Question 21B6

When NAVTEX broadcasts on MF (Medium Frequency) are degraded by atmospheric interference, especially in tropical regions prone to static from thunderstorms, the GMDSS operator should switch to a more robust, long-range system. Moving to **HF (High Frequency) Maritime Safety Information (MSI)** is the appropriate solution. HF signals can propagate over vast distances and, while also affected by atmospheric noise, offer more frequency options and often better reliability for long-range communication than MF under severe conditions. There are **8 designated HF MSI frequencies** used globally. The standard broadcast mode for these transmissions is **FEC (Forward Error Correction) TELEX**, also known as SITOR-B. FEC adds redundant information to the data stream, allowing the receiver to correct errors without needing a return channel, which is ideal for broadcast services where many receivers are listening simultaneously. Therefore, selecting one of the 8 HF MSI frequencies and setting the transceiver to FEC TELEX mode (B) directly addresses the problem of atmospheric interference affecting MF NAVTEX by using a more robust frequency range and error-correcting broadcast mode. Options A, C, and D are incorrect because: * A specifies 6 HF frequencies (there are 8) and ARQ TELEX (ARQ is for point-to-point, FEC for broadcast). * C and D suggest staying on MF, which is the frequency range experiencing interference, and D incorrectly references a "tropical MSI" MF frequency.