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
Use the approved single- or multi-frequency procedure.
UTC alone cannot select the band.
- 2
Note the received identity, position, time and distress information.
The alert and its context must remain clear.
- 3
Listen for five minutes on the associated voice frequencies.
Subsequent voice traffic uses frequencies different from DSC.
- 4
Inform a coast station or RCC by an appropriate means.
Shore coordination precedes an instructed relay.
- 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.
DSC and voice are paired, but they are not interchangeable.
- 2.
The watch keeps required frequencies and adds a suitable HF frequency.
- 3.
HF depends on path, daylight, season and solar conditions.
- 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.
6Practice case & exam keys
Situation
An HF DSC distress alert is received on 8414.5 kHz. In the isolated exercise, no coast acknowledgement or associated voice exchange is heard during the five-minute monitoring period. A learner immediately prepares an all-ships relay.
What should happen before a DSC distress relay in this situation?
Show the answer →
Inform an appropriate coast station/RCC through an effective available means and relay only if instructed, under the master's authority.
Why: The received-alert procedure is designed to connect information to coordinated assistance while avoiding unnecessary relays. The associated voice frequency is 8291 kHz. Not hearing a reply is information to assess and report; it is not a general authorization to retransmit an alert.
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.
7Full 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) |
|---|---|
| 2187.5 | 2182 |
| 4207.5 | 4125 |
| 6312 | 6215 |
| 8414.5 | 8291 |
| 12577 | 12290 |
| 16804.5 | 16420 |
Read the frequency pair as a change of task
The DSC/voice table is a relationship between two tasks. The digital frequency carries the selective call or alert. Its associated radiotelephony frequency carries the subsequent spoken distress communication. Similar numbers do not make the channels interchangeable.
For example, 8414.5 kHz and 8291 kHz belong to different parts of the exchange. A receiver being set to the first does not establish that the operator is listening to voice traffic on the second. Learn to state both the frequency and its function when describing a setup.
Separate a distress watch from a transmit-band choice
The prescribed watch and the choice of a useful transmission path answer different questions. The watch arrangement supports detection of incoming distress alerts. Selecting an HF band for a transmission considers the path and conditions as well as the authorized procedure.
| Decision | Relevant question |
|---|---|
| Required DSC watch | Which frequencies must the applicable installation monitor? |
| Transmission path | Which suitable service and band can reach the intended station? |
| Voice follow-up | Which radiotelephony frequency corresponds to the received alert? |
| Relay decision | Does the current procedure and coast-station/RCC instruction permit it? |
Do not remove the required 8414.5 kHz watch merely because another HF frequency seems promising for the path. Conversely, keeping the watch does not guarantee that any particular station will receive your transmission on that band.
Reason about propagation without a rigid rule
HF paths vary with ionospheric conditions, the daylight pattern along the path, season and other factors. A classroom phrase such as “try higher by day” is not a complete operational rule. It cannot replace current information, the approved equipment procedure and assessment of the actual path.
A received HF alert may concern a vessel far beyond local visual or VHF range. Its reception does not prove the receiving ship is the best unit to provide direct assistance. Read the position and time, consider the situation and avoid adding unnecessary traffic when better-placed units are already responding.
Work through an unacknowledged received HF alert
The current IMO flow separates monitoring, information to a coast station/RCC, and an instructed relay. Apply that sequence to the received-alert situation; do not confuse it with originating an alert for your own vessel.
- 1Note the received identity, position/time and distress information.
- 2Listen on the associated radiotelephony channel or channels for five minutes under the receiving procedure.
- 3Establish whether a coast station/RCC has acknowledged or relayed and whether distress traffic is in progress.
- 4When the procedure requires, inform a coast station/RCC by an appropriate means.
- 5Transmit the DSC distress relay only if instructed in this received-HF-alert situation, under the master's authority.
Preserve the information needed afterwards
Record the message information, reception time, relevant frequencies, communications with shore and instructions received. Separate observed events from assumptions about why a reply was absent. The absence of a reply you heard does not establish that no station received the alert.
Official references · 2026-09-09
8Test your knowledge
What relationship does the MF/HF table show?
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