What Is AIS? A Plain-English Guide to Vessel Tracking

Every live ship-tracking map you've ever seen is powered by the same underlying technology: AIS. If you've ever wondered how a vessel's position ends up on your screen — and why it's sometimes hours out of date — here's the plain-English version.

What AIS stands for

AIS is the Automatic Identification System. It's a radio system ships use to continuously broadcast who they are and where they are, primarily so vessels can see each other and avoid collisions. Under international SOLAS rules, most commercial ships over 300 gross tons are required to carry it.

What an AIS message contains

• Identity — the ship's name, callsign and MMSI number.
• Position — latitude and longitude from the ship's GPS.
• Movement — course over ground, speed and heading.
• Voyage data — destination and estimated arrival, when the crew enters it.

How the signal reaches your screen

AIS broadcasts are picked up two ways. Terrestrial receivers along coastlines catch ships near shore in near-real time. Satellite receivers cover the open ocean, but only pass over any given spot periodically.

Why “live” isn't always live

Out in mid-ocean a ship might not be seen by a satellite for hours. That doesn't mean anything is wrong — it just means the last known position is older than it would be near port. This is why an honest tracker tells you the age of a fix instead of pretending every dot is current.

Ready to try it? Here's how to track a ship by name.