Satellite communications (Inmarsat) systems
Architecture and GMDSS roles
Inmarsat uses geostationary satellites covering four ocean regions: Atlantic Ocean East, Atlantic Ocean West, Pacific Ocean, and Indian Ocean. A terminal communicates through a land earth station (LES), coordinated by a network coordination station (NCS). Inmarsat-C is a store-and-forward messaging terminal, not voice. It supports distress alerting, two-way messaging, position reporting, and Enhanced Group Call (EGC) SafetyNET for MSI. Distress priority is selected in the terminal and routed to a rescue coordination centre through the LES. FleetBroadband and Fleet services can carry voice and data, but only approved variants satisfy GMDSS carriage; verify the approval before relying on them.
Terminal setup
Select the correct ocean region and LES. Enter the ship's position and UTC accurately, preferably from GNSS. Enable EGC for the correct fixed areas so SafetyNET messages are received. Confirm the terminal is not shaded by the superstructure and that the radome is clear. Keep the distress alert procedure visible and know how to cancel a false alert through the terminal and by notifying the RCC.
Worked example
An A3 vessel has Inmarsat-C but no MF/HF. The maintainer must confirm that Inmarsat-C satisfies distress alerting and MSI, that EGC is enabled for the current ocean region, that position is updated, and that the LES is reachable. A test message to a shore subscriber checks the link without raising a distress. The reasoning: Inmarsat-C covers the A3 alerting and MSI functions, but EGC must be enabled and the terminal must know its position for a valid distress alert.
Traps
Inmarsat-C does not provide voice. SafetyNET is EGC, not ordinary email. Do not assume every FleetBroadband installation is GMDSS-approved. Do not leave the terminal in the wrong ocean region. Do not obstruct the radome with radar or mast shadowing. Do not use a routine test that raises a distress priority. Do not ignore LES routing failures; they can prevent an alert from reaching an RCC.
Key facts
Inmarsat is geostationary and has four ocean regions. Inmarsat-C is store-and-forward, not voice. EGC SafetyNET delivers MSI. Only approved Fleet variants count for GMDSS.