Global Maritime Distress and Safety System (GMDSS) Guide
The GMDSS is an international system that uses terrestrial and satellite technology and ship-board radio systems to ensure rapid alerting of shore-based rescue and communications authorities in the event of an emergency at sea.
GMDSS Sea Areas
Sea Area A1
An area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC alerting is available.
Range: Approximately 20-30 nautical miles from shore.
Sea Area A2
An area, excluding Sea Area A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available.
Range: Approximately 100-150 nautical miles from shore.
Sea Area A3
An area, excluding Sea Areas A1 and A2, within the coverage of an INMARSAT geostationary satellite in which continuous alerting is available.
Range: Between approximately 70°N and 70°S latitude (global coverage except polar regions).
Sea Area A4
An area outside Sea Areas A1, A2, and A3 (primarily the polar regions).
Coverage: Polar regions beyond INMARSAT coverage, primarily above 70°N and below 70°S latitude.
GMDSS Equipment Requirements by Sea Area
| Equipment | Sea Area A1 | Sea Area A2 | Sea Area A3 | Sea Area A4 |
|---|---|---|---|---|
| VHF Radio with DSC | ✓ | ✓ | ✓ | ✓ |
| VHF DSC Watch Receiver (Ch.70) | ✓ | ✓ | ✓ | ✓ |
| MF Radio with DSC | ✓ | ✓ | ✓ | |
| MF DSC Watch Receiver | ✓ | ✓ | ✓ | |
| MF/HF Radio with DSC | ✓* | ✓ | ||
| INMARSAT Ship Earth Station | ✓* | |||
| NAVTEX Receiver (518 kHz) | ✓ | ✓ | ✓ | ✓ |
| EPIRB (406 MHz) | ✓ | ✓ | ✓ | ✓ |
| SART (9 GHz) or AIS-SART | ✓ | ✓ | ✓ | ✓ |
| Two-way VHF Handheld Radios | ✓ | ✓ | ✓ | ✓ |
* Ships in Sea Area A3 must have either MF/HF with DSC or INMARSAT Ship Earth Station (or both).
GMDSS Communication Equipment and Frequencies
VHF Digital Selective Calling (DSC)
| Function | Channel/Frequency | Description |
|---|---|---|
| DSC Distress and Safety | Channel 70 (156.525 MHz) | For automated digital distress alerts and safety calls |
| Voice Distress, Safety, and Calling | Channel 16 (156.800 MHz) | International distress, safety, and calling channel |
| DSC Range | 20-30 nautical miles | Line-of-sight communications range |
MF/HF Digital Selective Calling (DSC)
| Band | DSC Frequency | Radiotelephony Frequency |
|---|---|---|
| MF | 2,187.5 kHz | 2,182 kHz |
| HF4 | 4,207.5 kHz | 4,125 kHz |
| HF6 | 6,312.0 kHz | 6,215 kHz |
| HF8 | 8,414.5 kHz | 8,291 kHz |
| HF12 | 12,577.0 kHz | 12,290 kHz |
| HF16 | 16,804.5 kHz | 16,420 kHz |
MF range: Approximately 100-150 nautical miles. HF range: Global depending on frequency, time of day, and ionospheric conditions.
INMARSAT Systems
| System | Frequency Bands | Services/Features |
|---|---|---|
| Inmarsat-C |
L-Band (1.5-1.6 GHz) Ship-to-Shore: 1626.5-1645.5 MHz Shore-to-Ship: 1525.0-1545.0 MHz |
- Text messaging - Distress alerting - EGC/SafetyNET - LRIT compliance |
| Fleet Broadband (FBB) |
L-Band (1.5-1.6 GHz) Ship-to-Shore: 1626.5-1660.5 MHz Shore-to-Ship: 1525.0-1559.0 MHz |
- Voice - Broadband data - Distress priority calling - Video conferencing - Internet access |
| Inmarsat Fleet Safety | L-Band (1.5-1.6 GHz) |
- GMDSS-compliant service - Enhanced Maritime Safety Data Service - Successor to Inmarsat-C for GMDSS |
Coverage: Between approximately 70°N and 70°S latitude (does not cover polar regions).
COSPAS-SARSAT System (EPIRBs)
| Component | Frequency | Description |
|---|---|---|
| EPIRB (Emergency Position Indicating Radio Beacon) | 406 MHz (primary) 121.5 MHz (homing) |
- Float-free, automatically activated - Transmits vessel identity and position - 48-hour minimum operating life - Integrated GPS in modern units - Detected by satellites |
| COSPAS-SARSAT Satellites |
LEOSAR (Low Earth Orbit) GEOSAR (Geostationary) MEOSAR (Medium Earth Orbit) |
- Global detection and location system - Processes distress signals from EPIRBs - Relays alert to Mission Control Centers |
The 121.5 MHz frequency is now only used for final homing by rescue units. Modern EPIRBs must operate on 406 MHz.
Radar Transponders
| Type | Frequency | Description |
|---|---|---|
| SART (Search and Rescue Transponder) | 9.2-9.5 GHz (X-band radar) |
- Responds to radar pulses - Creates distinctive line of dots on radar display - 96-hour standby + 8-hour active use - Range of approximately 5-8 nautical miles |
| AIS-SART (AIS Search and Rescue Transmitter) | AIS frequencies: 161.975 MHz (AIS1) 162.025 MHz (AIS2) |
- Transmits position data via AIS - Displayed on electronic chart systems - 96-hour standby + 8-hour active use - Range of approximately 5-10 nautical miles |
Two-Way VHF Portable Radios
| Type | Frequency | Description |
|---|---|---|
| GMDSS Two-Way VHF | Channel 16 (156.800 MHz) Channel 6 (156.300 MHz) |
- Waterproof, portable radios - For communication during abandonment - 8-hour operation at 0°C - Simplified operation for emergency use |
Maritime Safety Information (MSI) Systems
| System | Frequency | Information Type |
|---|---|---|
| NAVTEX |
518 kHz (International, English) 490 kHz (National, Local language) |
- Navigational warnings - Weather forecasts - Search and rescue information - Range of approximately 200-400 nautical miles |
| Enhanced Group Call (EGC) / SafetyNET | Via Inmarsat (L-band) |
- Navigational warnings - Weather information - Search and rescue coordination - Shore-to-ship distress alerts - Global coverage (except polar regions) |
| HF NBDP (Narrow Band Direct Printing) | HF bands (4, 6, 8, 12, 16 MHz) |
- Similar to SafetyNET - For regions outside Inmarsat coverage - Used in Sea Area A4 |
Newer Satellite Communication Systems
| System | Frequency Band | Features/Notes |
|---|---|---|
| Iridium | L-band (1616-1626.5 MHz) |
- Global coverage including polar regions - Recognized for GMDSS since 2020 - Voice and data capabilities - Smaller equipment than traditional INMARSAT |
| Starlink Maritime | Ku-band, Ka-band |
- High-speed broadband (up to 350 Mbps) - Not currently GMDSS-approved - Expanding maritime coverage - Used for crew communications and operations |
| VSAT (Very Small Aperture Terminal) | C-band, Ku-band, Ka-band |
- High-bandwidth satellite communications - Not a GMDSS-approved system - Used for supplementary communications - Requires larger antenna installations |
Note: While newer satellite systems like Starlink provide communication capabilities, they are not yet approved as primary GMDSS equipment and must be used alongside mandatory GMDSS systems.