Module 8 - Digital Selective Calling
MMSI Structures And DSC Call Types
What you will learn
Recognise current ship, coast and group identity patterns and use them as a check on station type rather than a licence to invent an address.
ITU-R M.585-10 defines nine-digit maritime identities. The leading pattern helps distinguish ship, coast and group identities, while the complete assigned number remains the only address to use.
A ship-station MMSI has the form MID plus six digits. The three Maritime Identification Digits identify the administration having jurisdiction over that ship station. A coast-station identity has the form 00, MID and four digits. A group ship-station call identity begins 0, followed by the MID and five digits.
The MID in a group identity reflects the assigning administration or geographical area; it does not require every ship in the group to have the same nationality. A group identity is assigned for a defined group call, such as an organised fleet. It is not a list of recently received individual MMSIs and does not replace each vessel's own identity.
Pattern recognition is a reason to stop and check an implausible entry, not permission to construct the missing digits. Use the exact assigned identity from a current authorised source or direct confirmation. Then select the call type and priority separately for the operational purpose.
| Station or call | Nine-digit pattern | Use |
|---|---|---|
| Ship station | MID + 6 digits | One assigned ship station |
| Coast station | 00 + MID + 4 digits | A shore station |
| Group of ships | 0 + MID + 5 digits | An assigned group call |
Use structure as a reasonableness check
A directory entry labelled 'Coastguard' contains a nine-digit number beginning with an ordinary ship MID rather than 00.
1. Flag the mismatch
The pattern is inconsistent with a coast-station identity and must be verified before use.
2. Find the assigned number
Use the current official station entry or another authorised source; do not repair the number by guesswork.
3. Confirm the call
Select the exact identity, individual call type and purpose-appropriate priority.
Sense check: A plausible prefix cannot prove the remaining digits or the station's current service; verify the complete address.
Common mistake or limitation
- Calling a coast station with a guessed number formed from 00 and a remembered MID.
- Treating a group MMSI as the identity of every member vessel.
- Assuming call type and priority are encoded by the MMSI alone.
Recap
- Ship: MID plus six digits; coast: 00 plus MID plus four; group: 0 plus MID plus five.
- Use the pattern to check station type, not to invent an address.
- Call type and priority remain separate operator decisions.
Optional quick check
Section 8 of 9
Which pattern represents an assigned coast-station MMSI?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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