Marine Radio

Marine VHF radio is installed on all large ships and most motorized small craft. It is used for a wide variety of purposes, including summoning rescue services and communicating with harbours and marinas, and operates in the VHF frequency range, between 156 to 174 MHz. more...

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Although it is widely used for collision avoidance, its use for this purpose is contentious and is strongly discouraged by some countries, including the UK.

A marine VHF set is a combined transmitter and receiver and only operates on standard, international frequencies known as channels. Channel 16 (156.8 MHz) is the international calling and distress channel. Channel 9 can also be used in some places as a secondary call and distress channel. Transmission power ranges between 1 and 25 watts, giving a maximum range of up to about 60 nautical miles (111 km) between aerials mounted on tall ships and hills, and 5 nautical miles (9 km) between aerials mounted on small boats at sea-level. Frequency modulation is used.

Marine VHF mostly uses "simplex" transmission, where communication can only take place in one direction at a time. A transmit button on the set or microphone determines whether it is operating as a transmitter or a receiver. The majority of channels, however, are set aside for "duplex" transmissions channels where communication can take place in both directions simultaneously . Each duplex channel has two frequency assignments. This is mainly because, in the days before mobile phones and satcomms became widespread, the duplex channels could be used to place calls on the public telephone system for a fee via a marine operator. This facility is still available in some areas, though its use has largely died out. In US waters, Marine VHF radios can also receive weather radio broadcasts, where they are available, on receive-only channels wx1, wx2, etc.

Types of equipment

Sets can be fixed or portable. A fixed set generally has the advantages of a more reliable power source, higher transmit power, a larger and more effective aerial and a bigger display and buttons. A portable set (often essentially a waterproof, VHF walkie-talkie in design) can be carried to a lifeboat in an emergency, has its own power source and is more easily water-proofed.

Marine radios can be "voice-only" or can include "Digital Selective Calling".

Voice-only equipment is the traditional type, which relies totally on the human voice for calling and communicating.

Digital Selective Calling equipment, a part of GMDSS, provides all the functionality of voice-only equipment and, additionally, allows several other features:

a transmitter can call a receiver automatically using Digital Selective Calling on Channel 70, using a telephone-type number known as a Maritime Mobile Service Identity or MMSI; a distress button, which automatically sends a digital distress signal identifying the calling vessel and the nature of the emergency; a connection to a GPS receiver allowing the digital distress message to contain the distressed vessel's position;

Read more at Wikipedia.org