47 C.F.R. § 80.1069 – Maritime sea areas.

(a) For the purpose of this subpart, a ship's area of operation is defined as follows:
1. (1) Sea area A1. An area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC alerting is available as defined by the International Maritime Organization.
2. (2) Sea area A2. An area, excluding sea area A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available as defined by the International Maritime Organization.
3. (3) Sea area A3. An area, excluding sea areas A1 and A2, within the coverage of an INMARSAT geostationary satellite in which continuous alerting is available.
4. (4) Sea area A4. An area outside sea areas A1, A2 and A3.
(b) Maritime sea areas are delineated in the International Maritime Organization Publication GMDSS Master Plan of Shore-Based Facilities. The Master Plan can be purchased from the International Maritime Organization, 4 Albert Embankment, London SE1 7SR, United Kingdom.