RTCM Report
Automatic Identification
Systems: A powerful tool
By Bob Markle, President
Let’s say you want to contact a vessel that you can see but not identify. You can make a Channel 16 call to the “white fishing vessel
off my port bow.” You could also publish a classified ad in the newspaper and get a similar result. But if both vessels were operating Automatic Identification Systems, you would know the identification of the vessel and its Maritime Mobile Service Identity
(MMSI), and could simply place a Digital Selective Calling (DSC) call. With the right navigation display, the call might only require
a touch of the finger to the screen.
The Automatic Identification System (AIS) was developed in the 1990’s as a navigation tool. It has developed into a system also used for mar- itime security. It is the technology that can link your communication and navigation systems
into an integrated system. Will everyone on the water
want one or need one?
A vessel’s Automatic Identification System (AIS) is a
VHF transceiver that broadcasts digital messages that
include information about the vessel’s identification,
speed, heading, and sometimes much more. It also
receives this same information about other nearby vessels.
Linked to a compatible electronic chart system or radar
display, this information can be displayed on that naviga-
tional display.
Class A
There are two basic types of AIS units for vessels, Class
A and Class B. Class A is the original version, intended for
larger ships. These typically broadcast MMSI, position,
speed, and a maneuvering report every three seconds.
Reports are more frequent if the ship is moving quickly or
turning, and less frequent if it is not moving. More details
about the ship are broadcast every 6 minutes. Class A units
include a display that shows the data for nearby vessels in
text format, but this is hard to use, and does not replace
the need for a connection to a compatible electronic chart
or radar system display for maximum utility.
In order to avoid interference, the AIS system uses two
channels, both divided into 2250 time slots every minute.
A shore station or a ship station acts as a “semaphore” to
establish a time reference, and all AIS units in the local
area use the Global Positioning System (GPS) for timing to
ensure that all in the immediate VHF reception area use
the same precise slot schedule. A Class A AIS reserves the
slots it needs to schedule the messages it has to broadcast
by sending a slot reservation message. Other Class A AIS
units won’t broadcast messages in those slots. The other
Class A AIS units do the same, reserving previously unre-
served slots for their own messages. Class A AIS units for
large ships worldwide were fitted between 2002 and
2008.
Class B
Class B AIS units are intended for smaller vessels,
including private, non-commercial vessels, as well as
smaller commercial vessels that might not be required to
carry Class A AIS under national law or international
treaty. The Class B system is designed to be “polite,”
deferring, if necessary to Class A broadcasts. Like polite
children, Class B AIS units listen to the Class A AIS “conver-
sation,” and don’t speak out of turn. By listening first, a
Class B AIS can determine the timing of the slots in the
area where it is operating and synchronizes to that sched-
ule. However, a Class B does not reserve slots for its mes-
sages. Instead when it is due to broadcast a message (nor-
mally every 30 seconds if it is underway), it picks a slot,
listens to the first few milliseconds in that slot, and if the
slot is not being used, it sends its message. Obviously, this
sets the stage for slot collisions if two Class B devices
decide to use the same time slot. As a result, other AIS
devices will not receive one or both of these collided mes-
sages. In fact, the number of available slots is large
enough that actual slot collisions are few.
Class B AIS units use a couple of other techniques to
limit their potential interference—they broadcast at a
lower power ( 2 W) than Class A’s ( 12 W), they don’t send
similar messages as frequently as Class A’s, and the mes-
sages they do send generally don’t require more than one
or two slots. Some Class A messages can be longer. Class B
AIS units will also reduce their broadcast frequency in an
especially busy AIS environment.
A safety concern is that vessels with Class A AIS could
turn off Class B returns on their displays, if the naviga-
tional display becomes too crowded with Class B AIS sym-
bols in a crowded waterway, thus negating the value of
operating a Class B as a collision avoidance tool. While it
is true that it may be possible to suppress the display of
Class B AIS symbols, responsible mariners will not do so
unless they are confident that their view of the waterway
is sufficient to detect small vessels visually.
Cost
When the Class A AIS was introduced, they typically
cost around $10,000, and that was before installation.
Since then, prices have fallen significantly. Class A’s are
now available for $2000 or so. Class B’s are being sold
for well under $1000. A third type of device, the AIS
receiver, allows you to see AIS transmitters in your area,
but does not broadcast your vessel’s information. Now
manufacturers are offering Class B AIS transceivers inte-
grated into VHF radios. So the barriers to owning and
operating AIS on almost any vessel have largely disap-
peared. Now that just about any vessel can be equipped
with AIS, the question is what vessels should be equipped
with AIS?
AIS as a security system
The terrorist attacks of 9/11/2001 forced an exami-
nation of the security vulnerabilities of the United States.
One of the identified vulnerabilities was maritime and
especially port security. Although not designed as a secu-
rity system, AIS was ready to be rolled out, so the Mar-
itime Transportation Security Act of 2002 required the car-
riage of AIS on certain vessels and gave the Coast Guard
the authority to require AIS on others. The Coast Guard
responded with regulations requiring AIS on certain com-
mercial vessels operating in Vessel Traffic Service (VTS)
areas, where vessels could be observed and identified by
the controllers in those ports. When Phase I of the
National Automatic Identification System (NAIS) was com-
pleted, the Coast Guard had complete receive-only cover-
age along the entire US coastline and certain inland rivers.
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