Area A1: Covered by VHF coast radio stations providing digital selective calling (DSC) alerting services (typically 20 NM from the coast).
Area A2: Covered by MF coast radio stations providing continuous DSC alerting (typically about 200 NM from the coast).
Area A3: Covered by the Inmarsat geostationary satellites (typically global coverage about 70N to 70S latitude 0 or DSC HF radio.
Area A4: For those outside the other areas (only required for vessels sailing in the polar regions out of Inmarsat satellite coverage). Served by DSC HF radio.