Regulations: GMDSS

Regulations: GMDSS

The core objectives of GMDSS are that a ship must be able to contact coastal stations, regardless of where it is, and that no ship must go down without automatically transmitting a distress alarm.

The essence of GMDSS can be traced back to the use of Morse code as a medium for conveying distress and safety telecommunications. The sinking of the passenger liner RMS Titanic in April 1912 dramatically underscored the need for ships across the board to adopt Morse code radiotelegraph equipment for distress calls, and Morse transmissions duly proved an indispensable aid in maritime distress situations over the ensuing decades.

However, Morse is subject to range limitations and misinterpretation. High-frequency long-range radiotelephony options, eg radio telex and radio telegrams, subsequently brought worldwide ship-to-shore communication within the bounds of possibility, but it took the inception of the International Convention on Maritime Search and Rescue in 1979 to address the requirement for a standardised and radically improved global search and rescue initiative.

To this end, the convention compelled the International Maritime Organisation (IMO) to devise a Global Maritime Distress and Safety System (GMDSS), combining satellite and terrestrial radio services and providing safety information broadcasts plus automatic distress alerting and locating.

A significant amendment to SOLAS in 1988 made the fitment of GMDSS equipment mandatory on all SOLAS vessels (cargo vessels over 300 gross tonnage, ocean liners, cruise ships, ferries), while further amendments required the vessels in question to carry satellite EPIRBs (Emergency Position-Indicating Radio Beacons) and Navtex (Navigational telex) by August 1993, and all other GMDSS equipment by February 1999.


Navtex (Navigational telex) is an international system which automatically provides navigating officers with Maritime Safety Information (MSI) including navigational and meteorological warnings, weather forecasts, alerts relating to missing vessels, and so on. These essential items of information are delivered as printouts from a radio receiver, generally installed on the bridge, and the area covered by Navtex can extend to 400NM from the broadcast station. Messages are transmitted over 518kHz in English, although 490kHz is sometimes used for broadcasts in a local language.

EPIRB (Emergency Position-Indicating Radio Beacon)

EPIRBs are deployed as an automatic means of alerting search and rescue services in cases of emergency. Jointly developed by the US, Canada, Russia and France via the international satellite-based Cospas-Sarsat search and rescue system, EPIRBs operate by transmitting a coded message over the 406MHz distress frequency to the nearest rescue co-ordination centre. More recent EPIRBs are equipped with built-in GPS, enabling rescue services to accurately home in to within 20 metres of the distress position.

SART (Search And Rescue Transponder)

A Search And Rescue Radar Transponder on a vessel – two are fitted on ships above 500 gross tonnage – is a self-contained unit which locates survival craft, or vessels in distress, by creating a series of 12 dots on the radar display of the rescuing ship. The average range for detection between SARTs and ships is approximately 8NM (15km), depending upon the height of the ship’s radar mast and the mounting height of the SART. Once the SART has been picked up by radar, it will visually and aurally alert those in distress.


GMDSS satellite services are currently provided by Inmarsat (Inmarsat B, C and F77), although Iridium is set to unveil a range of GMDSS services and equipment from 2020. GMDSS satellite systems offer ship-to-ship, ship-to-shore and shore-to-ship telex, telephone and data transfer services, with a special priority telex and telephone service specifically designated for communication with shore rescue centres.

DSC (Digital selective calling)

This calling service is allocated for ship-to-ship, ship-to-shore and shore-to-ship safety and distress information, predominantly over high or medium frequency and VHF maritime radio. Emergency communications with ships and rescue coordination centres can be achieved via DSC distress alerts, utilising a pre-formatted distress message.

Sea Areas

The four GMDSS Sea Area classifications, A1, A2, A3 and A4, have been demarcated in order to identify regions in which GMDSS services can be accessed, and also as a means of determining the radio equipment GMDSS vessels are required to carry. As a basis, all GMDSS-regulated vessels are mandated to possess a satellite EPIRB, a Navtex receiver, an Inmarsat C SafetyNET receiver, a DSC-equipped VHF radiotelephone, two 9GHz SARTs, and two VHF handhelds for vessels between 300-500 gross tonnage. (Vessels over 500 gross tonnage will require three handheld VHFs to be carried.)

Sea Area A1

This refers to an area of radiotelephone coverage, typically within 30-40NM from a VHF coast station, in which continuous DSC alerts (Ch70) and radiotelephony services are available.

Sea Area A2

This refers to an area of radiotelephone coverage of MF coast stations in which continuous alerting by DSC is available.

Sea Area A3

This refers to an area (excluding sea areas A1 and A2) which comes under the coverage of Inmarsat’s geostationary satellites. The area is approximately contained within 76˚ north and south.

Sea Area A4

This refers to the polar regions beyond areas A1, A2 and A3, beyond 76˚ of latitude north and south.

Towards the future

Cobham SATCOM offers a wide variety of SAILOR GMDSS systems and equipment, ranging from EPIRBs, SARTs and VHF handhelds to control panels, consoles and mini-C systems. The company is already engaged in the development of next-generation GMDSS, taking into consideration new SOLAS resolutions, revised definitions of sea areas and the entry of Iridium to the sector.

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