Inmarsat is migrating the GMDSS Services, Inmarsat C and Fleet 77, from the Inmarsat-3 satellite constellation to the Inmarsat-4 satellite constellation. During the migration manual intervention on Inmarsat Ship Earth Stations (SES) may be necessary. After each phase of the migration the coverage area of these GMDSS services will change.
The change of coverage area means that areas that were Sea Area A3 will now be Sea Area A4. This will affect the ships’ ability to transmit and receive...
https://safety4sea.com/changes-expected-to-inmarsat-satellite-services-including-gmdss/