The development of DSC was brought about to allow a mechanism that could automate some of the watch-keeping functions of marine radios, notably for vessels that were mandated to be fitted with a radio. The DSC function allows digital information regarding marine communication to be sent over an analogue link and effectively is a pager for a vessel, or in the case of distress, a pager for all vessels.
DSC equipped VHF transceivers are now common on small vessels, and many include connections to receive GNSS/GPS signals. This allows the benefit of single press distress functions that include the transmission of accurate position information at the time a distress alert is sent. This is exceptionally valuable at times of poor transmission as the data is sent as a DSC burst transmission and is recorded by the acknowledging station, removing some of the potential for human error.
To identify a DSC station, each radio is programmed with an MMSI that is linked to the vessel and the owner. This is a unique 9 digit code that preceeds the burst transmission, which then follows the rest of the transmission content. Each MMSI contains a code that correlates to the nation of registry.
MMSI numbers are issued by a responsible authority in each country that has been tasked with responsibility for managing VHF radio communications. An MMSI is not issued automatically, but must be applied for by a holder of a radio certificate of competency.
Once programmed into the radio, the MMSI is fixed and cannot easily be changed without returning to the manufacturer. If you purchase a vessel that a radio-telephony equipment that has already been programmed with an MMSI, you can re-assign the MMSI registration to your self through the relevant authority.
DSC Distress Alerts, when activated, will only be detected by vessels that have DSC capability that has been correctly setup.
With the incorporation of GPS into DSC capable VHF transmitters, the GPS position is encoded into a DSC burst transmission along with time stamps as to when the alert was activated, and when the position was last updated. This allows identification of your position even if the voice broadcast did not come through clearly. If the last update of your position exceeds a certain time limit, then the transmitter will send an invalid position signal.
The level of DSC capability is determined by manufacturer and sophistication of the unit that has been bought. The functionality often reflects the price tag of the radio. At its most basic level, the radio will provide a simple distress function only, others allow the operator to select the priority of the message, or others allow the user to add information to the nature of the distress.
In a standard distress alert the following details are sent as part of the alert: