AIS transponders explained: Class A, Class B and receive-only

An AIS transponder does two jobs. It listens to the AIS messages of vessels around it, and it transmits its own: who you are, where you are, which way you are heading and how fast. A receiver does only the first. That is the single most important distinction when buying AIS equipment, and the one sales listings blur most often.

Strictly, the transmitting units are transceivers: they transmit on a schedule rather than in response to an interrogation. Everyone calls them transponders, and so does this guide.

Transmit or only listen?

Start with the question the hardware answers.

• “I want to see ships around me.” A receive-only unit does that. It decodes nearby traffic and sends it to a chartplotter, a laptop or a phone app. It draws little power, needs no licence to transmit, and is invisible to everyone else.
• “I want other ships to see me.” Only a transponder does that. The bridge of a container ship sees your name, size, course and speed on its display, and so does its collision-warning software.

Many owners start with a receiver and add a transponder later. The second purchase usually replaces the first: every transponder also receives.

Class A, Class B/SO and Class B/CS

There are three kinds of shipborne transponder, defined by ITU-R M.1371 and type-approved against IEC standards. They share the two AIS channels, 161.975 MHz and 162.025 MHz, and every receiver can hear all three. What differs is how loudly they transmit, how they claim airtime, how often they report and what they say.

	Class A	Class B/SO	Class B/CS
Standard	IEC 61993-2	IEC 62287-2	IEC 62287-1
Transmit power	12.5 W	5 W	2 W
Channel access	SOTDMA — reserves slots	SOTDMA — reserves slots	CSTDMA — waits for a free slot
Position report, under way	every 2 to 10 s, faster when turning	every 5 to 30 s, by speed	every 30 s
Position report, below 2 knots	every 3 min	every 3 min	every 3 min
Navigational status	Yes	No	No
Destination, ETA, draught	Yes	No	No
Name, call sign, type, size	Every 6 min	Every 6 min	Every 6 min
Typical fit	Ships under SOLAS and national carriage rules	Faster or offshore leisure and small commercial craft	Most leisure boats

SOTDMA (self-organising time-division multiple access) means the unit listens, builds a map of which of the 2,250 slots per minute its neighbours are using, reserves its own and announces its next transmission in advance. CSTDMA (carrier-sense TDMA) means the unit checks whether a slot sounds free and transmits if it is. In quiet water the difference is hard to see. In a congested approach, where Class A ships have already claimed the slots, a CS unit defers — that is the design, not a fault. How AIS works covers the slot scheme in more depth.

What Class B leaves out, and why it shows

The table hides three practical consequences that explain a lot of what you see on any live AIS map.

No navigational status. Class B position reports (message 18) have no status field. A small boat that shows no “moored” or “under way” is not misconfigured; its equipment cannot send one. See the sixteen status codes for what Class A ships report instead.

No voyage data. Destination, ETA and draught travel in the Class A static and voyage message (message 5). Class B static data (message 24, in two parts) carries the name, call sign, ship type code and dimensions, and nothing about where the boat is going.

Names arrive late. Static data goes out every six minutes. A receiver that has just come into range of a Class B boat can show its position — and its MMSI — for several minutes before it knows the name. A nameless target that “becomes” a named one is the receiver catching up.

Who has to carry AIS

Carriage is set by treaty and by national rules, not by the size of the boat alone.

• SOLAS Chapter V, Regulation 19 requires Class A on ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 gross tonnage and upwards on other voyages, and all passenger ships regardless of size.
• In the European Union, fishing vessels of 15 metres overall length and above must carry Class A under Directive 2002/59/EC as amended.
• In US waters, 33 CFR 164.46 requires AIS on, among others, self-propelled commercial vessels of 65 feet and above, towing vessels of 26 feet or more with over 600 horsepower, and certain passenger vessels. Some of these may fit Class B; the regulation sets out which.

Other flag states and inland waterway authorities set their own thresholds. For most recreational boats, AIS is voluntary — which is why small craft are the patchiest part of any AIS picture.

Choosing a transponder

Once you know you want to transmit, these decisions narrow the field more than brand does.

SO or CS. A 5 W SOTDMA unit is heard further, reports more often at speed, and holds its slots in busy water. A 2 W CSTDMA unit uses less power and costs less. If you sail offshore, cross shipping lanes or move faster than about 14 knots, the SO unit’s extra reporting is worth having. On a quiet inshore boat, CS is often enough.

Antenna plan. A transponder needs a VHF antenna. You can fit a dedicated one or share the radio’s antenna through an active splitter. Never wire two transmitters to one antenna without one. Antenna height matters more than transmit power; AIS antennas and splitters works through the numbers.

Position source. Every transponder has its own GNSS receiver, either internal or with an external antenna that needs a clear view of the sky. It does not borrow position from your plotter, by design: the AIS position must not depend on another device staying healthy.

Data connection. Check what your display speaks — NMEA 2000, NMEA 0183, USB or Wi-Fi — and that the unit outputs it. A transponder that cannot reach your plotter still transmits, but you lose the receive half.

Silent mode. Many Class B units have a switch or menu setting that stops transmission while continuing to receive. Useful for privacy at anchor; easy to forget to turn off.

Power draw. A transponder runs whenever the boat is under way and often at anchor too. Check the stated average consumption against your battery budget; SO units draw more than CS.

Identity: the MMSI comes first

A transponder transmits an MMSI, the nine-digit identity that AIS and DSC radio share. You need one assigned by your national administration before you can transmit, and in many countries a ship radio licence as well. Rules differ between countries, so check with your national radio authority before you buy.

On many Class B units the MMSI can be entered only once by the owner. A typo then needs a dealer or the manufacturer to reset the unit. Enter it slowly, and check it with the MMSI decoder first — it will tell you if the digits decode to the wrong country or to a non-ship format.

What a transponder does not do

• It does not make you visible to vessels without AIS. Most small craft carry none. Radar, lights, sound signals and a proper lookout still apply.
• It does not guarantee anyone is watching. A ship’s officer may filter Class B targets to declutter a busy display.
• It does not put you on every tracking website. Terrestrial sites like this one see you only within VHF range of a shore receiver. AIS coverage explained shows why that range is patchy and why it ends offshore.
• It is not a distress beacon. If you want to be found after something goes wrong, that is a different device: see AIS MOB, AIS-SART, EPIRB and PLB.

Checking it works

After installation, the unit’s own status display and the receive side are the first check: if it is decoding nearby traffic, the antenna path works. For the transmit side, ask a nearby boat or a marina with an AIS display whether they can see you, and confirm the name, size and type are right.

A public map is a weaker test. If you are in range of a contributing shore receiver, your boat may appear on our live map within a few minutes, labelled with the feed that heard it and how old the position is. If it does not, that usually means no receiver is in range — not that the installation has failed.