You can search for a vessel by looking up MMSI, IMO, name, or call sign.

Results will come back with exact or close matches to your search term. However, search results with incorrect information, duplicate vessels, or duplicate MMSIs and/or call signs can occur. This is generally because:

  • AIS does not enforce data integrity

  • MMSIs, call signs, and vessel names can be changed and reused

  • Vessel names are not required to be unique

  • Information is entered into the AIS transponder by a technician and occasionally this can be done incorrectly.

Learn more: Understanding why incorrect information and duplicate vessels appear in the vessel search →

The following provides some background on each of the vessel identifiers.

MMSI (Maritime Mobile Service Identity)

An MMSI is associated with a vessel and is used for AIS transponders. All vessels with an AIS transponder(s) will have an MMSI. An MMSI is also used for handheld VHF transceivers with Digital Selected Calling (DSC) and Global Navigation Satellite System (GNSS). An MMSI is intended to be unique, but a vessel may have several MMSIs associated with it because Aids to Navigation (AtoN) often also have an MMSI assigned and this includes mobile AtoNs such as fishing buoys¹.

A vessel’s MMSI can change and this is required if a vessel reflags. It can also occur when a vessel’s ownership is changed. An MMSI can also be reused when it is reassigned to another vessel². The MMSI is issued by a country’s telecommunication authority, ship registry or relevant licensing authority³. The MMSI is overseen by the International Telecommunication Union (ITU)¹.

IMO (International Maritime Organisation)

The IMO ship identification number scheme is a way to enhance maritime safety and pollution prevention, and to facilitate the prevention of maritime fraud. The IMO ship identification number is intended to be unique. Not all vessels require a number, and the requirement depends on the size and type of vessel⁴.

The IMO ship identification number is intended to be a permanent number for each vessel. The number is never reassigned. There is a central manager for the scheme, IHS Maritime⁴.

Call sign

A call sign is required for radiocommunications. It is used when communicating over the ship’s radio and is generally associated with a vessel’s radio license. A vessel’s call sign is intended to be unique. An MMSI and call sign are often associated². However, an AIS transponder does not require a call sign, so not all vessels with an AIS transponder will have a call sign.

A vessel’s call sign can change, and this is required if a vessel reflags. It can also occur when a vessel's ownership is changed. A call sign can also be reused by reassigning it to another vessel. A call sign is issued by a country’s telecommunication authority, ship registry, or relevant licensing authority².


A vessel’s name is generally not required to be unique and a vessel name is not required for an AIS transponder.


  1. International Telecommunication Union. (2019). Assignment and use of identities in the maritime mobile service. Retrieved from!!PDF-E.pdf

  2. International Telecommunication Union. (2014). ITU Guidance for Administrations. Retrieved from

  3. United States Coast Guard. (2021). Maritime Mobile Service Identity. Retrieved November 13, 2021, from

  4. International Maritime Organization. (n.d.). IMO identification number schemes. Retrieved November 12, 2021, from

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