Radiotelephony and narrow-band direct-printing (NBDP) equipment
Modes and controls
GMDSS radiotelephony uses VHF FM and MF/HF single-sideband suppressed-carrier telephony. NBDP uses frequency-shift keying, commonly 100 baud with 170 Hz shift, and a seven-unit constant-ratio code. ARQ is a point-to-point telex mode with automatic repeat request; FEC is a broadcast mode with forward error correction, used for MSI and distress traffic. Typical controls are frequency or channel selection, mode, transmitter power, receiver gain, AGC, clarifier or RTTY tuning, and antenna tuner tune. The antenna tuner matches the transmitter to the whip or insulated backstay.
Operating practice
Listen before transmitting. Use the published paired NBDP channel, not a DSC distress channel. Tune the antenna into a dummy load when testing. Keep modulation within limits and use standard phonetic spelling. For NBDP, confirm the coast station's ARQ or FEC capability and the correct shift and baud rate. Store the ship's answerback and selcall identity correctly; these identify the terminal in telex procedures.
Worked example
An HF NBDP terminal will not connect to a coast station. The operator has selected J3E and is calling on 8414.5 kHz. Correct the setup: select the published NBDP channel, mode F1B, ARQ, 100 baud, 170 Hz shift, tune the antenna tuner, and call the coast station's telex number. If ARQ fails, try the coast station's FEC broadcast or an alternate channel. The reasoning is that 8414.5 kHz is a DSC distress and calling frequency, J3E is voice, and NBDP requires the correct mode and paired frequency.
Traps
Do not use 2182 kHz for NBDP. Do not select J3E for NBDP. Do not use an 85 Hz shift when 170 Hz is required. Do not transmit a continuous carrier for tuning on an operational frequency. Do not rely on the clarifier to correct a large frequency error. Do not confuse ARQ with FEC; ARQ needs a responding station. Do not overlook a high SWR caused by a disconnected antenna.
Key facts
NBDP is F1B, not voice. ARQ is point-to-point; FEC is broadcast. Standard NBDP is 100 baud with 170 Hz shift.