Glossary

What is AIS?

The Automatic Identification System is the radio heartbeat of the world's merchant fleet — and the raw material every vessel tracker is built on.

In one sentence

AIS (Automatic Identification System) is a VHF radio system through which ships automatically broadcast their identity, position, course, and speed every few seconds, so that other vessels — and anyone with a receiver — can see where they are.

How AIS works

Every AIS-equipped ship carries a transponder that transmits short radio messages on marine VHF frequencies: who the vessel is (its MMSI identity number), where it is (GPS position), and how it's moving (speed over ground, course, heading). Under the IMO's SOLAS convention, AIS is mandatory for cargo ships of 300 gross tonnage and up on international voyages — which covers essentially every container vessel you'll ever book cargo on.

Those broadcasts were designed for collision avoidance between ships. Tracking platforms exist because the same signals can be received and aggregated at scale.

Terrestrial vs satellite AIS

The signal is only useful if something hears it, and there are two kinds of ears:

  • Terrestrial AIS — networks of shore-based receivers. Excellent coverage near coasts, ports, and busy straits; range is roughly 40–50 nautical miles from a station.
  • Satellite AIS — satellites that pick up the same broadcasts from orbit. They cover the open ocean, where no shore station can hear, but access to that data is commercial and priced per use or by subscription.

This split is why mid-ocean coverage gaps exist: a ship crossing the Pacific is transmitting the whole way, but for days at a time only satellites can hear it. When a tracker relies on terrestrial data alone, the vessel appears to go dark — even though nothing is wrong.

What AIS can and can't tell you

AIS gives you measured facts: position, speed, course, and the vessel's own declared destination and ETA. It does not tell you whether your specific container is aboard — that's a separate question of carrier milestones and load confirmations, which is how transshipment and rollover surprises happen even while the "tracked" ship sails exactly on schedule.

How Calibrate Freight uses AIS

Live terrestrial AIS tracking is free in Calibrate Freight, and every AIS-measured fact wears an OBS badge — observed, with the sample count and the age of the last fix. Vessel-declared data (like the ETA the ship itself broadcasts) is badged DECL so you never mistake a claim for a measurement. When a ship crosses an open-ocean gap, satellite fixes are available as metered top-ups — and every one an agent buys lands on a receipt showing what it cost.

FAQ

Is AIS data free?
The broadcasts themselves are open radio, and terrestrial receiver networks make near-shore data widely available — Calibrate Freight includes terrestrial AIS tracking for free. Satellite AIS, which covers the open ocean, is commercial: someone has to operate the satellites, so that data is typically paid. We price it as metered per-fix top-ups instead of a subscription.
Can a ship turn AIS off?
Physically yes — the transponder has a switch, and crews may legally disable it in narrow security situations (for example, piracy risk areas). For mainstream container shipping it stays on: it's required by SOLAS, and port states take violations seriously. In practice, when a container ship "disappears," the overwhelmingly common cause is a receiver coverage gap, not a switched-off transponder.
Does AIS tell me where my container is?
Indirectly. AIS tells you where a vessel is. Your container's location is that vessel's position only if the box is actually aboard — which is why good trackers also watch carrier milestones for loading, discharge, and transshipment, and alert you when the cargo and the tracked hull stop matching.

Related terms

Gone darkWhen the AIS signal stops mid-ocean — what it usually means, and what an honest tracker should do about it. TransshipmentYour box quietly changes ships at a hub port — and the vessel you're watching arrives without it. RolloverBooked on one sailing, loaded on a later one — why it happens and how to catch it before the ETA lies to you. Demurrage & detentionThe per-day charges that turn a late pickup into a four-figure invoice — and how the free-time clocks actually run.
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