IMO SOLAS Chapter IV GMDSS Sea Areas
A1=Sea-area VHF coverage, 10-20NM offshore
A2=Sea-area MF coverage, 100-150NM offshore
A3=Sea-area Inmarsat/HF coverage, between latitudes 70N and 70S
A4=Sea-area north of 70N and south of 70S
GMDSS regulations are applicable to all ocean-going passenger vessels and all ocean-going cargo vessels of over 300GT on international voyages.
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