FCC GMDSS Maintainer cheat sheet

GMDSS sea areas: A1 within VHF DSC range of a coast station (nominally ~20-30 NM); A2 beyond A1 but within MF DSC range (~100-150 NM); A3 beyond A1/A2 within Inmarsat coverage (approx. 70°N-70°S); A4 outside A1-A3 (polar regions).

Carriage requirements are functional, not equipment-specific: SOLAS Chapter IV requires means to transmit ship-to-shore distress alerts by at least two independent means using different radiocommunication services.

SOLAS applies to passenger ships and cargo ships of 300 GT and above on international voyages; GMDSS full implementation date was 1 February 1999, with A4 remaining a mixed HF/MF environment.

Certificate structure: GOC (General Operator's Certificate) for A1-A3/A4 operation, ROC (Restricted Operator's Certificate) for A1 only, and the maintainer-level certificate covering installation, servicing and repair.

Distress, urgency and safety priority order: distress (highest), urgency, safety, then routine; DSC uses distress alert, all-ships, group, individual and geographic area call formats.

Radio Regulations define the distress frequencies: 2187.5 kHz (MF DSC), 8414.5 kHz plus 4207.5/6312/12577/16804.5 kHz (HF DSC), 156.525 MHz (VHF Ch 70 DSC), 406 MHz (EPIRB COSPAS-SARSAT).

False alert procedure: if a distress alert is transmitted inadvertently, immediately transmit a distress cancellation on the associated radiotelephony/NBDP frequency and notify the relevant coast station or RCC.

The maintainer is responsible for ensuring the installation meets SOLAS IV/15 performance standards; periodical surveys and the ship's Safety Radio Certificate depend on documented maintenance capability.

Distress call and message: MAYDAY (spoken three times), THIS IS, ship name/call sign/MMSI, position, nature of distress, assistance required, number of persons on board, any other information.

Urgency signal PAN PAN (three times) is used for urgent messages concerning safety of a ship or person; safety signal SECURITE (three times) precedes messages about navigational or meteorological warnings.

Distress relay: MAYDAY RELAY is used by a station not itself in distress to relay a distress message, or by a coast station acknowledging a distress alert on behalf of a ship.

Silence periods: 500 kHz (historic MF radiotelegraphy) and 2182 kHz radiotelephony are guarded for distress; 2182 kHz is the MF distress and calling frequency, with 2174.5 kHz used for NBDP distress traffic.

On VHF, Ch 16 is the distress, safety and calling channel; Ch 70 is DSC only and must never be used for voice; Ch 13 is bridge-to-bridge; Ch 06 is inter-ship.

Acknowledgment of a DSC distress alert should normally be made by a coast station or RCC; a ship station acknowledging should do so on the associated radiotelephony frequency and then assume control of distress traffic if appropriate.

On-scene communications: the On-Scene Coordinator (OSC) is normally the first ship to arrive, or a designated ship; SAR co-ordinators may use the SART/EPIRB homing signals to locate survivors.

Distress traffic must be kept brief and controlled by the station in control (normally the RCC or coast station); all other stations must maintain silence until the distress is terminated.

DSC is a digital selective calling system using FSK (frequency-shift keying) with a 170 Hz shift; the message format includes format specifier, address, category, type of distress/communication, position/time and a check sum.

VHF DSC (Ch 70) uses 1200 baud; MF/HF DSC uses 1200 baud with the same message structure but different transmission characteristics (e.g., 100 baud is used for some legacy NBDP, not DSC).

Distress alert format includes the nature of distress, position (from an associated GNSS), time, and the MMSI; a distress alert can be sent with a 'distress' key or a dedicated menu.

MMSI structure: ship stations begin with the MID (Maritime Identification Digits, e.g., 232-235 for the UK), followed by 6 digits; group stations and coast stations have distinct prefixes.

A DSC controller should be tested using the self-test or a test call to a coast station; routine test calls are addressed to a specific coast station and use the 'test' category.

DSC watchkeeping: VHF Ch 70 and MF/HF DSC frequencies must be continuously watched; the receiver scans the distress and calling frequencies, and an alarm sounds on receipt of a distress alert.

Distress alert retransmission: if no acknowledgment is received within 3.5-4.5 minutes, the alert may be retransmitted; a distress alert should be acknowledged by a coast station or RCC.

Common faults: no position input (check GNSS feed), incorrect MMSI (programmed in service mode by authorized personnel), and antenna/VSWR problems causing failed test calls.

MF/HF radiotelephony uses SSB (J3E) on 2182 kHz and the HF distress frequencies; VHF uses FM (G3E) on Ch 16 and working channels.

NBDP (Narrow-Band Direct Printing) uses FEC (forward error correction) for broadcast (e.g., MSI) and ARQ for point-to-point telex; it operates at 100 baud with a 170 Hz shift in the HF bands.

FEC modes: collective (CFEC) for broadcasts to all stations; selective (SFEC) for a specific group. ARQ requires a two-way link between two stations.

NBDP distress frequency: 2174.5 kHz (MF) and 8414.5 kHz (HF); NBDP is used for distress traffic and MSI broadcasts.

Radiotelephony modulation: SSB suppressed carrier (J3E) for MF/HF; FM (G3E) for VHF; AM (A3E) is used for aeronautical and some legacy services.

Transmitter power: MF/HF typically 150-400 W PEP; VHF 25 W (or 1 W low power); antenna tuning units (ATU) match the transmitter to the whip antenna.

Squelch and noise blanker settings affect receiver sensitivity; excessive squelch can mask weak distress signals; AGC should be on for SSB reception.

NBDP terminal equipment includes a keyboard, printer/display and modem; the modem converts digital data to audio tones for the transmitter.

Inmarsat services: Inmarsat-C (store-and-forward, GMDSS distress and MSI/SafetyNET), Inmarsat Fleet (Fleet 77/55/33, voice/data), and Inmarsat-B/M (legacy).

Inmarsat-C is a GMDSS requirement for A3; it provides distress alerting, two-way messaging, EGC (Enhanced Group Call) reception for SafetyNET, and position reporting.

Inmarsat satellites are geostationary; coverage is between approximately 70°N and 70°S, leaving polar regions (A4) outside coverage.

Inmarsat-C distress alert: press the distress button, select nature of distress, and the alert is sent to an RCC via the LES (Land Earth Station).

SafetyNET is the EGC service for MSI; it uses Inmarsat-C (and Fleet) to broadcast navigational and meteorological warnings, SAR information and other urgent safety information.

Inmarsat-C terminal setup requires the correct LES/ Ocean Region and the ship's Inmarsat-C ID; the terminal must be logged into the correct ocean region for EGC reception.

Fault finding: check antenna cable and LNA, verify the terminal is logged in, check the GPS input for position reporting, and confirm the LES is operational.

Inmarsat-C is not a real-time voice service; it uses store-and-forward messaging, so latency can be minutes, and it is unsuitable for live distress voice communications.

406 MHz EPIRBs transmit a digital distress message to COSPAS-SARSAT; the message includes the beacon's 15-hex-ID (or MMSI for some), position (if GNSS-equipped), and nature of distress.

EPIRB activation: manually or automatically by a hydrostatic release unit (HRU); the HRU releases the beacon at 1.5-4 m depth, and the beacon floats free and transmits.

EPIRB testing: use the self-test function; do not transmit a live distress alert during testing; check battery expiry date and HRU expiry date.

121.5 MHz is the homing frequency on EPIRBs (and some SARTs) used by SAR aircraft; 406 MHz is the satellite alerting frequency; 243 MHz is the military homing frequency.

SART (Search and Rescue Radar Transponder) operates in the 9 GHz (X-band) radar band; it responds to a radar interrogation with a series of 12 dots on the radar display, indicating range and bearing.

SART range is typically up to 5-10 NM depending on radar height and antenna; the SART should be mounted at least 1 m above sea level and switched on only when survivors are in the water.

SART battery life: typically 96 hours standby and 8 hours continuous operation; check the battery expiry date and test the SART using the built-in test function.

EPIRB registration: the beacon must be registered with the national authority; the registration data includes the vessel name, MMSI, owner and emergency contacts.

MSI (Maritime Safety Information) includes navigational warnings, meteorological warnings and forecasts, SAR information, and other urgent safety information.

NAVTEX: 518 kHz (international), 490 kHz (national), and 4209.5 kHz (HF); it uses a 100 baud FEC (forward error correction) broadcast with a 170 Hz shift.

NAVTEX message format: B1 (station ID), B2 (message type), B3/B4 (message number), and the message text, with a specific end-of-message sequence.

NAVTEX receiver setup: select the correct station, message types (A, B, D, etc.), and ensure the receiver is tuned to 518 kHz; the receiver should be left on continuously.

SafetyNET via Inmarsat-C EGC: the terminal must be logged into the correct ocean region and have the correct EGC receiver settings; messages are addressed to specific areas or all ships.

NAVTEX coverage is typically 200-400 NM from the transmitter; it is the primary MSI service for A1/A2 areas, while SafetyNET covers A3.

Message types: A = navigational warnings, B = meteorological warnings, C = ice reports, D = SAR information, E = meteorological forecasts, F = pilot service messages.

MSI reception must be logged and monitored; the receiver should be tested regularly, and the printer/display should be checked for correct operation.

GMDSS power requirements: a reserve source of energy (battery) must be provided for the radio installation, capable of operating the equipment for a specified period (e.g., 1 hour for emergency, 6 hours for normal operation depending on the ship).

Battery types: lead-acid (flooded or VRLA) and NiCd; check specific gravity (lead-acid) or voltage, and ensure the battery is charged and not sulfated.

Battery maintenance: check electrolyte level, terminals, and charging voltage; equalize charging for flooded lead-acid; avoid overcharging and deep discharge.

Antenna types: whip (MF/HF), vertical dipole, and VHF whip; the antenna must be mounted clear of obstructions and have a low VSWR (typically < 2:1).

VSWR measurement: use a directional wattmeter or SWR meter; high VSWR indicates a faulty antenna, cable, or connector, and can damage the transmitter.

Antenna tuning: MF/HF transmitters use an ATU to match the antenna impedance; the ATU must be tuned for each frequency, and a high VSWR may indicate a tuning fault.

Insulation and grounding: antenna insulators must be clean and dry; a good RF ground (e.g., copper strap to the hull) is essential for MF/HF operation.

Emergency power: the reserve battery must be independent of the main ship's power and be capable of powering the radio equipment, including the DSC and EPIRB (if applicable).

Installation: equipment must be installed per manufacturer instructions and SOLAS IV/15; cables should be routed away from high-power lines, and antennas should be mounted with proper clearance.

Periodic tests: daily tests of DSC, weekly tests of EPIRB/SART (self-test), monthly tests of batteries and antennas, and annual surveys by a recognized organization.

Fault-finding: use a systematic approach - check power, then antenna/VSWR, then cables/connectors, then the equipment itself; use built-in test functions and diagnostic menus.

DSC test call: use the 'test' category to a specific coast station; a successful test confirms the DSC controller and transmitter are working.

EPIRB/SART test: use the self-test function; check battery and HRU expiry; do not transmit a live alert; record test results in the radio log.

Antenna fault-finding: measure VSWR, check connectors for corrosion, inspect cable for damage, and verify the antenna is not shorted or open.

Power supply fault-finding: check battery voltage, charger output, fuses, and circuit breakers; verify the reserve battery is connected and charged.

Documentation: maintain a radio log with all tests, faults, and repairs; the log is required for surveys and demonstrates compliance with SOLAS IV/15.