Module 8 - Digital Selective Calling
MMSI
What you will learn
Use an MMSI as a controlled station identity, verify it before a call and avoid unsafe reprogramming or directory assumptions.
The Maritime Mobile Service Identity is a nine-digit radio identity. It links digital calls to a licensed station or authorised group, so accuracy and ownership matter.
An MMSI is transmitted over the radio path to identify a maritime station or group call. Ofcom assigns the relevant identity through a Ship Radio or Ship Portable Radio Licence. A ship-station identity remains with its assigned vessel; a portable identity follows the licensed equipment and its territorial conditions. Use only the identity authorised for the actual station arrangement rather than inventing, borrowing or casually altering one.
Before sending an individual DSC call, compare all nine digits with the intended station's current official or directly confirmed identity. A directory entry saves typing but does not make stale data correct. Label entries clearly enough that another operator can distinguish a vessel, coast station and group identity.
The MMSI sent by DSC does not replace spoken identity where the voice procedure requires vessel name, call sign or MMSI. It also does not prove that the displayed caller is safe to obey without checking the category, position, time, channel and surrounding circumstances.
Release an identity before releasing the call
A crew member selects an old directory entry labelled only 'Harbour' before making a routine DSC call.
1. Stop before send
Do not infer that the stored number still belongs to the required station.
2. Verify all nine digits
Check the current official or directly confirmed identity and the station type.
3. Correct the directory
Update the label and number under the set instructions before relying on it again.
Sense check: A successful transmit indication proves only that the set sent a digital call; it does not prove that the intended station received it.
Common mistake or limitation
- Treating an MMSI as a telephone number that may be guessed from a vessel name.
- Assuming every saved directory entry remains current and correctly labelled.
- Reprogramming the set to another vessel's identity without the licensing and equipment process.
Recap
- An MMSI is a nine-digit licensed digital identity.
- Verify the complete number and station type before sending.
- Digital identity supports but does not replace the required voice identity.
Optional quick check
Section 2 of 9
A stored DSC directory entry is ambiguous and has not been checked since the radio changed owner. What is the safe next step?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Continue studying Digital Selective Calling
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