Marine VHF & DSC
Digital Selective Calling
Basically the additon of DSC equipment to marine VHF radio completely changes the method of making initial contact with other stations.
If GPS is interfaced with DSC the position of the vessel is automatically transmitted with the distress alert giving sufficient information for a search to start . The majority of DSC VHF sets can be interfaced with GPS equipment that you have on board such as a plotter.
The DSC is linked to or is an integral part of the VHF radio. Just like a radio pager the DSC alert another DSC that it is being called. Thereafter communication is just like a conventional radio other than the radio link is duplex and not simplex.
Simplex uses one channel (standard VHF) and therefore only transmits and receives signals one way, if both operator press their transit keys at the same time the transmission is jammed and no one hears anything hence the keyword of “over” and the end of a transmission to let the operator at the other station know that you have finished and he or she can return the transmission.
Duplex uses two channels and therefore transmissions can be made and received from both stations at the same time just like using a telephone. Radio protocol should still be maintained when making a DSC call, however the system has its advantages when talking to someone who is not trained in communications such as a doctor, when you requested medical advice from your Pan Pan Call.
Distress Alerts and All Station calls can always be made using DSC as they are a Broadcast, however to make a ship to ship or a ship to shore call you need to have the MMSI (Maritime Mobile Service Identity) of the station you want to call.
DSC sets allow you to store MMSI in the radios memory similar to the phone book on a mobile phone, so as you would store a person name and phone number in your phone you would store a vessels name and MMSI in your radio. from this you can make groups of vessels and believe it or not make group calls so that several vessels can be in communication with each other at the same time on the same channel set. very handy if you have a fishing fleet, or a group of yachts in a regatta maybe.
Distress calls using DSC are quick and functional at the press of just one button you have transmitted a distress alert to all DSC stations that provides the time of your call your International call sign your MMSI and most importantly your exact position at the time of transmission. The very nature of a Mayday call is that “shits hit the fan” you are in “grave and imminent danger” and you require “immediate assistance” so using DSC the coastguard can start the ball rolling and get a lifeboat launched or a helicopter in the air and on its way to your position or direct vessels at sea to your location even whilst you are making your voice mayday.