MMSI: the maritime radio identifier

MMSI stands for “Maritime Mobile Service Identity.” It identifies a radio station in the maritime mobile service and is used, among other things, in AIS data. For an individual ship station, the MMSI consists of nine digits: a three-digit MID (Maritime Identification Digits) and six additional digits. Each digit in the six-digit sequence can range from 0 to 9.

Structure and administration

The format depends on the type of radio station. For an individual ship station, the pattern is MIDXXXXXX. Group ship stations use 0MIDXXXXX, while coast stations use 00MIDXXXX. The leading zeros therefore belong to these other station formats; they are not a general prefix for every ship MMSI.

The three-digit MID identifies the administration or geographic administrative area responsible for the radio station. The US Coast Guard lists MIDs in the range 201 to 775. This does not indicate a ship’s current location or the nationality of its owner: the MID associates the radio identifier with a responsible administration; it is not a real-time position report.

Ship MMSIs are assigned internationally and registered under an individual ship station license. Certain US radio stations are authorized under “license by rule.” The US Coast Guard also describes a US-specific transfer scenario in its FAQ: the MMSI should stay with the ship. A specific procedure for BoatUS registrations starts with the previous holder. They can cancel their account online or contact BoatUS to have the number released. BoatUS can then transfer the registration and set up access to a new account for the new owner. If the former owner is unknown, BoatUS explains that it can check whether the MMSI is listed in its database and may be eligible for transfer. This procedure applies to BoatUS registrations in the United States. BoatUS also notes that the license status must be taken into account for identifiers linked to the FCC; other administrations have their own procedures.

MMSI in AIS data

An AIS device transmits the MMSI as part of its identification data. The ship’s name and other associated information for the AIS transmission are programmed into the device. A change to the MMSI registration does not automatically update these device settings. The registry entry and the transmission programmed on board are therefore two separate places where data is maintained. AIS as an onboard source of informationtherefore also relies on the stored information of the transmitting radio station.

A change of ship namealso affects the information stored in the AIS device.

MMSI and ship identity

The MMSI identifies the radio station; the call signis recorded separately. The IMO ship identification number, by contrast, identifies the ship throughout its lifetime. It remains unchanged even if the ship’s flag, name, owner or type changes.