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The Maritime Mobile Service Identity (MMSI) is a number that uniquely identifies a vessel (or shore station). It can be regarded more or less as a phone number for maritime purposes. The number can be used to make a call to a specific vessel, or a group call to a group of vessels. The number has to be programmed in the maritime radio and is shared by all radios on board. The radio must support digital selective calling (DSC) to make a call to a specific vessel, but all radios do nowadays.
A MMSI number can be used to identify:
Structure of a MMSI number
The MMSI number starts with a few digits which give the kind of identification the number is used for. A MMSI number of an individual vessel starts with the MID, followed by 6 digits to identify a particular vessel. A MMSI number for a group of vessels has a leading zero before the MID, followed by 5 digits to identify a particular group of vessels. A MMSI number of a shore station or a group of shore stations has two leading zeros before the MID followed by 4 digits to identify a particular station or group of stations.
Search and Rescue (SAR) aircrafts, used to assist in search and rescue operations, use MMSI's with the block 111 as the leading three digits, followed by the country MID and then three digits to indicate the individual aircraft. Navigation aids, like light ships and beacons have a MMSI with 99 as the leading digits, followed by the country MID and then 4 digits. These navigation aids with an MMSI number are mainlyy used at sea. See also AIS.
The structure of the different kind of MMSI numbers is summarized in the following table.
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