Module 4 - Channels & Frequencies
VHF Channel 16 vs Channel 70: voice calling and DSC
What you will learn
Select the first digital and voice route from the message priority, then identify where the working traffic continues.
Channel selection is a sequence, not a single memorised number. Priority, the station identity and whether DSC is available determine the first contact and the next voice channel.
Start with the situation. Grave and imminent danger requiring immediate assistance is distress; concern for the safety of a person or vessel is urgency; navigational or meteorological warning traffic is safety; ordinary coordination is routine. The chosen label controls the procedure as well as the channel.
Channel 70 carries the digital selection or alert. Channel 16 carries the distress voice message and specified urgency, safety or initial-calling voice stages. A nominated, accepted or instructed working channel then carries the detailed non-distress traffic where the procedure calls for it.
The table is a decision aid for revision, not a substitute for the complete distress, urgency, safety or DSC module. In use, read the radio acknowledgement and current station instruction rather than forcing every case into one remembered route.
| Situation | First contact | Voice route | Critical guardrail |
|---|---|---|---|
| Distress | DSC distress alert on 70 when available | Mayday call and message on 16 | Do not delay justified distress traffic for routine channel etiquette |
| Urgency to all ships | DSC All Ships Urgency call | Announcement on 16, message on nominated working channel | Use the real urgency procedure, not a routine call with urgent wording |
| Safety information to HM Coastguard | Individual DSC safety call when supported | Working channel indicated in the acknowledgement | If unsupported, MCA guidance gives a voice call on 16 |
| Routine inter-ship, MMSI known | Individual DSC routine call | Accepted inter-ship channel | Do not speak on 70 |
| Routine inter-ship, MMSI unknown | Brief low-power voice call on 16 | Agreed inter-ship channel | If unanswered, the documented bridge-to-bridge fallback is 13 |
Choose the route before touching the channel control
A crew member reports an unlit object drifting in a busy harbour entrance. There is no casualty and no immediate danger to your vessel.
1. Classify
Decide whether this is safety information for the relevant authority rather than inventing distress or routine priority.
2. Select
Use the supported safety route and the current authority identity; if acknowledged digitally, read the indicated working channel.
3. Pass facts
Give the observed position, nature of the hazard and time without speculation, then continue the required watch.
Sense check: If the chosen route does not match the message priority, correct the classification before transmitting.
Common mistake or limitation
- Memorising Channel 16 and Channel 70 without learning when the exchange moves to a working channel.
- Assuming every safety message is voiced on Channel 16 even after a DSC acknowledgement specifies another channel.
- Using a high-priority label to obtain attention for routine traffic.
Recap
- Priority determines the procedure and channel sequence.
- Channel 70 is digital; Channel 16 and working channels carry the specified voice stages.
- Read the acknowledgement or current instruction rather than assuming the next channel.
Optional quick check
Section 4 of 9
An Individual DSC safety call to HM Coastguard is acknowledged with a working channel. What should the operator do next?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
Practice choosing the right VHF channel
The channels module links distress, DSC, inter-ship, port and working-channel choices to realistic call decisions.