GOC GMDSS Exam cheat sheet

Distress: MAYDAY (spoken) / MAYDAY on DSC and radiotelephony; indicates grave and imminent danger and immediate assistance required; absolute priority over all other traffic.

Urgency: PAN PAN; concerns safety of a vessel or person but no immediate danger; used for medical advice, broken-down vessel not in danger, or important navigational warning.

Safety: SÉCURITÉ; used for short important navigational or meteorological warnings and announcements of MSI broadcasts.

DSC distress alert format: nature of distress, position (lat/long or if unavailable), time, subsequent communication mode (radiotelephony); sent on VHF Ch 70, MF 2187.5 kHz or HF distress frequencies.

Distress alert acknowledgement: coast station acknowledges by DSC; ship stations normally do not acknowledge but may do so if no coast station responds within 5 minutes and distress traffic is not already coordinated.

MAYDAY RELAY is used when a station other than the one in distress transmits the distress message on its behalf (e.g. a ship relaying for an EPIRB activation).

Silence periods: 00-03 and 30-33 minutes past each hour on 500 kHz (historic) and radiotelephony silence periods on VHF Ch 16 are 00-03 and 30-33; distress traffic takes precedence.

On receipt of a distress alert from a coast station or RCC, ships should maintain radio watch on the indicated frequency and, if able, proceed to assist.

Cancel a false distress alert immediately by DSC or radiotelephony stating 'false alert' and inform the nearest coast station or RCC.

GMDSS functional requirements: alerting, SAR coordination, on-scene communications, locating, MSI, and general radiocommunications (bridge-to-bridge).

Sea area A1: coverage by VHF coast stations with continuous DSC alerting (approx. 20-30 nautical miles).

Sea area A2: coverage by MF coast stations with continuous DSC alerting, excluding A1 (approx. 100-150 nautical miles).

Sea area A3: coverage by an Inmarsat geostationary satellite with continuous alerting, excluding A1 and A2 (approx. 70°N to 70°S).

Sea area A4: remaining sea areas outside A1, A2 and A3, mainly polar regions; requires HF DSC and/or additional satellite coverage.

Equipment carriage by sea area: A1 requires VHF DSC, NAVTEX (if available), EPIRB, SART and reserve power; A2 adds MF DSC; A3 adds Inmarsat or HF; A4 adds HF DSC and HF radiotelephony.

Distress alerting paths: VHF Ch 70 DSC, MF 2187.5 kHz DSC, HF 4207.5/6312/8414.5/12577/16804.5 kHz DSC, Inmarsat, and 406 MHz EPIRB (COSPAS-SARSAT).

The GMDSS uses both terrestrial (DSC, NAVTEX, VHF/MF/HF) and satellite (Inmarsat, COSPAS-SARSAT) subsystems, providing redundancy.

A1: VHF coverage; A2: MF coverage; A3: Inmarsat coverage between 70°N and 70°S; A4: polar areas beyond satellite coverage.

All ships in all sea areas must carry a 406 MHz EPIRB, at least one SART (two on cargo ships ≥500 GT and passenger ships), and VHF radiotelephony with Ch 16.

A1-only ships: VHF DSC (Ch 70), NAVTEX receiver if in NAVTEX coverage, EPIRB, SART, and reserve source of energy.

A2 ships: add MF/HF DSC watch on 2187.5 kHz and MF radiotelephony on 2182 kHz.

A3 ships: add Inmarsat ship earth station (or HF DSC as alternative) capable of distress alerting and MSI reception (EGC/SafetyNET).

A4 ships: HF DSC and HF radiotelephony for distress alerting and watchkeeping on all HF distress frequencies.

Reserve source of energy must supply the radio installation for at least 1 hour (or 6 hours for ships with emergency generator) and be independent of the main and emergency generators.

Passenger ships and cargo ships ≥300 GT on international voyages must comply with SOLAS Chapter IV GMDSS carriage requirements.

VHF Ch 16 is the distress, safety and calling frequency (156.800 MHz); Ch 70 is DSC only (no voice); Ch 13 is bridge-to-bridge.

MF distress and calling: 2182 kHz radiotelephony; MF DSC: 2187.5 kHz.

HF distress radiotelephony: 4125, 6215, 8291, 12290, 16420 kHz; HF DSC: 4207.5, 6312, 8414.5, 12577, 16804.5 kHz.

DSC distress alert contains: format specifier (distress), nature of distress, position, time, and telecommand (radiotelephony).

DSC routine call: format (individual/group/all ships), address (MMSI), category (routine/safety/urgency), and proposed working channel.

MMSI structure: ship station = MID + 6 digits; coast station = 00 + MID + 4 digits; group = 0 + MID + 5 digits; SAR aircraft = 111 + MID + 4 digits.

DSC watchkeeping: VHF Ch 70 continuous; MF 2187.5 kHz continuous; HF distress frequencies scanned when HF DSC is fitted.

Radiotelephony distress procedure: MAYDAY ×3, THIS IS, station name/callsign/MMSI, MAYDAY, position, nature of distress, assistance required, persons on board, other information, OVER.

Inmarsat provides GMDSS services in sea area A3: distress alerting, MSI (SafetyNET/EGC), and general communications.

Inmarsat-C is a store-and-forward messaging terminal with a small omnidirectional antenna; supports distress priority (Priority 3) and EGC reception.

Inmarsat Fleet services (Fleet 77, FleetBroadband) provide voice and data; Fleet 77 supports GMDSS distress priority and is approved for A3.

Inmarsat-C distress alert is sent by selecting 'Distress' priority and including position, nature of distress and course/speed; it is routed to an RCC.

COSPAS-SARSAT is a separate satellite system for 406 MHz EPIRB detection, providing global coverage including polar regions (A4).

Inmarsat ocean regions: Atlantic Ocean Region (AOR-E, AOR-W), Indian Ocean Region (IOR), Pacific Ocean Region (POR); a terminal must be logged into the correct region.

EGC (Enhanced Group Call) via SafetyNET delivers MSI to Inmarsat terminals, including NAVAREA, METAREA, coastal warnings and SAR messages.

Iridium and other satellite systems may be used for non-GMDSS communications but are not part of the GMDSS satellite distress alerting architecture unless approved.

MSI includes navigational warnings, meteorological warnings and forecasts, SAR information, and other urgent safety information.

NAVTEX broadcasts on 518 kHz (international, English) using a dedicated narrow-band direct-printing receiver; 490 kHz is used for national language broadcasts.

NAVTEX message format: B1 (station ID), B2 (message type), B3/B4 (serial number), then text; message types include navigational (A), meteorological (B), ice (C), SAR (D).

NAVTEX receiver should be programmed with the correct station and message type selection; it prints messages automatically and stores them.

SafetyNET (Inmarsat EGC) covers ocean areas and NAVAREAs; it requires an Inmarsat terminal with EGC capability.

NAVAREAs are geographic areas for navigational warnings coordinated by a NAVAREA coordinator; METAREAs are for meteorological information.

MSI reception is a GMDSS functional requirement for all sea areas; ships must be able to receive MSI in the area in which they operate.

NAVTEX coverage is typically 200-300 nautical miles from the transmitter; outside coverage, SafetyNET or HF MSI (e.g. 518 kHz via HF NBDP) is used.

406 MHz EPIRB: transmits a digital distress message to COSPAS-SARSAT with the vessel's identity (MMSI/hex ID) and, if GPS-equipped, position; detected globally.

EPIRB also transmits a 121.5 MHz homing signal for SAR aircraft and vessels to locate the beacon.

EPIRB activation: manually or automatically by hydrostatic release when the vessel sinks; must float free and activate.

EPIRB registration: must be registered with the national authority; registration includes vessel details, MMSI and emergency contacts.

SART (Search and Rescue Transponder): responds to 9 GHz (X-band) radar, producing a series of dots on the radar display; range typically 5-10 nautical miles.

SART battery: typically 96 hours standby and 8 hours continuous operation; test using the built-in test function, not by activating in port.

AIS-SART: transmits AIS messages (Message 1 and 14) indicating a SART in distress; displayed on AIS-equipped vessels and shore stations; no radar response.

EPIRB testing: follow manufacturer instructions; test only during permitted periods (e.g. first 5 minutes of the hour) to avoid false alerts.

MF (2182 kHz) uses ground wave for short range (up to ~150 nm) and sky wave at night for longer range; MF propagation is more reliable at night.

HF sky wave propagation depends on ionospheric layers (D, E, F1, F2); F2 layer supports long-distance communication, especially during daylight.

Frequency selection: use higher HF frequencies during daylight and lower frequencies at night; MUF (maximum usable frequency) and LUF (lowest usable frequency) define usable bands.

VHF propagation is essentially line-of-sight; range depends on antenna height (radio horizon ≈ 1.23 × √height in feet, or 4.12 × √height in metres for nautical miles).

Antenna types: whip/vertical for VHF/MF/HF, dipole, and directional antennas; VHF antennas should be mounted as high as possible and kept clear of obstructions.

EMC: keep transmitting antennas away from other antennas, GPS receivers and sensitive electronics; use proper grounding and cable routing to avoid interference.

SWR (standing wave ratio) should be low (ideally <1.5:1); high SWR indicates antenna or cable fault and reduces radiated power.

DSC on VHF Ch 70 requires a dedicated antenna or a properly designed splitter; poor antenna isolation can cause interference.

Reserve source of energy must be independent of main and emergency generators and capable of supplying the radio installation for at least 1 hour (6 hours if no emergency generator).

Battery types: lead-acid (vented or sealed VRLA), NiCd, and lithium; GMDSS reserve batteries are typically lead-acid or VRLA.

Battery maintenance: check electrolyte level (vented lead-acid), specific gravity, terminal cleanliness, and charge voltage; equalize periodically.

Battery capacity test: discharge test at the manufacturer's specified rate; capacity should be at least 80% of rated capacity.

Daily tests: DSC self-test, VHF/MF/HF radiotelephony test (low power, dummy load or short test call), EPIRB and SART self-test (per manufacturer).

Weekly/monthly tests: battery voltage and charger operation; antenna and cable inspection; emergency power changeover test.

Fault-finding: check power supply, fuses, antenna connections, and SWR; use built-in test facilities and diagnostic menus before replacing equipment.

GMDSS equipment must be type-approved and maintained to ensure availability; spares and documentation should be carried as required.

SOLAS Chapter IV governs GMDSS carriage requirements; the ITU Radio Regulations govern frequencies and operating procedures.

Ship radio station licence: issued by the flag state; must be on board and available for inspection; includes call sign and MMSI.

Operator certificates: GMDSS General Operator's Certificate (GOC) for A1-A4; Restricted Operator's Certificate (ROC) for A1 only; certificate must be on board.

Watchkeeping: continuous DSC watch on VHF Ch 70, MF 2187.5 kHz, and HF distress frequencies as required by sea area; VHF Ch 16 listening watch.

Radio log (GMDSS logbook): record distress, urgency and safety communications, position reports, equipment tests, and watchkeeping details; retain as required by flag state.

Distress communications have absolute priority; all stations must cease transmissions that may interfere with distress traffic.

False alerts must be cancelled immediately and reported to the nearest coast station or RCC; repeated false alerts may lead to penalties.

Confidentiality: communications secrecy must be maintained; do not disclose the contents of intercepted messages except for distress or safety purposes.