So here's how it works: Every minute is broken into time slots, ACR's Wahler says. The system assigns a vessel a time slot so that its transponder can broadcast static (vessel name, size, etc.) and dynamic (speed, heading, etc.) data. Each vessel "talks" in turn while the others "listen." This happens at blinding speed, of course: More than 2,000 transmissions may occur in a one-minute period.
AIS transponders send VHF signals, so the same rules of range apply. Range depends on the height and gain of the antenna and the power of the transponder. Class A broadcasts at 12 watts, Class B at 2 watts.
Wahler says he has heard reports that some vessels with Class A may see each other 25 to 30 miles away. Boats with Class B may see commercial vessels out to 10 miles, but the commercial vessel may not see them until it's about five miles away. And some Class A users with older units that have not yet been upgraded by available software may not see Class B at all, the U.S. Coast Guard warns.
Broadcast rate is set by speed. The faster a vessel is moving, the more often it reports. Class B updates slower than A: The fastest B will update is 30 seconds, and some data may not update for six minutes.
Another distinctive difference between A and B: You can turn off Class B AIS completely or just shut down your transmission and continue to receive, says Simrad's marketing director Paul Comyns. The option allows boaters transiting areas of known pirating activity, such as remote parts of the Caribbean, or boaters competing in fishing tournaments, to go dark. In fact, several companies - including Raymarine (AIS250, $1,120) - sell receive-only AIS units.
But sometimes fishing may be enhanced. Simrad's AI50 ($1,595) features a tracking mode that allows you and your fishing buddies to watch each other. Of course, you could already do that using a VHF radio with position polling and DSC (digital selective calling) by interfacing it with your GPS/plotter. But Simrad's AIS tracking mode makes the process a snap and allows you to make a quick DSC call to a vessel on the screen.
AIS primarily comprises a transponder and two antennas - a VHF antenna tuned to the high end of the signal spectrum and a GPS antenna. The transponder often takes the shape of a black box (a belowdecks brain without an abovedecks display). The box can talk to most newer GPS/plotter/sounders - via NMEA 0183, NMEA 2000 or proprietary network connections - or to an onboard PC, which then displays the vessel markers on-screen.
Simrad offers either a black-box version - NAIS 300 ($1,200) - or the AI50 display unit. Furuno's FA50 ($1,895) is a black-box system that can connect to the company's NavNet 3D or NavNet vx2 networks, to other units via NMEA 0183 or via an Ethernet cable to a PC.
While anyone can hook up a Class B AIS unit, the FCC requires that the vendor or a qualified installer must enter the vessel's name, size and other static data into the unit. That's to keep users from entering erroneous information by mistake or deliberately.
Questions remain, and debate surges among AIS proponents and detractors. But as with many new systems, time will sort the answers, and lightning-fast technology changes will fix whatever's broken.