Digital Selective Calling (DSC)

Traditional VHF radio systems require users to listen until someone speaks, and then determine whether the call is for them. DSC ensures that calls are received by alerting or announcing the intended recipient(s) first so they are ready to listen to the subsequent message on the relevant channel.

Digital Selective CallingDSC is part of the Global Maritime Distress and Safety System (GMDSS), a maritime communications system for emergency and distress messages and all types of routine communications such as ship-to-ship or ship-to-shore.

DSC is a digital signalling system, which operates on VHF channel 70. DSC calls include other data such as your vessel’s identification number, purpose of the call, your position and the channel you want to speak on.

DSC calls can be divided into 4 categories and are prioritized as shown below:

  1. Distress

  2. Urgency

  3. Safety

  4. Routine

Distress

Distress callA Distress call should only be used when there is imminent danger to a vehicle or person that requires immediate assistance.

When making a distress call the following information is transmitted to all stations within range:

  • Vessel MMSI number.

  • Vessel position (must be input manually if no position data is available or the manual GPS source is missing).

  • Local time (must be input manually if no position data is available or the manual GPS source is missing).

  • Nature of the distress (when designated).

  • Transmission frequency.

The call is automatically repeated at approximately 4 minute intervals until it is acknowledged either by a coast radio station (CRS) or a vessel within radio range. Distress calls must be followed by a MAYDAY call on priority channel 16.

Urgency

Urgency callAn urgency call should be used when there is danger to a vehicle or person that does not require immediate assistance.

When making an urgency call the following information is transmitted to all stations within range:

  • Vessel MMSI number.

  • Vessel position (must be input manually if no position data is available or the manual GPS source is missing).

  • Local time (must be input manually if no position data is available or the manual GPS source is missing).

  • Transmission frequency.

Once an urgency call is sent it must be followed with a PAN PAN voice message on channel 16 and include the necessary details.

Safety

Safety callA safety call should be used when there is an important navigational warning or meteorological forecast/broadcast. Safety alerts can also used for communications during search and rescue operations.

When making a safety call the following information is transmitted to all stations within range:

  • Vessel MMSI number.

  • Vessel position (must be input manually if no position data is available or the manual GPS source is missing).

  • Local time (must be input manually if no position data is available or the manual GPS source is missing).

  • Transmission frequency.

Once a safety call is sent it must be followed with a SECURITE voice message on channel 16 and include the necessary details.

Individual (routine)

Routine call Individual callRoutine calls are used for contacting other vessels, marinas, or shore stations.

Routine calls are made on channel 70 using the dedicated Maritime Mobile Service Identity (MMSI) number of the station to be contacted, selecting a VHF working channel and sending the call. Both radios automatically switch to the chosen channel for conversation.

Routine calls can also be made to groups — When groups of ships need the same information (yacht races, club rallies etc.) a special group-call identity can be used to enable restricted broadcast calls.

Note

To transmit precise position data, the radio must be connected to a GNSS (GPS) receiver. Otherwise, regular manual position updating is required.