A marine radio is a radio intended to be carried onboard marine vessels for the purpose of communication. A boat equipped with a marine radio can use it to listen for important broadcasts, such as severe weather warnings or distress signals from other vessels.
Various types of radio equipment can be used aboard boats; however, the term marine radio usually refers to a Very High Frequency FM (VHF FM) radio. The VHF frequency range is between 156 and 162 MHz. A VHF marine radio is a transceiver - a combination of transmitter and receiver - and operates on standard, international frequencies or channels. As transceivers, marine radios mostly use simplex transmission, meaning they can only communicate in one direction at one time, determined by a transmit button. A few channels allow for 2-way calls.
A VHF marine radio ranges in transmission power from 1 to 25 watts. This gives the marine radio a maximum range of 25 nautical miles (46 km) between antennas on tall ships or structures, and 3 nautical miles (6 km) between smaller ships at sea level. A marine radio can be either fixed or portable. Fixed sets offer more reliable power sources, larger aerials and displays, and higher transmitting power. Portable sets can be more useful in emergencies and are easier to waterproof.
There are 104 VHF channels designated for marine radio service, 54 of which are to be used exclusively in United States waters. The International Telecommunications Union (ITU) has established three VHF marine radio stations to be recognized worldwide for safety reasons. Channel 16 (156.800 MHz) is the distress, safety, and calling frequency, monitored continuously by the U.S. Coast Guard. Channel 13 (156.65 MHz) is the bridge-to-bridge or “piloting” channel, used for communicating navigational information between vessels. Channel 70 (156.525 MHz) is for Digital Selective Calling.
A Digital Selective Calling (DSC) marine radio uses a two-tone digital signaling protocol to call or receive calls from other ships or coast stations, or all stations in a geographic area. It is part of the Global Maritime Distress and Safety System (GMDSS), a set of regulations established in 1996 by the Federal Communications Commission (FCC).
The FCC regulates all use of marine radio in U.S. waters. They require certain U.S. vessels, such as those over 20 meters (65.5 feet) in length or carrying more than six passengers for hire on the open sea, to carry a marine radio and to monitor channel 16 at all times. These vessels must have an FCC ship station license. Other FCC regulations for marine radio include limits on calling time, keeping channel 16 open by switching to another channel after making contact, and prohibiting false emergency calls and obscenity.