Global Maritime Distress and Safety System (GMDSS) – An Outline

by OldSailor on October 10, 2007



The purpose of this post is to give basic information regarding Global Maritime Distress and Safety System (GMDSS).

In 1979, the International Maritime Organization (IMO) recognized the need for an updated maritime communication system and helped create the Inmarsat system employing geostationary satellites positioned above the Atlantic, Indian, and Pacific Oceans. Shortly thereafter, a polar orbiting satellite system was established to locate Emergency Position- Indicating Radio Beacons (EPIRBs). The IMO also decided to commence a general upgrade of the distress and safety system to be known as the GMDSS. This system would provide rapid and automated distress reporting and improved telecommunications for the maritime community.

In 1988, the IMO amended its SOLAS convention to complete this upgrade of the maritime safety communications procedures and equipment for the GMDSS. GMDSS applies system automation techniques to the traditional maritime VHF, MF, and HF bands, which previously required a continuous listening watch.GMDSS incorporates the Inmarsat and the EPIRB satellite systems to improve the reliability and effectiveness of the distress and safety system on a global basis. GMDSS also provides for the timely dissemination of maritime safety information, including navigational and meteorological warnings and weather forecasts.

The purpose of Global Maritime Distress and Safety System (GMDSS) is to bring major improvements to maritime safety and communications. GMDSS is an automated ship to shore distress alerting system that relies on satellite and advanced terrestrial communications links. The system also provides some limited ship to ship communications capabilities.Every ship subject to the communications act or the safety convention must comply with GMDSS.

These vessels include:

  • All passenger ships regardless of size
  • Cargo ships of 300 gross tons and upward

GMDSS is a system based on the linking of Search and Rescue (SAR) authorities ashore with shipping in the immediate vicinity of a vessel in distress or in need of assistance. The primary purpose of GMDSS is to automate and improve emergency communications for the world’s shipping industry. GMDSS uses a number of frequencies, modes and systems to accomplish this mission. The Global Marine Distress and Safety System (GMDSS) uses both satellite and terrestrial radio systems because each system has its own individual limitations with respect to geographical coverage and services it can provide.The Basic concept of GMDSS is to alert SAR authorities ashore and vessels in the vicinity of a distress so they can assist in a coordinated search and rescue operation with minimum delay.


click to enlarge Image Courtesy: ICS Electronics

The IMO and the Coast Guard strongly encourage all vessels not required to participate in GMDSS to voluntarily carry selected GMDSS systems to enhance their safety. The primary systems would be a DSC equipped marine radio suitable for your area of operations and the 406 MHz satellite EPIRB. If you use GMDSS systems voluntarily, training is not required but is highly recommended.

Digital Selective Calling (DSC): DSC-equipped marine radios are used for routine communications and for transmitting, acknowledging, and relaying distress alerts. DSC allows a specific station to be contacted and indicates the method and channel on which to reply. It can also make “all ships” calls. Follow-up communications are made on an appropriate non-DSC frequency. DSC radio users need to understand the basic operation of the radio, how DSC acts as an automated watch, the importance of registering the radio identity and keeping it on and tuned to the DSC channel. DSC is on channel 70 VHF and on the frequency 2187.5 kHz MF. There are DSC calling channels in each HF band.


Satellite EPIRBs: One of the most useful GMDSS systems is the satellite EPIRB, a small device designed for automated transmission of distress alerts. The 406 MHz satellite EPIRB (which is far superior to the 121.5 MHz EPIRB) is a newer and more reliable device designed to provide rapid alerting, identification, and accurate location information to search and rescue authorities. Monitoring of 121.5 MHz distress alerting will cease in the near future.


Inmarsat: Inmarsat Global Ltd (Inmarsat) operates global voice and data services for ships at sea. Inmarsat terminals (B, C or F77) are formally accepted for the use of GMDSS. These terminals come in a variety of sizes, weights, and costs so that Inmarsat is feasible for use on smaller vessels.


Image Courtesy: Alphatelecom

Other elements of the GMDSS that small vessels should be aware of include:

  • The coastal NAVTEX broadcast system provides marine weather forecasts, navigation warnings, and search and rescue alerts
  • High seas SafetyNET broadcast system (delivered by the Inmarsat-C system) provides information similar to NAVTEX, but to vessels on the high seas.
  • The Search and Rescue Transponder (SART) for life rafts is used to aid in the location of survivors by enhancing the radar visibility of small targets.

Global Positioning System (GPS): GPS receivers, if connected, provide automatic accurate position to DSC radios and Inmarsat terminals in the event of a distress alert.


I hope you this post was useful for you, if you have any questions then contact us.

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