details of interest, the big-
gest dif erences between these
devices are their power, range,
transmitting rates, and who
uses which unit.
Class A AIS is typically
used by commercial vessels
(or fast-moving recreational
boats) and features 12.5 watts
of transmitting power. Class A
transponders have a variable
reporting rate of once every
two to 10 seconds, depend-
ing on the vessel’s speed. (This
rate drops to once every three
minutes if the vessel is trav-
eling at less than 3 knots, and
once every six minutes when
the vessel is anchored.) They
are required to have a ded-
icated display screen, and
contemporary Class A units
are designed to display both
Class A and Class B AIS target
information. Critically, Class A
units broadcast their transmis-
sions using the Self-Organizing
Time-Division Multiple Access
(STDMA) scheme, which is an
automated method that allows
dif erent users to ei ciently
share fi nite channel access.
Class B AIS is primar-
ily aimed at the recreational
marine market and will even-
tually be available in two
varieties: AIS Class B/SO,
which uses the same STDMA
scheme as Class A units, and
AIS Class B/CS, which uses a
Carrier Sense Time Division
Multiple Access (CSTDMA)scheme. Unlike the STDMA
scheme, which announces the
time slots for transmission
in advance, the CSTDMA
scheme continually monitors
a unit’s designated frequen-
cies and selects unused slots.
Moreover, Class B/SO units
feature 5 watts of transmit-
ting power and a reporting
rate of once every fi ve to
30 seconds, depending on
vessel speed, while Class B/
CS units feature 2 watts of
transmitting power and a
static reporting rate of once
every 30 seconds. Both units
reduce their reporting rate to
once every three minutes for
vessels traveling at less than
2 knots, and once every six
minutes when anchored.
Given that most cruising
sailboats travel at speeds well
below 20 knots (the speed at
which reporting rates become
germane, as fast boats can out-
pace their reporting rates when
using Class B/CS), most cruis-
ers can safely spec a Class B/CS
transponder. (As a side note,
while the Coast Guard has
approved AIS Class B/SO units,
as of this writing, they are not
yet commercially available.)
While Class B/SO units are
required to have a dedicated
display, separate screens are
optional for Class B/CS units.
Because of this, many AIS
manufacturers build black-
box Class B/CS units, which
can be networked with a ves-
sel’s multi function display
or plotter in order to graphi-
cally present a target list atop
the cartography. Most Class
B units are compatible with
NMEA 0183 (and sometimes
NMEA 2000) data backbones,
making it easy to network
them to an MFD.
A third type of Class B AIS
allows mariners to listen to
incoming AIS transmissions
but does not broadcast its
own navigational information.
These AIS receivers are pop-
ular, but it’s vital to remember
that while you can use them to
see other maritime trai c, that
trai c is blind to your position
unless the other mariners can
see you on their radar or with
their eyeballs.A Versatile Technology
All AIS signals can be “heard”
by nearby vessels and aircraft
as well as by shore-based facil-
ities such as the Coast Guard’s
Nationwide AIS (NAIS) base
stations, allowing the Coast
Guard to monitor maritime
trai c and use AIS as a Vessel
Tr a i c Service tool. Because
AIS uses line-of-sight VHF
communications, range is typ-
ically limited to 10 to 20 miles,
with masthead antennas deliv-
ering better performance
than obstructed antennas;
shore-based repeater stations
can substantially extend this
range. While an AIS unit’s
range is limited, it of ers
better propagation than radar,
allowing its signals to “peerA Raymarine plotter of ers one option for displaying
AIS information (opposite). An MOB alert appears
automatically on B&G’s MFDs (top). Vesper Marine’s
fi lters can eliminate tra c (center) so you can concentrate
on the boats that actually pose a threat (bottom).
AIS ManufacturersACR Electronics: 954-981-3333,
acrartex.com; from $900B&G: 603-324-2042,
bandg.com; from $930Digital Yacht: 978-277-1234,
digitalyacht.co.uk;
from $240 (receive-only)Em-Trak: 617-273-8395,
em-trak.com; from $580Furuno: 360-834-9300,
furunousa.com; from $1,100Garmin: 800-800-1020,
garmin.com; from $500
(receive-only)Icom: 800-872-4266,
icomamerica.com; from $450McMurdo: +44 (0) 23 9262
3900, mcmurdomarine.com;
from $650NKE Marine Electronics:
800-222-7712,
nke-marine-electronics.com;
from $620Raymarine: 603-324-7900,
raymarine.com; from $500
(receive-only)Standard Horizon: 714-827-
7600, standardhorizon.com;
from $670SI-TEX: 631-996-2690,
si-tex.com; from $330
(receive-only)Vesper Marine: 858-405-5810,
vespermarine.com; from $600may 2016cruisingworld.com69