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GMDSS carriage requirements and basic provisions

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Navigating the vast and unpredictable oceans has always posed significant challenges, making safety and effective communication paramount for maritime operations. The Global Maritime Distress and Safety System (GMDSS) represents a revolutionary advancement in ensuring the safety of vessels and their crews. Established under the framework of the 1974 International Convention for the Safety of Life at Sea (SOLAS), the GMDSS has transformed maritime communication, introducing standardized protocols and state-of-the-art technology to respond swiftly and efficiently to emergencies.

The implementation of the GMDSS has mandated several functional requirements tailored to enhance safety and coordination at sea. This includes the definition of specific sea areas, each with distinct communication needs and equipment requirements. Ships are required to carry appropriate radio equipment and maintain it to ensure reliability in distress situations. Additionally, the role of trained radio personnel has become critical, ensuring that the sophisticated equipment is operated correctly and that communication protocols are adhered to.

Introduction to Importance GMDSS

Radio at Sea

Radio has been the foundation of the distress and safety systems used bye ships at sea since the first instance of the use of radio to save lives at sea in 1899. It was soon realized that, to be effective, a radio-based distress and safety system has to be founded on internationally agreed rules concerning the type of equipment, the radio frequencies used and operational procedures. The first international agreements were established under the auspices of the predecessor to the International Telecommunication Union (ITU). Many of the operational procedures for Morse telegraphy established at the turn of the century have been maintained to the present day.

1974 SOLAS Convention

As more detailed regulations became necessary for the shipping industry, the most recent of International Conventions for the Safety of Life at Sea (SOLAS 1974) was adopted in 1974. The 1974 SOLAS Convention has become one of the main instruments of the International Maritime Organization (IMO).

The distress and safety system used by most of the world’s shipping until 1992, as defined by chapter IV of the 1974 SOLAS Convention and the ITU Radio Regulations, required a continuous Morse radiotelegraphy watch on 500 kHz for passenger ships, irrespective of size, and cargo ships of 1 600 gross tonnage and upwards. The Convention also required a radiotelephone watch on 2 182 kHz and 156,8 MHz (VHF channel) on all passenger ships and cargo ships of 300 gross tonnage and upwards. Although the system has proven itself reliable over many years, its limitations of short range, manual alerting and aural watchkeeping have become a matter of increasing concern. Advances of technology led the IMO member governments to develop a new system based on modern technology and automation.


The new system called the Global Maritime Distress and Safety System (GMDSS). This system was adopted by Information about Vessel Manoeuvring based on IMO StandardsIMO in 1988 and replaces the 500 kHx Morse code system. The GMDSS provides a reliable ship-to-shore communications path in addition to ship-to-ship alerting communications. The new system is automated and uses ship-to-shore alerting bye means of terrestrial radio and satellite radio paths for alerting and subsequent communications. The GMDSS will apply to call cargo ships of 300 gross tonnage and above, and to all passenger ships, regardless of size, on international voyages.

GMDSS Implementation

The GMDSS requirements for radiocommunications are contained in the new chapter IV of SOLAS 1974 adopted at the GMDSS Conference held in 1988. There is a transition period from the old to the new system in order to allow industry time to overcome any unforeseen problems in implementation of the new system. The transition period began on 1 February 1992 continues to 1 February 1999.

The phased implementation of the GMDSS started with a general requirement for the carriage of NAVTEX receivers for the reception of maritime and satellite EPIRBs (Emergency Position-Indicating Radio Beacons) from 1 August 1993. during the transition period, ships operating under the GMDSS will have to comply with the 1988 amendments to chapter IV of SOLAS 1974. Until 1 February 1999, both systems will require watchkeeping on 2 182 kHz and VHF channel 16.

Governments have undertaken to ensure that the necessary shore installations will be in place in order to provide the required communication services.

Digital Selective Calling – DSC

DSC Technology provides a method of calling a station or stations using digital techniques, and as such forms the basis of GMDSS communications on VHF, MF and HF.

DSC provides automated access to coast stations and ships, in particular, for the transmission and reception of both routine and distress calls, i. e., it is to be used as the initial means of contact with other stations.

Read also: General Provisions, Rules and Requirements for Safe Carriages of Cargoes

The DSC system allows for the name of the vessels in distress, the nature of the distress and the last recorded position to be displayed or printed out on receipt of a distress alert.

DSC receivers sound an alarm when a distress call is received. Distress priority Comprehensive Guide to Ship and Shore Preparation and Manifold Connection for LNG Cargo Operationsship-to shore DSC calls receive priority handling by coast stations and are routed to the nearest Rescue Co-ordination Centre (RCC).

Functional requirements

The GMDSS is a largely, but not fully, automated system which requires ships to have a range of equipment capable of performing the nine radiocommunication functions of the GMDSS, viz:

  1. transmission of ship-to-shore distress alerts by at least two separate and independent means, each using a different radiocommunication service;
  2. reception of shore-to-ship distress alerts;
  3. transmission and reception of ship-to-ship distress alerts;
  4. transmission and reception of search and rescue co-ordinating communications;
  5. transmission and reception of on-scene communications;
  6. transmission and reception of signal for locating;
  7. transmission and reception of maritime safety information;
  8. transmission and reception of general radiocommunications to and from shore-based radio systems or networks;
  9. and transmission and reception of bridge-to-bridge communications.

Sea areas

The GMDSS is based on the concept of using four marine communications sea areas to determine the operational, maintenance and personnel requirements for maritime radiocommunications,:

A1. An area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC alerting is available. Such an area could extend typically 30 to 50 nautical miles from the coast station.

A2. An area, excluding sea area A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available. For planning purposes this are typically extend to up to 150 nautical miles offshore, but would exclude any A1 designated areas. In practice, satisfactory coverage may often be achieved out to around 400 nautical miles offshore.

A3. An area, excluding sea areas A1 and A2, within the coverage of an Inmarsat geostationary satellite in which continuous alerting is available. This area lies between about latitudes 760 of latitude, but excludes any other areas.

A4. An area outside sea areas A1, A2 and A3. This is essentially the polar regions, north and south of about 760 of latitude, but excludes any other areas.

Carriage requirements

Equipment carriage requirements for ship at sea now depend upon the sea in which the ship is sailing. (In the past it was only dependant upon the type/or size of the ship). Furthermore, ships operating in the GMDSS are required to carry a primary and secondary means of distress alerting.

This means having VHF DSC as a primary system for a ship near coastal areas, backed up by a satellite Emergency Position-Indicating Radio Beacon (EPIRB). A ship operating in an offshore ocean area could have Medium-Frequency DSC, High-Frequency DSC or Inmarsat satellite communications as a primary system backed up by a satellite EPIRB.

The type of equipment used in the primary system is determined by the sea area in which the ship will be navigating. The carriage requirements are defined in SOLAS chapter IV for the four sea areas. Table 1 shows how the SOLAS Regulations would translate into the bare minimum carriage requirements for the four sea areas. The majority of ships will, however, be fitted with a more comprehensive radio installation.

Table 1. Minimum GMDSS carriage requirements
EquipmentSea area A1Sea area A2Sea area A3Sea area A4
EGC receiverBBBB
VHF portable (2 or 3)XXXX
2 182 kHz watch receiver (until 1 February 1999) The Administration may exempt ships constructed on or after 1 February 1997 from these requirements.xXXX
Inmarsat-A, -B or –CX or
HF R/T with DSC and telexXX


A. Required only in those areas where the NAVTEX service is available.

B. Required only in those area where the NAVTEX service is NOT available; also, the EGC receive facility is included in the standard Inmarsat-C terminal.


Maintenance requirements

The means of ensuring the availability of equipment are determined by the sea areas in which this ship sails (see chapter IV of SOLAS).

In sea areas A1 and A2, the availability of equipment shall be ensured by one of the following strategies:

(a) duplication of equipment;

(b) shore-based maintenance;

(c) et-sea electronic maintenance;

(d) or a combination of the above, as may be approved by the Administration.

In sea areas A3 and A4, the availability of equipment shall be ensured by using a combination of at least two of the above, as may be approved by the Administration.

Radio Personnel

Regulation IV/16 of the SOLAS Convention requires that:

Every ship shall carry personnel qualified for distress and safety radiocommunication purpose to the satisfaction of the Administration. The personnel shall be holders of certificates specified in the Radio Regulations as appropriate, any one of whom shall be designated to have primary responsibility radiocommunications during distress incidents.

The provisions of the Radio Regulations require that the personnel of ship stations and ship earth stations for which a radio installation is compulsory under international agreements The SOLAS Convention.x and which use the frequencies and techniques of the GMDSS shall include at least:

The combined effect of the requirements for maintenance and personnel in the four sea area is that there must be at least one GOC holder on board ships sailing in A2, A3 or A4 sea areas. The International Convention on Standards of Training, Certification an Watchkeeping foe Seafares, 1978, as amended in 1995, requires that all deck officers shal hold an appropriate qualification to operate VHF radiocommunication equipment; that is, ROC standard on GMDSS ships or whatever international/national requirement determine.

Global Maritime Distress and Safety System
Source: en.wikipedia.org

In those cases, particularly in sea area A1, where additional equipment, over and above the minimum carriage requirements, is fitted, a higher standard of operator certification may also be required in order to ensure that the operator knowledge requirements match the actual equipment comprising the radio installation.

Equipment Introduction

The exact equipment fitted will include a selection from the following list.

VHF Radiotelephone
Operated in the band 156-174 MHz. Duplex channels are available for Ship/Shore working and simplex channels for Ship/Ship and routine Ship/Shore calling. Maximum range around 30-40 nautical miles, dependent upon heights of antennas.


Operates on channel 70 and is used for both distress alerting and for routine calling.

VHF Portable Two-way Radiotelephones

Required for emergency communications from survival craft.


Search and rescue radar transpoder operating on the 3 cm radar X-band (9,3-9,5 GHz). Used to help search and rescue (SAR) units to locate survivors.

NAVTEX receiver

Used to receive maritime safety information (MSI) automatically by means of narrow-band direct printing from selected stations, using 518 kHz, 490 kHz and 4209,5 kHz.


Satellite emergency position-indicating radiobeacons operate on 406 MHz (including 121,5 MHz for homing by rescue aircraft) through the COSPAS-SARSAT network and on 1,6 GHz (L-band Inmarsat-E) through the Inmarsat network. DSC EPIRBs operating on VHF channel 70 may be used in sea areas A1. EPIRB transmission serve to identify the ship in distress, to inform the RCC of a distress incidents and to help to determine the position of survivors.

Note: EPIRB transmissions are regarded as a distress alert.


Used to monitor the DSC distress frequencies in the 2, 4, 6, 8, 12 and 16 MHz bands. Also for routine calling or replying on the 2, 4, 6, 8, 12, 16, 18, 22 and 25 MHz bands.

MF/HF transceiver

With full R/T and telex facilities on all the Marine bands.

Note: The DSC unit uses this equipment in order to transmit and to await a reply to a routine call.


Used for voice, telex, data, video and facsimile communications.


Provides telex, data, E-mail and polling on a store-and forward basis. Usually incorporates an EGC (Enhanced Group Call) receiver for the automatic reception of maritime safety information via the International SafetyNET service.

2 182 kHz Watchkeeping Receiver

Receiver, with a muted loudspeaker, which is used to listen for the two-tone alarm, upon reception of which the mute is lifted to enable the distress call and message to be heard.

2 182 kHz Radiotelephone Alarm Signal Generator

Fitted into the MF R/T transceiver, it produces the two-tone alarm signal for 1 minute to alert others that a distress call and message is about to follow.

Author photo - Olga Nesvetailova
  1. General operator’s Certificate for The Global Maritime Distress and Safety System, Course + Compendium, Model Course IMO 1.25, Printed by PMS UK Ltd London, 2004.
  2. European Radiocommunications Committee ERC Decision of 10 March 1999 on the harmonised examination syllabi for General Operator’s Certificate (GOC) and Restricted Operator’s Certificate (ROC)(ERC 99 01).
  3. IMO GMDSS-Handbook, London, U. K., 2004.
  4. Norcontrol Capella GMDSS Simulator, Technical documentations, Kongsberg Maritime Ship Systems, Norway, 2005.
  6. Seafarers’ Training, Certification and Watchkeeping CODE 1995 (STCW Code 95, published by IMO, London, 1996), – Part A Mandatory standards regarding provisions of the annex to the Convention Chapter IV Standards regarding radio personnel.
  7. Seafarers’ Training, Certification and Watchkeeping CODE 1995 (STCW Code 95, published by IMO, London, 1996), – Part B Mandatory guidance regarding provisions of the STCW and its annex; Chapter IV Guidance regarding radiocommunication and radio personnel.
  8. V. Pipirigeanu, M. Udrea, Introducere in GMDSS – Sistemul Mondial de Primejdie si Siguranta Maritima, Ed. Europolis, Constanta, 2002.
  9. Graham D. Lees, William G. Williamson, Handbook for Marine Radio Comunication, e d. LLOYD S OF LONDON PRESSLTD., 2004.
  10. ITU Manual for Use by The Maritime Mobile and Maritime Mobile Satellite Services, 2006.
  11. IAMSAR Manual – International Aeronautical and Maritime Search and Rescue Manual, 2001.
  12. C/S G.003 – Introduction to Cospas-Sarsat System, (G3OCT28.99D Issue 5 – Rev 1 October 1999), C/S Documents published by Cospas-Sarsat in Handbook of Regulations on 406 MHz and 121,5 MHz Beacons, (1999);
  13. Tor R. Kristensen – An Introduction to GMDSS, revised GOC Edition, – 7th edition, Leknes, Norway, 2007.
  14. C/S T.001 – Specification for Cospas-Sarsat 406 MHz Distress Beacon, (T1OCT30.99D – Issue 3-Rev. 2 October 1999), Documents published by Cospas-Sarsat in Handbook of Regulations on 406 MHz and 121,5 MHz Beacons (1999).
  15. IMO SOLAS (SAVE OF LIVE AT SEA), Consolidated Edition, London, 2001.


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