MF/HF Radiotelephony, DSC and NBDP Fundamentals
Frequencies and roles
MF distress DSC is on 2187.5 kHz and MF distress voice on 2182 kHz. HF DSC distress frequencies are 4207.5, 6312, 8414.5, 12577 and 16804.5 kHz. HF distress voice uses 4125, 6215, 8291, 12290 and 16420 kHz. NBDP distress traffic uses 2174.5 kHz and the HF pair 4177.5, 6268, 8376.5, 12520 and 16683 kHz. The exam expects you to keep DSC, voice and NBDP roles separate.
Propagation and band choice
MF ground wave gives reliable short-range coverage. HF skywave depends on the ionosphere: higher frequencies work better by day and over longer paths, lower frequencies by night. A DSC alert should be followed by the associated voice or NBDP frequency, and the operator should select a band likely to reach the coast station at that time.
NBDP
NBDP is narrow-band direct printing, used for telex over radio. SITOR-A is ARQ, a two-way link with error correction; SITOR-B is FEC, a broadcast mode. Distress telex normally uses the dedicated frequencies, not a general commercial channel.
Worked example: a vessel 600 nm offshore has a medical emergency at night. Send the HF DSC alert on 6312 kHz, then establish voice on 6215 kHz, with NBDP on 6268 kHz if a telex link is needed. The reasoning: 6 MHz is a reasonable night band for that range, and the three services sit on matched frequencies.
Traps
Do not send DSC on 2182 kHz; 2182 is voice. Do not assume the highest HF band is best at night. Do not start an NBDP distress link without checking that the coast station is listening on that frequency. Remember that HF DSC is a scanned watch, so the alert must be repeated or followed up correctly if no acknowledgement arrives.