Radiotelephony and NBDP / Telex Procedures
What this domain tests
This domain tests the spoken and printed sides of GMDSS: voice discipline on the distress frequencies, and the narrow-band direct-printing conventions that carry distress, urgent and MSI traffic where voice is unusable.
Radiotelephony
Use the ITU phonetic alphabet (Alfa, Bravo, Charlie) and standard prowords: THIS IS, OVER, OUT, ROGER, SAY AGAIN, I SPELL, WAIT, CORRECTION. Call on the calling or distress frequency, then move to a working frequency. Distress: MAYDAY; urgency: PAN PAN; safety: SECURITE. Distress frequencies are Ch 16 (156.8 MHz), 2182 kHz and HF 4125 / 6215 / 8291 / 12290 / 16420 kHz; bridge-to-bridge is Ch 13; on-scene and SAR aircraft working use Ch 6 and 16, with 3023 and 5680 kHz for aircraft. Silence is imposed by the controlling station with SEELONCE MAYDAY and lifted with SEELONCE FEENEE; PRUDONCE restricts working under urgency.
NBDP
Narrow-band direct printing uses a seven-unit constant-ratio code (four marks, three spaces), 100 baud, 170 Hz shift. Two modes matter:
- ARQ - two-station, point-to-point, error-correcting; used for ship-to-shore telex.
- FEC (CFEC) and selective FEC - one-way broadcast; the right choice when no acknowledgment can be expected and for MSI broadcasts.
Distress NBDP frequencies: 2174.5 kHz (MF) and 4177.5, 6268, 8376.5, 12520, 16695 kHz (HF); the associated DSC frequencies are 2187.5 kHz and 4207.5 / 6312 / 8414.5 / 12577 / 16804.5 kHz. NBDP carries no voice, and distress traffic begins with MAYDAY in the message text.
Worked example
You sent a DSC distress alert on 2187.5 kHz and no acknowledgment has arrived after five minutes. Reasoning: send the distress message by NBDP on 2174.5 kHz using FEC so any coast or ship station can read it, including MAYDAY, identity, position, nature of distress, assistance required and persons on board. At the same time keep trying radiotelephony on 2182 kHz and consider satellite alerting.
Traps
Using ARQ for a broadcast when no station has been contacted; expecting voice on 2174.5 kHz; entering an NBDP frequency in the DSC subsequent-communication field that does not pair with the alerting frequency; dropping the phonetic alphabet when spelling a name or position; leaving a frequency without announcing it.