GMDSS (Equipments and Procedures)

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IMO began work on the Global Maritime Distress and Safety System in the 1970s and its introduction marks the biggest change in maritime communications since the invention of radio. 

It has been introduced in stages between 1993 and 1st February 1999. The basic concept of the system is that search and rescue authorities ashore, as well as ships in the vicinity, will be rapidly alerted in the event of an emergency. 

The GMDSS makes great use of the satellite communications provided by INMARSAT but also uses terrestrial radio. 

The equipment required by ships varies according to the sea area in which they operate – ships travelling to the high seas will need to carry more communications equipment than those, which remain within reach of specified shore-based radio facilities. In addition to distress communications, the GMDSS also provides for the dissemination of general maritime safety information (such as navigational and meteorological warnings and urgent information to ships). 

Solas Regulations

The 1988 amendments to the 1974 SOLAS Convention replace the existing Chapter IV with a new Chapter IV, which covers the Global Maritime Distress and Safety System (GMDSS). 

This amended Chapter IV applies to all passenger ships, irrespective of size, and cargo ships of 300 tons Gross Tonnage and upwards engaged on international voyages. 

All ships complied with the regulations concerning NAVTEX and satellite EPIRB before 1s1 August 1993. 

All ships constructed on or after 1st February 1995 now comply with all the applicable requirements. 


Bridge to bridge communications: means safety communications between ships from the position from which the ships are normally navigated. 

Continuous watch: means that the radio watch concerned shall not be interrupted other than for brief intervals when the ship’s receiving capability is impaired or blocked by its own communications or when the facilities are under periodical maintenance or checks. 

Digital selective calling (DSC): means a technique using a digital code which enables a radio station to establish contact with, and transfer information to, another station or group of stations, and complying with the relevant recommendations of the ITU Radiocommunication Sector (1TU-R) 

Direct printing telegraphy: means automated telegraphy techniques, which comply with the relevant recommendations of the ITU-R 

General Radiocommunications: means operational and public correspondence traffic, other than distress, urgency and safety messages, conducted by radio. 

International NAVTEX service: means the co-ordinated broadcast and automatic reception on 518 kHz of maritime safety information by means of narrow-band direct-printing telegraphy using the English language. 

Locating: means the finding of ships, aircraft, units or persons in distress. 

Maritime safety information: means navigational and meteorological warnings, meteorological forecasts and other urgent safety related messages related to ships. 

Polar orbiting satellite service: means a service which is based on polar orbiting satellites which receive and relay distress alerts form satellite EPIRBs and which provides their position. 

Sea area A1: means an area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC alerting is available. 

Sea area A2: means an area, excluding sea area A1 , within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available. 

Sea area A3: means an area, excluding sea areas A1 & A2 within the coverage of an INMARSAT Geostationary satellite in which continuous ,  alerting is available. 

Sea area A4: means an area Outside sea areas Al , A2 and A3. 

Basic Requirements

The main purpose of GMDSS is to provide a comprehensive communication system for search and rescue operations e.g. to enable vessels in distress to transmit alerting signals to indicate that they require assistance. These signals alert shore-based authorities, usually a Rescue Co-ordination Centre (RCC) which then co-ordinates a SAR operation. 

Prior to the advent of GMDSS a ship in distress had to rely on other ships to come to its assistance after a distress message had been transmitted. This system relied on short-range inter-ship communication, which is effective in congested and well-frequented waterways but is not reliable in less populated sea areas. Moreover assistance from shore-based authorities was limited in certain parts of the world. 

The concept of GMDSS was to overcome the limitations of this kind of unreliable distress communications by introducing satellite communication which has the advantage of alerting and locating vessels during emergencies. So now the emphasis is away from ship to ship distress alerting using Morse code and instead on ship to shore alerting using automated systems that communicate via satellite. 

It must be noted that although satellite is the major communication medium, there are also other automated means such as digital selective calling (DSC) and radio-telex – both these services using the terrestrial maritime frequency bands. Thus there are alternative methods of communication available to vessels. 

Towards this end, all ships shall be capable of meeting the following basic Radio communications requirements of: – 

1, Transmitting ship-to-shore distress alerts by at least two separate and independent means, each using a different radiocommunication service. 

2. Receiving shore-to-ship distress alerts 

3. Transmitting and receiving ship-to-ship distress alerts 

4. Transmitting and receiving search and rescue co-ordinating communications 

5. Transmitting and receiving on-scene communications 

6. Transmitting and, as required, receiving signals for locating 

7. Transmitting and receiving maritime safety information 

8. Transmitting and receiving general radiocommunications to and from shore-based radio systems or networks 

9. Transmitting and receiving bridge-to-bridge communications 

Based on these requirements, and depending on the Sea Area that a ship sailing in the following is the GMDSS equipment required in each Area:  

Sea Areas

gmdss sea area

Under the 1974 solas Convention, radio equipment that was mandatory on ships depended solely on their tonnage. Under the new system, ships radio equipment has been considered in terms oft he range of the communications system in use and the areas in which the ship will trade. For this four distinct sea areas have been designated, and the corresponding mandatory radio equipment described therein. 

Area A1

  1. VHF Receiver/Transmitter with DSC Ch 70, and R/T Ch 6, 13, 16 — the VHF DSC Watch Receiver and the DSC Ch 70 may be separate or combined. 
  2. Satellite EPIRB – on 406 MHz band (Cospas-Sarsat) 
  3. Search and Rescue Transponder (SART)- 2 minimum for ships over 500 Gross Tons 
  4. Navtex Receiver 
  5. Portable VHF Receiver/Transmitters – 3 minimum for ships over 500 Gross Tons Area 

Area A2

  1. VHF Receiver/Transmitter with DSC Ch 70, and R/T- Ch 6, 13, 16 — the VHF DSC Watch Receiver and the DSC Ch 70 may be separate or combined. 
  2. Satellite EPIRB on 406 MHz band (Cospas-Sarsat) 
  3. Search and Rescue Transponder (SART)- 2 minimum for ships over 500 Gross 
  4. Navtex Receiver 
  5. Portable VHF Receiver/Transmifters – 3minimum for ships over 500 Gross Tons 
  6. MF Transmitter and Receiver with DSC on 2187.5 khZ, and R/T 2182 kHz

Area A3

  1. VHF Receiver/Transmitter with DSC Ch 70, and R/T Ch 6, 13, 16 — the VHF DSC Watch Receiver and the DSC Ch 70 may be separate or combined. 
  2. Satellite EPIRB – on 406 MHz band (Cospas-Sarsat) and/or INMARSAT
  3. Search and Rescue Transponder (SART)- 2 minimum for ships over 500 Gross Tons 
  4. Navtex Receiver and EGC Receiver (when outside Navtex coverage) 
  5. Portable VHF Receiver/Transmitters – 3 minimum for ships over 500 Gross Tons
  6. MF Transmitter and Receiver with DSC on 2187.5 kHz, and R/T 2182 kHz and radio telex on 2174.5 kHz
  7. INMARSAT Ship Earth Station or HF Receiver/Transmitter with DSC on 8414.5 kHz together with one of 4207.5 kHz, 6312 kHz, 12577 kHz, 16804.5 kHz and specified R/T, RadioTelex, and MSI frequencies. 

Area A4

  1. VHF Receiver/Transmitter with DSC Ch 70, and R/T Oh 6, 13, 16 — the VHF DSC Watch Receiver and the DSC Ch 70 may be separate or combined. 
  2. Satellite EPIRB – on 406 MHz band (Cospas-Sarsat) 
  3. Search and Rescue Transponder (SART)- 2 minimum for ships over 500 Gross Tons 
  4. Navtex Receiver 
  5. Portable VHF Receiver/Transmitters – 3 minimum for ships over 500 Gross Tons 
  6. MF Transmitter and Receiver with DSC on 2187.5 kHz, and R/T 2182 kHz and RadioTelex on 2174.5 kHz 
  7. HF Receiver/Transmitter with DSC on 8414.5 kHz together with one of 4207.5 kHz, 6312 kHz, 12577 kHz, 16804.5 kHz and specified R/T, RadioTelex, and MSI frequencies. 
  8. Every passenger ship should have a VHF transceiver working on the aeronautical frequencies 121.5 MHz and 123.1MHz, located on the navigation bridge.

GMDSS Services

GMDSS Services Each Contracting Government is required to undertake to make available, as it deems practical and necessary, either individually or in co-operation with other Contracting Governments appropriate shore based facilities for the following radio communication services: 

  1. A radio communication service utilizing Geostationary satellites in the Maritime Mobile-Satellite Service 
  2. A radio communication service utilizing polar orbiting satellites in the Mobile – Satellite Service. 
  3. The Maritime Mobile Service in the bands between 156MHz and 174 MHz (VHF) 
  4. The Maritime Mobile Service in the bands between 4000kHz and 27500kHz (HF) 
  5. The Maritime Mobile Service in the bands between 415kHz and 535kHz and between 1605kHz and 4000kHz (MF) 
Each Contracting Government is not required to provide all radio communication services, but should be adequate to cover the sea areas, which it has designated off its coasts. 

Navtex Receiver


NAVTEX is an international, automated system for instantly distributing maritime navigational warnings, weather forecasts and warnings, search and rescue notices and similar information to ships. The transmitters transmit on 518 Khz frequency and the transmitters in a particular area work on the principle of time sharing, i.e. when one transmitter is transmitting, the other transmitters in that area are quiet and do not transmit. When one transmitter completes transmission, the next transmitter starts transmitting as scheduled. Hence even though the same frequency is used, there is no clash between transmissions from various transmitters. A small, low-cost and self-contained “smart” printing radio receiver installed on the bridge checks each incoming message to see if it has been received during an earlier transmission, or if it is of a category of no interest to the ship’s master. If it is a new and wanted message, it is printed on a roll of adding-machine size paper; if not, the message is ignored. A new ship coming into the area will receive many previously broadcast messages for the first time; ships already in the area, which had already received the message, won’t receive it again. No person needs to be present during a broadcast to receive vital information 

For a NAVTEX receiver to function effectively, it is essential that the operator should know how to programme and operate his particular receiver. This is not difficult provided the following practical steps are followed: 

  1. Make sure that there are sufficient rolls of NAVTEX paper on board.
  2. Check that there is paper in the receiver. 
  3. Turn the NAVTEX receiver on at least four hours before sailing, or better still, leave it on permanently. This avoids the chance of losing vital information that could affect the vessel during its voyage. 
  4. Make sure that you are familiar with the Operating Manual for that particular equipment model, paying particular attention to the fact that your equipment may be programmed differently from other makes and models. 
  5. Programme your receiver to accept only those messages identified with the B1 character of the NAVTEX station which covers the area in which your vessel is currently sailing and the one covering the area into which you are about to sail. This will avoid the equipment printing information, which has no relevance to your voyage and will avoid unnecessary waste of paper. 
  6. Programme your receiver to accept only those messages identified with the B2 characters (type of message) you wish to receive. It is recommended that most B2 characters (A to Z) be programmed, but you may exclude those for nav aid equipments (Decca or Loran for example) with which your vessel is NOT fitted. 
  7. Be aware that the characters A, B and D MUST be included, as they are mandatory. 
  8. Take extra care not to confuse the Programming of B1 characters (station designators) with those of B2 characters (type of messages), it is very easy for an operator to believe that they are Programming B1 characters when in fact they are programming B2 characters. After programming ALWAYS CHECK the programme status to ensure that it is correct.
  9. If information is received incomplete/garbled, inform the relevant NAVTEX station, giving the time of reception (UTC) and your vessel’s position to enable you to obtain the information you require.



COSPAS-SARSAT satellite system is a satellite based search and rescue system designed to locate 3 types of distress beacons. The system operates on two frequencies 406MHz and 121.5MHz and relies on low altitude satellites in near polar orbits to provide global coverage. The satellites communicate with a network of earth stations known as Local User Terminals (LUTs) who can pass distress alerts and location data to rescue authorities via Mission Control Centres (MCCs). 

The significance of the polar orbit is that as the earth rotates, the path of the orbiting satellite will pass over a different part of the earth each time. This way, full earth coverage is possible with only 1 satellite as the satellite travels in the north-south direction and the earth in the east-west direction. 

However the COSPAS-SARSAT has several satellites in polar orbits in different orbital planes. This way the time between satellites passes at any fixed point on earth is reduced as compared to a single satellite model and a complete worldwide distress alert monitoring system is provided. 

When an EPIRB working on the COSPAS-SARSAT frequencies is activated, the distress signal is received by the nearest orbiting satellite and relayed to the most suitable LUT. The LUT processes this signal and determines the beacon’s location. 


Satellite EPIRBs use the INMARSAT satellite system for their search and rescue operations. These operate in the L band (1.6GHz). Like other INMARSAT based system this can be used in any one of the four ocean region satellites. 

This type of EPIRB can be fitted in place of the COSPAS-SARSAT type for ships sailing in areas A1 , A2 and A3. Ships sailing in area A4 must use the COSPAS-SARSAT EPIRB as the INMARSAT system does not reach vessels sailing in these high Latitudes. 

SOLAS Requirements

All ships must have a satellite EPIRB capable of transmitting a distress alert either through the polar orbiting satellite service operating in the 406 MHz band, or if the ship is engaged only on voyages within INMARSAT coverage, through the  INMARSAT geostationary satellite service operating in the 1.6 GHz band.  

The EPIRB should fulfill the following requirements: 

a) Installed in an easily accessible position 

b) Ready to be manually released and capable of being carried by one person into a survival craft 

c) Capable of floating free if the ship sinks and of being automatically activated when afloat 

d) Capable of being manually activated 

e) Should be installed close by, or by remote activation from, the position from which the ship is normally navigated. 

False Alerts


A rising number of false alerts caused by inadvertent or incorrect operation of GMDSS equipment are putting a significant burden on Search and Rescue Centres. Since such distress alerts are of a global nature, all search and rescue machinery including air and surface crafts have to be alerted and deployed. This results in substantial wastage in time and expenditure.


EPIRBS are activated either when they float free or when their ” AUTO/ON switches is manually switched on. must never be the EPIRB is working switched on. These Conditions must never be simulated,as there is a separate TEST switch, Which causes a lamp to flash, which denotes that the EPIRB is working satisfactorily. 

Each EPIRB set has a clear set of instructions, which must be followed for testing and for the actual activation of the set. 

In case of an accidental activation of the EPIRB on board “every reasonable attempt must be made to contact with the SAR authorities to advice them of the situation. Until SAR authorities have been contacted the beacon should be left ON. This will allow the vessel to be located and prevent unnecessary searching ” 


There is increasing number of false alerts caused by false MF/HF DSC distress calls. In order to avoid the increased burden imposed on search and rescue centers the following guidelines should be observed: 

If You Receive a DSC Distress Alert: 

  1. DO NOT relay a distress alert known to have been acknowledged unless you have personally determined that further help to the ship in distress is necessary. 
  2. DO wait 3 minutes for an acknowledgement from a coast station before sending a relay of a received distress call. If the call remains unacknowledged or unrelayed, and there is no distress traffic on the associated radiotelephone frequency, relay the call to the nearest coast station and attempt to contact the distressed vessel by radiotelephone. Once contact with the coast station is made, explain the situation by radiotelephone. 
  3. DO NOT relay a distress alert to all ships, unless a known distress situation requires such a relay. Certain DSC radios permit relays of both distress acknowledgements and relays or they may transpose the MMSI of the ship in distress when constructing a relay. DO NOT routinely relay these messages, and be certain the MMSI in your relay matches the MMSI of the ship in distress before transmitting. 

If You Inadvertently Transmit a DSC Distress Alert

  1. Reset the equipment immediately.
  2. Tune for radiotelephony on the associated distress and safety frequency in each band in which a false distress alert was transmitted, and 
  3. Transmit a broadcast message to “All Stations” giving the ship’s name, call sign, time the alert was transmitted and MMSI, and cancel the false alert on the distress and safety frequency in each band in which the false distress alert was transmitted. 

Mariners who transmit a false DSC distress alert are required to immediately cancel the alert. ITU regulations allow administrations to take appropriate steps against any ship or mariner who fails to cancel an inadvertently transmitted distress alert. 

Keeping radio Watches

Every  ship, While at sea, shall maintain a continuous watch:

a) On VHF DSC channel 70.

b) On MF DSC on 2187.5 khZ, if fitted with an MF radio installation.

c) On MF/HF DSC frequencies 2187.5 kHz and 8414.5 kHz, and on at least one of the following DSC frequencies of 4207 kHz, 6312 kHz, 12577 kHz or 16804.5 kHz, appropriate to the time of day and geographical position of the ship — if fitted with receiver, an MF/HF radio installation. This watch may be kept by means of a scanning receiver. 

d) For Satellite shore-to-ship distress alerts, i earth station. if the Ship is fitted with INMARSAT ship  earth station.

Every ship, while at sea, shall maintain a radio watch for broadcasts of maritime safety information on the appropriate frequency on which this is broadcast for the area area in which the ship is navigating.

Radio Equipment

Radio equipment must be so designed that the main units can be replaced readily, without major recalibration or readjustment. It should be constructed and installed in such a way that it is readily accessible for inspection and on-board maintenance. Sufficient tools, spares and information should be provided to enable the equipment to be properly operated and maintained, taking into account the recommendations of IMO. 

The Administration should ensure that the radio equipment is maintained to meet the functional requirements and recommended performance standards. For ships engaged on voyages in sea areas A1 and A2 this is ensured by duplication of equipment, shore-based maintenance or at sea electronic maintenance, or a combination of these, as approved by the Administration. Ships engaged on voyages in sea areas A3 and A4 should use a combination of at least two of the above methods. 

Malfunction of equipment for providing general radiocommunications is not considered as making a ship unseaworthy, neither should it be delayed in a port where repair facilities are not available, provided the ship can perform all distress and safety functions. 

Requirement of Qualified Personnel

Every ship must have personnel qualified for distress and safety radio communication purposes to the satisfaction of the Administration. The personnel shall be holders of certificates specified in the Radio Regulations as appropriate. 

One holder of a certificate specified in the Radio Regulations as qualified for distress and safety radiocommunications shall be designated to have primary responsibility for radio communications during distress incidents. In passenger ships, at least one such qualified person shall be assigned to perform only radiocommunication duties during distress incidents. 

Record Keeping

A record should be kept, to the satisfaction of the Administration and as required by the Radio Regulations, of all incidents connected with the radiocommunication service, which appears to be of importance to safety of life at sea. 

The GMDSS radio logbook consists of two sections as follows: 

Section A

a) Ship’s name 

b) Call sign 

c) Port of Registry

d) Gross Tonnage 

e) IMO number 

f) Sea areas in which the ship is authorised to operate 

g) Date of expiry of Safety Radio Certificate 

h) Methods used to ensure availability of radio facilities 

  • Duplication of equipment (DOE) 
  • Shore based maintenance giving details of name and address of Service Company. (SBM) 
  • At-sea maintenance capability. 

i) Name and address of owner, managing owner or agent. 

Section B- Details of radio personnel

a) Names 

b) Dates on board 

c) Certificate numbers 

d) Names of person designated for radio communications during emergencies 

e) Name of person nominated to carry out appropriate tests and checks and log entries. 

Required Enteries

The following events should be recorded together with the time of occurrence: 

a) A summary of communications relating to distress, urgency and safety traffic. 

b) A record of important incidents connected with the radio service. 

c) Were appropriate the position of the ship at least once a day. 

The master is to inspect and sign each day’s entries in the GMDSS logbook. 

Testing of GMDSS Equipment


1) Proper functioning of all DSC facilities to be tested. This is to be done without radiation of signals — a TEST switch is normally provided on the equipment. 

2) Batteries to be checked and if required brought up to the fully charged condition. 

3) Printer(s) to be checked to ensure there is adequate supply of paper. 

Weekly Test

1) Proper operation of the DSC facilities to be tested by a test call, when within communication range of a coast station with DSC equipment. If a vessel has been out of range of a similar station for over a week, a test call should be made on the first opportunity that the ship is in range. 

2) When the backup source of power on a ship is not a battery but an emergency generator, this should be tested at least once a week. 

3) Survival craft two-way VHF equipment should be tested on a frequency other than Ch 16. 

Monthly Test

1) EPIRB should be tested to determine its capability to operate properly using the TEST switch normally provided for this purpose. Under no circumstances should the EPIRB be activated, even for a short duration. 

2) SART should be checked for security and signs of damage.

3) Survival craft two-way VHF equipment should be tested on a frequency other than Ch 16. 

4) Check the securing and condition of the batteries — including the connections and the battery compartment. 

Videos on GMDSS Equipment Testing

Battery On/Off Load Test

VHF DSC Weekly Testing

MF/HF DSC Weekly Test

NBDP Shore Telex Monthly Test


SART & EPIRB Testing

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