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My modules›M3 GMDSS›14. MF/HF DSC and long-range radio
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MF/HF DSC and long-range radio

Lesson 4
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GMDSS≈ 8 min read

Module 4 — MF/HF radiotelephony and DSC

Selected propagation examples, not to scale: a direct VHF path, a ground wave following the surface, and a two-hop HF sky wave refracted in the ionosphere with an intermediate sea-surface reflection. Actual propagation depends on conditions; no fixed range is implied.

1The lesson in plain words

In one sentence

With MF/HF, DSC alerts and voice use different frequencies: read the right pair and follow the approved procedure.

What does MF/HF add beyond VHF?

An HF alert may concern a vessel beyond local VHF range, and each frequency has a job in the exchange.

How do you read the table?

Every DSC distress frequency has an associated radiotelephony frequency; read the full pair before changing the set.

How do you choose a band?

Path, daylight, season and solar conditions matter; UTC alone cannot select the band.

How do you test the set?

Follow the approved procedure; never send a live distress alert to check it.

2Step by step

HF alert: choose the band, listen and relay

  1. 1

    Use the approved single- or multi-frequency procedure.

    UTC alone cannot select the band.

  2. 2

    Note the received identity, position, time and distress information.

    The alert and its context must remain clear.

  3. 3

    Listen for five minutes on the associated voice frequencies.

    Subsequent voice traffic uses frequencies different from DSC.

  4. 4

    Inform a coast station or RCC by an appropriate means.

    Shore coordination precedes an instructed relay.

  5. 5

    Transmit the DSC relay only if instructed, under the master's authority.

    Receiving an alert does not authorize an automatic relay.

3Key data

DSC 2187.5 kHz
Voice: 2182 kHz
DSC 4207.5 kHz
Voice: 4125 kHz
DSC 6312 kHz
Voice: 6215 kHz
DSC 8414.5 kHz
Voice: 8291 kHz
DSC 12577 kHz
Voice: 12290 kHz
DSC 16804.5 kHz
Voice: 16420 kHz
MF/HF watch
2187.5 + 8414.5 kHz
Additional HF
At least one of four, by time/position
Unacknowledged alert
Five minutes on associated voice

4Key points

  1. 1

    DSC and voice are paired, but they are not interchangeable.

  2. 2

    The watch keeps required frequencies and adds a suitable HF frequency.

  3. 3

    HF depends on path, daylight, season and solar conditions.

  4. 4

    Unacknowledged alert: listen, inform shore and relay only when instructed.

5Frequent mistakes

  • Wrong: "I use the DSC frequency for voice traffic."

    Right: Subsequent voice uses the associated radiotelephony frequency, not the DSC frequency.

  • Wrong: "UTC alone tells me which band to choose."

    Right: Path, daylight, season and solar conditions also matter.

  • Wrong: "I send a live alert to test the set."

    Right: Follow an approved test; never use a live distress alert to check it.

6Exam keys & interpretation

Exam keys

  • When you see «DSC» and «voice», look for a frequency pair, not one frequency.
  • When «8414.5 kHz» appears, do not drop that watch because another band seems better.
  • For an «unacknowledged HF alert», listen, inform shore and relay only if instructed.

How to read it

  • Treat each row as a pair: one frequency carries the digital alert and the other carries subsequent voice.
  • Receiving an alert does not make you an automatic relay: listen, coordinate and respect the stated authority.

7Go deeper: GOC exam level

The detail behind each step for the General Operator's Certificate: the rules, the radio technique and worked cases.

Learn the frequencies in pairs

BandDSC distress/safetyVoice distress traffic
MF 2 MHz2187.5 kHz2182 kHz
HF 4 MHz4207.5 kHz4125 kHz
HF 6 MHz6312 kHz6215 kHz
HF 8 MHz8414.5 kHz8291 kHz
HF 12 MHz12577 kHz12290 kHz
HF 16 MHz16804.5 kHz16420 kHz

Choosing the HF band for an alert

ITU-R M.541 allows a distress alert attempt on a single HF frequency or a multi-frequency attempt on up to six DSC distress frequencies; many sets automate the sequence. The point is to raise the chance that some coast station hears you when propagation is uncertain.

8 MHz is a common first automatic choice on modern equipment, but the best band depends on the path. If acknowledgements arrive on more than one band, start distress traffic on one of them; if that fails, try the others.

A ship that receives an HF distress alert

On HF, ship stations do not acknowledge another ship's distress alert by DSC.

  1. 1Watch for a coast-station acknowledgement.
  2. 2Set the receiver and transmitter to the voice distress frequency of the same band (8291 kHz for an alert on 8414.5 kHz).
  3. 3If no coast station acknowledges within about five minutes and no distress traffic follows, contact a coast station or RCC by any available means and report what you received.
  4. 4Send a DSC distress relay only if that coast station or RCC asks for one, addressed to it alone. Never broadcast a relay to all ships.

SSB voice: what J3E means

MF/HF voice uses J3E: single sideband with suppressed carrier, normally the upper sideband (USB). Classic AM wastes power on the carrier and on a second sideband that carries the same information. SSB sends a single sideband and no carrier, packing useful energy into a narrow channel, which matters on power-limited HF links.

The receiver has to rebuild the carrier locally, so a slight mistuning makes voices sound too low or too high ("Donald Duck").

ClassPlain meaningWhere you meet it
A3EDouble-sideband AM with full carrierBroadcast/compatibility
H3ESSB with full carrierSpecific uses
R3ESSB with reduced carrierSpecific uses
J3ESSB with suppressed carrierMF/HF maritime voice
F3E / G3EFM / phase modulation telephonyVHF maritime voice
F1B / J2BDigital data, automatic receptionMF/HF DSC
G2BDigital data on a phase-modulated subcarrierVHF DSC
A1AMorse telegraphyHistorical

Receiver controls that change readability

  • AGC (automatic gain control) keeps the audio steady across strong and weak signals.
  • RF gain can be reduced to avoid overload and intermodulation; do not use it at random.
  • Filter / bandwidth: narrower can improve SNR and selectivity, but too narrow distorts voice.
  • Noise blanker / reduction helps but can add artefacts; it never replaces a good antenna or a hunt for on-board noise.
  • Sensitivity is the ability to detect a weak signal; selectivity is the ability to separate it from nearby frequencies.

HF noise: often made on board

Noise comes from atmospherics, thunderstorms, galactic noise, other transmissions and, very often, the ship itself: inverters, switch-mode supplies, LED drivers, variable-frequency drives, motors, chargers and data networks. A professional diagnosis compares noise with suspect equipment switched on and off, and across bands.

Changing band or mode on an MF/HF console

  1. 1Identify the service: distress, urgency, safety or routine.
  2. 2Select the band and the frequency or ITU channel from propagation and the publications.
  3. 3For voice, select J3E/USB.
  4. 4Press TUNE and let the ATU match after any frequency change.
  5. 5Wait for TUNED/READY and watch for alarms or high VSWR.
  6. 6Adjust RF gain, AGC or noise blanker only if reception needs it.
  7. 7Transmit with the power the link needs.
  8. 8If the exchange began by DSC, move to the traffic frequency set by the procedure or acknowledgement.

ACS: the automatic connection system

ACS uses DSC to automate setting up an MF/HF link: scanning, calling station, called station and choice of a working frequency are handled by the equipment under ITU-R M.541. It builds on the DSC equipment ships already carry.

References: ITU-R M.541 · ITU-R M.493

GOC exam check

  1. You receive a DSC distress alert on 8414.5 kHz with telephony as the follow-on. Which frequency do you set for the voice traffic?

    1. 8414.5 kHz
    2. 8291 kHz
    3. 6215 kHz
    4. 12290 kHz
    Show the answer

    B. 8291 kHz

    Why: 8291 kHz is the voice partner of 8414.5 kHz in the same band. Treating 8414.5 kHz as voice is a conceptual error, not just a wrong number.

  2. Which is the DSC distress and safety frequency in the 12 MHz band?

    1. 12290 kHz
    2. 16804.5 kHz
    3. 8414.5 kHz
    4. 12577 kHz
    Show the answer

    D. 12577 kHz

    Why: 12577 kHz is DSC; 12290 kHz is its voice partner. Learn each band as a pair so the voice number never gets picked as DSC.

  3. You received another ship's HF DSC distress alert. After about five minutes there is no coast-station acknowledgement and no distress traffic. What next?

    1. Send a DSC distress relay addressed to all ships
    2. Contact a coast station or RCC by any means and report it
    3. Acknowledge the alert by DSC on 8414.5 kHz
    4. Keep listening on 8291 kHz until traffic starts
    Show the answer

    B. Contact a coast station or RCC by any means and report it

    Why: Tell a coast station or RCC by any available means. Ships never ACK an HF alert by DSC, and a relay is sent only if that station asks, addressed to it alone.

  4. What does emission class J3E mean on an MF/HF set?

    1. Single sideband with full carrier
    2. Single sideband with reduced carrier
    3. Single sideband with suppressed carrier
    4. Double sideband with full carrier
    Show the answer

    C. Single sideband with suppressed carrier

    Why: J3E is SSB with suppressed carrier, the maritime MF/HF voice mode. H3E keeps a full carrier, R3E a reduced one, and A3E is double-sideband AM.

  5. On 4125 kHz in J3E the other station's voice sounds too high and squeaky. What is the usual cause?

    1. The receiver is slightly mistuned
    2. The AGC has been switched off
    3. The other station is using FM
    4. The RF gain is set too low
    Show the answer

    A. The receiver is slightly mistuned

    Why: In SSB the receiver rebuilds the carrier locally, so a small mistuning shifts the voice pitch. AGC and RF gain change level, not pitch.

  6. Propagation favours 8 MHz, but it shows S9 noise. 6 MHz has S2 noise and workable propagation. Which band will communicate better?

    1. 8 MHz, because propagation favours it
    2. 6 MHz, because its signal-to-noise ratio is better
    3. 8 MHz, with the RF gain turned fully up
    4. 4 MHz, because lower bands have less noise
    Show the answer

    B. 6 MHz, because its signal-to-noise ratio is better

    Why: Readability follows signal-to-noise ratio, not the band's reputation. Turning up RF gain lifts the noise with the signal.

  7. Loud noise appears on every HF band after new LED lighting was fitted on board. What is the best first step?

    1. Increase RF gain to lift the signal above the noise
    2. Narrow the filter as far as the set allows
    3. Move up to a higher band until the noise fades
    4. Switch suspect gear off and on, comparing noise
    Show the answer

    D. Switch suspect gear off and on, comparing noise

    Why: On-board noise is diagnosed by switching suspects off and on and comparing across bands. Filters and noise blankers never replace finding the source.

  8. Which MF watch does a SOLAS ship keep today?

    1. Automatic DSC watch on 2187.5 kHz
    2. Aural watch on 2182 kHz with silence periods
    3. Aural 2182 kHz and DSC on 2187.5 kHz
    4. Automatic DSC watch on 2182 kHz
    Show the answer

    A. Automatic DSC watch on 2187.5 kHz

    Why: The automatic DSC watch on 2187.5 kHz replaced the 2182 kHz listening watch on 1 February 1999. Silence periods are legacy; 2182 kHz is voice, not DSC.

  9. At 2200 UTC your ship, far offshore in A3 with MF/HF, receives a DSC distress alert on 12577 kHz from another ship, with telephony as the follow-on. Which frequency do you set for the voice traffic?

    1. 12577 kHz, the frequency the alert came on
    2. 12520 kHz, the old telex partner of the band
    3. 16420 kHz, the next band up for more range
    4. 12290 kHz, the voice partner of the band
    Show the answer

    D. 12290 kHz, the voice partner of the band

    Why: Each HF DSC frequency has a voice partner on the same band: 12577 kHz pairs with 12290 kHz in J3E. Staying on 12577 is the classic error, because DSC frequencies are for signalling, and 12520 kHz is the former NBDP frequency.

  10. Same scenario: a coast station acknowledges the alert and the MAYDAY traffic starts on the voice frequency. Your ship is 900 NM away and cannot help. What do you do?

    1. Acknowledge the alert by DSC so the casualty knows
    2. Keep listening on 12290 kHz and stay off the air
    3. Send a DSC distress relay to all ships on 12577 kHz
    4. Call the casualty on 12290 kHz to offer your help
    Show the answer

    B. Keep listening on 12290 kHz and stay off the air

    Why: With the shore already handling it, a distant ship keeps listening on the voice frequency, stays silent and logs what it hears. A ship never acknowledges an HF distress alert by DSC, and a relay is sent only if a coast station or RCC asks, never to all ships.

8Full lesson & sources

Open the full lesson (diagrams and official references)

MF/HF DSC alerts and subsequent voice traffic use different frequencies. The table pairs the six DSC frequencies with their associated radiotelephony frequencies. A compliant MF/HF distress watch includes 2187.5 and 8414.5 kHz plus at least one of the other four HF frequencies appropriate to time and position. It must not omit 8414.5 kHz merely because another band seems better.

HF propagation depends on the path, daylight, season and solar conditions; UTC alone cannot select a band. Use the approved single- or multi-frequency procedure. After receiving an unacknowledged HF DSC alert, listen on associated voice channels for five minutes, inform a coast station/RCC, and transmit a DSC distress relay only if instructed, under the master's authority. Follow approved test procedures; never test by sending a live distress alert.

DSC (kHz)Voice (kHz)
2187.52182
4207.54125
63126215
8414.58291
1257712290
16804.516420

Read the frequency pair as a change of task

On MF and HF every distress band has two frequencies: one where the DSC alert is sent and one where people then talk. Always give the frequency and its function together. 2187.5 kHz carries the MF DSC alert; 2182 kHz carries the MAYDAY by voice. On 8 MHz, 8414.5 kHz is the DSC frequency and 8291 kHz the voice frequency. They are 123.5 kHz apart, so tuning the receiver to one tells you nothing about what is happening on the other.

The pairing is fixed per band, so after a DSC alert you never have to guess: an alert received on 12577 kHz means voice traffic on 12290 kHz; one on 6312 kHz means 6215 kHz. There are six pairs, one per band. Learn them that way, and say "DSC" or "voice" every time you say a number.

Separate a distress watch from a transmit-band choice

Keep two decisions apart: the distress watch from a transmit-band choice. The watch is fixed by the rules: an MF/HF ship keeps DSC watch on 2187.5 kHz and 8414.5 kHz plus at least one of 4207.5, 6312, 12577 or 16804.5 kHz. A scanning DSC receiver does this for you. The transmit band is a judgement made for one call: which band will reach a coast station from here, now?

SituationBetter first choiceReasoning
150 NM from an MF coast station, any timeMF 2187.5 kHzGround wave covers this distance reliably
1,500 NM from the nearest HF coast station, early afternoon local time12 or 16 MHzDaytime ionosphere supports high bands; low bands are absorbed by the D layer
Same 1,500 NM path at 0300 local8 or 6 MHzAt night the usable maximum frequency falls; 16 MHz may skip over the station
Unsure of propagationMulti-frequency attemptITU-R M.541 allows one alert sent across up to six DSC distress frequencies

You may decide that 12 MHz is the best band to transmit on and still keep watching 8414.5 kHz. Choosing a transmit band never removes a watch frequency.

Why propagation beats power

HF radio reaches far because the ionosphere bends the signal back to earth. That creates a skip zone: beyond the ground wave (roughly a hundred miles at sea) and before the first sky-wave return, there can be a gap where a station hears nothing. If you are 300 NM from a coast station on 16 MHz and the signal returns 600 NM away, doubling your power does nothing. Changing band does. Use "higher by day, lower by night" as a first guess, then go by what you actually hear: the band where the coast station's signal is clear above the noise is the band to use.

Work through a received HF alert

Here is how a received HF alert plays out at 0312 UTC. The rule is to listen first, then tell the shore, and relay only if the shore asks you to.

  1. 10312: the receiver shows a distress alert on 8414.5 kHz from MMSI 538001234 at 12°40'S 064°20'E, nature flooding. You tell the master.
  2. 20312: you set receiver and transmitter to 8291 kHz, J3E, and listen. You do not acknowledge by DSC: on HF a ship station never does.
  3. 30317: after five minutes, no coast-station acknowledgement and no voice traffic heard.
  4. 40319: you call an MRCC by satellite phone and report the identity, position, time and nature you received.
  5. 50326: the MRCC asks you to relay. You send a DSC distress relay addressed only to that coast station, not to all ships, under the master's authority. You would send it only if instructed.

Keep a record you can rely on later

Log each step with its UTC time: the frequency you received on, the MMSI and position, the frequencies you monitored, whom you contacted and how, and the instructions you were given. Write "no acknowledgement heard on 8414.5 kHz between 0312 and 0317". Do not write "no station received the alert": a coast station 3,000 NM away may have acknowledged on a band you were not monitoring.

Official references · 2026-09-09

MSC.512(105) · MSC.1/Circ.1657/Rev.1 (2026-07-06)

Selected propagation examples, not to scale: a direct VHF path, a ground wave following the surface, and a two-hop HF sky wave refracted in the ionosphere with an intermediate sea-surface reflection. Actual propagation depends on conditions; no fixed range is implied.

Finish this unit

When you've read it, match the terms, then answer three questions. Get them all right to finish the unit; opening the page alone doesn't count.

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