How to track ships live

The Automatic Identification System was built so ships could see each other and avoid collisions. Under international rules, every passenger ship, and every other ship of 300 gross tonnes or more on an international voyage, must carry it, and many fishing boats and yachts carry a simpler version.

A transponder sends two kinds of message. Position reports, every few seconds under way and every few minutes at anchor, carry position, speed and course. Static reports, sent less often, carry the ship’s name, its type and its destination, which the crew types in.

Receivers ashore hear them, and aisstream.io relays what they hear. TAKIP keeps up to 20,000 ships at a time, asks for new positions every 10 seconds, and drops a ship it hasn’t heard from for 10 minutes.

Ships are coloured by type: cargo, tankers, passenger ships and ferries, fishing, military and law enforcement, tugs and service vessels, sailing and leisure, and other. Each type has its own switch under Maritime / Naval in the Layers panel, and Underway only hides anything slower than half a knot: anchored, moored or drifting.

A ship whose static report hasn’t been heard yet has no type, so it shows as Other / Unidentified, sometimes without a name.

The MMSI on the card is the ship’s nine-digit radio identity. Its first three digits usually say which country the ship is registered in.

A few narrow passages carry much of the world’s trade. The ship layer marks ten of them, among them Hormuz, Bab el-Mandeb, Suez, Malacca and Panama, and the Chokepoint Traffic layer adds how busy each has been.

Chokepoint Traffic comes from IMF PortWatch, which estimates transits from AIS. Each strait’s last seven days are compared with its own normal, the three months or so before. If each of the last three days ran below 85% of normal, it is marked disrupted. Click one for the share of normal, a 30-day chart and any disruption PortWatch has recorded.

PortWatch publishes weekly, so its latest day is often a week or more old. Use it for the trend, and the ships themselves for what is passing now.

AIS is mostly heard from the shore, so coverage is best near busy coasts and ports. Far out at sea, ships often drop off the map and come back as they near land.

Ships can switch AIS off, and warships often do. Nothing checks what a transponder says, so a position can be faked, and a type or destination typed in by the crew can be wrong or out of date.

A ship missing from the map is not proof that it isn’t there.