Automatic Identification SystemAIS
A VHF transponder system that broadcasts a ship’s identity, position, course and speed for collision avoidance and tracking.
The Automatic Identification System (AIS) is a transponder that automatically broadcasts a ship’s identity (MMSI and often IMO), position, course, speed and voyage details over VHF, received by other ships, shore stations and satellites. SOLAS requires it on most commercial vessels for safety and collision avoidance.
Because the signal can be received globally, AIS is also the backbone of commercial vessel tracking. A deliberate AIS gap — switching off the transponder — is a classic evasion tactic and one of the strongest signals in dark-fleet and sanctions screening.
On TheMaritime
Also known as: AIS, automatic identification system, transponder.
Related terms
Dark Fleet
Ageing, opaquely owned and often uninsured tankers that move sanctioned oil while disguising their movements.
AIS Spoofing
Falsifying a ship’s broadcast position or identity on AIS to disguise where she really is or what she really is.
Ship-to-Ship TransferSTS
Transferring cargo directly between two vessels moored alongside, often at anchor or offshore rather than at a terminal.
Maritime Mobile Service IdentityMMSI
A nine-digit number identifying a ship’s radio station, used by AIS and distress systems — distinct from the permanent IMO number.
Plain-English reference definition — our own explanation of a standard shipping concept, not a licensed source or legal advice. See the full glossary or the broader maritime dictionary.
Last reviewed: June 2026.