Marine and mobile radio telephony
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The Marine Radiotelephone Service or HF ship-to-shore operates on short wave 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.
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 become 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 teletype.