How AIS actually works

The Automatic Identification System is easy to misunderstand because of how it is usually presented. A website shows you a map of moving ships and it looks like surveillance. It is not. AIS is a broadcast: each vessel transmits its own position on an open VHF radio channel, in the clear, to anyone within radio range who cares to listen. There is no central tracker, no query, and no permission. If nobody is listening, the transmission still happens and simply goes unheard.

That single fact explains almost every quirk of live vessel maps, including this one’s.

What a transponder sends

An AIS transponder is a VHF radio bolted to two inputs: a GNSS receiver for position, speed over ground and course over ground, and a data-entry panel where the crew types the voyage details. It sends two very different kinds of message.

Dynamic data — position, speed over ground, course over ground, true heading, rate of turn, and navigational status — comes straight off the instruments and is sent every few seconds. This is the data that moves the dot.

Static and voyage data — name, MMSI, IMO number, call sign, ship type, the four hull dimensions, draught, destination and ETA — is typed in by a human and sent every six minutes. This is the data that is wrong. Destination fields say FOR ORDERS, SEE AGENT or a port the ship left three voyages ago. Draught is often the last laden figure. Nothing validates any of it.

When you see a ship on this site with a plausible position and an implausible destination, that is not a bug in the feed. That is the difference between a sensor and a text box.

Class A and Class B

There are two grades of transponder, and the distinction matters more than most maps admit.

Class A is the mandatory fit. Under SOLAS Chapter V, Regulation 19, it is required on ships of 300 gross tonnage and upwards on international voyages, cargo ships of 500 GT and upwards not on international voyages, and all passenger ships regardless of size. It transmits at 12.5 W and uses SOTDMA — Self-Organising Time Division Multiple Access. Each minute on each channel is cut into 2,250 slots of 26.67 ms. A Class A station listens, builds a map of which slots its neighbours are using, reserves its own, and announces in each transmission when it will next transmit. Stations negotiate the airwaves between themselves with no shore infrastructure at all, which is a genuinely elegant piece of engineering.

Class A reporting rate varies with what the ship is doing: roughly every 2 seconds at speed and turning, 2 to 6 seconds under way, 10 seconds at slow speed, and 3 minutes at anchor or moored. A fast-moving ship updates more often precisely because its position goes stale faster.

Class B is the voluntary fit found on small commercial craft, fishing boats and yachts. The common “Class B/CS” variant transmits at 2 W using carrier-sense TDMA: it does not reserve a slot, it checks whether one sounds free and transmits if so. Under way it reports every 30 seconds; below 2 knots, every 3 minutes. A newer “Class B/SO” variant transmits at 5 W with SOTDMA and reports every 5 to 15 seconds.

The practical consequences: a Class B vessel is quieter, updates less often, and in congested water where Class A traffic has already claimed the slots, it yields. In a crowded harbour, small craft are the ones that thin out on the map first. They have not gone anywhere.

Channels and message types

AIS uses two 25 kHz channels in the VHF maritime band, transmitting 9,600 bit/s GMSK:

Channel	Frequency	Common name
AIS 1	161.975 MHz	Channel 87B
AIS 2	162.025 MHz	Channel 88B

Stations alternate between the two, which doubles capacity and gives some resilience against local interference. Coast stations can also instruct ships in a region to move to different frequencies, though in practice the two defaults are what you will hear.

The data itself is carried in numbered message types. The ones that matter for reading a live map:

Type	Content
1, 2, 3	Class A position report — the dynamic data
4	Base station report — a shore station announcing itself and UTC
5	Class A static and voyage data — name, IMO, dimensions, destination, ETA
18	Class B standard position report
19	Class B extended position report (adds some static data)
21	Aid-to-navigation report — a buoy, beacon or light, real or virtual
24	Class B static data, sent in two parts (A: name, B: type and dimensions)
27	Long-range position report, a low-rate message intended for satellite reception

If a vessel page here shows a position but no name, that usually means we have heard its type 1/2/3 and not yet its type 5. Names arrive on a slower clock than positions.

Why the signal stops

AIS is VHF, and VHF is line of sight. The usable radio horizon between two antennas is approximated by:

d (nautical miles) ≈ 2.23 × (√h₁ + √h₂), with antenna heights in metres.

A ship with its antenna 20 m up talking to another at 20 m gets roughly 20 NM. The same ship talking to a shore receiver on a 100 m hill gets roughly 32 NM. A well-sited coastal station on a high mast can hear 40 to 60 NM on a good day, occasionally much further under ducting conditions when a temperature inversion traps the signal against the sea.

Beyond that, nothing. There is no fallback, no cell network, no relay. A terrestrial AIS network — which is what feeds this site — sees a coastal band, not an ocean. This is the single most important limit to understand about any free vessel map. Our receiver network is worldwide, but “worldwide” describes where the receivers are, not an ability to see every ship: coverage is dense around Europe, North America and East Asia, thin elsewhere, and effectively absent in mid-ocean.

Satellites can pick up AIS from orbit, but that is a different system with its own severe problems, covered in AIS vs radar vs satellite tracking.

What this means when you read a map

• A gap is usually range, not absence. A ship that disappears mid-ocean has most likely sailed out of the receivers’ reach — or was never in range of one.
• A stale position is not a stopped ship. Check the age we print next to every position. If it says 40 minutes, the vessel is somewhere on a 40-minute circle around that dot, not on it.
• Small craft are systematically under-represented. Class B is quieter and lower priority, and the Finnish feed we use as a fallback also filters fishing vessels out at source.
• The identity is self-declared. The position comes from a satellite navigation receiver. The name attached to it comes from whatever was typed into the box.

None of this makes AIS unreliable. It makes it a radio system with the properties of a radio system, which is a much more useful thing to know than “a website that shows ships”.