The lesson in plain words
- 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.
Step by step
VHF DSC distress alert
- Send the DSC alert following the installed radio's instructions. The digital alert gets attention and gives your identity and position.
- Go to channel 16 and make the MAYDAY by voice. Pressing the button doesn't explain the full circumstances.
- Give identity, position and its time, danger, assistance needed and people involved. A position without its time can mislead the response.
- Listen for replies and update the information. Acknowledgements and instructions come by voice.
- 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]
- [Other useful information, such as persons on board]
Key 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
Frequent 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.
Exam keys & interpretation
- "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.
Key points
- Channel 70 = DSC (data) · channel 16 = voice.
- DSC alert → MAYDAY by voice on 16 → listen.
- Position always with its time; without GNSS, alert anyway.
- False alert: cancel, tell stations on 16, inform the RCC and log it.
Test your knowledge
What is VHF channel 70 used for?
Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
Full 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 exists because people cannot listen to a loudspeaker all day. A DSC receiver watches Channel 70 continuously and decodes every call without a human: it shows who is calling, how urgent it is, where the caller is, and which voice channel to go to. Then it sounds an alarm. Voice on Channel 16 does the part a machine cannot: describing the situation, answering questions, coordinating help.
Channel 70 is for DSC data. Speaking on it does nothing useful: DSC receivers are listening for data, so your voice reaches nobody and can block a real alert. Channel 16 is the voice channel for distress, safety and calling; Channel 13 is bridge-to-bridge; Channel 6 is intership and on-scene SAR.
| Item | Carried by the DSC alert? | Must you add it by voice? |
|---|---|---|
| Ship identity (MMSI) | Yes, automatically | Yes, with name and call sign, because people work with names |
| Position and its time | Yes, from GNSS or manual entry | Yes, and say how old it is if it is not current |
| Nature of distress | Only if you selected one | Yes, in plain words |
| Assistance required | No | Yes |
| Persons on board, intentions | No | Yes |
Position and its time
The DSC alert carries the position and the UTC time it was valid. If the GNSS feed has failed, the set may be sending a position typed in hours ago. Work out what that means: a disabled ship drifting at 2 knots with a position 30 minutes old may be 1 NM away from it; a ship steaming at 12 knots with a position three hours old may be 36 NM away. Rescuers searching the wrong 36 miles lose hours.
So check the DSC display every watch: does it show a position, and is the time beside it recent? In a real emergency, do not hold back the alert to fix the position. Send what you have, then give the better position by voice.
A routine DSC call in practice
You want to call a ship whose MMSI is 224123450 to agree a passing arrangement. Select an individual call to that MMSI, routine category, telephony, and propose a working channel such as 72. Her set acknowledges and both radios switch to Channel 72, so neither of you has to call on Channel 16. An all-ships call addresses everyone in range and is for urgency and safety announcements, not for chatting to one ship whose MMSI you do not know.
Cancellation is communication
A false alert sets people moving: the coast station acknowledges, the RCC starts planning, nearby ships alter course. A cancellation is communication with those people. Switching the set off cancels nothing; the alert has already gone out. Use the set's self-cancel function if it has one, then go to Channel 16 and say, for example: "ALL STATIONS, ALL STATIONS, ALL STATIONS. THIS IS ALTAIR, ALTAIR, ALTAIR, call sign ECHO ALFA BRAVO CHARLIE, MMSI 224123450, POSITION 41 DEGREES 22 MINUTES NORTH, 002 DEGREES 11 MINUTES EAST. CANCEL MY DISTRESS ALERT OF 26 OCTOBER AT 1432 UTC." Then stay on Channel 16 to answer questions, inform the RCC, and log it.
Receiving another ship's alert
When your set sounds for another ship's VHF distress alert, your job is to listen, not to press buttons. Follow the current IMO receiving-distress procedure:
- 1Read the caller's MMSI, position, time and nature of distress, and tell the master.
- 2Do not acknowledge by DSC: that is the coast station's job.
- 3Listen on Channel 16 for five minutes to give the coast station time to acknowledge.
- 4If you can help, acknowledge by voice on Channel 16 and follow the coast station's or RCC's instructions.
- 5Send a DSC relay only if a coast station or RCC asks for one, and log everything with UTC times.
Practise all of this in a simulator or with the set's test functions. A real distress button is never a training aid.
Official references · 2026-09-09
MSC.511(105) · ITU RR Resolution 349 (Rev.WRC-23) · USCG DSC