The radio frequency of 2182 kilohertz (kHz) is the international calling and distress frequency for voice maritime communication (radiotelephony) on the marine MF bands. It is analogous to Channel 16 on the marine VHF band.
Transmissions on 2182 kHz should use single-sideband modulation (SSB) (upper sideband only). That said, Amplitude modulation (AM), and some variants in between the two, such as vestigial sideband, are still in use. These are used mainly by vessels with older equipment installed and by some coastal stations in an attempt to ensure compatibility with older and less sophisticated receivers afloat.
during the day and 500 (or more) nautical miles at night.
At night a well equipped station can achieve intra-continental communication, but this range can be severely limited in
summer because of static caused by lightning.
There are many other marine frequencies, including distress and calling frequencies, in the Marine HF bands, up to and including worldwide coverage with the right conditions. In addition to these, satellite technology can provide worldwide communications for vessels at sea.
Twice each hour, all stations listening on 2182 kHz are required to maintain a strictly enforced three-minute silence period, starting at h+00 and h+30. As a visual aide-memoire a typical clock in a ship's radio room would have these silence periods marked by shading the sectors between h+00 to H+03 and h+30 to h+33 in green. Similar sectors at h+15 and h+45 may be marked in red for a corresponding silence period on 500 kHz, although this is no longer an active marine distress or listening frequency.
In order to operate a marine radio transceiver on 2182 kHz, the operator must hold a GMDSS Long Range Certificate, which requires success on a wider syllabus than that of the GMDSS Short Range Certificate that is necessary for Marine VHF use. In either case, though, an unqualified operator would not be prosecuted for the use of either rig in what turns out to be a genuine distress situation. (In the United States comparable certificates would be the GMDSS Operator and the GMDSS Restricted Operator licenses.)