Module 3 — VHF radiotelephony and DSC

VHF is the most used equipment on board and the first one the student masters. It operates in the 156–174 MHz band by direct wave.

Main parts

Transceiver with screen and keypad, microphone with PTT, channel selector, high / low power control, squelch, DSC interface and connections to GPS and antenna.

Key channels

DSC in VHF

Digital Selective Calling automatically directs a call to a specific MMSI (ship or coast station), announces the traffic category (routine, safety, urgency, distress) and the working channel where the voice traffic will continue.

VHF-DSC distress alert procedure

False-alert cancellation

Cancelled by DSC and by voice on channel 16, identifying the vessel and confirming "cancel my distress alert of [UTC time]".

Channel 70 and nothing else for VHF DSC

Section 3.2.3 of the manual fixes channel 70 (156.525 MHz) as the only VHF channel for DSC distress, urgency, safety and general calling. The equipment watches this channel automatically and permanently. Channel 16 (156.8 MHz) is no longer subject to compulsory continuous watch on fully compliant GMDSS ships, but remains the associated voice channel after a DSC alert and the bridge-to-bridge channel.

Technical content of the alert (3.2.3.10–3.2.3.12)

The VHF-DSC distress alert transmits at a minimum:

• The ship's 9-digit MMSI.
• Nature of distress (selected by the operator or from the designated distress button).
• Position and UTC time: automatic via the GNSS interface, manual otherwise.
• Subsequent communication mode (by default J3E on ch 16).

Since July 2002 (3.2.3.22) automatic position input is a mandatory technical feature of installed equipment.

Key VHF channel plan (Appendix 18 RR)

The up-to-date list is published in ALRS Vol. 5.

Exact VHF-DSC alert sequence

1. 1Lift the hinged cover over the red DISTRESS button.
2. 2Press and hold ≥3 s (5 s on most makers); a countdown appears on screen or a sustained beep plays.
3. 3The set auto-fills position and UTC time from the GNSS feed; without automatic input, type lat/long and time before releasing.
4. 4If time permits, select nature of distress (sinking, fire, collision, listing, flooding, abandoning, piracy, man overboard, undesignated).
5. 5The set transmits the DSC alert on ch 70.
6. 6The transceiver auto-switches to ch 16 on radiotelephony.
7. 7Wait for DSC acknowledgement from a coast station (5-15 s if in coverage).
8. 8Transmit voice MAYDAY on ch 16 following the full script.

MAYDAY voice script on ch 16

1. 1MAYDAY, MAYDAY, MAYDAY.
2. 2THIS IS [name ×3, MMSI].
3. 3MAYDAY [ship name once].
4. 4POSITION [lat/long or bearing + distance from landmark].
5. 5NATURE OF DISTRESS [type].
6. 6ASSISTANCE REQUIRED.
7. 7NUMBER OF PERSONS ON BOARD [N].
8. 8[Further info: listing, abandoning, injuries, intentions].
9. 9OVER.

False-alert cancellation (MSC.514(105))

Once the error is detected, do not switch off the set; cancel by voice on ch 16:

1. 1ALL STATIONS, ALL STATIONS, ALL STATIONS.
2. 2THIS IS [name, call sign, MMSI].
3. 3Please cancel my distress alert of [UTC time] on [frequency/channel].
4. 4No distress, no distress.
5. 5Master, [surname]. OUT.

Log alert time, cancellation time, cause and signature in the radio log. Some administrations additionally require written notification to the RCC within 24 h.

Received-alert handling (ITU-R M.541)

A ship that receives a DSC distress alert must not acknowledge immediately on DSC. It waits about 5 min for a coast station to acknowledge. Only if no coast acknowledgement is heard after 5 min does the ship acknowledge by DSC (VHF) or by voice on ch 16. If the alert keeps repeating after the coast has acknowledged, a nearby ship may relay as MAYDAY RELAY, clearly identifying itself as the relay rather than the origin.

Set operational functions

• Dual Watch / Tri Watch: watches ch 16 plus one (or two) channels alternating.
• Scan: sweep of configured channels.
• Memory: favourites for quick access.
• Group Call / All Ships: selective call to a group or to all ships in coverage.
• Squelch: noise threshold; too high silences weak traffic, too low exhausts the operator.
• High/Low power (25 W / 1 W): 1 W is mandatory in port, ATIS inland waters, and whenever the coast station orders it.
• Received-alert log: review every watch and transfer to the radio log before clearing.

Full VHF false-alert cancellation script

On detecting a VHF-DSC alert transmitted in error (button pressed by accident, improper test, operator slip), cancel immediately by voice on ch 16 using the verbatim script: 'ALL STATIONS, ALL STATIONS, ALL STATIONS. THIS IS [ship name] [call sign] [MMSI]. CANCEL MY DISTRESS ALERT OF [UTC time] ON VHF CHANNEL 70. THERE IS NO DISTRESS. NO DISTRESS. MASTER [surname]. OUT.' Then, by phone, satellite or VHF contact the RCC responsible for the area with the same data plus the cause. Enter in the radio log the alert time, cancellation time, cause and operator signature. Cancellation follows resolution MSC.514(105) and Article 32 of the ITU Radio Regulations; administrations do not normally penalise a properly executed cancellation — what they sanction is silence.