FCC GMDSS Maintainer key terms

A sea area within VHF coverage of at least one shore station with continuous DSC alerting, typically about 20 to 30 nautical miles from the coast. Ships here must carry VHF DSC and a 406 MHz EPIRB.

A sea area covered by an Inmarsat geostationary satellite with continuous distress alerting, excluding A1 and A2 areas. It extends roughly between 70 degrees north and 70 degrees south latitude and requires satellite communication equipment.

A periodic inspection of a ship's radio installation by an approved surveyor or classification society to confirm compliance with SOLAS carriage requirements. It results in a Safety Radio Certificate and verifies equipment performance, documentation and battery condition.

The certificate issued to a ship after a satisfactory radio survey, listing the GMDSS equipment carried and any exemptions. It must be kept on board and produced during port State control inspections.

A digital signalling technique used on VHF, MF and HF to establish contact and transmit alerts, including distress alerts, without voice communication. It sends a formatted message containing the vessel's identity and the nature of the call.

The nine-digit Maritime Mobile Service Identity number programmed into DSC and AIS equipment that uniquely identifies a ship, coast station or group. The first three digits indicate the station's country or administration.

A DSC or satellite message indicating that a vessel or person is in grave and imminent danger and requires immediate assistance. It contains the vessel identity, position and nature of distress where available.

The spoken radiotelephony distress signal used to announce grave and imminent danger and request immediate assistance. It has absolute priority over all other transmissions and is repeated three times at the start of a distress call.

The radiotelephony urgency signal indicating a situation concerning the safety of a vessel or person that does not yet present grave and imminent danger. It is spoken three times and precedes an urgency message.

The radiotelephony safety signal used to announce an important navigational or meteorological warning. It is spoken three times and followed by the safety message, often on a working channel after an initial announcement.

Designated three-minute intervals, from 00 to 03 and 30 to 33 minutes past each hour on 500 kHz, historically reserved for distress traffic. Modern GMDSS practice retains similar watch discipline on DSC distress frequencies.

A store-and-forward satellite messaging terminal used for distress alerting, two-way messaging and Enhanced Group Call reception. It has no voice capability but provides reliable global coverage in Sea Area A3 and is a common GMDSS installation.

A family of maritime satellite services providing voice and data communications, including the FleetBroadband and Fleet One variants. These terminals can carry distress priority calls and are accepted as GMDSS equipment in some configurations.

The highest access level in the Inmarsat system, which pre-empts other traffic and routes the call to a rescue coordination centre. It is initiated by a dedicated distress button or a two-step confirmation sequence on the terminal.

A shore authority responsible for coordinating search and rescue operations within a defined area. It receives distress alerts, tasks rescue units and maintains communication with the vessel in distress.

An emergency position-indicating radio beacon operating on 406 MHz that transmits a coded distress alert to the Cospas-Sarsat satellite system. Modern units include a 121.5 MHz homing transmitter and often a GNSS receiver for position reporting.

An international satellite-based search and rescue system that detects and locates distress beacons on 406 MHz. It uses geostationary and low-earth-orbit satellites to relay alerts to associated rescue coordination centres.

A search and rescue radar transponder that, when interrogated by a 9 GHz radar, produces a distinctive line of dots on the radar display. It helps locate survival craft and operates for at least 96 hours in standby.

A low-power transmission, typically on 121.5 MHz, emitted by an EPIRB or carried by survival craft to allow rescue aircraft and vessels to direction-find the beacon using dedicated receivers.

A narrow-band direct-printing system that broadcasts Maritime Safety Information on 518 kHz in English from coastal stations to ships within about 200 nautical miles. Receivers automatically print or display messages for the vessel's sea area.

The Inmarsat Enhanced Group Call service used to broadcast Maritime Safety Information to ships fitted with Inmarsat-C or Fleet terminals. Messages can be addressed to fixed geographic areas or to individual vessels.

Navigational and meteorological warnings, forecasts and urgent safety information broadcast to ships. It includes navigational warnings, weather forecasts, search and rescue information and other urgent notices.

The Inmarsat broadcast mechanism that allows messages to be sent to all ships, to a defined geographic area, or to a selected group. SafetyNET uses EGC to distribute Maritime Safety Information.

Narrow-band direct-printing, a telegraphy technique that transmits text over a narrow radio channel using frequency-shift keying. It is used for distress traffic, radiotelex and Maritime Safety Information on MF and HF.

A store-and-forward messaging service carried over NBDP or satellite links, allowing typed messages to be exchanged with shore subscribers. It is used for distress and routine traffic when voice is unsuitable.

A DSC message sent by a coast station or another vessel to retransmit a distress alert when the original alert has not been acknowledged. It is used to ensure the alert reaches a rescue coordination centre.

A routine internal check of a DSC controller that verifies its transmitter, receiver and message memory without radiating a signal. It is normally performed daily and recorded in the radio log.

A performance test of the DSC installation, including transmission on a non-distress frequency, carried out at least once a year and recorded. It confirms the equipment meets the required standards between surveys.

The official record of a ship's radio communications, equipment tests, watchkeeping and distress traffic. It must be maintained and may be inspected by surveyors and port State control officers.

A periodic discharge test confirming that the emergency battery can supply the required load for the specified period, commonly one hour for VHF and MF/HF installations. Results are recorded and compared with the manufacturer's specification.

A dedicated battery or generator that supplies GMDSS equipment when the main and emergency ship supplies fail. It must be independent of the main switchboard and capable of powering the required equipment for the specified duration.

A measure of the electrolyte density in a lead-acid battery, used to assess state of charge. It is checked with a hydrometer and compared with the manufacturer's fully charged value.

The ratio of maximum to minimum voltage or current along a transmission line, indicating how well the antenna matches the transmitter. A high SWR suggests a mismatch, damaged cable or poor antenna connection.

An automatic or manual matching network that presents a suitable impedance to the transmitter across a range of frequencies. It is used with HF and MF installations where a single antenna serves several bands.

The attenuation of signal power along a coaxial feeder, increasing with frequency and cable length. Excessive loss reduces transmitted power and receiver sensitivity, so cable type and routing must be chosen carefully.

A radar operating around 9 GHz, used for navigation and for interrogating SARTs. Its shorter wavelength gives good resolution but reduced range in heavy rain compared with S-band radar.

A portable VHF transceiver carried in survival craft for on-scene communication with rescue units. It operates on the maritime VHF channels, is waterproof and has a battery with a defined minimum life.

Inspection of foreign ships in national ports to verify compliance with international conventions, including GMDSS carriage and equipment condition. Deficiencies can lead to detention until rectified.

The systematic process of locating a defect in a radio installation by checking power, connections, antennas and equipment self-tests. It follows a logical sequence from simple external causes to internal component failure.