GMDSS divides the world’s oceans into four “sea areas”, designated A1 through A4.
An area within the radiotelephone coverage of at least one VHF coast station in continuous digital selective calling (DSC), alerting is available.
Area could extend up to 30-50 Nautical Miles.
An area within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available.
Area could extend up to 200 nautical miles.
AREA A3 :-
An area, excluding area A1 and A2 within coverage of an Inmarsat geostationary satellite in which continuous alerting is available.
Range – Between latitude (76 degree N to 76 degree S)
IMO requirements for Range :(70 degree N to 70 degree S)
AREA A4 :-
An area outside , Sea area A1, A2 and A3. This is essentially the polar region.