MMSI
An MMSI (Maritime Mobile Service Identity) is a nine-digit number that uniquely identifies a ship (or coast station) in maritime radio communications. It is programmed into the vessel's radio equipment and is the identity broadcast by AIS and used in Digital Selective Calling (DSC) and GMDSS distress alerting, so that ships and shore stations can be automatically addressed and identified.
The MMSI's first three digits form the Maritime Identification Digits (MID) indicating the vessel's flag/country of registration. Because it is tied to the radio licence and flag, an MMSI can change when a ship changes flag or ownership — unlike the permanent IMO number. In practice, the MMSI is the identifier you see in AIS-based vessel tracking (identifying the transmitting ship), while the IMO number is the stable identifier of the hull itself; tracking systems often map between the two. Understanding the MMSI is useful for interpreting AIS data and vessel identification in tracking and communications. It is a key identifier in the technical plumbing of maritime radio and vessel tracking.
The MMSI is the identity a ship broadcasts over AIS and radio, so it is what appears in vessel-tracking feeds and distress alerts. But because it can change with flag or ownership, tracking systems map it to the permanent IMO number for a stable history. Understanding the MMSI is key to interpreting AIS data and identifying vessels in tracking and communications.
What do the first digits of an MMSI mean?
The first three are the Maritime Identification Digits (MID), indicating the vessel's flag or country of registration.
Can an MMSI change?
Yes — it is tied to the radio licence and flag, so it can change with flag or ownership, unlike the permanent IMO number.