The polar sea areas beyond GMDSS Area A3, not covered by geostationary satellites, where HF radio is used for distress alerting.
Quick facts
Frequency
406 MHz
In practice
HF radio propagation in polar regions is affected by ionospheric disturbances linked to geomagnetic activity, which can disrupt communications for extended periods. Radio operators on A4 voyages must be proficient in HF frequency management, selecting the appropriate working frequency for the time of day, season, and distance to a shore station. The GMDSS General Operator Certificate is required for HF operations, and operators are expected to maintain logs of all safety communications. Upcoming LEO satellite systems such as Iridium are expected to complement and eventually supersede HF radio for distress alerting in polar areas under future IMO GMDSS modernisation amendments.
Regulatory detail & full definition
GMDSS Sea Area A4 covers the polar sea areas not served by Inmarsat geostationary satellites, principally the Arctic Ocean above approximately 70°N and the Southern Ocean below approximately 70°S, where the geostationary satellite geometry is so poor that A3 satellite coverage cannot be relied upon. Vessels operating exclusively or partly in Sea Area A4 must be fitted with an HF radio installation providing DSC, radiotelephony, and narrow-band direct-printing telex on all GMDSS HF working frequencies, in addition to VHF, MF, and 406 MHz EPIRB. The IMO Polar Code, which became mandatory in 2017, adds further requirements for communication and navigation equipment in polar waters.
HF radio propagation in polar regions is affected by ionospheric disturbances linked to geomagnetic activity, which can disrupt communications for extended periods. Radio operators on A4 voyages must be proficient in HF frequency management, selecting the appropriate working frequency for the time of day, season, and distance to a shore station. The GMDSS General Operator Certificate is required for HF operations, and operators are expected to maintain logs of all safety communications. Upcoming LEO satellite systems such as Iridium are expected to complement and eventually supersede HF radio for distress alerting in polar areas under future IMO GMDSS modernisation amendments.