A Maritime Mobile Service Identity (MMSI) is a unique nine-digit number programmed into marine electronics, acting as a "phone number" for ships, AIS, and VHF radios with Digital Selective Calling (DSC). It is crucial for safety, allowing automatic distress alerts to send a vessel's identity and location to rescuers and nearby boats.

Key Aspects of MMSI:
Purpose: Identifies ship stations, coast stations, and AIS devices to improve maritime safety and communication.

Emergency Use: When a DSC distress button is pressed, the MMSI transmits the vessel's ID and position to authorities.

Structure: The nine-digit number begins with a 3-digit Maritime Identification Digit (MID), which indicates the country of registration.

How to Obtain: In the US, it is obtained via the FCC (for international) or authorized agents like BoatUS (for domestic). In Canada, via ISED.

Programming: Once programmed into a VHF radio or AIS unit, it is difficult to change, ensuring security.