Marine Communications (GMDSS)
GMDSS questions test sea areas, distress procedures, equipment frequencies and false-alert handling. The examiner wants to see that you can send the right alert on the right equipment and follow up on the right voice channel.
Sea areas and equipment
Sea area A1 is within VHF coverage of a coast station with DSC on VHF Channel 70 and voice on Channel 16. A2 is within MF coverage, using DSC on 2187.5 kHz and voice on 2182 kHz. A3 is within Inmarsat geostationary coverage. A4 is outside A1–A3 and relies on HF. GMDSS provides distress alerting, SAR coordination, on-scene communications, locating, maritime safety information and general communications.
Distress, urgency and safety
Mayday is for grave and imminent danger; Pan-Pan is urgency; Securite is safety. A DSC distress alert should include the nature of distress and position. Follow with a Mayday voice call if time allows. EPIRBs operate on 406 MHz with 121.5 MHz homing; SARTs operate in the 9 GHz band and are detected by X-band radar. NAVTEX international is 518 kHz; national is 490 kHz.
Worked example: DSC distress in A2
A vessel is sinking in sea area A2. What is the correct GMDSS distress procedure?
Transmit a DSC distress alert on 2187.5 kHz with the nature of distress and position. If time permits, follow with a Mayday voice call on 2182 kHz. Activate the 406 MHz EPIRB and the SART. Listen on 2182 kHz and VHF Channel 16 for acknowledgment and SAR communications. Do not speak on VHF Channel 70; it is a DSC channel only. If no acknowledgment arrives, repeat the alert or use an alternative HF distress frequency according to the equipment fitted.
False alerts
If a distress alert is transmitted accidentally, cancel it immediately by the same equipment or by voice, notify the nearest coast radio station or RCC, and record the action in the radio log. Do not simply switch off the equipment.
Traps
- Speaking on VHF Channel 70 or DSC 2187.5 kHz.
- Confusing the 406 MHz EPIRB alert with the 121.5 MHz homing signal.
- Forgetting that A2 uses MF, not Inmarsat.
- Testing an EPIRB outside test mode.
- Leaving a false alert uncancelled.
- Mixing NAVTEX 518 kHz international with 490 kHz national.