Chapter 9
GMDSS
Background and principles
Throughout most of the last century, the radio equipment ships were required to carry was based on their size and the number of passengers they carried. So a cross-channel ferry — quite a large vessel, carrying lots of passengers — was required to carry equipment capable of world-wide communications, and a fully-qualified radio officer to operate it, despite the fact that it was never much more than ten miles from port.
A small tramp steamer, on the other hand, could set off across the Pacific with an MF radio which would struggle to reach two hundred miles. If it had to send a distress message, it was largely a matter of luck whether there would be anyone within range or not.
In 1979, the International Maritime Organisation decided that modern technology could do better, and that a Global Maritime Distress and Safety System (GMDSS) was called for. New regulations came into force in 1992, phasing-in the system over a period of seven years.
The main idea was that all ships should carry at least two independent means of transmitting distress alerts directly to search and rescue authorities ashore. To this were added a number of supplementary requirements, including a means of communicating with other vessels nearby, and of receiving navigational and weather warnings.
To decide what equipment is required for any particular vessel, the world is divided into four areas.
A1 areas are within VHF coverage of a suitably- equipped Search and Rescue centre;
A2 areas are within MF coverage of a suitably-equipped Search and Rescue centre;
A3 areas are within Inmarsat satellite communication coverage;
A4 areas are beyond the coverage of Inmarsat satellite communication — i.e. within about 1200 miles of the north or south pole.
Sub-systems involved in GMDSS
The Global Maritime Distress and Safety System is not really a communication system: it’s a legal framework of equipment requirements and operating procedures, involving several different communication systems. Some of these, such as HF, MF, and VHF radio, existed before GMDSS was thought of: others were developed in the twenty years that elapsed between the decision to implement GMDSS, and the 1999 deadline for its full operation.
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