7.1 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.
DSC 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
A 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).
• Local time (must be input manually if no position data is available).
• 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
An 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).
• Local time (must be input manually if no position data is available).
• 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
A 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).
• Local time (must be input manually if no position data is available).
• 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.
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