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
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.
