VHF Radiotelephony and DSC Operation, Channels and Watchkeeping
Channel discipline
Ch 16 is the VHF voice distress, safety and calling channel. Ch 70 is DSC only and must never carry voice. Ch 13 is bridge-to-bridge navigation safety, Ch 06 is ship-to-ship, Ch 15 and 17 are on-board low-power channels, and AIS uses 87B/88B. Ch 75 and 76 are guard channels and are not for public correspondence. Port and marina working channels vary, so the DSC acknowledgement or the coast station should assign the working channel.
Watchkeeping
A GMDSS VHF installation keeps a continuous DSC watch on Ch 70 and a voice watch on Ch 16. If the set is in dual-watch, confirm that Ch 70 is included. Switching off the DSC watch or leaving Ch 16 for a private channel can miss a distress alert.
DSC calls
For a routine individual call, enter the coast station's MMSI, select routine priority, transmit on Ch 70, wait for the acknowledgement and then switch to the proposed working channel. For a distress alert, lift the protective cover and hold the distress button as specified; the set transmits the alert and usually waits for acknowledgement.
Worked example: you need a marina berth. Send an individual routine DSC call to the marina or coast station on Ch 70, receive the acknowledgement with a working channel, then conduct the voice conversation there. Do not call on Ch 16 and immediately ask for a private channel; the DSC link establishes the working channel first.
Traps
Voice on Ch 70 is a serious operating error. Do not use Ch 16 for DSC; it is the voice calling channel. Do not assume a DSC acknowledgement means the voice conversation has started. Confirm the working channel and ship's identification before exchanging operational detail.