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 If you've ever wondered how a vessel's position ends up on your screen — and why it's sometimes hours out of date — here's the plain-English version, start to finish.

What AIS stands for

AIS is the Automatic Identification System. It's a VHF radio system ships use to continuously broadcast who they are and where they are, primarily so vessels can see one another and avoid collisions. Under international SOLAS rules it's mandatory on most commercial ships over 300 gross tons and on all passenger ships — so the vast majority of meaningful traffic is broadcasting its position whether or not anyone is watching.

What problem AIS was built to solve

AIS wasn't designed for the tracking maps we use today — that's a happy side effect. It was built for safety at sea. Radar can show a nearby blip, but not what that blip is, who's steering it, or where it's headed. AIS fills that gap: every equipped ship announces its identity, position, course and speed, so a bridge crew (and any shore station in range) knows exactly what's around them — even in fog, at night, or around a headland where radar struggles.

What an AIS message contains

Each broadcast is a compact packet of data. The parts that matter for tracking are:

• Identity — the ship's name, callsign and MMSI number.
• Position — latitude and longitude from the ship's onboard GPS.
• Movement — course over ground, speed over ground, heading and rate of turn.
• Voyage data — destination, estimated arrival and draught, entered manually by the crew.

That last group is worth flagging: voyage data is typed in by a human, so it can be abbreviated, out of date, or left blank. The position and movement fields, by contrast, come straight from instruments and are far more reliable.

Class A vs Class B: not every transponder is equal

There are two grades of AIS transponder, and the difference shows up in how often — and how far — a ship is seen:

• Class A is the mandatory kit on large commercial and SOLAS vessels. It transmits at higher power and as often as every few seconds, so these ships appear frequently and at long range.
• Class B is a cheaper, lower-power unit used on fishing boats, workboats and leisure craft. It transmits less often (typically every 30 seconds) and at shorter range, and some feeds de-prioritise it.

In practice that means a large container ship updates far more often than a small pleasure craft — useful to know when a vessel seems to be updating slowly.

How the signal reaches your screen

AIS broadcasts are picked up two ways, and most tracking feeds stitch the two together:

• Terrestrial AIS — receivers along the coast catch ships in near-real time, but VHF is line-of-sight, so their reach is roughly 40–50 nautical miles from shore.
• Satellite AIS — low-orbit satellites pick up ships out on the open ocean where no coastal receiver can, but a satellite only passes over any given patch of sea periodically.

An aggregator combines both streams, removes duplicates, and serves the result to apps like this one. Near the coast you're mostly seeing fresh terrestrial data; mid-ocean, you're relying on whenever a satellite last flew over.

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. A vessel steaming across the Pacific can legitimately show a fix that's several hours old while behaving perfectly normally.

This is why an honest tracker tells you the age of a fix instead of pretending every dot is current. A three-hour-old position in the middle of an ocean is expected; the same gap just outside a busy port is the kind of thing worth a second look.

What AIS can't tell you

AIS is powerful, but it isn't magic, and a few limits are worth keeping in mind:

• It can be switched off. Crews sometimes turn AIS off — occasionally for legitimate reasons, such as security in high-risk waters — so a ship going dark isn't necessarily lost or in trouble.
• Some fields are manual. Destination and ETA are only as accurate as what the crew typed in.
• It shows the last message, not the present. The dot is where the ship was when it was last heard, not necessarily where it is this exact second.

Treat an AIS position as a very good, timestamped estimate rather than a live GPS pin — and always check how old it is.

Ready to try it? Here's how to track a ship by name — or, if you're tracking cargo, our guide for shippers shows how to spot delays early.