VHF channel 70 (156.525 MHz) carries DSC, never voice. Channel 16 (156.800 MHz) carries distress, urgency, safety and calling voice. Channel 6 is 156.300 MHz and channel 13 is 156.650 MHz. All-ships DSC addressing is a call format, not a special nine-digit ship MMSI.
In distress, follow the installed radio instructions to send the DSC alert, then communicate the MAYDAY on channel 16 and listen for replies. Give identity, position and its time, danger, assistance needed and persons involved. Do not delay urgent alerting for missing GNSS data. For a false alert, stop repetition, use supported DSC self-cancellation, cancel by voice on channel 16, inform the RCC and record the event.
Treat DSC and voice as complementary tasks
DSC sends structured digital information that can attract attention and identify the calling station. Radiotelephony lets people describe a situation, ask questions and coordinate action. The two functions belong to one communication process, but use different channels and different forms of information.
An individual routine call identifies its intended station and proposed communication arrangements. An all-ships format addresses the relevant receiving group; it is not a fictitious ship identity. Read the call category and address deliberately before following the installed equipment's procedure.
Prepare the information, not just the transmitter
| Information | Why the operator checks it |
|---|---|
| Ship identity | The receiving station must know who is calling |
| Position and its time | A coordinate without its age can mislead the response |
| Nature of the problem | The station needs to understand the danger |
| Assistance required | The request should describe what help is needed |
| People involved | Relevant numbers and circumstances help coordination |
Before practice, compare the radio's displayed identity and position with the exercise brief. If a position is entered manually, identify its source and time. During an actual urgent event, provide the best available information without delaying the alert merely to make every field perfect.
Move from the alert into communication
Follow the installed radio's distress procedure. After the DSC alert, make the associated voice communication on channel 16, listen for replies and provide requested updates. Do not assume that pressing the distress control has already explained the full circumstances.
Speak clearly and in short groups. Keep identity and position unambiguous, listen before adding non-essential traffic, and distinguish your own vessel's information from information about another casualty. Acknowledgement, instructions and important changes belong in the appropriate radio record.
Respond to an accidental alert
A cancellation is communication with the stations that may already be acting on the alert. Silencing a local alarm or closing a screen does not tell those stations that assistance is unnecessary. Use the installed set's supported cancellation procedure and the associated voice notification, inform the RCC and record the event.
Receive another ship's alert without creating confusion
Receiving an alert is not an invitation to make an immediate all-ships DSC relay. Follow the current IMO receiving-distress procedure, listen for the associated traffic and involve the appropriate coast station/RCC. Distress alert, acknowledgement and relay are different actions.
- 1Read the received identity, position/time and distress information.
- 2Move attention to the prescribed associated voice traffic and monitor the situation.
- 3Assess whether the ship can assist and communicate under the master's authority as the procedure requires.
- 4Keep the coast station/RCC informed and record the relevant facts.
Use isolated simulation or authorized test functions for practice. A real distress transmission is never a classroom test message.
Official references · 2026-09-09
MSC.511(105) · ITU RR Resolution 349 (Rev.WRC-23) · USCG DSC
Apply what you learned
During isolated practice, the radio shows an old manually entered position. The exercise then presents grave and imminent danger requiring immediate assistance. A learner wants to postpone all communication until a new position can be obtained.
How should the learner handle the missing or stale information?
Suggested answer: Use the best available information and the appropriate distress procedure without delaying urgent alerting solely to obtain a perfect position field.
Why this works: The age and uncertainty of a position matter and should be communicated clearly. They do not justify silence when immediate assistance is needed. The subsequent voice exchange allows the operator to explain the situation and supply updated information.
Continue with guided radio exercises.
Practise in the radio simulatorKey Takeaways
DSC supplies structured calling information; voice supplies explanation and coordination.
Channel 70 carries digital DSC traffic, never speech.
Check the identity and the age of the position information.
An accidental alert needs an effective cancellation and notification, not a silent power-off.
Receipt of an alert, acknowledgement and relay are distinct procedural decisions.
Common Mistakes
Attempting voice traffic on channel 70.
Assuming the digital alert communicates every detail needed by rescuers.
Treating a local alarm reset or switched-off screen as cancellation to the RCC.
Test Your Knowledge
Practise this topic and review the explanation for each answer.
More GMDSS lessons
- Module 5 — NBDP / radiotelexNBDP is narrow-band direct-printing telegraphy.
- Module 6 — Inmarsat C and EGCInmarsat-C provides data messaging, distress alerting and SafetyNET reception; it does not provide voice.
- Module 7 — NAVTEX and maritime safety informationInternational NAVTEX broadcasts on 518 kHz in English; national services may use 490 or 4209.5 kHz.
- Module 8 — EPIRB / 406 MHz beaconA 406 MHz EPIRB alerts the Cospas-Sarsat system; its 121.5 MHz signal helps nearby rescue units home in.
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