Module 4 — MF/HF radiotelephony and DSC

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.

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.

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.

1. 1Note the received identity, position/time and distress information.
2. 2Listen on the associated radiotelephony channel or channels for five minutes under the receiving procedure.
3. 3Establish whether a coast station/RCC has acknowledged or relayed and whether distress traffic is in progress.
4. 4When the procedure requires, inform a coast station/RCC by an appropriate means.
5. 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

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