Picture courtesy of Simrad

Background Information

What is GMDSS
The Global Maritime Distress and Safety System (GMDSS) is an international system that uses terrestrial, satellite, and ship-board radio systems to ensure rapid, automated alerting of shore-based communication and rescue authorities, in addition to ships in the immediate vicinity, in the event of a marine distress.

GMDSS regulations are now compulsory for all commercial vessels over 300grt, registered fishing vessels and craft carrying 13 or more passengers. These are called 'compulsory fit vessels'. Some smaller commercial vessels are strongly advised to fit VHF DSC. Pleasure craft are counted as 'voluntary fit' and may fit any component of the GMDSS.

VHF Digital Selective Calling (DSC)
Digital Selective Calling provides automated access to coast stations and ships. The message  information is stored in the VHF receiver and can be displayed or printed out following receipt. There are four levels of priority

  • distress
  • urgency
  • safety
  • routine

At the coast station, ship-to-shore distress calls receive priority handling and are routed to the nearest Rescue Co-ordination Centre. On board ship, DSC receivers sound an alarm when a distress call is received.

DSC Concept
DSC is a technique of transmitting digital codes which allow suitably equipped stations to:

(a) Transmit and receive distress alerts.
(b) Transmit and receive distress alert acknowledgments.
(c) Relay distress alerts.
(d) Announce urgency and safety calls.
(e) Initiate routine priority calls and set up working channels for subsequent general communications on R/T or telex.

DSC Call Format
All DSC calls automatically include phasing signals, error-checking signals and the identity (MMSI number) of the calling station.

Maritime Mobile Service Identity (MMSI) Numbers
An MMSI is a unique nine digit number that identifies a particular ship or shore station. Those for ship stations are issued free of charge by the licensing authority and entered into the set on purchase. Each MMSI contains the country code, UK vessels being identified with the numbers 232,233,234 or 235. If you wish to add your MMSI number and details to this National Directory then please follow the link at the top of page.

DSC General Calling
Procedures Routine calls should be made just once on a single frequency or channel. Coast stations may, however, transmit a second call within 45 seconds if no acknowledgement has been received. If the call is still not acknowledged, the coast station should normally wait at least another 30 minutes before repeating the call attempt on the same frequency, or 5 minutes on another calling frequency.