Digital Selective Calling (DSC) and MMSI Procedures
Anatomy of an MMSI
The Maritime Mobile Service Identity is a nine-digit number identifying a station in DSC and AIS. A ship station is MID plus six digits, where the MID identifies the flag state (UK: 232, 233, 234, 235). A group station begins with 0 (0 + MID + five digits) and serves a fleet or club; a coast station begins with 00 (00 + MID + four digits). The same MMSI is programmed into the VHF/DSC, the EPIRB, the AIS and any satellite terminal, so that an alert, a position report and a registration record all point to the same vessel.
DSC in practice
Beyond distress, DSC sends individual calls to one MMSI, group calls, all-ships calls and test calls. A routine call selects the target MMSI, proposes a working channel (Ch 6, 8, 9, 72 or 77, or a marina channel), and transmits on Ch 70. The called station acknowledges, automatically or manually, and both switch to the nominated channel for voice. Automatic acknowledgement should be off for routine calls, or the set answers on your behalf. Enter position and time manually when there is no GPS feed: an alert without a position wastes the most valuable information you have.
Registration and changes
The MMSI is issued with the ship radio licence and cannot be changed by the user; a dealer must reprogram the set. Keep the registration database current - owner, address, emergency contacts, EPIRB details - because that record is what a rescue coordination centre reads when an alert arrives. If you sell the vessel the new owner updates the licence; do not carry an MMSI across to a new boat, and never program two sets with the same number.
Worked example
You want to arrange a rendezvous with a friend's yacht whose MMSI you hold. Reasoning: there is no need to call by voice on Ch 16 when DSC will do it. Menu, individual call, enter their MMSI, select Ch 72, send. Their set acknowledges on Ch 70; both stations switch to Ch 72 and speak there. Ch 16 stays clear and the call is logged with time and identity.
Traps
- Pressing the distress button to "test" the DSC - that is a real alert.
- Attempting to speak on Ch 70.
- Using a group MMSI for a one-to-one call.
- Leaving the set with a stale manually entered position.