Inmarsat has announced that it will migrate Inmarsat-C, Mini C, and Fleet77 used for Global Maritime Distress & Safety System, Long Range Identification and Tracking and Ship Security Alerting Systems from their I-3 to their I-4 Network. This migration will begin 1000 UTC on April 16, 2018 and take approximately nine months to complete.
Source: Investigations & Casualty Analysis
Link: 4/9/2018: Inmarsat network migration impact on maritime distress and safety services