AMSA advises on automatic VHF channel switching

AMSA issued a Marine Notice to inform operators of how automatic VHF channel switching may interfere with the safe operation of vessel communications.

Vessel operators should consider the implications of using or not using the disable feature for automatic VHF channel switching during safety critical operations. If considered necessary, instructions on its use should be included in the ship’s bridge procedures, AMSA suggests.

As explained, VHF marine radios with digital selective calling (DSC) capability have a design function that enables automatic channel switching to VHF channel 16 when a DSC distress alert is received. This design function complies with the International Telecommunications Union (ITU) Recommendation ITU-R M.493-13 (2009).

The wide use of VHF DSC means that VHF DSC alerts may cause a VHF DSC transceiver to switch voice channels unexpectedly. This could create a safety risk for the vessel during safety critical operations. Vessel operators should be aware that the automatic channel switching function can be disabled.

Since 2004, VHF DSC equipment has been built to allow for automatic channel switching to be disabled, so that essential communications during safety critical operations can be maintained. Manufacturers implement the disabling feature in different ways, and usually include this in the user manual. If the user manual or documentation is not clear in this regard, advice from the supplier or service agent may need to be sought.

All VHF DSC equipment should provide visual indication when the automatic switching function is disabled.

Further, if automatic channel switching is disabled to prevent channel switching, transceivers will continue to detect DSC distress alerts and the operator may decide whether to accept the channel switch request.

AMSA suggests that the ship’s bridge procedures include instructions on how and when to disable and re-enable automatic channel switching in respect to safety critical operations.

In all circumstances users should ensure that they are operating on the correct working voice channel when using DSC capable VHF transceivers with automatic channel switching.