The provisions relating to GMDSS in the revised 1974 SOLAS Convention came into force on February 1, 1992. This new system takes advantage of modem technology to ensure immediate alerting, fast distribution, and effective communications in search and rescue operations at sea. A major principle of the GMDSS, is that any ship, in any sea area, shall be able to operate all means of communication regarded as important to itself and other ships in the same area.
- Means of Communication – definitions
- Alerting
- Distress communication
- Safety messages to ships
- Communication in general
- GMDSS system configuration
- GMDSS Implementation Schedule
- GMDSS – functional requirements
- GMDSS ships – radio watch
- Basic equipment – minimum requirements
- Maintenance requirements for GMDSS equipment
- The GMDSS Master Plan
Satellite and radio systems that are part of GMDSS, have individual limitations concerning range and availability. In order to ensure that the required communication functions are available at all times, duplicates of vital functions such as alerting, warning and communications are included.
Ships in distress shall be capable of alerting coast stations and Rescue Coordination Centres (RCC) automatically. These stations will then pass on warnings to ships located in that particular area.
Special systems within the GMDSS take care of inter-ship communication within the area in question.
Requirements with regard to The Structure, Functions and Details of Professional Marine Radio Communicationradio equipment on board ships depend upon which sea areas the ship is sailing in.
The GMDSS defines four sea areas:
A1 | An area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC alerting is available. Area which extends approx. 30 – 50 nautical miles from the coast station.x |
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. Area which extends approx. 150 nautical miles from the coast station.x |
A3 | An area excluding sea area A1 and A2 within the coverage of an INMARSAT geostationary satellite in which continuous alerting is available. Inmarsat range is between latitude 76 °north and latitude 76 °south. © POSEIDON, 1999. Unauthorised photocopying pr reproduction prohibited.x |
A4 | An area outside sea areas A1, A2 and A3. Polar areas north o f 76 °north and south of 76 °south.x |
Note: Continuous availability of a 1 erring is required in all sea areas. |
Means of Communication – definitions
Alerting
When NON-DSC Terrestrial Distress Communicationsdistress messages are sent to another ship, or to a Rescue Coordination Centre (RCC), the RCCs leads and coordinates the ensuing rescue operation.
Ship to ship alerting takes place on: | VHF DSC channel 70 |
MF DSC 21S7.5 kHz | |
Ship to shore alerting takes place on: | VHF DSC channel 70 |
MF DSC 2187.5 kHz | |
MF DSC 2187.5 kHz HF DSC 4207.5/6312.0/8414.5/12577.0/16804.5 kHz |
Additionally, an INMARSAT A/B or C terminal can be used depending on ship’s position and equipment.
As a secondary method of alerting, the Cospas/Sarsat EPIRB, INMARSAT (L-Band EPIRB), or VHF (DSC channel 70) Emergency Preparedness: The Role of EPIRBs and SARTs in Maritime SafetyEPIRB can be used.
Distress communication
Distress communication by radio means the communication between the station in distress, and the station(s) involved in the search and rescue work – on-scene communications.
Frequencies to be used:
Ship-ship: | VHF channel 16,06 (intership) MF 2 182 kHz |
Ship-aircraft: | 3 023 kHz, 4 125 kHz and 5 680 kHz |
Safety messages to ships
Safety messages to ships include navigational warnings, meteorological warnings, weather forecasts and messages of general importance. Maritime Safety Information (MSI) messages are sent via NAVTEX, EGC (Enhanced Group Call) via International SafetyNet service or by HF-telex.
Communication in general
Ordinary communication between ship stations, from Ship to Shore Access Guidelines for Gas Terminal and Vessel Operatorsship to shore or vice versa takes place on VHF, MF, HF or the INMARSAT network by means of voice, telex or data transmissions.
The figure below shows the INMARSAT System’s role in GMDSS.

GMDSS system configuration

GMDSS Implementation Schedule
This text outlines the phased implementation schedule for the Global Maritime Distress and Safety System (GMDSS) as mandated by the 1974/78 SOLAS Convention. It details the key milestones and requirements for ships to comply with the evolving GMDSS regulations.
FEBRUARY 1, 1992
Ships were given the option of installing GMDSS radio equipment in accordance with the provisions of the 1974/78 SOLAS Convention.
AUGUST 1, 1993
New requirements apply to all ships covered by the Convention. Ships covered by the SOLAS Convention: All passenger vessels sailing m international waters irrespective of their size, together with alt cargo vessels over 300 gross tons sailing in international waters. x
- NAVTEX receiver.
- Free float satellite EPIRB.
FEBRUARY 1, 1995
All new ships covered by the Convention were required to have a complete set of GMDSS installations. Furthermore, the following requirements were made on the equipment on board all existing ships covered by the Convention:
- Radar transponders:
- 2 on ships over 500 gr. t., and 1 on ships between 300 and 500 gr. t.
- Portable VHF transceivers:
- 3 on ships over 500 gr. t., and 2 on ships between 300 and 500 gr. t.
FEBRUARY 1, 1999
All ships covered by the Convention must be equipped with a complete set of GMDSS radio installations in accordance with the sea area in which they sail.
GMDSS – functional requirements
The GMDSS lays down nine specific functions which all ships must be able to perform, irrespective of which sea area they will be operating in.
GMDSS ships at sea must be able to:
a transmit ship-to-shore distress alerts by at least two separate and independent means, each using different radio communication services.
b transmit and receive ship to ship distress alerts.
c receive shore to ship distress alerts.
d transmit and receive SAR communications (SAR – Search And Rescue).
e transmit and receive on-scene communications – SAR communication.
f transmit and receive locating signals.
g receive maritime safety information (MSI).
h transmit and receive general radio communication to and from shore-based radio systems or networks.
i transmit and receive bridge to bridge communication.
GMDSS ships – radio watch
When at sea, ships equipped according to GMDSS specifications shall maintain continuous radio watch in accordance with the requirements of the sea area in which the ship is sailing:
A Ships fitted with a VHF radio installation shall keep continuous watch on VHF DSC channel 70.
B Ships fitted with an MF radio installation shall keep continuous watch on the MF DSC distress and safety frequency 2187.5 kHz.
C Ships fitted with an MF/HF radio installation shall keep continuous watch on the distress and safety DSC frequencies 2187.5 kHz and 8414.5 kHz and at least one other HF DSC frequency.(4207.5, 6312.0, 12577,0 or 16804.5 kHz).
The propagation characteristics of HF radio waves for the current sea son, time of day and ship’s position should be taken into account when choosing the frequency.
D Ships provided with a VHF installation in accordance with ITU Radio Regulations shall, until 1 February 2005, keep continuous watch on VHF channel 16.
E Ship stations should also keep a continuous watch for MSI (Informed for Safe Navigation at Sea with Critical Maritime InformationMaritime Safety Information) in the area in which the ship is sailing, by means of:
- NAVTEX receiver (518 kHz).
- EGC receiver/INMARSAT–C for reception of SafetyNet messages.
- HF telex.
F Wherever practicable, ships should maintain listening watch on VHF channel 13 (156.650 Mhz) for communications aimed at ensuring safe navigation.
Basic equipment – minimum requirements
Sea Area A1


Sea Areas A1+A2


Sea Areas A1+A2+A3 (INMARSAT)


Sea Areas A1+A2+A3 (HF) and A1+A2+A3+A4


Additional carriage requirements for A3/A4 ships using “equipment duplication”

- May be combined with required MF-transceiver in Sea Area 3 (INMARSAT).
- One is not required to install more than one MF/HF DSC watchkeeping RX.
Maintenance requirements for GMDSS equipment
GMDSS-certified ships shall meet the specific requirements concerning maintenance methods for radio installations which are prescribed in the SOLAS Convention.
Ships may choose from the following three methods in order to ensure the availability of radio equipment:
- shore based maintenance;
- on-board maintenance;
- duplication of equipment.
Ships in GMDSS sea areas A1 and A2 shall use one of the three methods, while ships in GMDSS sea areas A3 and A4 shall use a combination of at least two of the above listed options.
Read also: Empowering Global Communication with INMARSAT Satellites in shipping
Shore based maintenance: The ship/shipping company should have a written agreement with a service company or be able to present a written plan/declaration showing how shore-based maintenance should be carried out.
On-board maintenance: Qualified and authorised equipment servicing personnel must be present on board.
Spare parts and necessary instruments must be available on board for servicing mandatory equipment.
Duplication of equipment: Duplication can be used as a method of ensuring the availability of equipment.
For further details study figure: “Additional carriage requirements for A3/A4 ships using equipment duplication”.
A combination of shore-based maintenance and duplication of equipment is likely to be the most common method for most of the ships trading under GMDSS regulations.
The GMDSS Master Plan
The authorities of the different flag states are responsible for ensuring that their ships are equipped in accordance with the GMDSS (SOLAS 1974/88) and that coastal radio stations/coast earth stations are sufficiently updated to handle traffic under the new system.
The Master Plan contains information to users on shore-based facilities regarding space and terrestrial communications services for the GMDSS.
The Master Plan includes the following information:
- Status of shore-based facilities for the GMDSS.
- The list of VHF DSC coast stations covering sea area A1.
- The list of MF DSC coast stations covering sea area A2.
- The list of HF DSC coast stations covering sea areas A3 and A4.
- The list of INMARSAT Coast Earth Stations.
- The list of Rescue Co-ordination Centres using Ship Earth Stations.
- NAVTEX services.
- The international SafetyNET services.
- HF NBDP Marine Safety Information broadcast.
- The list of COSPAS/SARSAT Mission Control Centres and Local Usertenninals.
- Maps of shore-based facilities for the GMDSS.
- Satellite EPIRB registration information.
- List of 24-hour points of contact for MMSI ship’s identification.
- Questionnaire on shore-based facilities for the GMDSS (MSC/Circ. 684).
Note: Information about shore-based facilities in the Master Plan is obtained by means of a special questionnaire developed by IMO.