Digital Selective Calling DSC is simply a tone signaling system, which operates on VHF Channel 70 and is similar to the tone dialing on your phone, but with the ability to include data such as the vessel’s identification number, the purpose of the call, the vessel’s position, and the channel for further voice communications. In other words, vessels can call each other direct by use of their MMSIs (rather like a telephone number) without bothering other vessels or shore stations unless of course it is a Distress/Urgency call.

The present VHF radiotelephony system requires users to listen until someone speaks and to determine whether the call is for them more often than not, it won’t be. Distress alerting By pressing the red Distress Alert button on your VHF radio, you can send your boat’s identity, your position and the nature of distress.

The position given will be precise and the alert will be heard immediately by all DSC equipped vessels and shore stations within range. The distress message will be automatically repeated every 4 minutes until it is acknowledged either by a Coastguard rescue coordination centre or ship within radio range.

If circumstances allow, the distressed vessel is required to follow up the alert with a Mayday voice message on Channel 16 to give further details and alert any non DSC equipped vessels in the vicinity. A typical DSC distress alert is sent as follows: Press the red Distress button. The set will automatically switch to Ch 70. Press again for 5 seconds to transmit a basic distress alert with position and time. The set then reverts to Ch 16. If time permits, select the nature of the distress from the menu (eg Collision, Fire, Flooding) then press the Distress button for 5 seconds to send a full distress alert. When a DSC distress acknowledgement has been received on your set, the vessel in distress should transmit a MAYDAY message by voice on Ch 16 including its MMSI.

Once a rescue service receive your DSC message they will acknowledge it and will select which VHF channel they would like you to initiate a voice call on. Your set will automatically switch to the desired VHF channel. Wait for your set to display the channel response from the coastguard.

MMSI – A Maritime Mobile Service Identity (MMSI) number is only issued to vessels fitted with Digital Selective Calling (DSC) and/or (Satellite) Ship Earth Station (SES) equipment. In order to get an MMSI number you will need to contact your local maritime authority.

Ensure the rest of your vessel is safe and that you remain up to date with regulatory compliance, vessel procedures, and emergency operations, with Ocean Time Marine’s Safety Management templates.