VHF radiotelephony: channel usage, distress/urgency/safety procedures, and DSC operations on VHF
This domain tests practical VHF operation: which channel does what, how a DSC distress alert is sent and followed up, and how urgency and safety traffic is announced. The exam favors correct channel and priority discipline over long scripts.
Channel use
- Channel 16 (156.800 MHz) is the international distress, safety and calling voice channel. Initial voice contact is made here, then traffic moves to a working channel.
- Channel 70 (156.525 MHz) is the DSC channel. It is data only; no voice, no Morse, no modem.
- Channel 13 is bridge-to-bridge; channel 06 is intership and on-scene; channels 72 and 77 are common ship-to-ship working channels; port operations use local channels such as 09, 12 or 14.
- Channel 16 may be used to announce a DSC alert, but the alert itself goes on channel 70.
DSC distress and follow-up
A VHF DSC distress alert contains the vessel MMSI, position, time and nature of distress. Transmit it on channel 70, then keep watch. If no acknowledgement arrives, transmit a MAYDAY on channel 16 with the same details. After acknowledgement, switch to the channel directed by the coast station or use channel 16 briefly and move to a working channel. Urgency traffic uses PAN PAN; safety traffic uses SECURITE, both announced on channel 16 and then worked elsewhere.
Worked example
Smoke appears in the engine room and the vessel is in A1. The operator opens the DSC menu, selects distress, confirms position and nature, and transmits on channel 70. No acknowledgement is received within a few minutes, so the operator transmits on channel 16: MAYDAY, MAYDAY, MAYDAY, vessel name and call sign, MMSI, position, nature of distress and number of persons. A coast station replies and directs the vessel to channel 06 for on-scene coordination with a nearby vessel.
Traps that cost marks
Speaking on channel 70 is the classic error. Sending a DSC alert and never following with voice traffic is another. Using PAN PAN for a mayday situation, or SECURITE for a medical emergency, loses marks. Do not use channel 16 for routine working conversation after contact.