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: About the vessel identifiers that are used for searching →

The following provides answers to problems some users encounter when searching for a vessel.

Why does a vessel have an incorrect MMSI and/or call sign or appear as a duplicate with a different MMSI and/or call sign?

If a vessel reflags it might require a change in MMSI and call sign to comply with the new flags MMSI and call sign requirements.

If a vessel has a change in ownership the original owner needs to release the call sign and MMSI for the vessel to the new owners. If the original owner does not do this then the vessel may be assigned a new MMSI and call sign.

These changes can result in a vessel being duplicated or appearing with the wrong MMSI and/or call sign.

Why am I seeing the same vessel more than once in the search?

As above, this may be the result of an MMSI and/or call sign being updated when a vessel changes flag or ownership.

In some cases, however, different message types for the same vessel (e.g. the lower accuracy message type 27 which is designed for long-range detection¹) can result in a vessel appearing twice.

Why are there different vessels with the same MMSI and/or call sign?

A vessel owner may choose to retain their MMSI and/or call sign after they have transferred ownership of the vessel. They may reuse the MMSI and/or call sign for another vessel. After a period of non-use, MMSIs and/or call signs may also be recycled for new vessels.

Why is a vessel’s name incorrect?

As above, this may also be related to the reuse of an MMSIs and/or call signs.

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


  1. International Telecommunication Union. (2014). Technical characteristics for an automatic identification system using time division multiple access in the VHF maritime mobile frequency band. Retrieved from!!PDF-E.pdf

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