RYA SRC Part 4 — DSC and MMSI: How the Radio Calls Other Radios

21 May 2026 PersonalSRCRYAdsc

Syllabus coverage: CEPT/RYA section B2 — DSC call categorisation and priority, types of call, MMSI structure including MIDs, ship/coast/group/portable numbers, and DSC facilities and usage (distress button, data entry, position update, watchkeeping).

Digital Selective Calling is the layer that turns VHF from “shout into the air and hope” into “address a specific station digitally and then talk.” It is the single most important concept on the SRC syllabus, and the bit the exam drills hardest.

What DSC is

DSC is a digital signalling system on top of VHF. Instead of using voice to call another station, a DSC transmission sends a short data burst on CH70 (156.525 MHz). That burst carries:

  • The transmitting station’s MMSI (its 9-digit identity).
  • The intended recipient — a specific MMSI, a group, all ships, or all ships in a geographic area.
  • The category of the call — distress, urgency, safety, or routine.
  • Position and time (if known).
  • The voice channel both stations should switch to after the DSC handshake.
  • For distress, the nature of distress (designated) or an “undesignated” flag.

The receiving station’s radio decodes the burst, alarms (loudly, in the case of distress), displays the calling information, and — if the user accepts the call — tunes itself to the agreed voice channel. The point: DSC handles the calling; voice handles the message.

CH70 is reserved exclusively for DSC. No voice traffic, ever. Class D sets enforce this — they will not key voice on CH70 — but you should never even attempt it.

Class D — what an SRC holder operates

There are several DSC equipment classes:

  • Class A — full SOLAS-grade DSC, MF/HF/VHF, separate CH70 watch receiver.
  • Class B — non-SOLAS, voluntary fit, separate CH70 receiver.
  • Class D — VHF only, integrated CH70 receiver shared with the main set. This is what the SRC syllabus assumes and the standard for small craft.
  • Class E — older non-DSC VHF (the radio you have to upgrade).

A Class D set keeps a continuous listening watch on CH70 in the background while you’re working on any other channel.

Call categories — the four priorities

Every DSC call carries a priority. Memorise the order:

  1. Distress — grave and imminent danger. The set’s siren alarm sounds. The receiver cannot ignore it (and the syllabus expects you to know that any station receiving a distress alert must respond appropriately).
  2. Urgency — very urgent message concerning the safety of a vessel, aircraft, or person. Below distress. Examples: serious mechanical failure, MOB still recoverable, urgent radio medical advice.
  3. Safety — message concerning the safety of navigation or important meteorological warnings.
  4. Routine — everything else. Calling a friend’s boat, asking the marina about berth allocation, requesting a lock booking.

Higher priorities pre-empt lower ones. If you’re mid-routine call and a distress alert arrives, drop everything and listen.

Types of call — who it’s addressed to

Independent of priority, DSC calls have a type describing the addressee.

  • Distress — broadcast to all stations. Cannot be addressed to a single recipient. The system is built around the assumption that the more stations that hear a distress, the better.
  • All Ships — broadcast announcement to every DSC-equipped station within range. Used for safety and urgency announcements.
  • Individual — addressed to one specific MMSI. The receiving station’s set alarms and prompts the operator to accept or reject. Used for routine ship-to-ship and ship-to-shore.
  • Group — addressed to a defined group of MMSIs sharing a group MMSI. Useful for fleet/club coordination.
  • Geographic — addressed to all stations within a defined geographic area (latitude/longitude box). Available on some equipment; less common on small-craft Class D.

The SRC exam expects you to know that a distress is always broadcast, and that urgency and safety announcements may be either All Ships or addressed.

MMSI — Maritime Mobile Service Identity

Every DSC-equipped station has a unique 9-digit identifier called the MMSI. It is the address the digital layer uses. You learn an MMSI by heart only for your own vessel, but you must understand the structure for the exam.

The structure

MMSI = 9 digits. The first three digits are the MID — Maritime Identification Digits, a country code allocated by the ITU.

UK MIDs: 232, 233, 234, 235. Republic of Ireland: 250. France: 226, 227, 228. Netherlands: 244, 245, 246. United States: 366, 367, 368, 369. Australia: 503.

MMSI types — recognise the leading-digit patterns

The leading digits of the MMSI tell you what kind of station it is. The CEPT/RYA syllabus expects you to recognise these.

PatternTypeExample
MID + 6 digits (full 9-digit)Ship station232123456 (UK ship)
00 + MID + 4 digitsCoast station002320014 (UK Coastguard)
0 + MID + 5 digitsGroup call023212345
8 + MID + 5 digitsUK Ship Portable (handheld licensed to a person)235812345
111 + MID + 3 digitsSAR aircraft111232001
970 + MID + 4 digitsAIS-SART970232001
972 + MID + 4 digitsMOB device972232001
974 + MID + 4 digitsEPIRB-AIS974232001

The exam typically asks: “What does an MMSI starting with 00 mean?” (Coast station.) Or: “What MMSI prefix identifies an AIS-SART?” (970.)

How MMSIs are issued in the UK

A vessel-mounted radio gets its MMSI as part of the Ship Radio Licence issued by Ofcom (free, online, lifetime renewable). A handheld licensed to a person gets a portable MMSI under the Ship Portable Radio Licence, with the 235 8XXXXXX format. The MMSI is programmed into the radio once at installation, and most Class D sets allow only one programming — getting it wrong means a dealer reset.

The exam expects you to know:

  • The MMSI is issued with the licence.
  • It’s programmed into the set.
  • It identifies the station, not the operator.
  • The operator licence (SRC) is separate and personal.

ITU MARS

The ITU maintains a public database of MMSIs called MARS (Maritime mobile Access and Retrieval System), accessible via the ITU website. It’s how a coast station looks up the registered vessel name and details when it receives an MMSI it doesn’t know. Anyone can search it.

DSC facilities and usage

The CEPT/RYA syllabus expects you to be able to operate the DSC menu functions. Practical things you’ll do on the exam (and on the boat):

The distress button

A red button under a hinged flap. Press and hold for the manufacturer’s specified duration — typically 3 to 5 seconds. The set will:

  1. Sound a confirmation tone.
  2. Transmit the distress alert on CH70, including MMSI, position (if known), time, and nature.
  3. Tune itself to CH16, ready for your voice MAYDAY.
  4. Retransmit the alert at intervals (typically every 3.5 to 4 minutes) until acknowledged by a coast station.

Designated vs undesignated distress

The set offers two flavours of distress.

  • Undesignated — the default if you just press and hold. Means “I’m in distress; nature unspecified.” Fastest to send.

  • Designated — you select the nature of distress from a menu before sending. The CEPT list is standard:

    1. Fire / explosion
    2. Flooding
    3. Collision
    4. Grounding
    5. Listing / capsizing
    6. Sinking
    7. Disabled and adrift
    8. Abandoning ship
    9. Piracy / armed robbery
    10. Man overboard

The exam often asks you to recite this list, or to recognise which option fits a scenario.

Use designated when you have a few seconds to spare; use undesignated when you don’t. Either way, follow up with a voice MAYDAY on CH16.

Acknowledging a distress alert — and why a small vessel must NOT do this in A1

This is the question the exam catches people on more than any other.

When a DSC distress alert is received in Sea Area A1, the coast station is the preferred controlling station and will acknowledge the alert by DSC. Once the coast station acknowledges, the casualty’s set stops retransmitting and the operational traffic continues by voice.

If a small craft DSC-acknowledges a distress alert before the coast station has done so, it silences the casualty’s set and may prevent the coast station from receiving the alert at all. Therefore: in Sea Area A1, do NOT press your set’s “Distress ACK” button. Stay quiet, listen on CH16, wait for the coast station to take charge, and only respond if the coast station calls for assistance.

You CAN — and should — acknowledge by voice on CH16 if the casualty is calling for help and no coast station has responded after several minutes. That’s a different action from a DSC acknowledgement.

If you witness a distress and the casualty’s own radio is not working (or there’s no coast station response and the situation is urgent), send a MAYDAY RELAY (covered in Part 5). That’s the small-vessel’s mechanism for raising an alert on someone else’s behalf — not the DSC ACK button.

Sending an Individual call (routine example)

For a routine call to another vessel by DSC:

  1. Select “Individual Call” from the DSC menu.
  2. Enter the recipient’s MMSI (or select from your directory).
  3. Choose the voice channel you want to switch to (e.g. CH06).
  4. Send.

The recipient’s set alarms and displays your MMSI and proposed channel. If they accept, both sets switch to CH06 automatically, and you make the voice call there. This is a far more polite way to initiate a routine call than shouting on CH16 — and on the exam, demonstrating it correctly is part of the practical.

Manual position entry

If your set has no GNSS feed (and the boat is moving), you must update position manually. The exam wants you to know:

  • Manual position must be updated at least every 4 hours.
  • The set timestamps the entry; an alert sent with stale position carries the timestamp so RCCs know how old the data is.
  • A stale position is still better than no position at all.

Practical procedure: menu → “Position” → enter lat/long → save. Most sets prompt you when the entered position is older than 4 hours.

Reviewing received messages

Class D sets keep a log of received DSC messages — distress alerts, individual calls, urgency announcements. Use the menu to review the log, especially after extended periods away from the set. Some sets clear the log on power-off; check your manual.

Directory entries

Most sets let you store frequently-called MMSIs with friendly names. Programming your sailing club’s MMSI and your usual rescue-coordination centre’s number into the directory means you can call them without remembering nine digits under pressure.

Watchkeeping and log keeping

The CEPT syllabus expects you to know:

  • A Class D DSC VHF maintains an automatic listening watch on CH70 in the background — you don’t need to do anything; the radio listens.
  • A separate voice listening watch on CH16 is also expected (and effectively automatic, since most sets default to CH16 when idle).
  • The UK does not require small craft to keep a formal radio log. SOLAS vessels do. The syllabus expects you to know which side of that line you sit on.
  • If you do hear distress traffic, noting time/position/details in a notebook is good practice — RCCs may later ask what you heard.

What to take into the next post

You should leave Part 4 able to:

  • State what CH70 is for (DSC only, no voice).
  • List the four DSC call priorities and four call types.
  • Recognise MMSI formats by their leading digits (00, 0, 8, 111, 970, 972, 974).
  • Identify UK MIDs (232–235).
  • Recite the nature-of-distress menu options.
  • Explain why a small vessel must NOT DSC-acknowledge a distress alert in Sea Area A1.
  • Describe manual position entry and its 4-hour update rule.

Part 5 puts the DSC distress button to work — what you actually do, in order, when something has gone properly wrong.