What is AIS & how ship tracking works

If you have ever watched ships move in real time on a map, you have been watching AIS. The Automatic Identification System is a VHF radio system that every large ship carries, broadcasting a steady stream of short messages that say, in effect, "this is who I am, here is where I am, this is where I am going." It was never designed as a surveillance tool — its original purpose was collision avoidance, letting two ships and the shore see each other's intentions. But because those broadcasts are open, they have become the raw material of all modern ship tracking, including the Marifest live map.

What AIS broadcasts

An AIS message carries two kinds of information. Dynamic data changes constantly and is sent frequently — position (latitude and longitude), course over ground, speed over ground, heading and rate of turn. Static and voyage data changes rarely and is sent less often — the ship's name, its MMSI (the radio identity), its IMO number, type, dimensions, destination and estimated time of arrival. The MMSI is the identifier the radio actually transmits; the IMO number, where included, is what ties the broadcast back to a permanent vessel record.

Class A and Class B

Not every AIS unit is equal. There are two grades:

Type	Used by	Behaviour
Class A	SOLAS ships — cargo, tankers, large passenger ships	Higher transmit power, frequent updates (down to seconds at speed), full data set
Class B	Smaller craft — fishing boats, yachts, workboats	Lower power, less frequent updates, reduced data, often voluntary

Because Class A units transmit harder and more often, commercial ships are far more reliably tracked than small craft — which is one reason coverage of the fishing fleet is patchier than coverage of tankers and bulkers.

Terrestrial vs satellite reception

AIS is a VHF signal, and VHF travels roughly line-of-sight. That single fact shapes how the data is collected:

• Terrestrial AIS — receivers on masts, lighthouses and coastguard stations pick up ships within VHF range, typically a few tens of nautical miles offshore. Coverage is dense near busy coasts and ports and absent in mid-ocean.
• Satellite AIS — spacecraft in low Earth orbit listen for the same VHF bursts from above, extending coverage to the open ocean where no shore station can reach. The trade-off is that a satellite passing over a crowded shipping lane hears thousands of ships at once, causing message collisions that can drop or delay reports.

A live map is therefore a fusion of both feeds. Marifest sources its positions from AIS via AISStream, combining terrestrial and satellite reception to place vessels on the map. Where the feeds overlap, coverage is excellent; far out at sea, updates are sparser and older.

Why ships go dark

AIS is mandatory, but it is not tamper-proof — the transponder can simply be switched off. When a ship stops transmitting, it "goes dark", and there are both innocent and suspicious reasons:

• Legitimate — masters may reduce or disable AIS transiting piracy hotspots to avoid being targeted, and reception can fail for technical or coverage reasons far from any receiver.
• Suspicious — a deliberate gap timed around a ship-to-ship transfer, or a sudden silence in a sensitive area, is one of the clearest markers of the shadow fleet moving sanctioned cargo. Some vessels go further and spoof their position, broadcasting false coordinates to appear somewhere they are not.

A live map shows you where ships are saying they are. The interesting question in compliance is often the opposite: which ships have just stopped saying anything at all. Why AIS gaps are a screening signal

Coverage limits and reading the map honestly

The practical upshot is that a tracking map is a best effort, not a ground truth. A vessel may be missing because it is beyond receiver range, because satellite messages collided, because the transponder failed — or because someone switched it off. That is why AIS behaviour is read as a signal in any serious sanctions screen, alongside flag history and ownership, rather than taken at face value.

Marifest joins the live AIS feed to the permanent vessel record. Across 97,000+ vessels, each position on the map is tied to a file in the registry by IMO number, so a track sits next to the ship's specs, flag, owner and sanctions standing. Terms such as MMSI and ship-to-ship transfer are defined in the glossary.