Distress, urgency and safety

DSC: the red button explained

also: Digital Selective Calling

DSC is the digital calling layer of the GMDSS: a short data burst instead of a voice call. On VHF it lives on Channel 70, and its most famous part is the red DISTRESS button.

Key facts

DSC stands for Digital Selective Calling. The radio transmits a short digital message: who is calling, whom, why, and where.
On VHF, DSC uses Channel 70 and nothing else. No voice is ever transmitted there; the channel is reserved for the data bursts.
Every DSC radio is programmed with an MMSI: a nine-digit Maritime Mobile Service Identity, in effect the radio’s phone number.
The red DISTRESS button sits under a spring-loaded flap. Press and hold it (on most sets a few seconds) and the radio sends an alert with your MMSI, position and time, and repeats it automatically until someone acknowledges.
A DSC set fed by GPS puts your position into the alert by itself. Without that feed you type the position in by hand, or the alert goes out without one and is worth far less to the rescuers.
After the alert the set switches to Channel 16. Give the coast station a moment to acknowledge, then send the voice Mayday there: the alert rings the bell, the voice message tells the story.
DSC also does routine work: an individual call rings one specific MMSI and proposes a working channel, so the called ship’s radio alarms even if nobody was listening to 16.

Watch out

A false distress alert is not cancelled by switching the set off. Stop the transmission the way the manual says, then broadcast on Channel 16 to all stations: your name, call sign and MMSI, your position, and "cancel my distress alert of" with the date and time in UTC. Rescue services treat every alert as real until it is properly cancelled.

Listen: the standard phrases

Recordings from our SMCP trainer, in the accents you actually hear on the air.

Did you transmit a DSC distress alert? Mihaylo · Ukrainian accent

Yes, I transmitted a DSC alert. Manav · Indian accent

Yes, I transmitted a DSC alert by mistake. James · British accent

What is your MMSI number? Shelley · British accent

Where this comes from

SOLAS Chapter IV (GMDSS); ITU Radio Regulations, Articles 32 and 33 (DSC procedures).

This is not a legal text: when in doubt, the current wording of the Radio Regulations and the SMCP decides.

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