Single-Sideband Ship-to-Shore
For long-range voice communications, single-sideband is the primary marine system worldwide. Frequencies are between 2.0 MHz and 23.0 MHz and like VHF can broadcast in a simplex or duplex mode. Singlesideband works on the principle of taking a standard amplitude modulated (AM) signal, suppressing one side of the signal, and using the energy to increase the efficiency of the remaining sideband signal. Most units are capable of utilizing the upper sideband (USB) or lower sideband (LSB) for communications depending on the need of the user. There are still a few units with double-sideband capability (DSB) which is the same as AM. The AM ship-to-shore/ship-to-ship system was the first type of voice communication method used by mariners but has been almost completely replaced by single-sideband.
Most single-sideband users still must be familiar with designated frequencies for specific uses. The frequencies now have specified channels associated with them called ITU channels or International Telephone Units. ITU channels begin at 401 and go through 2240 with the first one or two numbers of the channel corresponding to the frequency range in megahertz. Not every number in this range has a designated marine use. The international calling and distress frequency for SSB is 2182 kHz or 2.182 MHz. There are no channel designations for the 2-3 MHz band, which is considered medium frequency, and the 4-23 MHz bands, which are considered high frequency.
The U.S. Coast Guard also monitors the SSB calling and distress frequency as well as having several designated working frequencies. High seas radiotelephone service is also available on various frequencies.
