NIMASA GMDSS GOC Exam cheat sheet

GMDSS is a global system using terrestrial and satellite communications to ensure distress alerting, SAR coordination, and MSI dissemination; it covers all sea areas A1–A4.

Functional elements: distress alerting, SAR coordination, distress and safety communications, MSI, and general radiocommunications (including bridge-to-bridge).

Sea area A1: within VHF coverage of a coast station with continuous DSC alerting (typically ~20–30 NM).

Sea area A2: beyond A1, within MF coverage of a coast station with continuous DSC alerting (typically ~100–150 NM).

Sea area A3: beyond A1 and A2, within coverage of an Inmarsat geostationary satellite (approx. 70°N–70°S).

Sea area A4: remaining sea areas outside A1, A2, and A3 (mainly polar regions).

Distress alert must be sent by the equipment available; if no response, try other equipment or relay via another station.

GMDSS equipment is powered from the ship's main supply and a dedicated reserve source (battery) for specified duration.

The GMDSS obligates ships to render assistance; the master retains ultimate responsibility for distress communications.

All ships on international voyages must carry VHF with DSC (Ch 70), and at least two SARTs (one per side) and two portable VHF radios for survival craft (if applicable).

Ships in A1: VHF DSC, NAVTEX (or EGC), SART, EPIRB (any type), and reserve power.

Ships in A1–A2: add MF/HF DSC and radiotelephony, or Inmarsat if A3.

Ships in A1–A3: Inmarsat-C (or Fleet) or MF/HF with DSC and telex, plus EGC receiver (or NAVTEX if within NAVTEX service).

Ships in A1–A4: MF/HF DSC and radiotelephony/telex with full frequency coverage, plus NAVTEX/EGC.

Cargo ships 300–500 GT: at least one SART and two portable VHF; 500 GT and above: two SARTs and three portable VHF.

Passenger ships: additional requirements including two-way VHF for each fast rescue boat and increased reserve power duration.

Reserve source for cargo ships: 6 hours for emergency power (or 1 hour if emergency generator available).

EPIRB: float-free, automatically activated, 406 MHz with 121.5 MHz homing; must be registered with the national authority.

Frequency bands: VHF 156–174 MHz; MF 300–3000 kHz; HF 3–30 MHz; Inmarsat L-band 1.5/1.6 GHz.

DSC distress frequencies: VHF Ch 70 (156.525 MHz), MF 2187.5 kHz, HF 4207.5, 6312, 8414.5, 12577, 16804.5 kHz.

Radiotelephony distress: VHF Ch 16 (156.8 MHz), MF 2182 kHz, HF 4125, 6215, 8291, 12290, 16420 kHz.

Ground wave propagation follows the Earth's curvature; MF ground wave range is typically 100–150 NM.

Skywave propagation via ionospheric refraction (F layer) enables HF long-distance; range varies with frequency, time of day, and solar activity.

VHF propagation is essentially line-of-sight; range ≈ 1.23 × √(antenna height in feet) nautical miles to horizon.

Frequency and wavelength: λ (m) = 300 / f (MHz); e.g., 156.8 MHz → λ ≈ 1.91 m.

Modulation: VHF/MF/HF voice uses AM (A3E) or SSB (J3E for HF); DSC uses FSK (F1B).

Antenna length for quarter-wave: L (m) = 75 / f (MHz); for half-wave: L = 150 / f.

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

DSC distress alert format: nature of distress, ship's MMSI, position, time, and communication mode (R/T or telex).

DSC distress alert must be sent on one frequency; if no acknowledgment after 5 minutes, repeat or try another frequency.

DSC acknowledgment (distress) is sent by coast station or ship in vicinity; it includes MMSI of acknowledging station.

After DSC distress alert, follow with distress message on associated R/T frequency (e.g., Ch 16 after Ch 70).

MF/HF DSC uses specific frequencies; watchkeeping on 2187.5 kHz (MF) and 8414.5 kHz (HF) is mandatory for A2/A3/A4.

DSC routine calls: use Ch 70 for VHF, 2177 kHz for MF, and appropriate HF DSC frequencies; follow with working channel.

Radiotelephony distress signal: MAYDAY (spoken); urgency: PAN PAN; safety: SÉCURITÉ.

DSC test calls should be addressed to a coast station and use the test format; avoid Ch 70 for routine voice.

Inmarsat provides global satellite coverage except polar regions beyond ~70°N/S; used for A3.

Inmarsat-C: store-and-forward, low-speed data, used for distress alerting, EGC, and two-way messaging.

Inmarsat Fleet (F77, F55, F33): voice and data; FleetBroadband for higher speeds.

Distress alert via Inmarsat-C: press distress button, select nature of distress, position, and send; includes priority 3.

Inmarsat-C EGC (Enhanced Group Call) receives SafetyNET and FleetNET messages; SafetyNET for MSI.

Inmarsat-C terminal must have a dedicated distress button and be connected to a GNSS for position updating.

Inmarsat ocean regions: Atlantic Ocean Region-East/West, Indian Ocean, Pacific Ocean; each has a satellite.

Satellite EPIRB (Inmarsat-E) is no longer supported; 406 MHz COSPAS-SARSAT is the primary satellite EPIRB system.

Inmarsat-C LES (Land Earth Station) provides distress alert routing to RCCs; two LESs must be accessible.

NAVTEX: 518 kHz international, 490 kHz national, 4209.5 kHz HF; uses narrow-band direct printing (NBDP).

NAVTEX message format: B1 (station ID), B2 (message type), B3 (serial), B4 (time), then text; each message has a header.

NAVTEX message types: A=Navigational warnings, B=Meteorological, C=Ice, D=SAR, E=Weather forecast, F=Pilot, etc.

NAVTEX receiver should be set to receive all relevant message types; avoid rejecting type D (distress).

SafetyNET is Inmarsat EGC service for MSI; used in areas outside NAVTEX coverage (e.g., A3).

SafetyNET messages are addressed to fixed areas (circular or rectangular); receiver must be configured with correct area.

NAVTEX coverage is typically 200–400 NM from coast; beyond that, use SafetyNET or HF NBDP.

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

NAVTEX receiver must be able to print or display messages; paper printouts are not mandatory if display is available.

Distress: MAYDAY (spoken), SOS (Morse), DSC distress alert; indicates grave and imminent danger requiring immediate assistance.

Urgency: PAN PAN; concerning safety of a ship or person but not immediate distress.

Safety: SÉCURITÉ; concerning navigational safety or important meteorological warnings.

Distress message contents: MAYDAY, ship's name/call sign/MMSI, position, nature of distress, assistance required, number of persons, etc.

Distress traffic has absolute priority; all stations must cease transmissions that may interfere.

Silence periods: 00–03 and 30–33 minutes on 500 kHz (historical) and on 2182 kHz (00–03, 30–33) for distress watch.

DSC distress relay: if unable to communicate with coast station, another station may relay the alert.

On receiving a distress alert, keep watch on the associated frequency and acknowledge if not acknowledged by coast station.

False distress alerts must be cancelled immediately by transmitting a DSC cancellation and a voice message on the associated R/T channel.

406 MHz EPIRB: transmits digital distress alert to COSPAS-SARSAT; includes MMSI, position, and nature of distress.

121.5 MHz homing transmitter on EPIRB allows SAR aircraft to home in; newer EPIRBs also use 406 MHz for location.

EPIRB activation: manual or automatic (float-free) when submerged; test using built-in test function only.

EPIRB registration: must be registered with national authority; update if ownership or vessel details change.

SART (Search and Rescue Radar Transponder): operates in 9 GHz (X-band) marine radar band; responds to radar interrogation.

SART range: typically 5–10 NM depending on radar height; displays as a line of 12 dots on radar screen.

SART battery: typically 96 hours standby, 8 hours operational; check expiry date.

SART should be mounted at least 1 m above sea level in survival craft; do not operate inside a liferaft canopy.

AIS-SART: transmits AIS messages, shown on AIS display; alternative to radar SART.

Reserve battery capacity must be sufficient for 1 hour (with emergency generator) or 6 hours (without) for cargo ships; passenger ships 36 hours.

Battery types: lead-acid (vented or VRLA), NiCd; check electrolyte level and specific gravity for vented lead-acid.

Battery maintenance: keep terminals clean and tight, charge regularly, avoid deep discharge, ventilate compartment.

Antenna maintenance: inspect for corrosion, loose connections, and physical damage; SWR should be checked periodically.

VHF antenna: vertically polarized, mounted as high as possible; coaxial cable impedance 50 ohms.

MF/HF antenna: often long wire or whip; tuner matches impedance; insulation and insulators must be intact.

Emergency power: must be independent of main power; automatic switchover within 30 seconds (or manual).

Fault finding: check power supply, fuses, antenna connections, and equipment settings before assuming equipment failure.

Battery testing: use load test or specific gravity; record results in radio log.

Radio license: ship station license issued by flag state; operator certificate (GOC) required for GMDSS operations.

Watchkeeping: continuous DSC watch on VHF Ch 70, MF 2187.5 kHz, and HF 8414.5 kHz (and others as required).

Radio log: must record distress, urgency, safety communications, and equipment tests; keep for 3 years.

Interference: report harmful interference to coast station or administration; avoid causing interference.

Frequency use: transmit only on authorized frequencies; use minimum power necessary.

Calling procedures: identify station with call sign/MMSI; use standard phrases; keep calls brief.

Silence periods: observe on 2182 kHz (00–03, 30–33) for distress; avoid transmissions during these periods.

Distress frequencies must be kept clear; do not test on distress frequencies.

Licensing: GMDSS operator certificate (GOC) required for vessels equipped with GMDSS; ROC for restricted.