Distress, urgency and safety communications procedures
Priority and signal structure
GMDSS uses three priority signals. MAYDAY is distress, for grave and imminent danger requiring immediate assistance. PAN PAN is urgency, for serious but not immediate danger, including urgent medical advice. SECURITE is safety, for navigation or meteorological warnings. DSC categories mirror these priorities. Voice distress frequencies include VHF channel 16, MF 2182 kHz, and HF 4125, 6215, 8291, 12290, and 16420 kHz. DSC distress frequencies are VHF channel 70, MF 2187.5 kHz, and HF 4207.5, 6312, 8414.5, 12577, and 16804.5 kHz.
Distress call content
A radiotelephone distress call contains MAYDAY three times, THIS IS, the vessel name, callsign or MMSI, position, nature of distress, assistance required, persons on board, and OVER. A DSC distress alert is followed by a radiotelephone distress message on the associated voice frequency. The distress alert has priority over all other traffic and must not be used for testing. Coast stations and RCCs normally acknowledge DSC alerts; ships should not routinely acknowledge another vessel's DSC alert by DSC.
Worked example
A DSC distress alert appears on 2187.5 kHz from another vessel, with position and nature FIRE. Keep watch on 2182 kHz and on DSC. Do not send a DSC acknowledgement. If no coast station acknowledges within the prescribed period and you are in a position to assist, acknowledge by radiotelephony on 2182 kHz, inform the vessel and the RCC, and relay the distress if required. Do not transmit your own MAYDAY unless you are relaying or are yourself in distress. The reasoning is that the shore authority coordinates the response and duplicate DSC acknowledgements can jam the alerting channel.
Traps
MAYDAY is not for urgent medical advice; PAN PAN is. Channel 70 is DSC only and must not carry voice. Do not cancel a false alert only by switching off; cancel on the same system and notify the coast station or RCC. Do not use 2182 or 156.8 MHz for routine tests. Do not omit position and time in a DSC distress alert. Do not assume a distress relay is the same as the original distress alert.