During the 1970’s very large ships were built to transport cargo and thanks to computer technology, marine VHF radios became smaller and easier for bridge watch keepers to operate. This meat that trained radio operators were no longer employed to listen for distress call at specific times.

The International Maritime Organisation (IMO) became so concerned that radio distress messages were not always heard by other ships and as a result, introduced the Global Maritime Distress and Safety System. This system requires that ships over 300 gross  register tons, known as ‘compulsory fir’ vessels are fitted with certain items of safety equipment and most importantly, can send a digital and very audible alarm to others when they are in distress.
These vessels must carry:-
VHF Digital Selective Calling (DCS) marine radio
Emergancy Position Indicateing Radio Beacon (EPIRB’s)
Search and Rescue Transponder (SART’s)
Automatic Identification System (AIS)

Smaller vessels not covered by GMDSS rules are referred to as ‘voluntary fit’ vessels  but are strongly advised to fit DSC sets so they can also summon help quickly and  efficiently. Whilst large ships are fitted with Class A or B DCSC sets, Leisure craft are usually fitted with the less sophisticated and less expensive Class D sets.

DSC is simply a method of alerting another ship and subsequent voice communication is just as important as it was before the introduction of DSC. Merchant Ship and HM Coastguard will maintain a listen watch on Channel 16 for the foreseeable future but are very keen that small craft fit DSC where possible to reduce the congestion of channel 16 and maintain an effective emergency service.