Global Maritime Distress and Safety System

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Introduced as amendments to the Safety of Life at Sea convention by the International Maritime Organization, the Global Maritime Distress and Safety System (GMDSS) went into effect on 1 February 1999. It required all passenger ships, and all cargo ships of 300 gross tons or larger, to carry specified electronic distress communications equipment, including satellite emergency position indicating radio beacons (EPIRBs) and search and rescue transponders (SARTs) for the location of the ship or survival craft. For oceangoing communications systems use UHF radio via satellite relay. Their MF radio must support digital selective calling (DSC), a "telephone number" system that can identify specific vessels, and also be addressed to groups of vessels.

Evolution of maritime technology was recognized by the GMDSS waiving the requirement for ships to have continuous coverage of radiotelegraphy distress channels, on which Morse code was the means of communication. Radiotelegraphy is considered obsolete for maritime use.

Ships covered by SOLAS also are required to carry a NAVTEX navigational and weather receiver. Supplemental systems using digital selective calling on VHF radio supplement the satellite system for coastal waters. It is strongly recommended, by the United States Coast Guard

Again, all vessels over 300 tons displacement, as well as passenger vessels of any size, must comply with GMDSS requirements. Smaller vessels are wise to do so, in the interest of safety. EPIRBs, priced at under USD $900, can be found on small pleasure craft; their default mode of operation is to float free if the vessel sinks. Depending on the EPIRB, it may be possible to trigger it manually for a safety-of-life situation where the vessel is not sinking.

Rather than triggering a GMDSS, an equally appropriate means of distress signaling can be actuating the distress signal on a DSC-equipped radio, which is usually a matter of holding down a guarded button. If, as is strongly recommended by search and rescue organizations, a GPS receiver is connected to the DSC-equipped radio, the DSC distress message will carry the vessel's identity and exact location.

Communications segment

The system uses both terrestrial and satellite radio.


Emergency broadcasts are monitored with digital selective calling on medium frequency radio at 2182 MHz, and VHF marine radio on Channel 16, 156.800 MHz. There is also an emergency notification capability through the VHF Automatic Identification System.

All vessels and aircraft with receivers for these services are required to monitor them continuously, and relay them until a rescue center responds.

Note: the equivalent air distress frequency is 121.5 MHz for civilian and 243.0 MHz for military aircraft.


Their space communications, which use a worldwide reserved 406 MHz signal, are part of the COSPAS-SARSAT, which includes:

  • Inmarsat B
  • Inmarsat C
  • Inmarsat F77

These satellites give good coverage in other than polar latitudes, where additional devices are needed.

  • Secondary payloads on government satellites such as the GOES earth observation orbiters.