Nine-digit numerical address used to identify ships, coast stations, and devices in DSC, AIS, and satellite distress alerting.
A Maritime Mobile Service Identity (MMSI) is a unique nine-digit number assigned to maritime mobile radio stations and certain associated devices in accordance with ITU-R Recommendation M.585. The first three digits form the Maritime Identification Digits (MID), which denote the country or territory of registration; the remaining six digits identify the individual station or vessel.
MMSI numbers serve multiple functions across the GMDSS: in Digital Selective Calling (DSC) they identify the originating and addressed station in distress, urgency, and routine calls; in AIS they identify individual transponders and are embedded in every position report broadcast on the VHF data link. Special MMSI formats are defined for specific purposes — numbers beginning with 970 are assigned to AIS Search and Rescue Transmitters (AIS-SARTs), those beginning with 972 to Man Overboard AIS devices, and those beginning with 974 to AIS AtoN (Aids to Navigation) stations. SAR aircraft DSC units use identities beginning 111 followed by the MID.
MMSI assignment is co-ordinated nationally by administrations and internationally through the ITU Maritime Mobile Access and Retrieval System (MARS) database. Correct registration is essential for SAR authorities to identify a vessel following a DSC distress alert.