VHF is the most-used equipment on board and the first one you have to master. It works in the 156–174 MHz band by direct wave: if the other station's antenna isn't "in sight", your signal doesn't reach.
Equipment parts
A marine VHF-DSC reduces to seven pieces you'll recognise on any brand:
- Transceiver with screen and keypad.
- Microphone with PTT (Push-To-Talk).
- Channel selector (rotary or numeric).
- Power control — high (≈25 W) / low (1 W).
- Squelch — silences background noise when no signal is present.
- DSC interface with its red DISTRESS button under a cover.
- Connections to GPS (automatic position) and antenna.
Key channels
| Channel | Frequency | Use | Mode |
|---|---|---|---|
| 16 | 156.800 MHz | Calling & distress by voice | Simplex |
| 70 | 156.525 MHz | DSC only — never voice | Simplex |
| 6 | 156.300 MHz | Ship-to-ship SAR coordination | Simplex |
| 13 | 156.650 MHz | Bridge-to-bridge (intership safety) | Simplex |
| 67 | 156.375 MHz | Supplementary distress/safety (UK) | Simplex |
| 72 / 77 | 156.625 / 156.875 MHz | Inter-ship working | Simplex |
What does DSC actually do?
Digital Selective Calling is a short data message on channel 70 that:
- 1Is addressed to a specific MMSI (a vessel, a coast station, or "all ships" via a special 9-digit MMSI).
- 2Announces the traffic category: routine, safety, urgency or distress.
- 3Indicates the working channel where voice traffic will continue.
- 4If distress, includes MMSI + position + UTC time + nature of distress.
The receiver processes it automatically: it silently watches channel 70 all the time, and only "wakes" the operator if the call is addressed to its MMSI or is general distress/urgency.
VHF-DSC distress alert procedure
The official sequence is 5 steps. You have to be able to execute them without thinking — the practical exam times the first two.
- 1Lift the red cover on the DISTRESS button.
- 2Press and hold for 3–5 seconds until the set confirms transmission. If time allows, preselect nature of distress from the menu (fire, flooding, collision, etc.).
- 3The set transmits the alert on channel 70 with MMSI, position, UTC time and nature.
- 4After the alert, the set switches automatically to channel 16 for voice traffic.
- 5Broadcast the Mayday by voice with: identification, position, nature of distress, number of persons on board and assistance required.
False-alert cancellation
If an alert goes out by mistake (button pressed by accident, badly configured drill), it must be cancelled immediately by two channels:
- 1By DSC: send a cancellation message if the set supports it.
- 2By voice, on channel 16, with the formula:
> All stations, all stations, all stations, this is [vessel name, MMSI, callsign]. Cancel my distress alert of [UTC time]. Out.
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)
| Channel | Frequency | Primary use |
|---|---|---|
| 70 | 156.525 MHz | DSC distress/urgency/safety/calling — DSC only |
| 16 | 156.800 MHz | Distress/safety voice and bridge-to-bridge |
| 13 | 156.650 MHz | Bridge-to-bridge navigation safety (worldwide) |
| 6 | 156.300 MHz | On-scene SAR communications |
| 67 | 156.375 MHz | SAR channel in most European states |
| 72, 77 | 156.625 / 156.875 MHz | Intership only |
| 23–28, 84–87 | — | Duplex for coast-station link |
The up-to-date list is published in ALRS Vol. 5.
Exact VHF-DSC alert sequence
- 1Lift the hinged cover over the red DISTRESS button.
- 2Press and hold ≥3 s (5 s on most makers); a countdown appears on screen or a sustained beep plays.
- 3The set auto-fills position and UTC time from the GNSS feed; without automatic input, type lat/long and time before releasing.
- 4If time permits, select nature of distress (sinking, fire, collision, listing, flooding, abandoning, piracy, man overboard, undesignated).
- 5The set transmits the DSC alert on ch 70.
- 6The transceiver auto-switches to ch 16 on radiotelephony.
- 7Wait for DSC acknowledgement from a coast station (5-15 s if in coverage).
- 8Transmit voice MAYDAY on ch 16 following the full script.
MAYDAY voice script on ch 16
- 1MAYDAY, MAYDAY, MAYDAY.
- 2THIS IS [name ×3, MMSI].
- 3MAYDAY [ship name once].
- 4POSITION [lat/long or bearing + distance from landmark].
- 5NATURE OF DISTRESS [type].
- 6ASSISTANCE REQUIRED.
- 7NUMBER OF PERSONS ON BOARD [N].
- 8[Further info: listing, abandoning, injuries, intentions].
- 9OVER.
False-alert cancellation (MSC.514(105))
Once the error is detected, do not switch off the set; cancel by voice on ch 16:
- 1ALL STATIONS, ALL STATIONS, ALL STATIONS.
- 2THIS IS [name, call sign, MMSI].
- 3Please cancel my distress alert of [UTC time] on [frequency/channel].
- 4No distress, no distress.
- 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.
STCW Bridge Watch Lens
Decide applicability before manoeuvring: Rules 4-10 apply in any visibility, Rules 11-18 only when vessels are in sight, and Rule 19 governs radar-only encounters in restricted visibility.
Build the traffic picture with sight, hearing, radar/ARPA and chart context.
Do not let AIS or one isolated bearing replace systematic observation.
After manoeuvring, keep monitoring bearing, range, CPA/TCPA and passing distance until the other vessel is finally past and clear.
Exam Focus
Start every scenario by classifying the encounter: overtaking, head-on, crossing, narrow channel, traffic separation, or restricted visibility.
If two rules seem to conflict, check the order carefully: overtaking duties still apply, and Rule 2 still requires ordinary seamanship.
Key Takeaways
Channel 70 is DSC only; channel 16 is voice.
The DSC alert does NOT replace the voice Mayday — it precedes it.
An accidental alert must be cancelled immediately by voice.
Use high power only when needed: low power in port.
Channel 70 (156.525 MHz) is the only VHF DSC distress channel; associated voice moves to channel 16.
The VHF-DSC alert carries MMSI, position, UTC time, nature of distress and subsequent mode.
Any DSC set installed after July 2002 must inject position automatically from the navigation interface.
False alerts must be cancelled on channel 16 following resolution MSC.514(105).
Ch 16 voice, ch 70 DSC, ch 13 bridge-to-bridge, ch 6 on-scene SAR; ch 70 does not carry voice.
Dual/Tri Watch and Scan keep 16 alive alongside other channels; squelch must be adjusted, not maximized.
1 W/25 W: 1 W is mandatory in port, restricted waters and whenever the coast station orders it.
DSC received-alert logs are reviewed every watch and copied into the radio log before being cleared.
Sequence: cover → button ≥3-5 s → position/time → nature → DSC on ch 70 → voice MAYDAY on ch 16.
Cancellation: voice on ch 16 with 'ALL STATIONS ×3' + identity + alert time + 'no distress, no distress' (MSC.514(105)).
On a received alert, wait ~5 min before acknowledging to let the coast station respond first (ITU-R M.541).
Common Mistakes
Transmitting voice on channel 70.
Forgetting the voice Mayday after the DSC alert.
Leaving a false alert uncancelled.
Wrong or unprogrammed MMSI.
Trusting the position will populate automatically without verifying the GNSS interface.
Pressing the red button 'to test' without knowing the cancellation sequence and generating a false alert.
Trying to talk on channel 70: it is DSC only.
Replying by voice immediately without giving the coast station time to acknowledge first.
Using 25 W inside a port and congesting other stations instead of switching to 1 W.
Maxing out the squelch 'to kill noise' and missing weak but real calls.
Clearing the DSC received-alert log without transferring entries to the radio log.
Releasing the button before the 3-5 s hold completes and unknowingly leaving the alert unsent.
Acknowledging someone else's alert on DSC immediately without waiting the 5 min and clogging the coast station.
Cancelling only on DSC and forgetting the voice cancellation on ch 16, or vice versa.
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