MF/HF SSB
Single-side-band radio in MF (1.6–4 MHz) and HF (4–27 MHz) bands used for medium- and long-range maritime voice and DSC.
In practice
Long-range voice + DSC + NBDP. Older equipment but still required by SOLAS; routine testing on Sundays under the GMDSS schedule.
Regulatory detail & full definition
Single-side-band radio in the MF band (1.6–4 MHz) and HF bands (4–27 MHz) used for medium-range and long-range maritime voice and DSC. MF reaches up to 200–300 nautical miles in daylight; HF can reach worldwide with appropriate frequency selection. SOLAS Ch. IV requires MF and MF/HF DSC equipment for vessels operating in GMDSS Sea Areas A2 and A3 / A4 respectively. ITU Radio Regulations allocate maritime frequencies; the GMDSS distress frequencies are 2182 kHz (MF) and 4125 / 6215 / 8291 / 12290 / 16420 kHz (HF voice).
Authoritative source: ITU Radio Regulations ↗
Related
Source: ITU Radio Regulations
High confidence
Editorial confidence: how we grade.