FCC GMDSS key terms

A digital signalling system used on VHF, MF and HF marine radios to send automated alerts and calls to specific stations or groups. It transmits a formatted digital message containing the vessel's identity and the nature of the call before voice communication begins.

A unique nine-digit number permanently assigned to a ship station, coast station, or group that identifies the caller in DSC and other digital marine communications. It is programmed into the radio equipment and used for selective calling and distress alerting.

A priority DSC transmission sent to announce that a vessel or person is in grave and imminent danger and requires immediate assistance. It typically includes the vessel identity and, when time permits, the position and nature of distress.

The international radiotelephony distress signal spoken three times to indicate grave and imminent danger and a request for immediate assistance. It precedes the distress message on voice channels and is reserved exclusively for genuine distress situations.

The international radiotelephony urgency signal indicating a very urgent message concerning the safety of a vessel or person but not an immediate distress situation. It is spoken three times and precedes the urgency message on the appropriate channel.

The international radiotelephony safety signal spoken three times to announce a message containing important navigational or meteorological information. It has lower priority than distress and urgency traffic and precedes the safety message.

The spoken message transmitted after a Mayday call that gives the vessel's identity, position, nature of distress, assistance required, and any other relevant details such as persons on board. It is sent on a distress channel and repeated as necessary.

A transmission by a station other than the vessel in distress that forwards a distress alert or message on behalf of a vessel unable to transmit its own. It is sent only when the distressed vessel cannot transmit or when a station observes a distress that has not been acknowledged.

All messages relating to a distress situation, including the original alert, acknowledgements, relays, and communications with rescue coordination authorities. It has absolute priority over all other communications and requires silence of other stations on the working frequency.

A radiotelephony procedure word meaning all stations must stop transmitting on the frequency because distress traffic is in progress. It is used to clear the channel for the distressed vessel and rescue authorities, and is lifted with the word 'seelonce feenee'.

An internationally mandated system of satellite and terrestrial radio equipment and procedures that allows ships to send distress alerts and receive safety information automatically. It defines equipment carriage requirements according to the sea area in which a vessel operates.

A sea area within the coverage of at least one VHF coast station equipped with DSC alerting, generally about 20 to 30 nautical miles from shore. Vessels operating only in A1 need VHF DSC and associated equipment but no satellite or MF/HF distress capability.

A sea area outside A1 but within the coverage of at least one MF coast station equipped with DSC alerting, typically extending to about 100 nautical miles from shore. It requires MF DSC distress capability in addition to VHF equipment.

A sea area outside A1 and A2 that is within the coverage of an Inmarsat geostationary satellite capable of distress alerting, roughly between 70 degrees north and 70 degrees south latitude. It requires satellite communication equipment or HF DSC capability.

The remaining sea area outside A1, A2 and A3, mainly the polar regions beyond Inmarsat coverage. Vessels operating in A4 must carry HF DSC and HF radiotelephony equipment for distress alerting and communications.

A narrow-band direct printing system that automatically receives maritime safety information broadcasts on 518 kHz in English, and on other frequencies for national languages. It provides navigational warnings, meteorological warnings and forecasts within coverage of coastal transmitters.

Navigational and meteorological warnings, forecasts and other urgent safety information broadcast to ships. It is received through NAVTEX, SafetyNET or HF NBDP and is essential for safe navigation and voyage planning.

An Inmarsat service that broadcasts maritime safety information to ships equipped with Inmarsat terminals, using enhanced group call addressing. It delivers MSI to areas where NAVTEX is unavailable, such as open ocean and polar regions.

A satellite communications provider whose geostationary satellites support maritime distress alerting, voice, and data services. Its coverage underpins Sea Area A3 and provides the SafetyNET service for broadcasting maritime safety information.

A distress alert transmitted through a satellite system such as Inmarsat or a 406 MHz EPIRB, which is routed to a rescue coordination centre. It provides vessel identity and position information and can be initiated automatically or manually.

A float-free beacon that transmits a distress signal on 406 MHz to the Cospas-Sarsat satellite system, and often includes a 121.5 MHz homing transmitter. It is activated automatically on immersion or manually, and must be registered with the national authority.

An international satellite system that detects and locates 406 MHz distress beacons and forwards the alert data to rescue coordination centres. It supports EPIRBs and other beacons worldwide and relies on beacon registration to identify the vessel.

A radar transponder carried by vessels that, when interrogated by a search radar, displays a series of dots on the radar screen to help locate a survival craft. It operates in the 9 GHz band and is part of GMDSS survival craft equipment.

Digital selective calling equipment operating in the medium and high frequency bands, used for distress alerting and general calling beyond VHF range. It is required for Sea Areas A2, A3 and A4 depending on the vessel's operating area.

A dedicated receiver that continuously monitors the DSC distress and calling frequency, such as VHF channel 70 or the MF/HF distress frequencies, so alerts are detected even when the main radio is not in use. It is a required GMDSS carriage item.

The VHF marine channel reserved exclusively for digital selective calling; voice transmissions are prohibited on it. It is the primary VHF distress and calling frequency monitored by DSC watch receivers.

The VHF radiotelephony distress, safety and calling frequency, monitored by ships and coast stations for voice communications. It is used to initiate calls and for distress traffic, with working channels used for extended conversations.

Voice communication by radio, used in the maritime service for distress, urgency, safety and routine traffic. It follows standardized procedures including priority signals, call formats and phonetic alphabet usage.

A telex-like data transmission system used in the maritime service for automated printed communications, including MSI broadcasts on HF. It provides error-corrected text messages and is used where voice or satellite services are unavailable.

A narrow-band direct printing service on high frequency that provides automated telex communications, including maritime safety information broadcasts. It is used in Sea Area A4 and other areas beyond NAVTEX and satellite coverage.

A standardized set of words used to spell out letters clearly over voice radio, such as Alfa, Bravo, Charlie. It reduces errors when transmitting call signs, names and coded information in maritime communications.

A license issued by the FCC that authorizes a vessel to operate marine radio equipment. It specifies the station's call sign and the frequencies and equipment permitted, and must be available aboard for inspection.

An FCC license that authorizes a person to operate marine radiotelephone equipment on board a vessel. It is required for certain operators and must be carried or available when the station is in use.

A unique identifier assigned to a radio station by the licensing authority, used to identify the station during communications. For ship stations it is issued with the ship station license and is spoken using the phonetic alphabet.

The regulations in Title 47 of the Code of Federal Regulations that govern radio communications in the United States, including the maritime service. They set licensing, equipment, operating and frequency requirements for ship and coast stations.

A DSC transmission addressed to a group of stations sharing a common group MMSI, used to distribute information efficiently. It is commonly used by coast stations to alert fleets or by organizations to reach multiple vessels at once.

A message concerning the safety of a vessel or person that is very urgent but does not involve immediate danger. It is preceded by the Pan-Pan signal and has priority over routine traffic but below distress traffic.

A message containing important navigational or meteorological information, announced with the Securite signal. It has lower priority than distress and urgency traffic and is used for warnings that affect safe navigation.