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2182 kHz is analogous to channel 16 on the marine VHF band, but unlike VHF which is limited to ranges of about 20 to 50 nautical miles (40 to 90 km) depending on antenna height, [3] communications on 2182 kHz and nearby frequencies have a reliable range of around 50 to 100 nautical miles (90 to 190 km) during the day and 150 to 300 nautical miles (280 to 560 km) or sometimes more at night.
For much of the 20th century, 500 kHz was the primary international distress frequency. Its use has been phased out in favor of the Global Maritime Distress Safety System. Use of some distress frequencies is permitted for calling other stations to establish contact, whereupon the stations move to another frequency.
The frequencies which would have been the second frequencies on half-duplex channels are not used for marine purposes and can be used for other purposes that vary by country. For example, 161.000 to 161.450 MHz are part of the allocation to the Association of American Railroads channels used by railways in the US and Canada.
DSC was intended to eliminate the need for persons on a ship's bridge or on shore to continuously guard radio receivers on voice radio channels, including VHF channel 16 (156.8 MHz) and 2182 kHz now used for distress, safety and calling. A listening watch aboard GMDSS-equipped ships on 2182 kHz ended on February 1, 1999.
2182 kHz is the international calling and distress frequency for SSB maritime voice communication (radiotelephony). It is analogous to Channel 16 on the marine VHF band. 500 kHz was for many years the maritime distress and emergency frequency, and there are more NDBs between 510 and 530 kHz.
The carrier frequency is 1.25 MHz above the lower edge of the 6 MHz wide channel. This effectively makes the system AM at low modulation frequencies and SSB at high modulation frequencies. The absence of the lower sideband components at high frequencies must be compensated for, and this is done in the IF amplifier.
Australia allows use of the standard 26.965–27.405 MHz AM/SSB 40 channel CB band for land mobile (and marine) communications plus 10 marine-only AM channels in the 27.68–27.98 MHz band. Many coast guard stations monitor 27 Meg channel 88 (27.880 MHz) in addition to VHF channel 16 (156.800 MHz) for distress, safety information and calling.
Marine radiotelephony originally used AM mode in the 2-3 MHz region before the transition to SSB and the adoption of various higher frequency bands in addition to the 2 MHz frequencies. One of the most important uses of marine radiotelephony has been to change ships' itineraries, and to perform other business at sea.