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1 The Global Maritime Distress and Safety System is the technical, operational and administrative structure for maritime distress and safety communications worldwide. It was established in 1988 by the International Maritime Organization (IMO) which adopted a revised text of Chapter IV of the International Convention for the Safety of Life at Sea, 1974, (SOLAS) - dealing with Radiocommunications - and was implemented globally between 1992 and 1997. The GMDSS establishes the radiocommunications equipment that ships are required to carry, how this equipment shall be maintained and how it is used, and provides the context within which governments should establish the appropriate shore-based facilities to support GMDSS communications.

Basic concept of the GMDSS

2 In the days since the very first radio equipment was used at sea, most famously in the sending of a distress message from the Titanic, vessels in distress relied almost exclusively on their ability to alert other ships in order to obtain assistance. The GMDSS, for the first time, changed this procedure and established a new fundamental principle that a ship in distress should send its alert to a shore, which would then accept the responsibility of co-ordinating the necessary rescue efforts. Thus the GMDSS became inextricably linked with the parallel implementation of the International Search and Rescue Convention (SAR Convention) and the development of shore facilities within the structure of the World-Wide SAR Plan.

3 In addition to improving the capability of ships to declare their distress and receive assistance co-ordinated from the shore, the GMDSS also provided for the broadcast of essential safety-related information – Maritime Safety Information (MSI) – which could be received automatically on board ships at sea and would offer ships the chance to navigate more safely on a routine basis.

Functional Requirements

4 The GMDSS therefore provides that every ship, while at sea, shall be able to perform the following nine basic communication functions:

  • transmitting ship-to-shore distress alerts (by at least two separate and independent methods);
  • receiving shore-to-ship distress alerts;
  • transmitting and receiving ship-to-ship distress alerts;
  • transmitting and receiving search and rescue co-ordinating communications;
  • transmitting and receiving on-scene communications;
  • transmitting and receiving signals for locating;
  • transmitting and receiving maritime safety information;
  • transmitting and receiving general communications; and
  • transmitting and receiving bridge-to-bridge communications.

Ship Requirements

5 Area of Operations Concept

The SOLAS Convention provides that “… every ship shall be provided with radio installations capable of complying with the functional requirements … throughout the intended voyage …”. Thus every ship has to carry a core installation of basic equipment that is applicable to all waters, supplemented by additional equipment that extends the ships communications capabilities according to the specific waters in which she will sail. These supplementary requirements are defined by the distance offshore the ship will travel:

Sea Area A1: the area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC (Digital Selective Calling) alerting is available;

Sea Area A2: the area, excluding Sea Area A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC (Digital Selective Calling) alerting is available;

Sea Area A3: the area, excluding Sea Areas A1 and A2, within the coverage of an Inmarsat geostationary satellite in which continuous alerting is available; and

Sea Area A4: an area outside sea areas A1, A2 and A3.

6 In practical terms, this means that ships operating exclusively within about 35 miles from the shore may be able to carry only equipment for VHF-DSC communications; those which go beyond this distance, up to about 150 to 400 miles from shore, should carry both VHF-DSC and MF-DSC equipment; while those operating further from the shore but within the footprints of the Inmarsat satellites should additionally carry approved Inmarsat terminal(s).

Sea Area A4 cannot, by definition, be covered by the Inmarsat satellites and so ships operating in those waters – essentially the northern waters of the Arctic region – need to make provision for using HF communications.

7 In addition to the general communications equipment they are required to fit, ships also have to carry equipment for primary distress alerting – an Emergency Position Indicating Radio Beacon (EPIRB) and for the receipt of MSI. EPIRBs are small floating (some can automatically float free from a sinking ship) buoys and work through the COSPAS-SARSAT 406MHz satellite system. They can send a distress alert to the shore automatically to alert a Rescue Co-ordination Centre (RCC) that a ship is in distress in a particular location, but they cannot be used for two-way voice or data communications.

8 MSI is a broadcast of essential safety-related information (navigational warnings, meteorological warnings and forecasts, and other vital information) from the shore to all ships. The shore authorities co-operate to structure this broadcast in such a way that intelligent receivers can discriminate between information that is relevant to a particular ship and other information that is not, automatically discarding those messages that are not relevant to the ship in which the particular receiver is carried. This, of course, requires some specific user choices to be applied in the software before the equipment can operate in this way. There are two systems a ship can use to receive MSI: NAVTEX and SafetyNET:

  • NAVTEX is a terrestrial MF system, used to make broadcasts up to around 450 miles from the transmitter. Broadcasts are co-ordinated in time, coverage and content so that ships can limit what they receive to only the specific information relevant to the waters in which they are operating. NAVTEX transmissions are provided for most of the highly populated coastal waters of the globe;
  • SafetyNET is a satellite-based system that is used to broadcast MSI to all other waters of the globe, except Sea Area A4. SafetyNET is a service provided via the Inmarsat C system, and is used to address MSI to the 23 NAV/MET Areas of the world. A ship’s terminal will normally select automatically the area in which she is currently located, and the ship’s staff have the option to receive also information addressed to the area in to which she is next heading.

Information broadcast on NAVTEX is not normally also broadcast via SafetyNET.

9 The SOLAS Convention also places responsibilities on ships to maintain watches on specific frequencies for distress and safety communications, so some element of ship-to-ship alerting is retained in the new system; and specifies in general terms to means to be employed to preserve the operation of the radio equipment through reserve sources of electrical supply. It deals also with the IMO Performance Standards – IMO’s series of operational criteria that each type of equipment must meet; plus how the equipment shall be maintained and the basic qualifications needed to operate the equipment safely and reliably.

Undertakings by Governments

10 While the requirements for ships are detailed and explicit, governments cannot be held by international regulation to provide particular shore facilities in quite the same way. The GMDSS therefore includes a provision whereby each government that chooses to sign the SOLAS Convention (called a Contracting Government) “… undertakes to make available, as it deems practical and necessary either individually or in co-operation with other Contracting Governments, appropriate shore-based facilities for space and terrestrial communications …”.

11 This undertaking establishes one of the really significant features of the GMDSS infrastructure: some communication systems will use facilities that are essentially international in nature and not under the control or supervision of any single government. It is this that gives rise directly to the need for the international community to establish special arrangements for the international supervision of satellite-based communication facilities for the GMDSS, which have been developed and are implemented by IMSO.


12 The GMDSS generally applies to all ships over 300 gross tons and upwards on international voyages. However, GMDSS systems are equally valuable for other vessels, including recreational and other “voluntary fit” vessels. GMDSS carriage requirements are also applied to some fishing vessels by national legislation.