping traffic. It’ll show you a target that you’ll be able to track, but it can’t see around corners or through land, bad weather and sea clutter can render it almost impotent, and using it well requires both practice and a degree of intuition on the user’s part. Shore-based AIS repeaters let you receive an AIS signal from a ship on the other side of a headland or an island. An ever-increasing number of navigation aids are also being equipped with AIS transmitters, which provides another level of security for cruising sailors. On the other hand, there are many boats and other hazards that aren’t equipped with AIS and that radar will (hopefully) be able to detect. If you have to make a choice between AIS and radar, these are the things to take into account. If your boat is already equipped with a late-model radar, you should be able to integrate the AIS into the system so that AIS targets are displayed on the radar screen. Talk about the best of both worlds…
Why a dedicated antenna?
You can buy an antenna splitter that allows your AIS to share your VHF radio’s antenna. Some of these splitters work better than others. AIS manufacturers almost all recommend, however, that you use a dedicated antenna that’s tuned to the AIS frequencies. A VHF radio antenna has to work across a wider range of frequencies. Since an AIS transmission is limited by law to 2 watts, it makes sense to use an antenna that’s made to transmit and receive on the two AIS frequencies. Such antennas are now widely available from specialist manufacturers like Shakespeare and V-Tronix—and some AIS makers also supply their own-brand antennas. Some even have integrated GPS antennas, further simplifying installation; these are often cheaper than an antenna splitter.
Class B AIS was only approved by the FCC for use in U.S. waters a few months ago, but it’s been operational in most of the rest of the world for years. There’s a wide range of equipment available to suit just about any budget; you can mix and match, or buy complementary equipment from just one maker. AIS data can be displayed on laptops, plotters, radar screens, or on dedicated AIS screens. Almost any plotter manufactured during the last two or three years should have AIS capability. Big names like Simrad, Raymarine, Icom, Furuno and ACR have all come out with AIS-B receivers and transceivers, and other companies like SevenStar, SR, Comar, SeaCas, Shine Micro, and SiTex are also making equipment.
Here’s a rough guide to choosing an AIS system:
Good: With an AIS receiver, you’ll be able to see other AIS-equipped vessels within the range of your VHF antenna—up to 40 miles or so, depending on antenna height, though some users report seeing targets as far away as 70 miles. If you already use a laptop with electronic charts, you can set up a receive-only AIS system for relatively little cost. You'll need a receiver, a GPS antenna, an antenna splitter that lets you use your existing VHF antenna, and the cables to connect it all together. This will display data from vessels equipped with AIS A and B on your laptop—if, that is, your charting software is AIS-compatible. You can buy a single channel receiver for less than $200; dual-channel receivers go for two to three times that.
Better: Laptops are great, but since they’re not waterproof you’ll have to go below to look at the screen, and this isn’t good if you’re the only one on deck. Better to be able to see ships displayed on a plotter in the cockpit so you can stay at the helm, ready to act. And it’s also better to let the ships know where you are. Even better is to have the ability to overlay AIS targets on a radar screen. You’ll want a Class B transceiver, dedicated GPS and AIS antennas, and the relevant cables. Most transceivers will come with all of this gear. Check that your plotter is AIS-compatible. Dual-channel receivers vary in complexity and features, which accounts for the wide disparity in prices, from under $500 up to around $1,000.
Best: I think the best set-up for a short-handed cruising boat is this: a transceiver, all the other gear, and a dedicated cockpit AIS display like the Simrad A150 or AISWatchMate. Priced from around $500 to over $1,000, these units let you keep a close eye on the movements of approaching vessels, while leaving your plotter or laptop free for other duties. They'll make it easier to calculate the chances of a collision, which is what AIS is all about.
To B or not to B
Who needs AIS? If your cruising grounds are bisected by busy shipping lanes, or if they are prone to fog, you should certainly think about it. If you’re planning to do some overnighters or offshore racing, AIS would be a worthwhile investment. If you sail in areas where there’s little or no commercial shipping activity, don’t bother.
The dangers of AIS
Like any electronic gizmo, AIS can foster a sense of dependence. It’ll let you know about Class A-equipped commercial ships and Class B-equipped sailboats and powerboats; but there are plenty of boats out there that don’t have AIS. The Coast Guard also warns that some commercial ships have outdated AIS software that won’t pick up Class B transmissions, so you can’t be sure every ship will see you. If you treat AIS as an aid to safe sailing that backs up your own senses, rather than an oracle to be obeyed to the exclusion of all else, you should have no problems.