FCC Restricted GMDSS (RG) cheat sheet

GMDSS is an IMO-mandated, SOLAS-based system: it combines satellite and terrestrial communications so that distress alerts can be sent automatically and shore-side SAR authorities can be alerted and coordinated; it also provides MSI broadcasts and locating signals.

Sea Area A1: within VHF coverage of at least one coast station providing continuous DSC alerting (typically about 20–30 nautical miles offshore). A1-only vessels must carry VHF with DSC, a 406 MHz EPIRB (or equivalent satellite alerting), NAVTEX or another MSI receiver, and a SART (one per vessel, two on vessels of 500 GT and above).

Sea Area A2: beyond A1 but within MF coverage of coast stations with continuous DSC alerting (approximately 100–150 nm). Requires MF/HF DSC or Inmarsat, plus the A1 equipment.

Sea Area A3: beyond A1 and A2, within coverage of a recognized mobile-satellite service (Inmarsat) supporting alerting (roughly 70°N–70°S). Requires an Inmarsat ship earth station or MF/HF DSC with NBDP, plus A1/A2 equipment.

Sea Area A4: the remaining sea areas (polar regions outside Inmarsat coverage). Requires HF DSC and NBDP radiotelegraphy capability on the 4/6/8/12/16 MHz bands, in addition to A1–A3 equipment.

A vessel is rated for the highest sea area it operates in; an A1-only certificate limits operation to within VHF DSC coast-station coverage, and the operator's endorsement (Restricted Operator's Certificate) applies accordingly.

Carriage is certified by the flag administration: the ship's radio licence, the Safe Manning Document, and the GMDSS Radio Survey/Record of Equipment define the required installation, power supplies, and duplication for the area of operation.

Sources of energy: VHF installations must have a reserve source (battery) capable of powering the installation for at least 1 hour (6 hours for A1/A2 ships with emergency generator arrangements per SOLAS); batteries must be tested and logged.

Channel 16 (156.800 MHz) is the international distress, safety, and calling channel; it is monitored for distress, urgency, and safety, and routine working traffic must move to a working channel after initial contact.

Channel 70 (156.525 MHz) is reserved exclusively for DSC; voice transmission on Channel 70 is prohibited because it would interfere with digital alerting.

Working channels: Ch 6 (intership/safety), Ch 13 (bridge-to-bridge navigation safety), Ch 9 (marina/port operations in some areas), Ch 12/14 (port operations), Ch 67 (small craft/safety in some regions), Ch 15/17 (onboard communications, low power).

Distress call and message: 'MAYDAY, MAYDAY, MAYDAY' + vessel name/call sign (repeated three times), then MAYDAY + name, position, nature of distress, assistance required, persons aboard, and any other useful information; end with 'OVER'.

Urgency: 'PAN PAN' (three times) for urgent messages concerning the safety of a vessel or person but not immediate distress; Safety: 'SECURITE' (three times) for important navigational or meteorological warnings.

VHF DSC distress alert: lift the protective cover, press and hold the DISTRESS button for about 5 seconds; the alert transmits on Ch 70 with the vessel's MMSI and position (if a position source is connected). Wait for acknowledgement, then switch to Ch 16 for the distress traffic.

DSC acknowledgement: a coast station or another station acknowledges the distress alert on Ch 70; if no acknowledgement is received, the alert may be retransmitted after a short interval, and the distress should also be broadcast by radiotelephony on Ch 16.

False alerts: cancel a false DSC distress alert by transmitting a DSC 'distress cancellation' on Ch 70 to all stations, then announce the false alert on Ch 16; log the event and report to the coast station.

Alerting vs. coordination: the DSC alert (Ch 70) or EPIRB alerts SAR authorities; the subsequent radiotelephony distress traffic on Ch 16 provides the details and coordinates the response.

Distress traffic is controlled by the SAR authority (RCC) or the on-scene coordinator (OSC); other stations must maintain silence unless they can assist, and must not transmit on the distress channel during the emergency.

MAYDAY RELAY: used by a station (e.g., a ship or coast station) to relay a distress message on behalf of another station that cannot transmit; the relay is prefixed 'MAYDAY RELAY' three times and includes the original station's details.

On-scene communications: use Ch 16 for initial contact, then move to a working channel (often Ch 6 or a designated channel) for coordination between the OSC, aircraft, and assisting vessels; keep transmissions short and follow the OSC's instructions.

SAR coordination: the RCC may designate an OSC (usually the first vessel on scene or a SAR unit); the OSC coordinates search patterns, communications, and rescue operations, and reports to the RCC.

Urgency (PAN PAN) and safety (SECURITE) messages are broadcast on Ch 16 (or a working channel) and may be preceded by DSC calls on Ch 70 to alert specific stations or all stations.

Silence periods: during distress traffic, stations must observe radio silence on the distress channel; the MAYDAY call and message have priority over all other transmissions.

Logging: distress, urgency, and safety communications, including times, frequencies, and stations involved, must be recorded in the radio log (or official logbook).

NAVTEX is a narrow-band direct-printing (NBDP) system transmitting MSI on 518 kHz (international, English) and 490 kHz (national, local language) plus 4209.5 kHz for some regions; coverage is typically 200–400 nautical miles from the transmitter.

Message categories: A = navigational warnings, B = meteorological warnings, C = ice reports, D = search and rescue information, E = meteorological forecasts, F = pilot service messages, G = AIS messages, H = Loran-C messages, I = GNSS messages, J = SATNAV messages, K = other electronic navaid messages, L = additional navigational warnings, V = notice to mariners, X = special services, Z = no message.

Each NAVTEX message begins with a header: station identifier (letter), message type (letter), and a sequential message number (two digits); the receiver can be programmed to accept or reject specific stations and message types.

NAVTEX broadcasts are scheduled at fixed times (typically every 4 hours for scheduled broadcasts, with unscheduled broadcasts for urgent warnings); the schedule is published in the Admiralty List of Radio Signals and on the NAVTEX receiver's station list.

MSI includes navigational warnings, meteorological warnings and forecasts, SAR information, and other urgent safety information; it is broadcast via NAVTEX, SafetyNET (Inmarsat), and HF NBDP.

NAVTEX receivers must be capable of receiving 518 kHz and, where required, 490 kHz; the receiver should be tested and the message log checked regularly, and received messages should be retained for the required period.

SafetyNET (Inmarsat Enhanced Group Call) provides MSI in Sea Areas A3 and A4 (and A2 where NAVTEX is unavailable); it uses a dedicated receiver or an Inmarsat ship earth station.

Receiving MSI: operators must ensure the NAVTEX/SafetyNET receiver is switched on and tuned, monitor the message log, and act on navigational and meteorological warnings relevant to the vessel's route.

406 MHz EPIRB: transmits a digital distress alert to the COSPAS-SARSAT satellite system, including the vessel's identity (MMSI/hex ID) and, if interfaced, position; it also transmits a 121.5 MHz homing signal for SAR aircraft and vessels.

EPIRB registration: the beacon must be registered with the flag administration (or the administration of the vessel's country) with the vessel's details and emergency contacts; registration must be updated when the vessel or ownership changes.

EPIRB testing: perform the self-test (usually a short button press) monthly and after any activation; do not activate the beacon accidentally - a false activation must be reported to the coast station and the beacon switched off.

EPIRB battery and HRU: the battery has a specified replacement interval (typically 5 years, marked on the label); the hydrostatic release unit (HRU) must be replaced by its expiry date, and the beacon must be mounted in a float-free position.

SART (Search and Rescue Radar Transponder): operates in the 9 GHz (X-band) radar band; when interrogated by a radar, it responds with a series of dots on the radar display, allowing the vessel to be located.

SART operation: switch on the SART when abandoning ship or when the vessel is in distress; it has a range of about 5–10 nautical miles depending on radar height; the SART should be mounted at least 1 metre above sea level in the survival craft.

SART testing: use the built-in test function (usually a button) to verify operation; do not test near other vessels' radars as it may cause interference; the SART battery has a specified life (typically 96 hours standby/8 hours operation).

AIS-SART: an alternative to the radar SART, transmitting an AIS message that appears on AIS displays; it is carried in addition to or instead of a radar SART depending on the vessel's equipment.

Survival craft VHF: a portable, battery-powered VHF radiotelephone (typically 121.5/156.8 MHz or dedicated channels) carried in survival craft for on-scene communications with SAR units; it must be waterproof and capable of operating in the marine band.

Channel 16 is used for distress and on-scene communications; the survival craft radio should be capable of transmitting on Ch 16 and at least one working channel, and often includes 121.5 MHz for homing.

Battery: the survival craft radio battery must have sufficient capacity for the required operating period (typically 8 hours or as specified); batteries should be checked and replaced per the manufacturer's schedule.

Two-way on-scene communications: use short, clear transmissions; identify the survival craft and its position; follow the OSC's instructions; conserve battery power by keeping transmissions brief.

121.5 MHz is the aeronautical emergency frequency used for homing by SAR aircraft; the survival craft radio may transmit on 121.5 MHz to attract attention, but it is not a general working channel.

Portable VHF operation: extend the antenna fully, hold the radio upright, and use the highest power setting only when necessary; keep the radio dry and warm to preserve battery life.

On-scene coordination: the survival craft should maintain a radio watch on Ch 16 and the designated on-scene frequency, and should report its position, number of persons, and condition to the OSC.

Emergency power: survival craft radios are independent of the ship's power; they must be stowed in the survival craft or in a readily accessible location, and their batteries must be maintained.

The ship radio station must be licensed by the flag administration; the licence specifies the equipment, frequencies, and operating conditions, and must be kept on board.

The Restricted Operator's Certificate (ROC) authorizes operation of GMDSS equipment on vessels operating in Sea Area A1 (and, where endorsed, other areas); the operator must hold a valid certificate and comply with its scope.

Watchkeeping: vessels must maintain a continuous radio watch on the appropriate distress and safety frequencies (VHF Ch 16 and DSC Ch 70 in Sea Area A1, plus NAVTEX/MSI reception) as required by SOLAS and the flag administration.

Radio log: a radio log (or official logbook) must record all distress, urgency, and safety communications, watch periods, equipment tests, battery checks, and any malfunctions; entries must be timed and signed.

Silence and priority: distress communications have absolute priority; stations must not transmit on a distress frequency except for distress, urgency, or safety purposes, and must observe radio silence when required.

False alerts: a false distress alert must be cancelled immediately by DSC and radiotelephony, and reported to the coast station; failure to cancel may result in unnecessary SAR operations and penalties.

Operator obligations: maintain a listening watch, respond to distress alerts when able, keep the station in good order, and report equipment failures to the master and the coast station.

Licensing and inspection: the radio installation is subject to survey and periodic inspection; certificates and logs must be available for inspection by authorized officials.

Batteries: GMDSS reserve batteries must be tested at intervals (typically weekly for voltage/electrolyte, and annually for capacity); keep terminals clean and tight, and ensure the battery is charged per the manufacturer's instructions.

Battery types: lead-acid batteries require distilled water top-up and ventilation; sealed/gel batteries require no topping up but must not be overcharged; record all checks in the radio log.

Reserve source capacity: VHF installations must have a reserve source capable of operating for at least 1 hour (6 hours for certain A1/A2 ships with emergency generators); verify the changeover switch works.

Basic fault-finding: check power supply (fuse, breaker, battery switch), antenna connections (corrosion, loose connectors), and microphone/handset; many faults are due to poor connections or discharged batteries.

Antenna and cable: inspect the VHF antenna and coaxial cable for corrosion, water ingress, and physical damage; a high SWR or poor range often indicates a faulty antenna or cable.

DSC faults: if DSC calls fail, check the MMSI is correctly entered, the position input is connected, and the DSC controller is powered; a common error is an unprogrammed or incorrect MMSI.

Equipment testing: perform daily/weekly tests (DSC test call, radiotelephony test on Ch 16 with a coast station or another vessel, NAVTEX self-test, EPIRB/SART self-test) and record results.

Reporting and repair: equipment failures must be reported to the master and, where required, to the coast station; repairs should be carried out by qualified personnel, and the operator should not attempt repairs beyond their scope.

GMDSS is an IMO-mandated, SOLAS-based system: it combines satellite and terrestrial communications so that distress alerts can be sent automatically and shore-side SAR authorities can be alerted and coordinated; it also provides MSI broadcasts and locating signals.

Sea Area A1: within VHF coverage of at least one coast station providing continuous DSC alerting (typically about 20–30 nautical miles offshore). A1-only vessels must carry VHF with DSC, a 406 MHz EPIRB (or equivalent satellite alerting), NAVTEX or another MSI receiver, and a SART (one per vessel, two on vessels of 500 GT and above).

Sea Area A2: beyond A1 but within MF coverage of coast stations with continuous DSC alerting (approximately 100–150 nm). Requires MF/HF DSC or Inmarsat, plus the A1 equipment.

Sea Area A3: beyond A1 and A2, within coverage of a recognized mobile-satellite service (Inmarsat) supporting alerting (roughly 70°N–70°S). Requires an Inmarsat ship earth station or MF/HF DSC with NBDP, plus A1/A2 equipment.

Sea Area A4: the remaining sea areas (polar regions outside Inmarsat coverage). Requires HF DSC and NBDP radiotelegraphy capability on the 4/6/8/12/16 MHz bands, in addition to A1–A3 equipment.

A vessel is rated for the highest sea area it operates in; an A1-only certificate limits operation to within VHF DSC coast-station coverage, and the operator's endorsement (Restricted Operator's Certificate) applies accordingly.

Carriage is certified by the flag administration: the ship's radio licence, the Safe Manning Document, and the GMDSS Radio Survey/Record of Equipment define the required installation, power supplies, and duplication for the area of operation.

Sources of energy: VHF installations must have a reserve source (battery) capable of powering the installation for at least 1 hour (6 hours for A1/A2 ships with emergency generator arrangements per SOLAS); batteries must be tested and logged.

Channel 16 (156.800 MHz) is the international distress, safety, and calling channel; it is monitored for distress, urgency, and safety, and routine working traffic must move to a working channel after initial contact.

Channel 70 (156.525 MHz) is reserved exclusively for DSC; voice transmission on Channel 70 is prohibited because it would interfere with digital alerting.

Working channels: Ch 6 (intership/safety), Ch 13 (bridge-to-bridge navigation safety), Ch 9 (marina/port operations in some areas), Ch 12/14 (port operations), Ch 67 (small craft/safety in some regions), Ch 15/17 (onboard communications, low power).

Distress call and message: 'MAYDAY, MAYDAY, MAYDAY' + vessel name/call sign (repeated three times), then MAYDAY + name, position, nature of distress, assistance required, persons aboard, and any other useful information; end with 'OVER'.

Urgency: 'PAN PAN' (three times) for urgent messages concerning the safety of a vessel or person but not immediate distress; Safety: 'SECURITE' (three times) for important navigational or meteorological warnings.

VHF DSC distress alert: lift the protective cover, press and hold the DISTRESS button for about 5 seconds; the alert transmits on Ch 70 with the vessel's MMSI and position (if a position source is connected). Wait for acknowledgement, then switch to Ch 16 for the distress traffic.

DSC acknowledgement: a coast station or another station acknowledges the distress alert on Ch 70; if no acknowledgement is received, the alert may be retransmitted after a short interval, and the distress should also be broadcast by radiotelephony on Ch 16.

False alerts: cancel a false DSC distress alert by transmitting a DSC 'distress cancellation' on Ch 70 to all stations, then announce the false alert on Ch 16; log the event and report to the coast station.

Alerting vs. coordination: the DSC alert (Ch 70) or EPIRB alerts SAR authorities; the subsequent radiotelephony distress traffic on Ch 16 provides the details and coordinates the response.

Distress traffic is controlled by the SAR authority (RCC) or the on-scene coordinator (OSC); other stations must maintain silence unless they can assist, and must not transmit on the distress channel during the emergency.

MAYDAY RELAY: used by a station (e.g., a ship or coast station) to relay a distress message on behalf of another station that cannot transmit; the relay is prefixed 'MAYDAY RELAY' three times and includes the original station's details.

On-scene communications: use Ch 16 for initial contact, then move to a working channel (often Ch 6 or a designated channel) for coordination between the OSC, aircraft, and assisting vessels; keep transmissions short and follow the OSC's instructions.

SAR coordination: the RCC may designate an OSC (usually the first vessel on scene or a SAR unit); the OSC coordinates search patterns, communications, and rescue operations, and reports to the RCC.

Urgency (PAN PAN) and safety (SECURITE) messages are broadcast on Ch 16 (or a working channel) and may be preceded by DSC calls on Ch 70 to alert specific stations or all stations.

Silence periods: during distress traffic, stations must observe radio silence on the distress channel; the MAYDAY call and message have priority over all other transmissions.

Logging: distress, urgency, and safety communications, including times, frequencies, and stations involved, must be recorded in the radio log (or official logbook).

NAVTEX is a narrow-band direct-printing (NBDP) system transmitting MSI on 518 kHz (international, English) and 490 kHz (national, local language) plus 4209.5 kHz for some regions; coverage is typically 200–400 nautical miles from the transmitter.

Message categories: A = navigational warnings, B = meteorological warnings, C = ice reports, D = search and rescue information, E = meteorological forecasts, F = pilot service messages, G = AIS messages, H = Loran-C messages, I = GNSS messages, J = SATNAV messages, K = other electronic navaid messages, L = additional navigational warnings, V = notice to mariners, X = special services, Z = no message.

Each NAVTEX message begins with a header: station identifier (letter), message type (letter), and a sequential message number (two digits); the receiver can be programmed to accept or reject specific stations and message types.

NAVTEX broadcasts are scheduled at fixed times (typically every 4 hours for scheduled broadcasts, with unscheduled broadcasts for urgent warnings); the schedule is published in the Admiralty List of Radio Signals and on the NAVTEX receiver's station list.

MSI includes navigational warnings, meteorological warnings and forecasts, SAR information, and other urgent safety information; it is broadcast via NAVTEX, SafetyNET (Inmarsat), and HF NBDP.

NAVTEX receivers must be capable of receiving 518 kHz and, where required, 490 kHz; the receiver should be tested and the message log checked regularly, and received messages should be retained for the required period.

SafetyNET (Inmarsat Enhanced Group Call) provides MSI in Sea Areas A3 and A4 (and A2 where NAVTEX is unavailable); it uses a dedicated receiver or an Inmarsat ship earth station.

Receiving MSI: operators must ensure the NAVTEX/SafetyNET receiver is switched on and tuned, monitor the message log, and act on navigational and meteorological warnings relevant to the vessel's route.

406 MHz EPIRB: transmits a digital distress alert to the COSPAS-SARSAT satellite system, including the vessel's identity (MMSI/hex ID) and, if interfaced, position; it also transmits a 121.5 MHz homing signal for SAR aircraft and vessels.

EPIRB registration: the beacon must be registered with the flag administration (or the administration of the vessel's country) with the vessel's details and emergency contacts; registration must be updated when the vessel or ownership changes.

EPIRB testing: perform the self-test (usually a short button press) monthly and after any activation; do not activate the beacon accidentally - a false activation must be reported to the coast station and the beacon switched off.

EPIRB battery and HRU: the battery has a specified replacement interval (typically 5 years, marked on the label); the hydrostatic release unit (HRU) must be replaced by its expiry date, and the beacon must be mounted in a float-free position.

SART (Search and Rescue Radar Transponder): operates in the 9 GHz (X-band) radar band; when interrogated by a radar, it responds with a series of dots on the radar display, allowing the vessel to be located.

SART operation: switch on the SART when abandoning ship or when the vessel is in distress; it has a range of about 5–10 nautical miles depending on radar height; the SART should be mounted at least 1 metre above sea level in the survival craft.

SART testing: use the built-in test function (usually a button) to verify operation; do not test near other vessels' radars as it may cause interference; the SART battery has a specified life (typically 96 hours standby/8 hours operation).

AIS-SART: an alternative to the radar SART, transmitting an AIS message that appears on AIS displays; it is carried in addition to or instead of a radar SART depending on the vessel's equipment.

Survival craft VHF: a portable, battery-powered VHF radiotelephone (typically 121.5/156.8 MHz or dedicated channels) carried in survival craft for on-scene communications with SAR units; it must be waterproof and capable of operating in the marine band.

Channel 16 is used for distress and on-scene communications; the survival craft radio should be capable of transmitting on Ch 16 and at least one working channel, and often includes 121.5 MHz for homing.

Battery: the survival craft radio battery must have sufficient capacity for the required operating period (typically 8 hours or as specified); batteries should be checked and replaced per the manufacturer's schedule.

Two-way on-scene communications: use short, clear transmissions; identify the survival craft and its position; follow the OSC's instructions; conserve battery power by keeping transmissions brief.

121.5 MHz is the aeronautical emergency frequency used for homing by SAR aircraft; the survival craft radio may transmit on 121.5 MHz to attract attention, but it is not a general working channel.

Portable VHF operation: extend the antenna fully, hold the radio upright, and use the highest power setting only when necessary; keep the radio dry and warm to preserve battery life.

On-scene coordination: the survival craft should maintain a radio watch on Ch 16 and the designated on-scene frequency, and should report its position, number of persons, and condition to the OSC.

Emergency power: survival craft radios are independent of the ship's power; they must be stowed in the survival craft or in a readily accessible location, and their batteries must be maintained.

The ship radio station must be licensed by the flag administration; the licence specifies the equipment, frequencies, and operating conditions, and must be kept on board.

The Restricted Operator's Certificate (ROC) authorizes operation of GMDSS equipment on vessels operating in Sea Area A1 (and, where endorsed, other areas); the operator must hold a valid certificate and comply with its scope.

Watchkeeping: vessels must maintain a continuous radio watch on the appropriate distress and safety frequencies (VHF Ch 16 and DSC Ch 70 in Sea Area A1, plus NAVTEX/MSI reception) as required by SOLAS and the flag administration.

Radio log: a radio log (or official logbook) must record all distress, urgency, and safety communications, watch periods, equipment tests, battery checks, and any malfunctions; entries must be timed and signed.

Silence and priority: distress communications have absolute priority; stations must not transmit on a distress frequency except for distress, urgency, or safety purposes, and must observe radio silence when required.

False alerts: a false distress alert must be cancelled immediately by DSC and radiotelephony, and reported to the coast station; failure to cancel may result in unnecessary SAR operations and penalties.

Operator obligations: maintain a listening watch, respond to distress alerts when able, keep the station in good order, and report equipment failures to the master and the coast station.

Licensing and inspection: the radio installation is subject to survey and periodic inspection; certificates and logs must be available for inspection by authorized officials.

Batteries: GMDSS reserve batteries must be tested at intervals (typically weekly for voltage/electrolyte, and annually for capacity); keep terminals clean and tight, and ensure the battery is charged per the manufacturer's instructions.

Battery types: lead-acid batteries require distilled water top-up and ventilation; sealed/gel batteries require no topping up but must not be overcharged; record all checks in the radio log.

Reserve source capacity: VHF installations must have a reserve source capable of operating for at least 1 hour (6 hours for certain A1/A2 ships with emergency generators); verify the changeover switch works.

Basic fault-finding: check power supply (fuse, breaker, battery switch), antenna connections (corrosion, loose connectors), and microphone/handset; many faults are due to poor connections or discharged batteries.

Antenna and cable: inspect the VHF antenna and coaxial cable for corrosion, water ingress, and physical damage; a high SWR or poor range often indicates a faulty antenna or cable.

DSC faults: if DSC calls fail, check the MMSI is correctly entered, the position input is connected, and the DSC controller is powered; a common error is an unprogrammed or incorrect MMSI.

Equipment testing: perform daily/weekly tests (DSC test call, radiotelephony test on Ch 16 with a coast station or another vessel, NAVTEX self-test, EPIRB/SART self-test) and record results.

Reporting and repair: equipment failures must be reported to the master and, where required, to the coast station; repairs should be carried out by qualified personnel, and the operator should not attempt repairs beyond their scope.