Module 4 - Channels & Frequencies
Simplex, Duplex, And Channel Purpose
A simplex channel uses the same working frequency arrangement for both directions, so only one station speaks at a time. A duplex channel uses paired frequencies for ship and coast directions and may not support ship-to-ship contact. The channel number is an allocation with a purpose, not just a convenient number.
Distress and calling, DSC signalling, inter-ship work, port operations, ship movement, public correspondence, on-board communication, and national small-craft safety services use different allocations. Always use the channel named by current local information or the station, and move routine traffic away from Channel 16 promptly.
Distress, urgency, very brief safety and initial calling
Guardrail: Keep routine traffic brief and move off
DSC digital signalling
Guardrail: No voice
Bridge-to-bridge navigation safety; fallback for an unanswered unknown-MMSI ship call
Guardrail: Not a general chat channel
Common inter-ship allocations
Guardrail: Use the appropriate available working channel
Operations, movement, and local services
Guardrail: Use only the locally assigned channel and purpose
| Channel or group | Core use | Guardrail |
|---|---|---|
| 16 | Distress, urgency, very brief safety and initial calling | Keep routine traffic brief and move off |
| 70 | DSC digital signalling | No voice |
| 13 | Bridge-to-bridge navigation safety; fallback for an unanswered unknown-MMSI ship call | Not a general chat channel |
| 6, 8, 72, 77 | Common inter-ship allocations | Use the appropriate available working channel |
| Port/VTS/marina | Operations, movement, and local services | Use only the locally assigned channel and purpose |
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