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It¶s a timeless scenario: Two vessels converging in the night. Offshore

this generally isn¶t an issue, provided that both watches are attentive.

Trouble brews, however, when tired eyes, confined waters and myriad

recreational vessels are involved. Necessity sparked innovation, which

led to the Automatic Identification System (AIS), an electronic system

that automatically broadcasts ² and tracks ² germane navigational

information between vessels. While AIS has undoubtedly added a

significant safety margin, several important questions emerge for 

yachtsmen: Namely, how visible is your yacht¶s AIS signal to a ship¶s

 bridge, and how much safety margin does it provide?

The answer depends largely on where you¶re operating and whether 

you use a Class A or Class B AIS system. While both systems were

designed to increase situational awareness and to facilitate direct

communication between vessels, they achieve this in different ways,

with differing results.

Some background: Class A AIS was primarily designed to help

 professional mariners avoid ship-to-ship collisions, and the

International Maritime Organization (and some local authoritiescontrolling commercial vessels) has mandated that it be installed

aboard all SOLAS-class (Safety of Life at Sea) ships. Class A AIS

relies on the self-organized, time-division, multiple-access

(SOTDMA) system for channel access, which is an automated method

that allows different users to efficiently share finite channel access.

Class A AIS transmits at 12 watts and updates position and other 

dynamic navigational data every two to 10 seconds, depending on

vessel speed. While Class A information can be overlaid onto someships¶ radars and/or plotters via their electronic chart display and

information systems (ECDIS), all Class A units must carry a dedicated

minimum keyboard display (MKD) monitor.

Class B AIS was designed as a less expensive alternative to Class A

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for both commercial and recreational vessels. This black-box system

transmits at 2 watts and utilizes the carrier-sense, time-division,

multiple-access (CSTDMA) system for channel access. Provided that

open channel slots exist, Class B units update their vessel¶s

navigational information every 30 seconds. Class B units are not

required to carry a dedicated monitor; instead, Class B information

(NMEA 0183 compatible and sometimes NMEA 2000 compatible) is

overlaid onto a multifunction display (MFD). This technology is

currently being used widely by some commercial traffic in other 

countries, and it¶s expected that the U.S. Coast Guard will mandate its

use on certain types of commercial traffic this year.

Jorge Arroyo, a Coast Guard civilian engineer who has been

instrumental in writing AIS regulations, says one of AIS¶s main

functions is to act like a caller ID on a phone. Capt. Skip Strong of the

Penobscot Bay and River Pilots Association wholeheartedly agrees,

noting that this is the biggest benefit to commercial traffic. ³Instead of 

 just seeing a [radar] target, I have something with a name on it,´

Strong says. ³I¶m not just calling a sailboat off my bow ² I¶m calling

a [specific] vessel.´

 Not all professional mariners share enthusiasm for Class B AIS,

however. Capt. R. William ³Bill´ Hughes III, an expert on radar and

navigation projects with the Washington State Ferry System, says the

introduction of Class B into the AIS system brought with it

uncertainty for Class A users. ³A fear of Class B is that a false sense

of security may lure small-boat operators into riskier navigation

decisions, by virtue of the fact that they¶re now seen,´ he says. ³Theability to be seen doesn¶t change the maneuverability of larger 

vessels.´

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