VHF and MF/HF Radiotelephony and DSC Operations
Channels and frequencies
VHF channel 16 is the radiotelephony distress, safety and calling channel; channel 70 is reserved for DSC and must never carry voice. MF distress and calling is 2182 kHz with DSC on 2187.5 kHz. HF uses the 4, 6, 8, 12 and 16 MHz bands, with 8414.5 kHz as the primary DSC distress frequency and radiotelephony distress frequencies such as 8291 kHz. Traps include using channel 70 for voice, confusing 2182 with 2187.5, and forgetting that HF propagation varies with time of day, so a frequency that works at noon may fail at night.
DSC operation
A DSC distress alert is transmitted by pressing the distress button, usually under a protective cover, and should be sent only when immediate assistance is required. The alert contains the vessel's MMSI, position and time, and a nature-of-distress code. After transmission, the operator should switch to the associated radiotelephony channel and send the MAYDAY message. Routine DSC calls use individual or group MMSIs and are followed by a working channel. Questions often test that the DSC alert must be followed by voice, and that a distress alert should not be sent for a situation that is not grave and imminent.
Radiotelephony technique
Use standard phraseology, speak clearly, and identify with vessel name and callsign. For distress, MAYDAY is repeated three times; for urgency, PAN PAN; for safety, SECURITE. Keep exchanges brief and use the phonetic alphabet for callsigns and difficult words.
Worked example
A vessel with engine failure and no immediate danger should send a PAN PAN urgency call on channel 16, not a DSC distress alert. If the vessel begins taking on water and the crew is in grave and imminent danger, the operator then sends a DSC distress alert on channel 70, switches to channel 16, and transmits the MAYDAY message.