Digital selective calling lets one radio ring another, the way a phone does. You talk by voice afterwards; DSC only makes the call. In distress the same system sends who you are and where you are in a single burst of data.
1 · What DSC does
A data call on 70, then voice
Will my DSC radio hear a distress alert while I'm talking on another channel?
Yes: a fixed DSC set has a second receiver that listens on channel 70 all the time, whatever voice channel you are using and whether or not dual watch is on.
A DSC call is a short digital message on channel 70, which carries data only, never voice. It names the station called, the station calling, the priority and the channel for the conversation. A fixed DSC set has a second receiver that listens on 70 all the time, whatever voice channel you are using and whether or not dual watch is on.
Call
Goes to
For
Distress alert
All stations
Grave and imminent danger
All ships
All stations
Urgency and safety only
Individual
One MMSI
Routine calls, and testing
Group
A group MMSI
A flotilla or a fleet
2 · MMSI
The yacht's nine-figure number
Can I put my own MMSI number into a charter yacht's radio?
No: the MMSI is issued to the vessel with her ship radio licence, not to a person. It is programmed into the set once and stays with the yacht.
The MMSI is the number DSC calls. It always has nine figures. A ship's MMSI begins with the three-figure MID, the code of her flag state, so a rescue centre sees at once where she is registered. A coast station's number begins with 00, a group's with a single 0.
Reading an MMSI
238 123450 · a ship
238 = MID: the flag state, here Croatia
00 238 0100 · a coast station
0 238 12345 · a group of ships
Example numbers: the zeros in front tell you what kind of station it is.
The MMSI is issued to the vessel with her ship radio licence, not to a person. It is programmed into the set once and stays with the yacht: a guest cannot put in his own. A set with no MMSI can send neither a call nor an alert, so on a charter yacht have the base put it right before you sail.
3 · Routine calls
Ringing one yacht, or your flotilla
How do I test the DSC on my VHF without sending a false alert?
Send a routine individual call to a friend's MMSI and see that it arrives. Never test DSC with the red DISTRESS button.
Choose individual call, enter or pick her MMSI and propose an intership channel such as 72.
Send. Her set rings and shows who is calling.
She acknowledges, both sets go to the proposed channel, and you talk by voice.
A group call rings every set that holds the group MMSI and sends them all to the channel named: go there and listen for the lead boat.
DSC: individual call
To: MMSI 238123450 (Nova)
Category: ROUTINE
Working channel: 72
ACKNOWLEDGED · both sets go to 72
An all-ships call sounds an alarm on every set in range. It is kept for distress, urgency and safety, never used to ask the fleet for a spare part. To test DSC, send a routine individual call to a friend's MMSI and see that it arrives. Never test with the red button.
4 · Distress alert
The red button
How do I send a DSC distress alert with the red button?
Lift the cover, press and hold the DISTRESS button for about five seconds until the set confirms the alert has gone. The set switches to 16: send your voice MAYDAY there and keep listening.
The DISTRESS button sits under a cover so that it cannot be pressed by accident. Lift the cover, press and hold for about five seconds until the set confirms the alert has gone.
The alert carries
From
Who: your MMSI
Programmed into the set
Where and when: position, time
The GPS feed, or entered by hand
What: nature of distress
The menu; otherwise "undesignated"
With a minute in hand, pick the nature of distress first: fire, flooding, collision, grounding, sinking, man overboard. Rescuers then know what to bring. With no time, the button alone sends an undesignated distress. The set repeats the alert by itself about every four minutes until a station acknowledges it. It also switches to 16: send your voice MAYDAY there and keep listening.
5 · Before you sail
No position, no rescue address
What does NO POSITION on my DSC radio display mean?
No GPS data reaches the set, so an alert would say who you are but not where. Switch the GPS or plotter on, or enter position and time by hand and update it regularly.
An alert is only as good as the data behind it. Without a GPS feed and with no position entered, it says who you are but not where. Read the DSC display before you leave the berth.
Display
Meaning
You do
NO POSITION
No GPS data reaches the set
Switch the GPS or plotter on, or enter position and time by hand
MMSI NOT SET
No call or alert can go out
Have it programmed before sailing
A position entered by hand goes stale as you sail: update it regularly.
6 · Alerts received, false alerts
When the alarm sounds on your set
What should I do if my VHF sends a false DSC distress alert?
Do not just switch the set off: cancel the alert on it if it has that function, then tell all stations by voice on 16, with your name, MMSI, position and the time it was sent.
Your set sounds an alarm and shows a distress alert: an MMSI, a position, a nature of distress. Silence the alarm, write the details down and listen on 16 for the voice MAYDAY. Do not acknowledge by DSC: that is normally left to the coast station, whose acknowledgement stops the casualty's set repeating.
If your own set sends an alert by mistake, do not just switch it off: a search may already be starting. Cancel the alert on the set if it has that function, then tell everyone by voice on 16.
Cancelling a false alert, on 16
ALL STATIONS, ALL STATIONS, ALL STATIONS
THIS IS Nova, MMSI 238123450
Position 43°10.5′N 015°58.0′E
CANCEL MY DISTRESS ALERT OF 0915 UTC
Rule
A false alert is nothing to hide. Cancel it at once, with your name, MMSI, position and the time it was sent.
Practice questions
Questions from this topic, as the app asks them. Open one to see the answer.
DSC: incoming call
GROUP CALL to your flotilla
From: the lead boat
Category: ROUTINE · channel 72
Your set rings and shows this. What do you do?
Ignore it — group calls are only used for tests
Call the lead on 16 to ask what it is about
Reply to the lead by voice on channel 70
Go to 72 (the set may switch itself) and listen for the lead
Show the answer
D. Go to 72 (the set may switch itself) and listen for the lead
A group call reaches every set with that group MMSI and sends them all to the proposed channel.
All 20 questions of this topic are in the app: practice by topic with the reason after every answer, your mistakes brought back, and a timed mock exam. Free, no sign-up, in the browser or in Telegram.
