Global Maritime Distress and Safety System (GMDSS)

(Information for the Fisherman and Recreational Boaters)

What is GMDSS?

The Global Maritime Distress and Safety System (GMDSS) is an international system which uses improved terrestrial and satellite technology and ship-board radio systems. It ensures rapid alerting of shore-based rescue and communications authorities in the event of an emergency. In addition, the system alerts vessels in the immediate vicinity and provides improved means of locating survivors.

GMDSS was developed through the International Maritime Organization (IMO) and represents a significant change in the way maritime safety communications are conducted. While it is mandatory for all ships subject to the International Convention for the Safety Of Life At Sea (SOLAS) (cargo ships 300 gross tons or greater and all passenger vessels, on international voyages), GMDSS will impact on all radio-equipped vessels, regardless of size. The global implementation of GMDSS services became effective on February 1, 1999.


GMDSS was developed to SAVE LIVES by modernizing and enhancing the current radiocommunications system. By utilizing satellite and digital selective calling technology, GMDSS provides a more effective distress alerting system. It improves the current system by:

  • increasing the probability that an alert will be sent when a vessel is in distress;
  • increasing the likelihood that the alert will be received;
  • increasing the ability to locate survivors;
  • improving rescue communications and coordination; and
  • providing mariners with vital maritime safety information.

GMDSS Equipment

Digital Selective Calling (DSC)

The traditional marine radio (VHF/MF/HF) has been enhanced with the addition of a feature known as DSC. This feature enables vessels to automatically maintain the required watch on distress and calling channels instead of the current aural listening watch. A DSC receiver will only respond to the vessel's unique Maritime Mobile Service Identity number (MMSI#), similar to a telephone number, or to an "All Ships" DSC call within range. Once contact has been made by DSC, follow-up communications take place by voice on another frequency.

Satellite Communications

The Inmarsat satellite network provides global communications, except for the polar regions. In areas without any VHF or MF DSC shore facilities, Inmarsat A, B or C terminals are used for distress alerting and communications between ship and shore. Inmarsat provides an efficient means of routing distress alerts to Search and Rescue (SAR) authorities.

Emergency Position Indicating Radiobeacon (EPIRB)

GMDSS makes use of the COSPAS-SARSAT Satellite System which provides global detection of 406 Megahertz (MHz) EPIRBs. These beacons are small, portable, buoyant, and provide an effective means of issuing a distress alert anywhere in the world. Float free EPIRBs (class 1) have been required on most Canadian commercial vessels 20m or more in length since 1989, and are highly recommended for all vessels. Owners must register these EPIRBs in the national beacon database (1-800-727-9414).


Search And Rescue Transponder (SART)

SARTs are portable radar transponders used to help locate survivors of distressed vessels, which have sent a distress alert. They are detected by radar and therefore operate in the same frequency range as radars carried onboard most vessels. SARTs transmit in response to received radar signals and show up on a vessel's radar screen as a series of dots, accurately indicating the position of the SART. In the event that a ship must be abandoned, SARTs should be taken aboard survival craft.

SART dots on radar screen
SART dots on radar screen
Examples of SARTs
Examples of SARTs

Maritime Safety Information (MSI)

Maritime Safety Information broadcasts, which comprise distress alerts, SAR information, navigational and weather warnings, as well as forecasts, can be received in three different ways in GMDSS:

  • NAVTEX receivers are fully automatic and receive broadcasts in coastal regions up to 300 nautical miles offshore.
  • Inmarsat-C terminals receive Enhanced Group Call - SafetyNET (EGC) broadcasts for areas outside NAVTEX coverage.
  • HF Narrow Band Direct Printing (NBDP) receivers can be used where service is available as an alternate to EGC.

GMDSS Sea Areas - International

Although ship-to-ship alerting is still an important function in GMDSS, the emphasis is on two way communications between ships and shore facilities. All GMDSS ships must be capable of communicating with the shore and transmitting a distress alert by two different means. The equipment carried by a GMDSS ship is therefore determined by its area of operation and the availability of shore-based communications services.

There are four "Sea Areas" defined internationally in the GMDSS:

Sea Area A1 Within range of shore-based VHF DSC coast station (40 nautical miles)
Sea Area A2 Within range of shore-based MF DSC coast station (excluding sea areas A1)(150 nautical miles)
Sea Area A3 Within the coverage of an Inmarsat geostationary satellite (approximately 70°N to 70°S) (excluding sea areas A1 & A2)
Sea Area A4 The remaining areas outside sea areas A1, A2 & A3 (polar regions)

GMDSS Sea Areas - Canada

In Canada, as a result of consultations with the Canadian marine industry it has been decided to implement sea areas A1 on the east and west coasts.  Outside of A1 will be an A3 sea area with an A4 sea area in the Arctic.

Canada Map

Communications Between GMDSS Vessels & Non-GMDSS Vessels

GMDSS ships maintain an automated listening watch on VHF DSC ch 70 and MF DSC 2187.5 kHz. This creates the situation, during the transition to GMDSS, where vessels fitted with traditional, non-GMDSS radio equipment, may have difficulties alerting or contacting a GMDSS ship. The Coast Guard is addressing this by monitoring both GMDSS and traditional distress frequencies during the transition. Although the final date for the cessation of mandatory watchkeeping on VHF ch 16 by SOLAS ships is under review by the IMO, all vessels should fit VHF DSC as soon as practicable to keep the transition period short.

Canadian Coast Guard Marine Communications and Traffic Services (MCTS) Centres

To help ease the transition to GMDSS and bridge the communication gap between the two systems, Canadian Coast Guard MCTS Centres will continue to monitor the current distress and safety channels VHF Ch16 and MF 2182 kHz for the foreseeable future. Once Canada's sea areas have all been implemented, lower cost DSC equipment is available, and it is determined that these services are no longer required, these listening watches will be discontinued. This decision will be evaluated at that time.

To supplement the broadcasting of Maritime Safety Information (MSI) on NAVTEX and INMARSAT EGC, MCTS Centres will continue safety broadcasts using the existing VHF continuous marine broadcast system and on the MF and HF radio bands.

Canadian Joint Rescue Co-ordination Centres (JRCC) and Maritime Rescue Sub-Centres (MRSC)

Canadian JRCCs and MRSCs will continue to receive distress alerts transmitted by vessels and relayed via MCTS or satellite. When a GMDSS distress alert is received, the centre must re-issue an "all ships" broadcast in the vicinity so that vessels in the immediate area are aware and can respond. Search and Rescue will task aircraft and vessels at this time. If a distress alert is sent in error, the Coast Guard MCTS Centre or JRCC/MRSC should be notified immediately so that these resources can be "stood-down".

More information can be obtained by contacting

Transport Canada - Marine Safety Regional Offices
Marine Communications and Traffic Services Centres