In a perfect world, skippers of all boats-large and small-would know the position, speed and other data pertaining to every nearby vessel in enough time to determine the potential for collision and maneuver accordingly.
Electronics companies and maritime regulators took a big step toward that goal with the development of class-A automatic identification systems (AIS), which are now required aboard oceangoing commercial vessels. Today, the marine industry is developing class-B AIS for small commercial vessels and a wide range of recreational yachts.
In essence, an AIS is an electronic device that continuously exchanges identification and position information with other vessels. The hardware of a class-A system comprises one VHF transmitter, two VHF TDMA (time-division multiple access) receivers, one VHF DSC (digital selective calling) receiver and a standard communications link to shipboard displays and sensors. A class-A AIS system-at this writing, the only system type-approved by the U.S. Coast Guard-also includes an internal GPS or a means for collecting position and timing information.
The system’s TDMA receivers enable many users to access a single radio-frequency channel without interference. The DSC receiver assigns each VHF transmission an identification code known as an MMSI, or Maritime Mobile Service Identity.
The AIS unit reaches out to other onboard systems for information such as heading and course, speed over ground, rate of turn, draft, angle of heel, pitch and roll, destination and ETA.
All AIS systems within one another’s signal range swap information. Because AIS units communicate on line-of-sight VHF marine frequencies, range-typically about 20 miles on large vessels, 3 to 9 miles on smaller boats-depends on the height of antennas.
AIS hardware and compatible displays indicate the position of each AIS-equipped vessel in range, as well as the vessel’s name, course, speed, classification, call sign, registration number, MMSI number, navigation status, rate and direction of turn, speed over ground, draft, position accuracy, course over ground and true heading. Other information, such as destination and ETA, can be broadcast and displayed at the skipper’s discretion. Most important, the AIS unit computes closest point of approach (CPA) and time to CPA, which can help the skipper plan avoidance maneuvers.
While AIS displays do not replace radar-based ARPA and MARPA displays, they certainly enhance the observer’s ability to identify hazards and plot avoidance strategies. They are not affected by sea clutter or precipitation, and they can even “see around some corners that obscure radar returns. AIS broadcasts can be monitored by shore- and ship-based vessel-traffic controllers and homeland security installations. That’s why the U.S. Coast Guard requires the installation of class-A AIS on all large commercial vessels.
Eric Kunz, senior product manager at Furuno, said he believes most recreational vessels over 40 feet LOA eventually will have AIS units. The company is at work on a class-B system.
As of this writing, a group within the International Electrotechnical Commission (IEC) is developing specifications for class-B AIS hardware-systems intended to cost less than class-A units and expected to be popular in the recreational boating market. Mark Johnson, president of Shine Micro Systems and a member of the IEC working group, said class-B units could be available by the end of this year. Initial price will be about $2,000, but that could drop to about $500-especially if the Coast Guard decides to mandate AIS carriage in critical port areas frequented by recreational boats.
Eventually, AIS systems will be available for search-and-rescue aircraft, enabling them to respond quickly to stricken vessels. AIS also may be installed on nav aids, replacing radar transponders. And shore-based AIS stations will monitor traffic and broadcast text messages, time synchronization, meteorological and hydrological information, navigation information and position of other vessels. Clearly, AIS is a technology to watch.