1The lesson in plain words
In one sentence…
On VHF, channel 70 is DSC only: the digital alert gets attention, then you speak on channel 16.
DSC or voice?
DSC sends data: who is calling, whom and where. Voice explains and coordinates. Channel 70 is never voice; channel 16 carries distress, urgency, safety and calling.
What do you prepare?
Identity, position and the time of that position, the danger, the help you need and the people involved. If GNSS is missing, don't delay the alert: use the best information you have.
Sent an alert by mistake?
Switching the radio off doesn't cancel it. Stop the repetition, use the set's cancellation, tell stations by voice on channel 16, inform the RCC and log it.
Receiving another ship's distress?
Receiving is not relaying. Read the data, listen to the voice traffic, assess whether you can help under the master's authority and inform the coast station or RCC.
2Step by step
VHF DSC distress alert
- 1
Send the DSC alert following the installed radio's instructions.
The digital alert gets attention and gives your identity and position.
- 2
Go to channel 16 and make the MAYDAY by voice.
Pressing the button doesn't explain the full circumstances.
- 3
Give identity, position and its time, danger, assistance needed and people involved.
A position without its time can mislead the response.
- 4
Listen for replies and update the information.
Acknowledgements and instructions come by voice.
- 5
Log acknowledgements, instructions and changes in the radio record.
It keeps a record of what is known and what was decided.
- MAYDAY
- This is [vessel name], MMSI [number]
- Position [latitude and longitude] at [time]
- [What is happening: fire, flooding…]
- I require [the assistance you need]
- [Number] persons on board
3Key data
- Channel 70
- 156.525 MHz · DSC only
- Channel 16
- 156.800 MHz · voice
- Channel 6
- 156.300 MHz
- Channel 13
- 156.650 MHz
- All-ships call
- A format, not an MMSI
- False alert
- Cancel + ch 16 + RCC
4Key points
- 1.
Channel 70 = DSC (data) · channel 16 = voice.
- 2.
DSC alert → MAYDAY by voice on 16 → listen.
- 3.
Position always with its time; without GNSS, alert anyway.
- 4.
False alert: cancel, tell stations on 16, inform the RCC and log it.
5Frequent mistakes
Wrong: "I can talk on channel 70 if 16 is busy."
Right: Channel 70 is for DSC data only; distress voice goes on 16.
Wrong: "Pressing the distress button explains everything."
Right: The alert gets attention; the details are given by voice on channel 16.
Wrong: "If I switch the radio off, the false alert is cancelled."
Right: You must cancel, tell stations by voice on 16, inform the RCC and log it.
6Practice case & exam keys
Situation
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?
Show the answer →
Use the best available information and the appropriate distress procedure without delaying urgent alerting solely to obtain a perfect position field.
Why: 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.
Exam keys
"Channel 70" together with "voice" is always the wrong option: 70 is DSC only.
If asked what goes with the position, look for "its time": a position without time can mislead.
Alert, acknowledgement and relay are different decisions: receiving an alert doesn't oblige you to relay it to all ships.
How to read it
Think of DSC as the doorbell and voice as the conversation: in distress you need both.
In a real distress, speed beats perfection: alert with what you have and update afterwards.
7Full lesson & sources
Open the full lesson (diagrams and official references)
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
8Test your knowledge
What is VHF channel 70 used for?
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