Chapter 10
Digital Selective Calling – Introduction & routine calls
Digital Selective Calling is a facility which automates many of the procedures which would otherwise be carried out by voice on Channel 16.
It’s an important part of GMDSS because it allows distress alerts to be transmitted simply by pressing a button, but this is not its only purpose: DSC can also speed up and simplify routine calls, so long as both vessels have the right equipment.
DSC Controllers
“The right equipment” means a DSC controller, which may either be built into a radio, or added as a separate component.
All radios sold since April 2000 have been required either to have a DSC controller built in, or to be capable of being connected to a DSC controller.
On vessels which are not required by law to carry VHF, it is still legitimate to continue to use an older radio without DSC, or to install and use a new radio without its associated DSC controller.
A DSC controller works by creating a coded message. It’s a bit like Morse code, except that instead of dots and dashes it uses ones and zeros; and instead of a flashing light it uses musical notes. If you could hear the message, it would sound like a high-pitched warble, but at 1200 notes per second, it is far too fast for any human ear to pick out individual tones.
The digital message can include several different types of information. In the case of a routine ship-to-ship call, for instance, it would include:-
A “format specifier” identifying the type of call;
A nine-digit number identifying the vessel being called;
A similar nine-digit number identifying the vessel that is calling;
A suggested working channel.
This burst of data, lasting about half a second, is passed from the DSC controller to the radio, which transmits it on Channel 70 (see page 29).
Any vessel with DSC that is within range will receive the call, but in most cases, nothing will happen.
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