RYA SRC Part 8 — Protecting Distress Frequencies: False Alerts, Testing, and Guard Bands

21 May 2026 PersonalSRCRYAfalse-alerts

Syllabus coverage: CEPT/RYA section C2 — avoidance of harmful interference, transmissions during distress traffic, prevention of unauthorised transmissions, test protocols, guard bands, and the procedure to follow when a false or inadvertent distress alert is transmitted.

GMDSS works because every alert is treated as real until proven otherwise. That credibility is fragile: a small percentage of false alerts ties up SAR resources, wastes Coastguard time, and conditions operators to second-guess. The CEPT regulations protect the distress system with rules covering channel use, equipment testing, and what to do when something goes wrong. The exam tests every one of them.

CH16 — what you can and cannot do

CH16 (156.800 MHz) is the international distress, safety, and calling channel. The rules:

  • Listening watch. Every DSC-equipped vessel underway maintains a listening watch on CH16. Class D sets default to CH16 when idle.
  • Distress traffic takes priority over everything else.
  • Safety traffic is permitted — SECURITE broadcasts and the announcement of follow-up traffic on a working channel.
  • Calling is permitted — making initial contact with another station before moving to a working channel. Move quickly. Don’t have your conversation on 16.
  • No routine traffic. No casual chat. No “how was your sail this morning.”
  • No testing of voice transmissions on 16. Use a working channel and another vessel/coast station for radio checks.
  • No music, no profanity, no jokes, no station ID by call sign alone without a purpose.

The exam often asks: “What is CH16 used for?” The correct answer is distress, safety, and calling — in that order — and nothing else.

CH70 — DSC only

CH70 (156.525 MHz) is reserved exclusively for DSC. The rules:

  • No voice transmission, ever. Class D sets refuse to key voice on CH70, but never even attempt it.
  • The set listens on CH70 continuously, even when you’re tuned to another channel for voice — the Class D’s CH70 watch receiver runs in parallel with the voice receiver.
  • CH70 is not a backup distress channel. A distress voice call goes on CH16, not 70. CH70 carries the DSC alert burst only.

The exam catches people who, faced with “Where do you make the voice distress call?”, say “CH70” because that’s where the DSC alert went. Voice distress = CH16. Always.

Guard bands — CH15, 17, 75, 76

The channels immediately adjacent to CH16 and CH70 are protected. The exam asks why and how.

Channels 15 and 17 — guarding CH16

CH15 (156.750 MHz) and CH17 (156.850 MHz) are the channels sitting 50 kHz either side of CH16. Strong transmissions on 15 or 17 can splash into 16, especially with old or poorly-designed equipment. To minimise this:

  • CH15 and CH17 are restricted to 1 W maximum output (low power only).
  • Used for on-board communications (within a single vessel) — for example, between a yacht’s helm and a tender at short range — and limited port operations.
  • Class D sets enforce low power on these channels automatically.

Channels 75 and 76 — guarding CH70

Similarly, CH75 (156.775 MHz) and CH76 (156.825 MHz) sit either side of CH70. They are:

  • Restricted to low power, with usage limits.
  • Used for very short-range port operations and on-board communications.
  • Treated as guard bands to keep adjacent-channel splatter off the DSC frequency.

Memorise: 15, 17, 75, 76 — low power only. The exam can frame this as “Which channels are restricted to 1 W?” or “What are CH75 and CH76 used for?”

Testing — radio checks and DSC test calls

You need to know your radio works before you need it. The CEPT syllabus is specific about how to test.

Voice radio checks

The traditional voice radio check goes:

[Coast station, OR another vessel] [3x]
THIS IS [your vessel] [3x]
RADIO CHECK
OVER

The other station responds with reception quality (loud and clear, weak but readable, broken, etc.). Rules:

  • Not on CH16. Use a working channel — CH06, CH08, or one of the small craft safety channels. Some coast stations in the past offered radio checks on a specific channel; check current Coastguard guidance for your region.
  • In the UK, HM Coastguard no longer routinely offers radio checks, in part because volume was tying up shore staff. Use another vessel, a marina station, or a commercial radio check service (such as SeaStart). Routine ship-to-ship radio checks on CH73 or similar are common.
  • Don’t request a radio check at the start of a busy port channel — it interrupts working traffic.
  • A reply of “loud and clear” confirms both transmit and receive. A “no reply” doesn’t necessarily mean transmit failure; could be propagation, could be no station listening.

DSC test calls

Most Class D sets have a built-in DSC Test function that sends a DSC test call to a coast station MMSI. The coast station’s set acknowledges by DSC. This tests the full digital chain — encoder, RF, decoder, receiver — without any voice traffic and without inconveniencing other VHF users.

Rules:

  • Send the test to a designated coast station MMSI that accepts test calls. UK Coastguard MMSIs are listed in the Maritime Safety publications and on Ofcom’s website. Don’t test against a random vessel.
  • Don’t run multiple tests in succession. Once is enough to confirm the chain.
  • The test does NOT route distress traffic — it’s a separate function on a separate menu path from the distress button.

What you must NEVER do as a test

  • Press the distress button “to see what happens.” That’s a real distress alert. Coast stations will respond, SAR may be tasked, and you’ll be reported.
  • Voice MAYDAY on CH16 “as a drill.” Same problem.
  • Voice on CH70. It’s DSC-only.

The exam asks: “How do you test the voice transmit function of your VHF set?” Answer: by making a radio check call on a working channel to another vessel or commercial radio check service.

Unauthorised transmissions

The CEPT regulations require:

  • Marine VHF radio is only operated by, or under the direct supervision of, holders of an Operator’s Certificate (SRC, ROC, LRC, or GOC for VHF).
  • A non-SRC holder may use the radio in a distress situation (the syllabus allows this — “in emergency you do what you must”), but routine operation by an unlicensed person under non-emergency conditions is unauthorised.
  • The master of the vessel must authorise distress transmissions. In practice this is the person sending them, but the master is accountable.

Other forms of unauthorised use:

  • Transmission without proper identification (failing to give callsign / MMSI / vessel name).
  • Transmission with the wrong MMSI (a borrowed radio with someone else’s MMSI programmed in is illegal — the MMSI is part of the station licence).
  • Profanity, music, false signals, deliberate interference.
  • Personal correspondence on safety/calling channels.

The exam asks: “Who may operate a marine VHF radio?” Answer: a holder of the SRC (or higher), or someone under the direct supervision of an SRC holder, with the master’s authorisation.

Transmissions during distress traffic

When distress traffic is in progress on CH16:

  • No station may transmit on CH16 except as part of the distress.
  • This includes routine, urgency (if not yet upgraded to distress), and safety traffic.
  • The controlling station may impose SEELONCE MAYDAY (covered in Part 5) to formalise this.
  • Communications on other channels may continue — distress only locks CH16 (or whichever channel the distress is being worked on).

If you are mid-conversation on CH06 and a distress lights up on CH16, you do NOT need to stop your CH06 traffic — but you must not transfer that conversation back to 16 until SEELONCE FEENEE.

The false alert — what to do when you press the wrong thing

Inadvertent activation of the distress button is the most-tested scenario on the exam after the MAYDAY format itself. Procedure:

1. Do NOT just power off the radio.

Powering off doesn’t cancel anything. The alert has already been transmitted; the satellite payload and any DSC-equipped station in range have heard it. And if the alert was triggered while in a distress-retransmit cycle, the set may resume transmitting when powered back on.

2. Use the set’s “Distress Cancel” function

Most Class D sets have a specific cancellation function in the DSC menu (e.g. “Cancel Distress” or similar). This sends a DSC cancellation message addressed to “All Stations.” Use it.

3. Make a voice cancellation on CH16

Even after the DSC cancel, make a voice broadcast on CH16:

ALL STATIONS ALL STATIONS ALL STATIONS
THIS IS [your vessel] [your vessel] [your vessel]
[Your callsign or MMSI]
POSITION [position at time of false alert]
PLEASE CANCEL MY DSC DISTRESS ALERT OF [time HHMM] UTC
PLEASE CANCEL MY DSC DISTRESS ALERT OF [time HHMM] UTC
THIS IS [vessel name] [vessel name]
OUT

The repetition of “cancel” matters — coast stations and other vessels need to be in no doubt this is a cancellation, not another distress.

4. Call the Coastguard directly

If you have phone signal, call HM Coastguard (the MRCC for your area) and confirm the cancellation. They will already be working the alert; the direct call lets them stand down quickly.

5. If it was an EPIRB

If you accidentally activated the EPIRB (not just the DSC distress button), follow the EPIRB cancellation procedure from Part 7: turn it off, then call the MRCC.

What you should NEVER do

  • Pretend it didn’t happen and hope no-one noticed.
  • Power off the set and walk away.
  • Send a flippant “ignore that” without using the proper cancellation phrasing.

The exam gives you a scenario (“you’ve sent a distress alert by mistake”) and expects you to recite the procedure: DSC cancel, voice broadcast cancellation on CH16, direct contact with MRCC if possible.

Why this matters

A high false-alert rate degrades the system. The Coastguard published, some years ago, the statistic that a significant fraction of EPIRB activations and DSC distress alerts received were false — most through unfamiliarity with the equipment or accidental button presses. Each one costs lifeboat hours, helicopter fuel, and operator attention that should be available for the next genuine distress.

The CEPT regulations build in the testing protocols and cancellation procedures specifically because the system depends on operator competence. The SRC is the certification that you know them.

What to take into the next post

You should leave Part 8 able to:

  • State that CH16 is for distress, safety, and calling only.
  • State that CH70 is DSC only, no voice, ever.
  • Identify CH15, 17, 75, 76 as low-power guard bands.
  • Explain how to test a voice radio (working channel, another station, not CH16).
  • Explain how to test a DSC set (test call function to a coast station MMSI).
  • Identify the steps to cancel a false distress alert (DSC cancel + voice on CH16 + call MRCC).
  • Explain who may operate a marine VHF radio.

Part 9 takes us into the regulations themselves — the licences you need, what they cover, and who makes the rules.