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ability of Class A vessels make this frequent
updating necessary to avoid collisions.
■ Class B transceivers were specifically
designed to be used on recreational vessels
and are significantly less powerful ( 2 watts)
but also significantly less expensive (about
$500-$1,000). Class B transceivers broadcast their course and speed once every 30
seconds at any speed over 1 knot.
■ Class B “receive-only” units will receive
signals from other vessels, but don’t transmit
your own vessel’s information. Thus you
can see other AIS-equipped vessels, but they
won’t see you on their AIS. These are the
least expensive units ($300-$500).
The range of AIS signals will depend on
both the antenna height and the transmit
power. Generally you can see Class A units
on ships at a range of about 50 nautical miles
(although you occasionally see them at more
than 100 nautical miles) and Class B on plea-
sure vessels at a range of about 12 nautical
miles. Some AIS units have their own built-
in screen for displaying the position of other
vessels, but most are meant to display AIS
information as overlaid data on chartplotter
or radar screens. This usually entails a bit of
not-too-difficult wiring from the AIS box to
your plotter or radar screen. Some AIS units,
including some Class B units, also broadcast
their data on Wi-Fi; this data can be picked
up by tablets like the iPad and displayed on
the iPad plotter program, iNavX, with no
Several websites, such as marinetraffic.
com, track vessels worldwide and show their
current location and course on Google maps.
INSTALLING AN AIS
Each vessel in the AIS system is identified by a unique MMSI (maritime mobile service identity) number. When you purchase an AIS in the U.S., the first thing you’ll need to do is provide the manufacturer with an MMSI, which they’ll program into the unit. If
you have a DSC radio onboard, then you probably already have an MMSI, and that’s the one
you should use in the AIS as well. If you don’t know what it is, you’ll have to look it up on your
radio, or find the slip of paper you wrote it down on when you programmed the radio. If you
don’t already have an MMSI, you can get one for your vessel either from the FCC or, if you do
not intend to leave U.S. waters, from BoatU.S. ( www.BoatUS.com/mmsi) for free.
AIS transceivers will need both a VHF antenna and a GPS antenna, while receivers need
only the VHF antenna; most take a GPS feed from another GPS unit onboard.
You can either “split” your current VHF antenna between your VHF and AIS, or install a
separate independent antenna for the AIS. If you use your current VHF antenna, you’ll need to
install an antenna splitter (available from West Marine, www.westmarine.com). The VHF antenna cable plugs in one side of the splitter box, and the VHF and AIS plug in the other side. The
AIS will transmit its signals at regular intervals, except when you key the VHF microphone to
talk. Using a splitter is usually a quicker and simpler installation job, but the splitter will slightly
degrade the AIS and VHF transmissions. Whether you use a splitter or not, it is best to install
the AIS antenna as high as possible to maximize the AIS range. — E.S.