What is an MMSI number used for?

An MMSI (Maritime Mobile Service Identity) number is a unique identification assigned to vessels and shore stations that participate in maritime communication. This number consists of nine digits and serves primarily to identify a ship in radio communication, particularly in Digital Selective Calling (DSC) and AIS (Automatic Identification System) systems. When a vessel transmits an MMSI number, it allows other vessels, coast stations, and authorities to identify its specific communication and location details quickly and accurately.

The significance of the MMSI number lies in its role in enhancing safety at sea and facilitating effective communication. It allows for the communication of vital information such as distress signals, positioning reports, and vessel identification, which are essential for maritime navigation and safety.

The other options do not accurately describe the function of the MMSI number. While it does not determine the size of the vessel, register with port authorities, or track speed, it fundamentally relates to identification and communication within maritime operations.