DSC
What Is DSC on a VHF Radio? Digital Selective Calling
Published · Last checked October 2026
DSC (Digital Selective Calling) is a digital calling system built into modern marine VHF radios. It sends short data messages on channel 70 to a specific station, a group or all stations. Its key feature is the red distress button: hold it for about five seconds and the radio transmits your MMSI and GPS position to every DSC station in range, including the coastguard, and keeps repeating until acknowledged. You then follow up with a Mayday by voice on channel 16.
How DSC works
Every DSC radio is programmed with a unique nine-digit MMSI number. Channel 70 is reserved for DSC data only: you never talk on it. A DSC call is like a phone ringing: it alerts the station you want, tells them which voice channel to use, and then you speak on that channel. Radios watch channel 70 continuously, even while you listen to another channel.
Types of DSC call
| Call | Use |
|---|---|
| Distress alert | Grave and imminent danger. Sends MMSI, position and (optionally) the nature of distress |
| Urgency (all ships) | Precedes a Pan-Pan voice call |
| Safety (all ships) | Precedes a Sécurité voice call |
| Individual / routine | Calls one vessel or a marina directly, without using channel 16 |
| Group | Calls a set of boats sharing a group MMSI, such as a flotilla |
| Position request | Asks another DSC radio for its position |
Sending a distress alert
- Lift the cover on the red DISTRESS button.
- If time allows, select the nature of distress (fire, flooding, sinking, man overboard and so on).
- Press and hold for around five seconds until the alert sends.
- Wait for an acknowledgement (usually up to 15 seconds), then make a Mayday on channel 16.
The radio repeats the alert every few minutes until a coast station acknowledges it. If you send one by mistake, do not switch off: cancel it on the radio, then call the coastguard on channel 16 and say so. There is no penalty for an honest false alert.
What you need
- A DSC-capable VHF (fixed sets have it as standard; many handhelds do too).
- An MMSI, issued free with an Ofcom Ship Radio Licence and programmed into the set.
- A GPS feed so your position is sent automatically.
- An operator qualification: in the UK, the RYA Short Range Certificate covers DSC.
The VHF DSC lesson walks through each call type, and the SRC guide explains the course.