Module 18 - Signals & Communications
DSC Distress Alerts
What you will learn
Execute the Class D VHF distress-alert sequence and verify which identity, position, time and distress information the set can actually send.
The protected DSC control sends a digital alert, but the alert and the voice MAYDAY are one procedure. Valid input data and a working antenna path matter as much as the red button.
Digital Selective Calling (DSC) sends an electronic distress alert from the VHF set. A DSC distress alert normally includes the vessel MMSI, position if the radio is connected to GNSS, time, and the selected nature of distress. It alerts coastguard stations and DSC-equipped vessels in range.
The red distress button is not a replacement for the voice call. On receipt of a DSC acknowledgement, or after about 15 seconds, transmit the spoken MAYDAY call and message on VHF Channel 16 unless the crew are unable to do so. The spoken message gives the human detail that rescuers need: position, problem, assistance required, people on board, and other useful information.
Use DSC when the position input may be stale
Flooding is increasing. The set shows a position time from earlier in the watch and the operator has a fresh charted position written beside the radio.
1. Alert
Select the nature of distress if time permits and press and hold until the set confirms the alert has been sent.
2. Follow with voice
On acknowledgement or after about 15 seconds, transmit the MAYDAY call and message on Channel 16.
3. Correct the picture
Give the current written position and state useful information rather than assuming the digital position was current.
Sense check: The operator can say what the set sent, what remains uncertain and what must be supplied by voice.
Common mistake or limitation
- Tapping the distress control may not send the alert; hold it until the set confirms transmission.
- Seeing coordinates on a screen does not prove that the radio has a live, correctly formatted position input.
- DSC does not replace the Channel 16 voice message or the immediate actions that protect life.
- Switching the set off after an accidental alert is not cancellation; follow the radio's cancellation procedure and inform the Coastguard.
Recap
- Press and hold until sent.
- Voice MAYDAY follows on acknowledgement or after about 15 seconds.
- MMSI identifies the station; live GNSS can add position and time.
- Check the displayed data and keep an independent written position available.
Optional quick check
Section 2 of 18
The DSC alert is confirmed sent, but the position time on the radio is stale. What should the operator do next?
Sources and factual review
Reviewed 20 August 2026 - Compass Revision editorial review.
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