GMDSS equipment, coverage and procedures
These 14 modules are independent study material. The displayed hours are a suggested study allocation, not an official national syllabus or approved practical training. Check your administration’s certificate and examination requirements.
Phase 1 — Fundamentals
8 h theory · 5 h practicePhase 2 — Equipment
29 h theory · 47 h practice- 3Module 3 — VHF radiotelephony and DSC
- 4Module 4 — MF/HF radiotelephony and DSC
- 5Module 5 — NBDP / radiotelex
- 6Module 6 — Inmarsat C and EGC
- 7Module 7 — NAVTEX and maritime safety information
- 8Module 8 — EPIRB / 406 MHz beacon
- 9Module 9 — SART, AIS-SART and survival VHF
- 10Module 10 — GPS, antennas, batteries and reserve power
Phase 3 — Procedures
11 h theory · 13 h practicePhase 4 — Integrated drills
1 h theory · 5 h practiceGMDSS equipment, coverage and procedures
GMDSS combines ship equipment, shore services and procedures. SOLAS IV/4 distinguishes nine GMDSS functions from general radiocommunications. Ship-to-shore distress alerting needs two separate, independent means using different services. Distress, urgency, safety and routine traffic have decreasing priority. MAYDAY concerns grave and imminent danger to a vessel or a person requiring immediate assistance; PAN-PAN concerns a very urgent safety message; SÉCURITÉ introduces safety information.
A1 is government-declared VHF radiotelephone coverage with continuous DSC alerting. A2 is corresponding MF coverage outside A1. Since 2024, A3 excludes A1/A2 and depends on continuous alerting coverage of the recognized satellite service supported by the ship earth station carried. A4 is outside A1, A2 and that A3. Polar waters can be A3 with a suitable approved global service. Missing MF equipment does not turn an A2 coverage area into A4.
These 14 modules are independent study material. The displayed hours are a suggested study allocation, not an official national syllabus or approved practical training. Check your administration’s certificate and examination requirements.
GMDSS sea areas at a glance
| Area | Coverage | Reference |
|---|---|---|
| A1 | Government-declared VHF radiotelephone coverage with continuous DSC alerting. | SOLAS IV/2.1.15; IV/8 |
| A2 | Outside A1, government-declared MF radiotelephone coverage with continuous DSC alerting. | SOLAS IV/2.1.16; IV/9 |
| A3 | Outside A1/A2, continuous alerting coverage of a recognized satellite service supported by the ship earth station carried. | SOLAS IV/2.1.17; IV/10 |
| A4 | Outside A1, A2 and the A3 defined for the ship. Polar waters are not automatically A4. | SOLAS IV/2.1.18; IV/11 |
Most-asked GMDSS modules
- VHF-DSC alerting (Channel 70)VHF channel 70 (156.525 MHz) carries DSC, never voice. Channel 16 (156.800 MHz) carries distress, urgency, safety and calling voice. Channel 6 is 156.300 MHz and channel 13 is 156.650 MHz. All-ships DSC addressing is a call format, not a special nine-digit ship MMSI.
- MF/HF-DSC (2187.5 / 4207.5 / 8414.5 kHz)MF/HF DSC alerts and subsequent voice traffic use different frequencies. The table pairs the six DSC frequencies with their associated radiotelephony frequencies. A compliant MF/HF distress watch includes 2187.5 and 8414.5 kHz plus at least one of the other four HF frequencies appropriate to time and position. It must not omit 8414.5 kHz merely because another band seems better.
- Inmarsat-C and SafetyNETInmarsat-C provides data messaging, distress alerting and SafetyNET reception; it does not provide voice. Other approved satellite equipment may support different services, including Iridium GMDSS with SafetyCast. Approval, installed terminal compatibility, actual coverage and service availability all matter. A consumer satellite messenger does not establish SOLAS GMDSS compliance.
- EPIRB 406 MHz and SARTA 406 MHz EPIRB alerts the Cospas-Sarsat system; its 121.5 MHz signal helps nearby rescue units home in. The applicable modern IMO standard adds GNSS and AIS locating capability for EPIRBs installed from 1 July 2022. First-generation beacon identifiers have 15 hexadecimal characters; second-generation identifiers have 23. Registration and national equipment approval must match the actual beacon.