What is the Automatic Identification System (AIS)?
Picture a radar-chartpotter display, with overlaid electronic chart data, that includes an icon for every significant ship sithin VHF radio range, each with a velocity vector (show-icon that vessel’s speed and heading). Each ship icon reflects the actual size of the ship, with positions accurate to GPS precision.By “clicking” on a ship mark, you can learn the ship’s name, course and speed, classification, call sign, MMSI, and other information. Maneuvering information, closest point of approach (CPA), time to closest point of approach (TCPA) and other navigation data is also available. Information previously visible only to the Vessel Traffic Service is now available to every AIS-equipped vessel.
TheAutomatic Information System is a digital VHF radio-based transponder system that can prevent collisions, and can protect your boat from being run down by a huge, fast moving ship. It is like a digital radar with precise position information. AIS uses GPS, VHF radio and sophisticated digital processing to automatically communicate between vessels without any operator interaction. The system’s rangeis similar to that of your VHF radio, essentially depending on the height of the antenna. Its propagation is slightly better than that of radar, due to the longer wavelength, so it’s possible to “see” around bends in rivers or over islands if the landmasses are not too high. AIS information is no degraded by rain-clutter like radar, so it works the same in all weather. The consensus among safetyexperts is that AIS is extremely valuable, especially if you travel at night or in crowded shipping lanes with restricted visibility.
What AIS shows you: Go to this website for an entertaining view of current real-time AIS data: marinetraffic.com/ais/. You can see what vessels are moving about in your local harbor and get a feel for what AIS does. To watch a video about AIS on our website, go towestmarine.com and search for 10393460.
Three Classes of AIS Units
Class A: Full-function transceivers are required for ships over 300 gross tons that travel internationally, and for passenger ships. Class A AIS units include both transmitter and receivers, and automatically transmit every 2 to 10 seconds when the ship is under way, and every three minutes when anchored. Transmitting at 20watts, they consume substantial amounts of electrical power. Class A units are also often pricey, Furuno’s FA150 (costing about $ 4,500) is a Class A unit.
Class B: This offers a somewhat reduced set of functions at a much lower cost, and is intented for recreational vessels and others not required to carry Class A AIS. Class B was approved for use throught the U.S. in September 2008. It isnearly identical to Class A, except that Class B:
• Has a less frequent reporting rate than Class A ( every 30 seconds when travelling at more than two knots, as opposed to 10 seconds for Class A)
• Does not transmit vessel destination., ETA, draft, IMO Number or rate of turn.
• Consumes less electrical energy than Class A (transmits at 2 watts of power instead of 20)
Receive-Only:Units like the Icom MXA-5000 and Raymarine AIS250 are “receive-only”, not two-way transceivers, and let you identify what ships are in the neighborhood while you remain off the AIS grid. Think of a receive-only unit more like radar than a radar unit with radar reflector. These receivers, containing only the two-channel VHF receiver and decoing software, are not Class B, since they are not activetransmitters the AIS system.
The West Marine AIS-1000, Raymarine’s AIS500 and ACR’s Nauticats B are true Class B transceiver that offer both send and receive functions. You can turn the transmit function off and operate in Silent Mode (one AIS authority calls it “Pirate Mode” – perhaps it is yours setting of choice when operating your boat in piracy –infested waters). Both of these are “black...