Class B AIS

Enlarge Photo

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 — and tracks — 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 authorities controlling 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 some ships’ 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 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. “Instead of just seeing a [radar] target, I have something with a name on it,” Strong says. “I’m not just calling a sailboat off my bow — I’m calling a [specific] vessel.”

Not all professional mariners share enthusiasm for Class B AIS, however. Capt. R. William “Bill” 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. “A 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. “The ability to be seen doesn’t change the maneuverability of larger vessels.”