Covered by at least one VHF coast radio station providing DSC alerting services (typically 20 NM from the coast)
Sea area A2
Areas outside A1 and covered by at least one MF coast radio station providing continous DSC alerting (typically about 100 NM from the coast)
Sea area A3
Areas outside A1 and A2 covered by the Inmarsat geostationary satellites (typically global coverage from about 70N to 70S latitude) or DSC HF radio
Sea area A4
Areas outside A1, A2 and A3 (only required for vessels sailing in the polar regions out of Inmarsat satellite coverage). Served by DSC HF radio