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]
- [Other useful information, such as 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
In a simulator exercise, the VHF DSC display shows a position entered manually three hours ago; the ship has been making 12 knots. A fire is now out of control and the crew need immediate assistance. The learner wants to fix the position before sending anything.
What should the learner do, and in what order?
Show the answer →
Send the DSC distress alert at once with the information available, then make the MAYDAY on Channel 16 giving the best current position and its time.
Why: The stored position could be 36 NM out, so it must be corrected, but correcting it by voice costs no time and delaying the alert does. The voice message adds the current position, the fire, the help needed and the number of people, which the DSC alert cannot carry.
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.
7Go deeper: GOC exam level
The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.
The VHF channels you cannot get wrong
| Channel | Frequency | Essential use |
|---|---|---|
| 70 | 156.525 MHz | DSC only: distress, urgency, safety and calling. Never voice. |
| 16 | 156.800 MHz | Voice distress, safety and calling; VHF distress traffic |
| 13 | 156.650 MHz | Bridge-to-bridge navigational safety |
| 6 | 156.300 MHz | Intership and SAR on-scene communication, including with aircraft |
| Working channels | Various | Port, VTS, ship-to-ship and correspondence according to the channel plan and local rules |
The DSC watch on Channel 70 is kept by a dedicated watch receiver. Channel 16 stays critical for voice; keep it clear of unnecessary chatter.
The controls, and the traps in each
| Control | What it does | Typical mistake |
|---|---|---|
| CH selector / 16 key | Changes channel / jumps straight to 16 | Holding a routine conversation on 16 |
| HI/LO | Transmit power (typically 25 W / 1 W) | Always using full power when low power would do |
| VOL | Audio level | Confusing volume with receiver sensitivity |
| SQL (squelch) | Mutes audio below a threshold | Setting it so high that weak stations disappear |
| SCAN / DW | Scanning or dual watch | Not knowing what is actually being monitored |
| DSC menu / address book | Digital calls | Confusing the addressee with the priority |
| DISTRESS (guarded) | DSC distress alert | Pressing it "to test", causing a false alert |
| GNSS input | Automatic position and time | Not noticing a frozen or missing position |
Range: height beats power
VHF range is set mainly by the radio horizon of both antennas. A common rule of thumb is d (NM) ≈ 2.2 × (√h₁ + √h₂), with heights in metres. Two ships with antennas at 16 m reach roughly 2.2 × (4 + 4) ≈ 18 NM; a coast station on a 400 m hill reaches much further.
Identities: MMSI, MID, call sign and MARS
An MMSI (Maritime Mobile Service Identity) is a nine-digit number used by DSC and AIS. It lets equipment select a station without anyone interpreting a spoken name. Formats are set by ITU-R M.585 (current version M.585-10).
| Station | Format |
|---|---|
| Ship station | MIDXXXXXX: the first three digits are the MID of the Administration |
| Group of ships | 0MIDXXXXX |
| Coast station | 00MIDXXXX |
| SAR aircraft | 111MIDXXX |
| AIS aid to navigation | 99MIDXXXX |
| AIS-SART | 970XXYYYY |
| MOB device | 972XXYYYY |
| EPIRB-AIS | 974XXYYYY |
The MID (Maritime Identification Digits) identifies the Administration that assigned the identity (for example 224 and 225 for Spain). The call sign is the international radio callsign. The ship's name is the human identity. In an emergency you use several of them together so automated systems, operators and databases agree.
MARS (Maritime mobile Access and Retrieval System) is the ITU database where ship stations, coast stations and MMSIs can be looked up.
Anatomy of a DSC call
You do not need the bits, but you must know what each field means.
| Field | Meaning |
|---|---|
| Format specifier | The kind of call: distress, all ships, individual, group, geographic area |
| Address | Who it is for, when the format needs one |
| Category | Distress, urgency, safety or routine |
| Self-identification | The MMSI of the calling station |
| Telecommand | The type of follow-on communication (for example telephony) |
| Information | Channel or frequency, position, time and other data |
| EOS | End of sequence; tells the receiver what reply is expected |
| ECC | Error check character, lets the receiver detect transmission errors |
VHF DSC uses a 1200 baud subcarrier (emission class G2B); MF/HF DSC uses 100 baud (F1B or J2B). That is why an HF DSC sequence takes longer, and why Channel 70 is a signalling channel, not a conversation channel.
The distress alert's content
A DSC distress alert should carry the last known position and the UTC time at which it was valid, normally from the connected GNSS. If they are missing, enter them manually when time allows. The nature of distress and the follow-on communication can also be selected.
| Nature of distress | Meaning |
|---|---|
| Fire, explosion | Fire or explosion |
| Flooding | Water ingress |
| Collision | Collision |
| Grounding | Grounding |
| Listing | Dangerous list, danger of capsizing |
| Sinking | Sinking |
| Disabled and adrift | No steering or propulsion, drifting in distress |
| Abandoning ship | Abandoning the vessel |
| Piracy / armed robbery | Attack |
| Man overboard | Person in the water (on equipment that supports it) |
| Undesignated | No nature selected |
Receiving a VHF distress alert on another ship
The modern rule avoids a storm of digital acknowledgements. A ship station that receives another ship's alert normally does not acknowledge it by DSC; that is the coast station's or RCC's job.
- 1Keep watch on Channel 70 for the acknowledgement.
- 2Tune the voice receiver to Channel 16.
- 3Within coast-station coverage, listen on Channel 16 for about five minutes before any voice acknowledgement, to give the shore time to respond.
- 4If you can help, acknowledge by voice on Channel 16 and follow the RCC or coast station's instructions.
Only if the alert keeps repeating and there is no other acknowledgement should a ship acknowledge by DSC, and then after consulting a coast station or RCC.
Telecommands and end-of-sequence codes
Each DSC call carries telecommands that tell the receiver what follows, and an end-of-sequence (EOS) symbol that says what reply is expected. The set shows them as words; the numbers are the ITU-R M.493 symbols you may meet in exams.
| Telecommand | Meaning |
|---|---|
| 100 | Telephony, F3E/G3E (VHF voice) |
| 109 | Telephony, J3E (MF/HF voice) |
| 118 | Test |
| 104 | Unable to comply |
| 121 | Ship position or location registration update |
| EOS | Meaning |
|---|---|
| 117 | Acknowledgement required (RQ) |
| 122 | Acknowledgement given (BQ) |
| 127 | Other calls, no acknowledgement expected |
A routine DSC call, step by step
- 1Select the correct MMSI.
- 2Select the routine category and the appropriate telecommand.
- 3Propose a working channel when the equipment asks for one.
- 4Transmit the call.
- 5Wait for the acknowledgement and move to the agreed channel.
- 6Keep the exchange short and close it properly.
GOC exam check
What is the frequency of VHF Channel 70?
8Full 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
9Test your knowledge
What is VHF channel 70 used for?
Keep studying…
Reviewed by Ramon Curto, Nautical teacher at IECNPC · Last reviewed: