VHF and MF/HF radiotelephony including DSC
Voice and DSC roles
VHF Ch16 is the voice distress, safety and calling channel; Ch70 is the DSC channel and must never carry voice. Ch13 is intership navigation safety; Ch06 is ship-to-ship. MF 2182 kHz and HF 4125/6215/8291/12290/16420 kHz carry voice distress traffic after DSC alerting on the paired DSC frequencies. DSC is used for distress alerts, urgency/safety, routine individual/group calls and position polling; it carries an MMSI, position, time and proposed working channel.
Distress alert sequence
On VHF, lift the cover and press the distress button for about 5 seconds. The alert goes on Ch70. If no acknowledgement arrives within about 5 minutes, transmit the Mayday call and message on Ch16. On MF/HF, select the distress frequency, ensure position/time are correct, and send the DSC alert; wait for acknowledgement on the associated voice frequency before sending the Mayday message. A coast station normally acknowledges and assumes control.
Worked example
You need to call a ship by DSC for a routine matter. Reasoning: enter the other ship's MMSI, select routine priority, propose a working channel such as Ch06 or Ch16 (then move off 16), and send. The called ship's DSC alarms and it acknowledges on DSC, then both switch to the working channel. Do not send a distress or all-ships call for a routine matter.
Traps
Do not speak on Ch70. Do not press DISTRESS without confirming position and nature; a false alert must be cancelled. Do not send a Mayday on HF voice before the DSC acknowledgement unless immediate voice alerting is required. Do not use Ch16 for routine chat.