EXPLAINER
What is AIS?
The open radio system behind every ship tracker on Earth. How vessels broadcast their identity and course to the world — and why the data was public from day one.
Built for safety, open by design
AIS — Automatic Identification System — is the maritime world's answer to a simple problem: in fog, at night, or in a crowded strait, how does every ship know where every other ship is? The answer: make every ship say so, constantly, over the radio.
A transponder on each equipped vessel broadcasts its identity, position, speed, heading and status on maritime VHF — openly and unencrypted, because encrypting it would defeat the purpose. Nearby ships, port authorities and coastal stations all listen, and collisions have fallen as a result.
But once the signal is in the air, anyone can hear it. Networks of shore receivers and satellites now collect those broadcasts globally, and ship-tracking maps are what happens when you visualize the aggregate. It's the same architecture as ADS-B in aviation — a safety broadcast that became a public window onto global movement.
From transponder to your screen
The ship reports itself
An AIS transponder on the vessel packages its identity, position, speed, heading and status — updated continuously while underway.
It broadcasts over VHF
The data goes out on maritime VHF radio frequencies, unencrypted, every few seconds for fast vessels and every few minutes for those at anchor.
Receivers and satellites listen
Shore-based receivers near coasts and satellite payloads over open ocean pick up the broadcasts and feed them into global aggregation networks.
The map draws the oceans
Those reports become live vessel markers: position, speed, heading, type and destination, plotted alongside ports and chokepoints.
What a broadcast carries
Who it is: the vessel's unique identity number, name, dimensions and type — container ship, tanker, ferry, tug, fishing boat and more.
What it's doing: GPS position, speed over ground, course, heading and navigation status (underway, at anchor, moored, fishing).
Where it's going: destination, estimated time of arrival and cargo information — entered by the crew, and only as accurate as the last person who typed it in.
The limitations
Not every vessel carries it
International rules require AIS on large commercial ships and passenger vessels, but small private boats, fishing craft in some regions and many older vessels operate without it — so the map shows the professional fleet, not every hull on the water.
Coverage varies by region
Near busy coastlines with dense receiver networks, positions are fresh and frequent. In remote ocean areas the picture depends on satellite reception and updates more slowly.
Transponders can go dark
AIS can be switched off, malfunction, or — in cases that make the news — be deliberately disabled to hide a vessel's movements. A quiet area of the map isn't necessarily an empty one.
It's a signal, not proof
Because broadcasts are unencrypted and self-reported, identity data can be mistyped or spoofed. For navigation and awareness it's excellent; for investigations, corroborate it.
AIS FAQ
What does AIS stand for?
Automatic Identification System. It was originally designed as a collision-avoidance tool: ships broadcast who they are, where they are and where they're going so nearby vessels and port authorities can see the traffic picture. The public ship-tracking industry is a side effect of that openness.
Which ships are required to carry AIS?
Under international maritime rules, large commercial vessels — generally ships of 300 gross tonnage and above on international voyages, plus all passenger ships — must carry AIS transponders. That's why the global map is dominated by container ships, tankers, bulk carriers and ferries.
What information does AIS broadcast?
Static data (the ship's identity number, name, dimensions and vessel type), dynamic data (position, speed over ground, course, heading and navigation status), and voyage data the crew enters manually (destination, estimated arrival, cargo type).
Can AIS track small boats?
Usually not. Small private and recreational boats aren't generally required to carry AIS, though some do voluntarily (a simpler variant called Class B). If a boat isn't broadcasting, it won't appear on any AIS-based tracker.
Is AIS data delayed or filtered?
Public feeds can carry a small delay, and some providers withhold or anonymize certain vessel types at the request of authorities or operators. What reaches a public map is a real but curated view of global traffic.
Now go watch the seas
Vessel traffic, ports and chokepoints on one globe — the maritime feed is part of WorldEye Pro.
Explore the ship trackerCurious about the aviation equivalent? Read what ADS-B is
Explore more
Ship tracker
Live vessel traffic on a 3D globe, powered by the broadcasts this page explains.
What is ADS-B?
The aviation twin of AIS — how aircraft broadcast their position in the sky.
Satellite tracker
Thousands of satellites overhead — the ISS, Starlink and more, live.
Pricing
Weekly, monthly and annual Pro plans — the maritime feed is included.