An Overview of the Global Maritime Distress & Safety System (Source: U. S. Coastguard) GMDSS portal
Since the invention of radio at the end of the 19th Century, ships at sea have relied on Morse code,
invented by Samuel Morse and first used in 1844, for distress and safety telecommunications. The
need for ship and coast radio stations to have and use radiotelegraph equipment, and to listen to a
common radio frequency for Morse encoded distress calls, was recognized after the sinking of the
liner Titanic in the North Atlantic in 1912. The U.S. Congress enacted legislation soon after,
requiring U.S. ships to use Morse code radiotelegraph equipment for distress calls. The
International Telecommunications Union (ITU), now a United Nations agency, followed suit for ships
of all nations. Morse encoded distress calling has saved thousands of lives since its inception almost
a century ago, but its use requires skilled radio operators spending many hours listening to the
radio distress frequency. Its range on the medium frequency (MF) distress band (500 kHz) is
limited, and the amount of traffic Morse signals can carry is also limited.
Over fifteen years ago the International Maritime Organization (IMO), a United Nations agency
specializing in safety of shipping and preventing ships from polluting the seas, began looking at
ways of improving maritime distress and safety communications. In 1979, a group of experts
drafted the International Convention on Maritime Search and Rescue, which called for development
of a global search and rescue plan. This group also passed a resolution calling for development by
IMO of a Global Maritime Distress and Safety System (GMDSS) to provide the communication
support needed to implement the search and rescue plan. This new system, which the world's
maritime nations, including the United States, are implementing, is based upon a combination of
satellite and terrestrial radio services, and has changed international distress communications from
being primarily ship-to-ship based to ship-to- shore (Rescue Coordination Center) based. It spelled
the end of Morse code communications for all but a few users, such as Amateur Radio. The GMDSS
provides for automatic distress alerting and locating in cases where a radio operator doesn't have
time to send an SOS or MAYDAY call, and, for the first time, requires ships to receive broadcasts of
maritime safety information which could prevent a distress from happening in the first place. In
1988, IMO amended the Safety of Life at Sea (SOLAS) Convention, requiring ships subject to it fit
GMDSS equipment. Such ships were required to carry NAVTEX and satellite EPIRBs by 1 August
1993, and had to fit all other GMDSS equipment by 1 February 1999. US ships were allowed to fit
GMDSS in lieu of Morse telegraphy equipment by the Telecommunications Act of 1996.
The GMDSS consists of several systems, some of which are new, but many of which have been in
operation for many years. The system will be able to reliably perform the following functions:
alerting (including position determination of the unit in distress), search and rescue coordination,
locating (homing), maritime safety information broadcasts, general communications, and bridge- to-
bridge communications. Specific radio carriage requirements depend upon the ship's area of
operation, rather than its tonnage. The system also provides redundant means of distress alerting,
and emergency sources of power.
The GMDSS consists of many separate systems which are being implemented in a coordinated and
agreed- upon manner. Some of these systems are discussed on the following page.
