Marine and mobile radio telephony
Encyclopedia
The Marine Radiotelephone Service or HF ship-to-shore operates on shortwave
radio frequencies, using single-sideband modulation
. The usual method is that a ship calls a shore station, and the shore station's marine operator connects the caller to the public switched telephone network
. This service is retained for safety reasons, but in practice has been made obsolete by satellite telephones (particularly INMARSAT
) and VoIP telephone and email via satellite internet.
Short wave radio is used because it bounces between the ionosphere
and the ground, giving a modest 1,000 watt transmitter (the standard power) a world-wide range.
Most shore stations monitor several frequencies. The frequencies with the longest range are usually near 20 MHz, but the ionospheric weather (propagation) can dramatically change which frequencies work best.
Single-sideband (SSB) is used because the short wave bands are crowded with many users, and SSB permits a single voice channel to use a narrower range of radio frequencies (bandwidth), about 3.5 kHz
. In comparison, AM radio uses about 8 kHz, and narrowband
(voice or communication-quality) FM
uses 9 kHz.
Marine radiotelephony first became common in the 1930s, and was used extensively for communications to ships and aircraft over water. In that time, most long-range aircraft had long-wire antennas that would be let out during a call, and reeled-in afterward.
One of the most important uses of marine radiotelephony has been to change ships' itineraries, and to perform other business at sea.
Some ships, including almost all military ships, carry teletypewriters
, and use them to communicate over short wave. This is called "marine radiotelegraphy", but in practice the equipment is a normal shortwave radio with an attachment that generates and receives audio tones in order to drive the teletypewiter.
Shortwave
Shortwave radio refers to the upper MF and all of the HF portion of the radio spectrum, between 1,800–30,000 kHz. Shortwave radio received its name because the wavelengths in this band are shorter than 200 m which marked the original upper limit of the medium frequency band first used...
radio frequencies, using single-sideband modulation
Single-sideband modulation
Single-sideband modulation or Single-sideband suppressed-carrier is a refinement of amplitude modulation that more efficiently uses electrical power and bandwidth....
. The usual method is that a ship calls a shore station, and the shore station's marine operator connects the caller to the public switched telephone network
Public switched telephone network
The public switched telephone network is the network of the world's public circuit-switched telephone networks. It consists of telephone lines, fiber optic cables, microwave transmission links, cellular networks, communications satellites, and undersea telephone cables, all inter-connected by...
. This service is retained for safety reasons, but in practice has been made obsolete by satellite telephones (particularly INMARSAT
Inmarsat
Inmarsat plc is a British satellite telecommunications company, offering global, mobile services. It provides telephony and data services to users worldwide, via portable or mobile terminals which communicate to ground stations through eleven geostationary telecommunications satellites...
) and VoIP telephone and email via satellite internet.
Short wave radio is used because it bounces between the ionosphere
Ionosphere
The ionosphere is a part of the upper atmosphere, comprising portions of the mesosphere, thermosphere and exosphere, distinguished because it is ionized by solar radiation. It plays an important part in atmospheric electricity and forms the inner edge of the magnetosphere...
and the ground, giving a modest 1,000 watt transmitter (the standard power) a world-wide range.
Most shore stations monitor several frequencies. The frequencies with the longest range are usually near 20 MHz, but the ionospheric weather (propagation) can dramatically change which frequencies work best.
Single-sideband (SSB) is used because the short wave bands are crowded with many users, and SSB permits a single voice channel to use a narrower range of radio frequencies (bandwidth), about 3.5 kHz
Hertz
The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....
. In comparison, AM radio uses about 8 kHz, and narrowband
Narrowband
In radio, narrowband describes a channel in which the bandwidth of the message does not significantly exceed the channel's coherence bandwidth. It is a common misconception that narrowband refers to a channel which occupies only a "small" amount of space on the radio spectrum.The opposite of...
(voice or communication-quality) FM
FM broadcasting
FM broadcasting is a broadcasting technology pioneered by Edwin Howard Armstrong which uses frequency modulation to provide high-fidelity sound over broadcast radio. The term "FM band" describes the "frequency band in which FM is used for broadcasting"...
uses 9 kHz.
Marine radiotelephony first became common in the 1930s, and was used extensively for communications to ships and aircraft over water. In that time, most long-range aircraft had long-wire antennas that would be let out during a call, and reeled-in afterward.
One of the most important uses of marine radiotelephony has been to change ships' itineraries, and to perform other business at sea.
Some ships, including almost all military ships, carry teletypewriters
Teleprinter
A teleprinter is a electromechanical typewriter that can be used to communicate typed messages from point to point and point to multipoint over a variety of communication channels that range from a simple electrical connection, such as a pair of wires, to the use of radio and microwave as the...
, and use them to communicate over short wave. This is called "marine radiotelegraphy", but in practice the equipment is a normal shortwave radio with an attachment that generates and receives audio tones in order to drive the teletypewiter.
See also
- ASTRA2Connect Maritime BroadbandASTRA2Connect Maritime BroadbandASTRA2Connect Maritime Broadband is a two-way satellite broadband Internet service for use on private boats and commercial ships throughout European waters....
- AT&T High Seas Service
- InmarsatInmarsatInmarsat plc is a British satellite telecommunications company, offering global, mobile services. It provides telephony and data services to users worldwide, via portable or mobile terminals which communicate to ground stations through eleven geostationary telecommunications satellites...