## GMDSS Concepts

The Global Maritime Distress and Safety System (GMDSS) is an internationally agreed-upon set of procedures, equipment, and protocols designed to enhance maritime safety and facilitate the rescue of ships in distress. Its primary purpose is to provide a rapid and automated means of distress alerting and communication, ensuring that search and rescue (SAR) authorities ashore are quickly informed of an emergency and can coordinate assistance. GMDSS integrates satellite and terrestrial radio communication systems.

Functional Requirements of GMDSS

All GMDSS-equipped vessels must be capable of performing specific functions, depending on the sea area they operate in. These include:

• Distress Alerting: Transmitting an alert from ship-to-shore, shore-to-ship, and ship-to-ship.
• SAR Coordination: Communicating for coordination of SAR operations.
• On-Scene Communications: Communicating during SAR operations at the scene of the distress.
• Locating: Pinpointing the position of survivors or the vessel in distress.
• Maritime Safety Information (MSI): Receiving urgent information like navigation warnings and weather forecasts.
• General Communications: Routine ship-to-shore and ship-to-ship communications.
• Bridge-to-Bridge Communications: For navigation safety.

## GMDSS Sea Areas

The GMDSS divides the world's oceans into four Sea Areas (A1, A2, A3, A4), each defining the mandatory carriage requirements for GMDSS equipment based on the communication services available. A vessel's GMDSS equipment must be suitable for the most restrictive sea area it intends to operate in.

• Sea Area A1: An area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC (Digital Selective Calling) alerting is available. Generally extends up to 20-30 nautical miles from the coast.

• Sea Area A2: An area, excluding A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available. Generally extends up to 100 nautical miles from the coast.

• Sea Area A3: An area, excluding A1 and A2, within the coverage of an Inmarsat geostationary satellite (which is typically between 70° North and 70° South latitude) in which continuous distress alerting is available. This area relies on satellite communications for distress and safety.

• Sea Area A4: The remaining sea areas outside A1, A2, and A3. This essentially covers the polar regions (above 70° North and below 70° South latitude), where Inmarsat geostationary satellites do not provide coverage. Communication in this area typically relies on HF radio for long-range terrestrial communications and polar-orbiting satellites (e.g., Cospas-Sarsat for EPIRBs).

## Distress Procedures

Distress signifies an imminent threat to a vessel or person's life. The primary method for initiating a distress alert in GMDSS is via Digital Selective Calling (DSC). A DSC distress alert transmits critical information like the vessel's MMSI, position, and nature of distress automatically to Rescue Coordination Centres (RCCs) and other vessels.

Upon sending a DSC distress alert, a voice distress message must follow on the corresponding radiotelephony distress frequency (VHF Ch 16, MF 2182 kHz, or HF distress frequencies). This message begins with "MAYDAY, MAYDAY, MAYDAY", followed by the vessel's name (3 times), call sign, MMSI, position, nature of distress, type of assistance required, number of persons on board (POB), and any other pertinent information.

Acknowledgement of Distress: A Coast Station (or RCC) will acknowledge a DSC distress alert. If no acknowledgement is received, other vessels may acknowledge after a suitable delay. Voice distress messages are acknowledged by "MAYDAY (name of vessel in distress) (name of acknowledging vessel)".

Distress Traffic: All communications relating to a distress are known as distress traffic. It is controlled by the Coast Station or RCC. Other stations must maintain radio silence on the distress frequency. The controlling station may impose silence by transmitting "SEELONCE MAYDAY" (Silence Mayday). When distress traffic is no longer required, the controlling station will transmit "SEELONCE FEENEE" (Silence Finished) or "PRUDONCE" (Prudence).

Relaying Distress: If a vessel hears a distress message that has not been acknowledged, or if they are not in a position to render assistance but believe others might be, they should relay the distress using "MAYDAY RELAY".

## Urgency Procedures

Urgency indicates a very serious situation concerning the safety of a vessel or person, but which is not immediately life-threatening. The urgency signal is "PAN PAN". An urgency message can be initiated by a DSC urgency call, followed by a voice message on the appropriate frequency. The voice message begins with "PAN PAN, PAN PAN, PAN PAN", followed by the vessel's name (3 times), call sign, MMSI, position, and the nature of the urgency. Medical transports often use PAN PAN.

## Safety Procedures

Safety refers to messages concerning important navigational or meteorological warnings, or other safety-related information. The safety signal is "SECURITE". A safety message can be initiated by a DSC safety call, followed by a voice message. The voice message begins with "SECURITE, SECURITE, SECURITE", followed by the vessel's name (3 times), call sign, MMSI, and the nature of the safety message. Maritime Safety Information (MSI) is broadcast via systems like NAVTEX and SafetyNET.

## VHF DSC and Radiotelephony: GOC Revision

The Global Maritime Distress and Safety System (GMDSS) relies heavily on VHF DSC (Digital Selective Calling) and Radiotelephony for short-range communications. Understanding their correct use is critical for safety at sea.

VHF Digital Selective Calling (DSC)

VHF DSC provides an automated method for initiating distress, urgency, safety, and routine calls. The dedicated DSC channel is Channel 70. Every GMDSS-equipped vessel has a unique Maritime Mobile Service Identity (MMSI), a 9-digit number used to identify the station in DSC calls.

• Distress Alerts: Initiated by pressing a dedicated button (often for 3-5 seconds). These alerts transmit the vessel's MMSI, position, and time (if interfaced with GPS), and potentially the nature of distress. Distress alerts are acknowledged by a coast station via DSC, followed by radiotelephony on Channel 16.
• Urgency, Safety, and Routine Calls: Can be made to individual stations, groups of stations, or all ships. These calls notify the receiving station(s) of the type of message to follow on a designated working channel.

VHF Radiotelephony

VHF Radiotelephony allows for direct voice communication. The primary channel for distress, urgency, safety, and calling is Channel 16. Vessels must maintain a continuous watch on both Channel 16 (RT) and Channel 70 (DSC).

Communication priority is strictly observed:

1. Distress: Signalled by MAYDAY. Indicates a vessel or person is in grave and imminent danger and requires immediate assistance.

2. Urgency: Signalled by PAN PAN. Indicates a vessel has a very urgent message concerning the safety of a ship, aircraft, or person.

3. Safety: Signalled by SECURITE. Indicates a vessel is about to transmit a message concerning navigational warnings or important meteorological warnings.

4. Routine: All other communications.

When making radiotelephony calls, use the International Phonetic Alphabet for clarity and standard Pro-words (e.g., "OVER," "ROGER," "OUT"). For short-range communications and in congested areas, use low power (1W); use high power (25W) only when necessary for longer distances.

## MF/HF Terrestrial Communications (DSC, RT, NBDP)

MF/HF terrestrial communications are crucial for long-range maritime communication, extending beyond the line-of-sight limitations of VHF. They enable ships to communicate with coast stations and other vessels across oceans, vital for the Global Maritime Distress and Safety System (GMDSS).

Digital Selective Calling (DSC)

DSC is the primary method for initiating distress, urgency, safety, and routine calls. It uses a digital signal to alert a specific station, group of stations, or all ships within range. Each vessel is identified by a unique Maritime Mobile Service Identity (MMSI).

Key DSC Frequencies:

• MF Distress/Safety: 2187.5 kHz
• HF Distress/Safety: 4207.5, 6312, 8414.5, 12577, 16804.5 kHz

Ships are required to maintain a continuous DSC watch on 2187.5 kHz and at least one appropriate HF DSC frequency. DSC calls can be specific (individual, group) or general (all ships call) and are categorised by priority: Distress, Urgency, Safety, and Routine.

Radiotelephony (RT)

Radiotelephony (RT) provides voice communication, typically used after an initial DSC alert to exchange detailed information. It's the primary method for distress traffic, urgency messages, and safety broadcasts once contact is established.

Key RT Frequencies:

• MF Distress/Safety: 2182 kHz (international distress, urgency, and calling frequency)
• HF Distress/Safety: 4125, 6215, 8291, 12290, 16420 kHz

RT procedures involve using standard phonetic alphabet, numbers, and distress phrases like "MAYDAY", "PAN-PAN", and "SECURITE". Communications can be simplex (one direction at a time) or duplex (simultaneous two-way).

Narrow-Band Direct Printing (NBDP)

Narrow-Band Direct Printing (NBDP), also known as Radio Telex, provides a reliable text-based communication method over MF/HF. It's used for routine messages, administrative traffic, and sometimes for safety information. NBDP systems often employ error correction techniques to ensure message integrity:

• Forward Error Correction (FEC): Used for broadcasts (e.g., weather, safety messages) where no return path exists for retransmission requests.
• Automatic Repeat Request (ARQ): Used for ship-to-ship or ship-to-shore communication, where the receiving station requests retransmission of corrupted characters, ensuring high accuracy.

NBDP frequencies are often paired with RT frequencies for follow-up or specific services. The propagation characteristics of MF/HF frequencies vary significantly with time of day, season, and solar activity, requiring operators to select appropriate frequencies for effective communication.

## Inmarsat Satellite Communications for GMDSS

The Inmarsat system provides global mobile satellite communications, playing a crucial role in the Global Maritime Distress and Safety System (GMDSS). It uses geostationary satellites positioned over the equator, providing coverage to four main Ocean Regions: Atlantic Ocean Region-East (AOR-E), Atlantic Ocean Region-West (AOR-W), Pacific Ocean Region (POR), and Indian Ocean Region (IOR). These satellites relay communications between shipboard terminals (Mobile Earth Stations - MES) and Land Earth Stations (LES), which connect to terrestrial networks. A Network Co-ordination Station (NCS) manages traffic within each ocean region.

Inmarsat-C

Inmarsat-C is a crucial component of GMDSS. It is a store-and-forward, data-only system, offering two-way messaging (similar to telex), data reporting, and polling. Its terminals are small, low-cost, and use omni-directional antennas, making them robust.

Key GMDSS functions of Inmarsat-C include:

• Distress Alerting: A dedicated button transmits a distress alert with the ship's identity and, if connected to a GPS receiver, its position.
• Enhanced Group Call (EGC) reception: This service includes SafetyNET and FleetNET.
• SafetyNET is mandatory for GMDSS vessels and broadcasts Maritime Safety Information (MSI) such as navigational warnings, meteorological warnings and forecasts, and SAR information, targeted to specific geographical areas or all ships.
• FleetNET is a commercial service for broadcasting information to selected groups of ships.

Inmarsat FleetBroadband (FBB)

Inmarsat FleetBroadband (FBB) is a modern service providing voice, high-speed IP data, and streaming IP. While primarily a commercial service, it is approved for GMDSS distress alerting and follow-up communications. FBB terminals have a dedicated distress button for transmitting alerts and can be used for subsequent voice or data communications with rescue coordination centres. Some FBB terminals are specifically designated as Maritime Safety Data (MSD) terminals for GMDSS.

Distress Alerting and Follow-Up

All GMDSS-approved Inmarsat terminals (Inmarsat-C, FBB) feature a dedicated distress button. Activating this button automatically transmits a distress alert, including the ship's identity, position (if GPS connected), and time. This alert is routed with the highest priority to an LES, then to a Rescue Coordination Centre (RCC). Subsequent communications, such as voice calls via FBB or telex messages via Inmarsat-C, are used for follow-up and coordination with SAR authorities, always maintaining distress priority.

Operational Considerations

Vessels must ensure their Inmarsat terminals are always operational, powered, and connected to a reliable GPS source. Regular testing of the distress function (via a non-distress test call) is essential. Proper log-on procedures ensure the terminal is registered with the NCS for the relevant Ocean Region.

## Maritime Safety Information (MSI)

Maritime Safety Information (MSI) is crucial for the safety of life at sea and efficient navigation. It encompasses navigational warnings, meteorological warnings and forecasts, and urgent search and rescue (SAR) related information. Under GMDSS, MSI is primarily broadcast via two automated systems: NAVTEX and SafetyNET.

## NAVTEX

NAVTEX (NAVigational TEXt) is an international automated direct-printing service for broadcasting MSI to ships in coastal waters (Sea Area A1 and A2). It operates on specific frequencies:

• 518 kHz: The primary international NAVTEX frequency, broadcasting in English.
• 490 kHz: Used for national language broadcasts.
• 4209.5 kHz: A long-range/tropical NAVTEX frequency, often used for extended coverage.

NAVTEX transmissions have a typical range of 250-400 nautical miles. Ships are equipped with dedicated NAVTEX receivers or integrated GMDSS consoles that automatically receive and print relevant messages. Receivers can be programmed to reject certain message categories (e.g., meteorological forecasts if a ship has other sources) and transmissions from specific stations. Messages are identified by a B1B2B3B4 format, where B1 is the transmitter identity, B2 is the subject indicator (e.g., A for Navigational Warnings, B for Meteorological Warnings, D for SAR information, E for Meteorological Forecasts, L for additional Navigational Warnings), and B3B4 is the message serial number.

## SafetyNET

SafetyNET is an international satellite-based service for the promulgation of MSI to ships operating in Sea Areas A3 and A4. It uses the Inmarsat-C satellite system to provide a global broadcast capability. Unlike NAVTEX, SafetyNET allows for geographical targeting of messages, meaning information can be sent only to ships within a specific circular or rectangular area, or to all ships in a designated sea area, or even nationally. This ensures that ships receive only information relevant to their current location, reducing information overload.

SafetyNET broadcasts include navigational warnings, meteorological warnings and forecasts, and SAR-related information. The service is managed by an International SafetyNET Service Provider (ISSP), which coordinates the broadcast of MSI from various national authorities. Ships require an Inmarsat-C terminal to receive SafetyNET messages, which are displayed on the terminal's screen or printed out.

## Survival Craft and Rescue Equipment: EPIRB and SART

The Global Maritime Distress and Safety System (GMDSS) relies heavily on dedicated equipment to alert rescue authorities and facilitate the location of distressed vessels or survival craft. Among the most critical are the Emergency Position-Indicating Radio Beacon (EPIRB) and the Search and Rescue Transponder (SART).

## Emergency Position-Indicating Radio Beacon (EPIRB)

An EPIRB is designed to transmit a distress alert and position information to rescue coordination centres (RCCs) via satellite. It is a vital component for initiating a distress response.

• Primary Frequencies: It transmits a digitally coded distress message on 406 MHz to COSPAS-SARSAT satellites. This message contains the vessel's unique identification number (MMSI encoded) and, if GPS-enabled, its precise position.
• Homing Signal: Most EPIRBs also transmit a continuous signal on 121.5 MHz, which is used by aircraft and vessels for final homing to the distress scene.
• Activation: EPIRBs are typically Category 1 (float-free), meaning they are designed to automatically release from a sinking vessel and activate when submerged. They can also be manually activated.
• Power Source: Powered by an internal battery, designed for a minimum of 48 hours of continuous operation.
• Maintenance: Regular checks include battery expiry date, hydrostatic release unit (HRU) expiry date, and a self-test function.

## Search and Rescue Transponder (SART)

A SART is a portable radar transponder used to locate survival craft or distressed vessels. Unlike an EPIRB, it does not initiate a distress alert but assists in pinpointing the location once search and rescue (SAR) operations are underway.

• Operation: A SART is activated manually in a survival craft. When interrogated by an X-band radar (typically 9 GHz) from a searching vessel or aircraft, it transmits a series of 12 equally spaced dots on the radar display, indicating its range and bearing. The dots become arcs as the searching vessel approaches.
• Frequency: Operates on the X-band (9 GHz) radar frequency.
• Placement: For optimal performance, a SART should be mounted at least 1 meter above sea level in the survival craft to ensure an unobstructed line of sight to searching radars.
• Power Source: Internal battery designed for a minimum of 96 hours in standby mode plus 8 hours of continuous transmission when interrogated.
• Maintenance: Check battery expiry date and perform a self-test.

## Key Differences

While both are vital rescue tools, the EPIRB alerts rescue authorities and provides initial position, whereas the SART assists in the final location of the survival craft by responding to radar. Both are mandatory equipment for GMDSS-compliant vessels and crucial for ensuring safety at sea.

## GMDSS Equipment Overview

Reliability is paramount for GMDSS. Key equipment includes VHF DSC, MF/HF DSC, Inmarsat C SES, EPIRBs, SARTs, and NAVTEX receivers. Many critical components are duplicated to ensure continuous operation, even if one unit fails. For example, two independent VHF DSC radios are often required for A3/A4 sea areas. All equipment must be type-approved and regularly maintained.

## Power Sources

GMDSS equipment must be supplied from three independent sources of electrical power:

• Main Power Source: The ship's primary electrical supply.
• Emergency Power Source: An emergency generator, typically starting automatically upon main power failure.
• Reserve Power Source: Dedicated batteries, specifically for GMDSS equipment. This is the most crucial independent power source for GMDSS. It must be capable of operating the GMDSS installation for a specified duration after failure of the main and emergency power supplies.

Reserve Power Duration Requirements:

• Ships < 500 GT: 1 hour (if emergency generator available) or 6 hours (if no emergency generator).
• Ships ≥ 500 GT: 6 hours (regardless of emergency generator availability).

These batteries must be automatically charged to maintain full capacity and be located as high as practicable and readily accessible for maintenance.

## Batteries and Maintenance

Common battery types for GMDSS reserve power are lead-acid or nickel-cadmium (NiCad). Regular maintenance is essential:

• Visual Inspection: Check for corrosion on terminals, secure connections, and physical damage.
• Electrolyte Levels: For lead-acid batteries, ensure electrolyte covers the plates. Top up with distilled water only, never acid.
• Specific Gravity: Use a hydrometer to check the specific gravity of each cell. This indicates the state of charge. Readings should be consistent across all cells.
• Ventilation: Battery compartments must be well-ventilated to prevent accumulation of explosive hydrogen gas.
• Charging System: Verify the automatic charging system is functioning correctly.

## Onboard Maintenance & Antennas

Routine onboard maintenance includes daily checks (e.g., visual inspection of equipment, power indicators), weekly checks (e.g., battery specific gravity, EPIRB/SART self-test), and monthly checks (e.g., EPIRB test via self-test function). A supply of essential spare parts and basic tools should be carried. Antennas are vital; ensure they are clear of obstructions, securely mounted, and free from damage or corrosion. Regular cleaning and inspection prevent signal degradation.