Class A vs Class B AIS: Which AIS Transponder Do You Need?

AIS (Automatic Identification System) lets boats see each other on a chartplotter: name, position, course and speed, sent automatically over VHF radio. Choosing a unit comes down to four questions: which class you need, how much transmit power, whether you only want to receive, and how many antennas you want on the boat. This guide walks through each one, then matches the answer to a specific em-trak model.

Quick answer

• Commercial vessel that must carry AIS: usually Class A, such as the em-trak A200. Some vessels may use Class B instead (see the rules below).
• Cruising or offshore recreational boat: a 5W SOTDMA Class B unit such as the em-trak B953 or B951.
• Coastal and inland boating on a budget: a 2W CSTDMA Class B unit such as the em-trak B921.
• Want a new VHF radio as well: the em-trak X100 is a VHF/DSC radio with a 5W Class B AIS built in.
• Only want to see other boats: a receiver such as the em-trak R300.

Class A vs Class B

Class A transceivers transmit at 12.5W, report most often and are built for ships and commercial craft. Class B transceivers are for leisure craft and smaller work boats. They transmit at 2W or 5W and report less often.

In US waters, 33 CFR 164.46 requires a Coast Guard type-approved Class A device on, among others:

• self-propelled commercial vessels of 65 feet or more;
• commercial towing vessels of 26 feet or more with more than 600 horsepower;
• vessels certificated to carry more than 150 passengers;
• dredges working in or near a commercial channel;
• vessels moving certain dangerous or bulk flammable cargo.

The same rule allows a type-approved Class B device instead on fishing industry vessels, on dredges, and on some smaller passenger vessels that stay out of VTS areas and under 14 knots. For most recreational boats under 65 feet, AIS is voluntary, according to the Coast Guard Navigation Center. Check the rule for your vessel before you buy.

Class B power: 2W CSTDMA vs 5W SOTDMA

There are two kinds of Class B transceiver:

• 2W CSTDMA ("carrier sense") listens for a free moment before it transmits.
• 5W SOTDMA ("self-organized") reserves its time slots the way Class A does. That gives it more range, more frequent position reports at speed and priority in busy waters.

Nominal reporting rates under way are:

If you run fast or spend time in shipping lanes, 5W SOTDMA is worth the difference. For slower boats in quieter waters, 2W CSTDMA is a sensible way to be seen for less money.

Transceiver or receiver?

A transceiver shows you other vessels and also broadcasts your own position, so ships can see you. A receiver, such as the R300, only shows other vessels. Receivers cost less and draw little power, but nobody can see you on AIS. For small craft and tenders that need to be visible without a full installation, the em-trak I100-X is a self-contained, battery-powered Class B tracker.

Antennas: one or two?

AIS uses the marine VHF band, so it needs a VHF antenna. You have three options:

• Built-in splitter: the B923, B924, B953, B954 and the X100 let AIS and your VHF radio share one antenna.
• External splitter: the em-trak S300 does the same job for any transceiver, and also feeds an FM radio.
• Separate antenna: fit a dedicated AIS antenna instead of using a splitter.

For GPS, the B900 series and X100 have an internal antenna. The A200, B400 and B200 ship with an external GPS antenna. The GPS100 is available as an external antenna or a spare.

Connecting to your chartplotter and phone

Every em-trak transceiver outputs NMEA 0183 and NMEA 2000, so it works with almost any chartplotter or multifunction display. Models with Wi-Fi (the A200, B400, B200, B922, B924, B952, B954 and X100) also send AIS data to phones, tablets and navigation apps. The A200 and B400 have their own 5-inch color screen, so they can show AIS targets without a separate display.

Before you install: get an MMSI

An AIS transceiver broadcasts your vessel's 9-digit MMSI number, so you need one before you install. If you only boat in US waters, the Coast Guard Navigation Center points recreational boaters to approved organizations such as BoatUS. If you also travel to foreign waters, or you run a commercial vessel, you need an FCC ship station license, which includes an MMSI.

Which em-trak AIS is right for you?

Shop the full range in our AIS transponders and AIS receivers, splitters and accessories collections. Every em-trak product carries em-trak's 3-year warranty. Not sure which fits your boat? Call us at 917-540-6070.

Sources

• 33 CFR 164.46, Automatic Identification System (Legal Information Institute, Cornell Law School)
• U.S. Coast Guard Navigation Center: MMSI and AIS guidance
• AIS nominal reporting intervals as published in marine-electronics manufacturer documentation (ITU-R M.1371)
• em-trak 2026 product specifications and product guide