Maritime Mobile Service Identity

The Maritime Mobile Service Identity is the digital identity of a ship or shore station in modern maritime radio. It is a nine-digit number assigned to a vessel, and it functions in the radio world rather like a telephone number, uniquely identifying the station so that automated equipment can address, call and recognise it. The MMSI is central to the digital systems that have replaced manual radio procedures, and it is programmed into the ship's radio equipment when that equipment is installed and licensed.

The MMSI is used across several systems. In the Automatic Identification System, every transmission a ship makes carries its MMSI, so other ships, shore stations and tracking services can identify exactly which vessel each AIS target represents. In digital selective calling, part of the GMDSS, the MMSI allows a station to make an automated call to a specific ship or group of ships, or to send a distress alert that identifies the vessel in distress. Satellite EPIRBs and other distress equipment can also be linked to a ship's identity. The structure of the number carries meaning: the first three digits form the Maritime Identification Digits that indicate the country of registration of the station.

Because so much automated safety and identification depends on it, the MMSI must be correctly assigned and programmed. It is issued by the ship's flag administration as part of the radio licence, and it should be updated if the ship changes flag or ownership, since an incorrect or reused MMSI can cause confusion in distress situations and in traffic monitoring. The MMSI works alongside the ship's permanent IMO number and its call sign as one of the identifiers that together fix a vessel's identity, but the MMSI is specifically the identity used by digital radio and AIS, making it fundamental to safety communications and to the tracking of ships worldwide.