WPC GMDSS ROC (India) key terms

A sea area within radiotelephone coverage of at least one VHF coast station offering continuous DSC alerting, normally taken as about 20 to 30 nautical miles from the coast.

A sea area outside A1 but within MF radiotelephone coverage of at least one coast station offering continuous DSC alerting, typically extending about 100 to 150 nautical miles offshore.

A sea area outside A1 and A2 covered by an Inmarsat geostationary satellite with continuous alerting capability, roughly between 70 degrees North and 70 degrees South latitude.

The remaining sea areas outside A1, A2 and A3, mainly the polar regions, where GMDSS carriage relies on HF radiotelephony and HF DSC rather than satellite coverage.

The Global Maritime Distress and Safety System: an internationally agreed set of radio equipment, frequencies and procedures allowing ships to alert shore authorities and other vessels and to receive maritime safety information automatically.

A transmission sent by DSC, Inmarsat or other means to announce that a vessel is in grave and imminent danger and requires immediate assistance, normally followed by a distress call and message by radiotelephony.

The spoken radiotelephony signal spoken three times to indicate distress, used in the distress call and repeated before the distress message giving position, nature of danger and assistance required.

The radiotelephony urgency signal, spoken three times, used when a vessel has a very urgent message concerning safety but is not in immediate grave and imminent danger, such as serious illness or machinery breakdown.

The radiotelephony safety signal, spoken three times, announcing an important navigational or meteorological warning such as a gale warning, drifting hazard or navigational change.

A digitally encoded alert sent on VHF Channel 70, MF 2187.5 kHz or the HF DSC frequencies, containing the vessel's MMSI and usually the nature of distress and position, which automatically alerts coast stations and other ships.

Maritime Mobile Service Identity: a unique nine-digit number programmed into DSC, AIS and other equipment that identifies a ship, coast station or group and is used to address and route alerts and calls.

The dedicated VHF DSC channel used only for digital selective calling alerts and calls; voice transmissions are prohibited on it because they would interfere with DSC signalling.

The continuous listening watch a GMDSS radio operator maintains on the DSC frequencies appropriate to the sea area, such as Channel 70 in A1 and 2187.5 kHz in A2, so distress alerts are not missed.

Digital selective calling on medium and high frequencies, using 2187.5 kHz for MF distress watch and designated HF frequencies, providing alerting beyond VHF range and in Sea Areas A2, A3 and A4.

The international VHF radiotelephony distress, safety and calling channel, watched continuously at sea; initial distress, urgency and safety traffic is normally established here before moving to a working channel.

A VHF or MF/HF voice channel, other than the calling and distress channel, to which stations move to conduct routine or distress traffic so that the calling channel stays clear.

Voice communication on high frequencies between roughly 4 and 26 MHz, using ionospheric propagation for long-range ship-to-shore and ship-to-ship traffic, including distress and safety working in Sea Area A4.

The practical distance over which voice communication is reliable, roughly line of sight for VHF, a few hundred nautical miles for MF ground wave, and intercontinental for HF depending on frequency and ionospheric conditions.

A store-and-forward satellite messaging terminal used for distress alerting, two-way text messaging and reception of SafetyNET maritime safety information, meeting Sea Area A3 carriage requirements.

The Inmarsat enhanced group call service that broadcasts maritime safety information such as navigational and meteorological warnings to ships fitted with Inmarsat terminals in selected sea areas.

An Inmarsat broadcast addressing method that lets messages be sent to all ships, to a group or to a defined geographic area, and which underpins the SafetyNET maritime safety information service.

Navigational warnings, meteorological warnings and forecasts, search and rescue information and other urgent safety broadcasts that GMDSS equipment must be able to receive automatically in all sea areas.

A narrow-band direct-printing system broadcasting maritime safety information on 518 kHz in English, and on other frequencies nationally, to ships within about 200 to 400 nautical miles of a coastal transmitter.

The international NAVTEX frequency used for English-language maritime safety information broadcasts; receivers are normally set to this frequency for automatic reception of navigational and weather warnings.

Emergency position-indicating radio beacon: a float-free transmitter, usually operating on 406 MHz via the Cospas-Sarsat system, that alerts search and rescue and identifies the vessel through its registered beacon hex ID.

The international satellite system that detects and locates 406 MHz distress beacons, passes alerts to rescue coordination centres, and provides a homing signal for search aircraft.

The frequency used by modern satellite EPIRBs to transmit a digital distress message carrying beacon identity and, with GNSS input, position, detected by Cospas-Sarsat satellites.

Search and rescue transponder: a radar transponder, normally operating in the 9 GHz band, that responds to radar pulses with a series of dots on a rescuing vessel's radar display to mark the survivor's position.

A search and rescue transmitter that broadcasts a distress position using AIS message format, appearing as a marked target on nearby AIS displays, offered as an alternative to a radar SART.

A transmission by a station that is not itself in distress, passing on a distress alert or message it has received on behalf of a vessel unable to transmit, using MAYDAY RELAY procedures.

An unintended distress alert, often from accidental DSC activation or beacon testing; it must be cancelled promptly by notifying the relevant coast station or rescue coordination centre so rescue resources are not wasted.

The official record, kept as required by regulation, of distress, urgency and safety communications, watchkeeping arrangements, position reports and equipment faults or failures on board.

The regulatory duty to maintain continuous radio watch on the appropriate distress and calling frequencies for the sea area, using qualified operators and functioning equipment throughout the voyage.

The national authorisation issued for a vessel's radio installation, listing permitted equipment, frequencies and the assigned identities such as call sign and MMSI, which must be kept on board.

Restricted Operator's Certificate: the GMDSS operator qualification covering VHF and, depending on national rules, limited MF/satellite operation, required for radio personnel on vessels in Sea Area A1.

A periodic test confirming that the reserve source of energy can supply the radio installation for the required period, typically 6 or 24 hours depending on the vessel, by discharge or specific gravity measurement.

The dedicated battery or emergency generator that powers GMDSS equipment if the main and emergency electrical supplies fail, sized to sustain the required radio services for the specified period.

A defect such as a broken whip, corroded connector, water ingress in coaxial cable or poor earth that reduces transmitted power or receiver sensitivity and is found by meter readings and visual inspection.

The way radio waves travel, by ground wave at MF and by ionospheric sky wave at HF, varying with frequency, time of day and solar conditions and strongly affecting which HF band works at a given time.