What is Global Maritime Distress and Safety System or GMDSS and its details?

GMDSS Stands for Global Maritime Distress and Safety System This System was adopted by IMO in 1988 and replaced the 500 Khz MORSE CODE SYSTEM

Limitation of Morse Code System :-
  1. Short Range
  2. Manual Alerting
  3. Aural Watch keeping.
Points to pounder regarding GMDSS:

The Global Maritime Distress and Safety System  or GMDSS is an international system using advanced communications technology. Development of GMDSS was initiated by the International Maritime Organization (IMO) and the system represents a significant improvement in maritime communications.

GMDSS is designed to enhance ship-to-shore communications and provide rapid, automated distress alerting, with positional information if available. While compliance is mandatory for large cargo and passenger ships on international voyages or in the open sea, it is voluntary for recreational vessels. GMDSS will however have an impact on recreational boaters, and it is recommended that recreational boaters become familiar with its features. To help with the transition to GMDSS, Coast Guard stations will continue to monitor VHF channel 16 and MF frequency 2182 kHz for the foreseeable future. The mandatory equipped vessels however, discontinued monitoring MF frequency 2182kHz on February 1st, 1999, and are only obligated to monitor VHF channel 16 until February 1st, 2005. The GMDSS equipment on these vessels will instead be monitoring for digital data on VHF channel 70 and MF frequency 2187.5 kHz.


Which types of vessel GMDSS apply to:

  • GMDSS will apply to all Cargo Ship of 300 GT and above, and
  • To all passenger ship regardless of size on international voyage.

GMDSS is largely but not fully automated system. The GMDSS emphasizes the ability to alert SAR authorities ashore as well as shipping in the vicinity in order to receive a rapid co-ordinated response to distress situations.

Functional Requirements of GMDSS equipments :-

  1. Transmission of ship to shore distress alert by at least two separate and independent means, each using a different radio communication.
  2. Reception of shore – to ship distress alerts.
  3. Transmission and reception of Ship to ship distress alert.
  4. Transmission and reception of SAR (Search and rescue) coordinating communication.
  5. Transmission and reception of on-scene communication (At the point distress location)
  6. Transmission and reception of signals for locating.
  7. Transmission and reception of MSI (Maritime safety Information).
  8. Transmission and reception of general radio – communication to and from shore based radio system or networks; and
  9. Transmission and reception of bridge to bridge communications.


GMDSS divides the world’s oceans into four “sea areas”, designated A1 through A4.


An area within the radiotelephone coverage of at least one VHF coast station in continuous digital selective calling (DSC), alerting is available.

Area could extend up to 30-50 Nautical Miles.


An  area within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available.

Area could extend up to 200 nautical miles.

AREA  A3 :-

An area,  excluding area A1 and A2 within coverage of an Inmarsat geostationary satellite in which continuous alerting is available.

Range – Between latitude (76 degree N to 76 degree S)
IMO requirements for Range :(70 degree N to 70 degree S)

 AREA A4 :-

An area outside , Sea area A1, A2 and A3. This is essentially the polar region.


Carriage requirements for Sea Areas :

Sea Area A1:-

VHF DSC,  VHF RT, EPIRB, SART, Portable VHF Transceiver, Navtex receiver 518 kH3, EGC receiver (If Navtex not available)

Sea Area A2:-

VHF DSC, VHF RT, MF DSC, MF RT, EPIRB, SART, Portable VHF, transceiver, Navtex receiver 518 khz, EGC receiver (where Navtex not available)

Sea Area A3:-

VHF DSC, VHF RT, MF/HF DSC, MF/HF RT, (*) EPIRB, SART, Portable VHF transceiver, Navtex received 518 khz, EGC receiver means (* Telex).

Sea Area A4:-

VHF DSC/ VHF RT, MF/HF DSC, MF/HF RT, Telex, EPIRB, SART, Portable VHF Transceiver and Navtex receiver 518 kHz.


Maintenance requirements of GMDSS Equipments  (Or Commanly known As “Back UP” maintenance )

Three options :-
  1. Duplications of equipments
  2. Shore – based maintenance. (SBM)
  3. At – Sea election maintenance.

In Sea Area (1) and A (2)
Vessel must use one option.

In Sea area (3) and A (4)
Vessel must use any two options.

  • Duplication of equipments mean availability of spare equipments on board.
  • A shore based maintenance contract must be acceptable to vessel’s flag state administration.


  1. It provides a worldwide warning system ship-shore, which does not depend on vessels in the vicinity.
  2. Simplifies the radio operations; alerts can be transmitted by “pressing a button”
  3. Assures the redundancy of communications; requires two separate systems to alert function.
  4. Strengthens the search and rescue; operations are coordinated by land centres.

What does “DISTRESS” signal mean:

Distress signal :-

The distress signal mayday indicates that a ship, aircraft or other vehicle is threatened by grave or imminent danger and require immediate assistance.

Distress alert :-

The transmission  of distress alert in GMDSS indicate a ship or aircraft or any other vehicle or other person is threatened by grave and imminent and requires immediate assistance.

The Radio telephone distress procedure consist of :-
  1. The alarm signal (whenever possible) followed by
  2. The distress call
  3. The distress message with full implementation of GMDSS in Feb 1999.

The automatic alarm device used on 2182 Khz are no longer required.

FORMAT of RADIO Telephony distress call :-

Mayday (x3)
This is
Name (x3) / Call sign / MMSI no. of ship.
Radiotelephony distress message
Name/ Call sign/ MMSI
Nature of distress .

Type of assistance required any other information which might facilitate the rescue.

Format of NBDP distress call and message

Name/ Call Sign
Nature of distress
Type of distress/ assistance required
Additional information that may facilitate rescue.
NAME/ Call sign
Time (UTC)

What is Mayday ( distress)  Relays:

Stations not themselves in distress which learn that a ship, aircraft or other vehicle is in distress may transmit a distress relay message but only in the following circumstances.

  1. When the station in distress in not itself in position to transmit the distress message.
  2. When the master or other person responsible for the station not in distress consider the further help is required/ necessary.
  3. When, although not in position to render assistance, the station has received a distress message which is believed not to have been acknowledge.

Such message/ distress relay would normally be sent on 2182 kHz or channel 16 or any other R/T distress frequency.

  •  Never substitute the name of the relaying station for that of the station in distress, even identification is difficult.
  • If the station in distress cannot be identified, it should be referred to as “Unidentified Traveler” or unidentified helicopter” or “Unknown Vehicle”.

What is DSC or Digital Selective Calling :

The calling sub system in GMDSSC being used as “Primary alerting system” of VHF, MF, HF and terrestrial service. DSC provides automated access to coast stations and ships.

Level of priority:-
  1. Distress
  2. Urgency
  3. Safety
  4. Routine

All level of priority are available for DSC calls.

Basic of DSC :-

DSC of technique of transmitting digital codes, which allows suitable equipped stations to :-

  1. Transmit and receive distress alert
  2. Transmit and receive distress alert acknowledgements.
  3. Announce urgency and safety calls
  4. Relay distress alert.
With the help of DSC call to
  1. All stations
  2. Stations in a Geographical area.
  3. An individual station 
  4. A group of station
  • DSC channels have been allocated in 2 MHz, 4MHz. on MF/HF bands.
  • CH-70 on VHF.
  • All DSC calls automatically includes phasing signals, error-checking signals and identity (MMSI) of the calling stations.

NBDP procedures during distress working :-

Silence Mayday :-

Controlling station may impose silence on interfering stations by sending the radio telex message.

Silence Distress :-

Any other stations may also impose silence, if it is necessary to do so, by using this.

Silence fini :-

Normal working may be resumed.

What are Cancellation procedures for false distress Alert :

  1. Switch off the transmitter immediately if the false alert is detected during transmission.
  2. Now switch on equipment and set to CH – 16.
  3. Make broad cost to “all stations” giving the ship’s name, call sign, MMSI number  and cancel the false distress alert.
Example –

All stations (x3)
This is
Name  (x3)/ Call sign/ MMSI No.
Cancel my distress alert of date, Time (UTC).
Name of ship/ call sign/ MMSI date/ time (UTC)0

On MF :-
  1. Switch off transmitter immediately if the false alert is detected during transmission.
  2. Switch on equipment and set to 2182 kHz.
  3. Make broadcast to “All stations”, giving the ship’s Name, call sign, MMSI No and cancel the false distress alert.
ON HF :-

We will use same procedure as for MF band, but the alert will be cancelled on all the frequency bands on which the alert was transmitted. The transmitter should therefore be tuned consecutively to the radio telephone distress frequency in the 4, 6, 8, 12 and 16 MHz bands.


  • Avoid sending false alerts, the DSC distress alert will be received at greater distances than the cancellation by the wider transmission.
  • If the sea area of receiving vessel is not mentioned then assume, vessel (it is in the same area as the alerting vessel).
  • Never (Under Any circumstances), attempt to use the DSC equipments to relay a relayed distress alert. Such action can initiate a chain of misleading relays and responses to several RCCs.

What is Urgency and Safety communications:

Urgency alert:-

Alert is to be transmitted on the DSC distress frequencies. Urgency message normally to be transmitted on RT (Distress), 2182 kHz or NBDP frequency. Moreover, in case of a long message, a medical call, or in areas of heavy traffic when the message in being repeated, then it is send on a working frequency after an announcement using the urgency signal on an appropriate distress frequency.

What does urgency signal mean:

Urgency signal PAN – PAN indicates that a very important message is to follow concerning the safety of a ship, aircraft or other vechile or the safety of a person.

Urgency Alert :-

In GMDSS urgency indicates that a very important message is to follow concerning the safety of ship, aircraft or other vechile or the safety of a person. Urgency call may be address to All Stations or to an Individual Station.

Example :

PAN PAN (x3)
All stations (x3) or Name of the station (x3)
This is
Name (x3)/ call sign/ MMSI of the station send message.
This is
Ship’s Name / Call sign/ MMSI
Nature of urgency
Require of Tug/ Tow/ Medical advise
Name of ship
Date/ Time (UTC)

Note:- Time in UTC

Urgency communication has highest priority except distress

A station which receives the urgency alert/ signal which is not followed by the urgency message shall:-

  1. Continue to listen for minimum 05 minutes on the appropriate frequency.
  2. At the end of the period, if urgency message is heard, a land station, if possible be notified of the receipt of the urgency signal.
  3. Thereafter normal working may be resumed.

Man Over Board :-

Distress priority for DSC (alerting) purpose but the subsequent RT procedure still requires the urgency signal PANPAN.


About the author

Amit Sharma

Graduated from M.E.R.I. Mumbai (Mumbai University), After a brief sailing founded this website with the idea to bring the maritime education online which must be free and available for all at all times and to find basic solutions that are of extreme importance to a seafarer by our innovative ideas.

1 Comment

  • Man Overboard is a distress situation. Even the RT message will be Distress message . Information given that RT message will be Urgency is absolutely wrong. Students following this in GOC examination will fail. Request you to correct this. You may consult ITU manual for maritime mobile & maritime mobile satellite services.
    Thanking you

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