Digital Selective Calling is a form of radio communication that was introduced in recent years as part of the Global Maritime Distress Safety System (GMDSS) – an international protocol designed to co-ordinate and facilitate rescue at sea.
DSC is in effect a paging system that uses a transmitted burst of information from one radio to alert another. That information automatically contains a unique number (MMSI – marine mobile service identity) that identifies the transmitting craft, and indicates the purpose of the call. Thereafter, two-way communication can be established. The initial broadcast takes place on the dedicated VHF channel 70. Communications thereafter are conducted on an alternative channel. All DSC radios can be connected to a GPS antenna, and the vessel’s position at the time of transmission included in the initial broadcast.
The DSC function has two main purposes:
1. It allows a distress signal complete with GPS position and vessel identification to be broadcast at the touch of a single button. All DSC radios within reception range will then automatically sound an alert to their crew.
2. For direct boat-to-boat communication a signal can be sent to a specific vessel if that vessel’s MMSI is known, much like dialling a phone number. One station can also use that number to automatically request the GPS position from another, a process known as polling. Both these facilities can be useful for boats sailing or fishing as a group, the latter function particularly so in reduced visibility.
All craft that are likely to sail independently should carry a DSC radio. It should also be noted that the historical distress VHF channel – 16 – is no longer continuously monitored by search and rescue authorities due to the universal adoption of DSC.