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Understanding why vessel information may appear incorrect
Understanding why vessel information may appear incorrect
Kelly Rummins avatar
Written by Kelly Rummins
Updated over a week ago

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

  • MMSIs can be shared by vessels

  • Vessel names are not required to be unique

  • Information is entered into the AIS transponder by a technician may be incorrect due to mistakes, intentional spoofing, or message corruption

  • Fishing buoys can also arbitrarily be assigned unregistered MMSIs

The following provides answers to problems some users may come across when searching for a vessel.

Why does a vessel have an incorrect MMSI and/or call sign or appear more than once with different MMSIs and/or call signs?

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.

When searching for a vessel using an IMO number you may find multiple matches which reflect changes to the vessel’s other identifiers over time.

The way shared MMSIs were managed by Starboard in the past also means some vessels were incorrectly split into multiple instances.

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.

This means that in some cases one or more AIS transceivers may broadcast subsequent messages with the same MMSI, but from positions that are physically impossible given required travel times. Starboard marks these with unique vessel IDs to avoid their details being jumbled together.

Why is a vessel’s name incorrect?

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

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

Why is a vessel not correctly matched to a register?

Where Starboard has marked a vessel with unique vessel IDs and caused genuine tracks to split into "two" vessels, we mark only the vessel with the most positions as "In [Register]" if a match was made to two vessels with identical MMSIs.


  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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