AIS for Sailors

Any sailor who has made extended passages along coastlines or across oceans has had at least a few close callswith big ships whose course and intentions can be difficult to discern until the last minute. The introduction of AIS (Automatic Identification System) has taken a lot of the guesswork (not to mention terror) out of these close-quarters situations. For just a
Author:
Publish date:
Updated on
ais_communications

Any sailor who has made extended passages along coastlines or across oceans has had at least a few close calls

with big ships whose course and intentions can be difficult to discern until the last minute. The introduction of AIS (Automatic Identification System) has taken a lot of the guesswork (not to mention terror) out of these close-quarters situations. For just a few hundred dollars, you can buy equipment that will display the names, positions, course, and speed of all ships within VHF range on your laptop or chartplotter screen. For a few hundred dollars more, you can set up a system that will also transmit your own details and position to all other AIS-equipped vessels within VHF range. The safety benefits for small-boat sailors are obvious.

What exactly is AIS?

AIS is a VHF-based automatic position reporting system that sends and receives details of a ship’s course, speed, position, name, size, type, and other more-or-less-pertinent bits of information to other ships and to shore-based stations. Ships use it as an adjunct to radar to prevent collisions at sea; in busy coastal areas it can be used with a shoreside base station and VTS (Vessel Traffic System) to manage shipping traffic, somewhat like an Air Traffic Control system; and authorities use it to keep tabs on who’s going where and when. At the bottom of the totem pole are we leisure sailors, whose primary concern is our own safety.

There are two types of AIS, one purely for big ships and the other for everyone else. Class A is the high-end variant. It’s powerful, complex and expensive, and it’s compulsory for nearly all commercial vessels over 65 feet overall. Class B is a less sophisticated and much more cost-effective system that’s marketed for smaller commercial vessels and the leisure market. You can buy a receiver that’ll show you where other AIS-equipped ships are, or a Class B transceiver that sends your position to other vessels.

It works like this

simrad_ais150

AIS signals are transmitted in packets—brief bursts of data—over two dedicated VHF channels. Each transmission is automatically allocated a time slot, so that hundreds of ships can use the system at the same time. AIS receivers decode the transmissions and send them to a laptop or plotter using the NMEA 0183 protocol. Class A transmits via the SOTDMA (Self Organizing Time Division Multiple Access) protocol on both VHF channels at the same time, but Class B alternates between the two. Being lower in the pecking order, Class B messages use a different protocol and must wait for “spare” transmission slots that aren’t being occupied by Class A transmissions; this accounts for B’s slower transmit/receive speeds.

Since there are 2,250 AIS time slots, in theory you could have that many vessels within VHF range of each other all sending messages almost simultaneously. How will your voice be heard above such a multitude? In practice, there are other variables that affect Class B transmission and reception, and signal degradation due to heavy traffic should not be a significant issue. Class B sets report at 30-second intervals as opposed to Class A’s 2 to 10-second intervals.

The cheapest AIS receivers are single-channel—that is, they switch between channels and monitor the one with the strongest signals. These are invariably Class A signals, so a single channel receiver may not catch Class B transmissions. Dual-channel receivers monitor both channels all the time.

If you buy a transceiver, your vendor will program a MMSI (Maritime Mobile Service Identity) number into it. This allows other ships to identify your boat by name, LOA and other details, and to contact you by name via VHF. If you already have a DSC radio, you should have an MMSI. If not, acquiring one is a simple matter.

Related

101218BTSC-9887

Just Launched: Little Big Boat

Peter Nielsen looks at Beneteau’s latest entry-level boat and a new cruiser from Tartan Group Beneteau’s commitment to entry-level boats has been reaffirmed over the last year with the assimilation of the sporty Seascape line of pocket cruisers and the ...read more

TOTW_PromoSite

SAIL's Tip of the Week

Presented by Vetus-Maxwell. Got a tip? Send it to sailmail@sailmagazine.com No chafe, safe stay  If you’re leaving the boat unattended for a longish period, there’s a lot to be said for cow-hitching the shorelines, as this sailor did. They’ll never let go, and so long as the ...read more

belize600x

Charter Special: Belize

It would be hard to imagine a more secure spot than the Sunsail base on the outskirts of the beachside community of Placencia, Belize. The entire marina is protected by a robust seawall with a channel scarcely a few boatlengths across. It’s also located far enough up Placencia ...read more

DSC00247

DIY: a Top-to-Bottom Refit

I found my sailing “dream boat” in the spring of 1979 while racing on Lake St. Clair in Michigan. Everyone had heard about the hot new boat in town, and we were anxiously awaiting the appearance of this new Pearson 40. She made it to the starting line just before the race ...read more

01-oysteryachts-regattas-loropiana2016_063

Light-air Sails and How to Handle Them

In the second of a two-part series on light-air sails, Rupert Holmes looks at how today’s furling gear has revolutionized sail handling off the wind. Read part 1 here. It’s easy to look at long-distance racing yachts of 60ft and above with multiple downwind sails set on roller ...read more

HanseCharles

Video Tour: Hanse 348

“It’s a smaller-size Hanse cruiser, but with some big-boat features,” says SAIL’s Cruising Editor, Charles J. Doane. At last fall’s Annapolis Boat Show, Doane had a chance to take a close look at the new Hanse 348. Some of the boat’s highlights include under-deck galleries for ...read more

amalfitown

Charter Destination: Amalfi Coast

Prego! Weeks after returning from our Italian flotilla trip last summer, I was still feeling the relaxed atmosphere of the Amalfi Coast. It’s a Mediterranean paradise, with crystal-clear waters, charming hillside towns and cliffside villages, plenty of delicious food and wine, ...read more

image005

Inside or Outside When Sailing the ICW

Last April, my wife, Marjorie, and I decided to take our Tartan 4100, Meri, north to Maryland from her winter home in Hobe Sound, Florida. This, in turn, meant deciding whether to stay in the “Ditch” for the duration or go offshore part of the way. Although we had both been ...read more