Distress, Urgency and Safety Communications Procedures
Priority and the three signals
Every GMDSS transmission sits in one of three priority bands. Distress (highest) uses MAYDAY on radiotelephony, SOS on Morse, and DSC category Distress with a nature-of-distress code and, where fitted, a position. Urgency uses PAN PAN and DSC category Urgency; it covers messages about the safety of a vessel or person that is not yet in grave and imminent danger. Safety uses SECURITE and DSC category Safety, typically for navigational or meteorological warnings. A frequent trap is confusing the trigger for each: a man overboard or a vessel drifting with engine failure is normally urgency, not distress, unless life is in grave and imminent danger.
Structure of a distress call and message
A distress alert is sent by DSC on VHF channel 70 or MF/HF, or by Inmarsat, then followed by a distress call and message on a radiotelephony distress channel. The call nominally contains MAYDAY three times, THIS IS, the vessel name and callsign repeated three times. The message adds position, nature of distress, assistance required, number of persons on board and any other useful information. Know the difference between an alert (the automated DSC or satellite signal that summons help) and the message (the spoken detail). Questions often test the order of items and the fact that position and time must be included.
Control of distress traffic
The station in distress, or the station controlling the traffic, imposes silence with the spoken SEELONCE MAYDAY (or the DSC/RT equivalent). Distress traffic ends with SEELONCE FEENEE; urgency and safety traffic are closed with PRUDONCE and SILENCE respectively. A trap: any station may acknowledge a distress alert, but the responsible coast station or the vessel best placed normally takes control. Relay of a distress alert is permitted when the source cannot transmit itself, and must be clearly marked as a relay. False alerts must be cancelled promptly by voice and, where required, by DSC or satellite notification to the coast station.
Rule of thumb
When unsure of priority, ask: is life in grave and imminent danger now? If yes, send distress; if the situation is serious but not immediately life-threatening, send urgency; if it is purely advisory, send safety.