For the purpose of GMDSS, four operational zones have been established loosely based on distance from shore and in range of different communication systems.
- SEA AREA A1: the area within the radiotelephone coverage of at least one VHF coast station in which continuous DSC (Digital Selective Calling) alerting is available;
- SEA AREA A2: the area, excluding Sea Area A1, within the radiotelephone coverage of at least one MF coast station in which continuous DSC alerting is available;
- SEA AREA A3: the area, excluding Sea Areas A1 and A2, within the coverage of an approved satellite constellation in which continuous alerting is available; and
- SEA AREA A4: an area outside sea areas A1, A2 and A3.
In practical terms, this means that ships operating exclusively within about 35 nautical miles from the shore may be able to carry only equipment for VHF-DSC communications; those which go beyond this distance, up to about 150 to 400 nautical miles from shore, should carry both VHF-DSC and MF-DSC equipment; while those operating further from the shore but within the footprints of an approved satellite service should additionally carry approved satellite terminal(s). Previously and until approved Iridium terminals are available, this has meant that only Inmarsat connected vessels met this requirement.
In the early days of GMDSS, Inmarsat C was the preferred option and minimum requirement where satellite services were mandated. The larger Inmarsat A and B systems were also approved but these were quite expensive and considered as ‘overkill’ by many shipowners.
Current compliant services include Inmarsat B, Inmarsat C, Mini C and Fleet 77. Inmarsat’s L-Band satellite network is available in areas A1 to A3 but does not extend to area A4 which is effectively waters in Polar regions.
In these areas HF communications are required although vessels equipped with Iridium communication systems can communicate with shore and ship to ship providing both vessels have the equipment.
In 2018, after more than four years of lobbying, Iridium was finally given the green light as an authorised GMDSS supplier at MSC 99 so ending Inmarsat’s monopoly on safety service provision. Before Iridium can commence providing services it must first enter into a Public Services Agreement with IMSO and begin production of suitable equipment either directly or in conjunction with equipment suppliers. Iridium is expected to be in a position to begin services in 2020. At the same MSC meeting in May, Inmarsat’s Fleet Broadband service was also given approval for use in GMDSS. The IMO document, Resolution MSC.451(99) – statement of recognition of maritime mobile satellite services provided by Iridium satellite LLC officially ended the Inmarsat monopoly under GMDSS.