The Global Maritime Distress and Safety System (GMDSS)

The introduction of the GMDSS began in February 1992 and was fully in place by 1999 and applies to all vessels over 300 Gross Tons, passenger vessels carrying thirteen or more passengers and fishing vessels over 12m.

There are several classes of equipment fitted to vessels depending on their operational area and type:

Class A equipment is fitted to vessels that operate in all Sea Areas. ALBs are fitted with Class A equipment although it is not a legal requirement. Class B equipment is the minimum requirement for vessels that operate up to 300 miles offshore. Class D equipment provides minimum VHF DSC facilities for voluntary-fit vessels. Most leisure craft will fit Class D equipment it is also fitted to all ILB's.

Digital Selective Calling

The cornerstone of the GMDSS is the technique known as Digital Selective Calling, (usually referred to as DSC). Before the introduction of DSC, an operator using VHF would have to call another radio station using the Distress, Safety and Calling channel, (Channel 16). On hearing their name, the called station would acknowledge the call and both parties would agree to move to a Working Channel. This method of communication relies on everyone constantly listening to Channel 16 in case someone wishes to speak to them.

DSC acts more like a telephone pager. Using DSC, a radio operator can now send a digital signal, (known as an Alert), to a selected radio station before starting any voice transmission. This Alert will automatically sound an alarm on the selected radio station's DSC receiver and therefore the receiving station no longer has to listen constantly to Channel 16

Under the SOLAS agreement, GMDSS compulsory-fit ships must be able to

  • Transmit Ship to Shore Alerts by two separate and independent systems, each using a different radio-communication service.
  • Receive Shore to Ship Distress Alerts.
  • Transmit and receive Ship to Ship Distress Alerts and Bridge to Bridge communications.
  • Transmit and receive SAR Coordinating Communications and On-Scene Communications.
  • Transmit and receive homing or locating signals.
  • Transmit and receive Maritime Safety Information.
  • Transmit and receive general shore-based radio communications.

The Concept:

The basic concept of the GMDSS is that the search and rescue authorities, as well as shipping in the immediate area, are rapidly alerted to a distress so that they can assist in a coordinated search and rescue operation with the minimum of delay. The system also provides for Urgency and Safety communications and the broadcasting of Maritime Safety Information, (NAVTEX).

MMSI Numbers:

Transmission via DSC are achieved by giving every radio station a unique nine digit identification number, known as its Maritime Mobile Service Identity number, (or MMSI), which works in the same way as a telephone number. By including a station's MMSI number in the Alert, only that station's receiver will sound an alarm. All other DSC receivers within range will remain silent.