Inmarsat and satellite communication services
Inmarsat in the GMDSS
Inmarsat provides geostationary satellite communications for A3, generally between about 70°N and 70°S. Inmarsat-C is a store-and-forward, data-only terminal used for distress alerting, EGC/SafetyNET reception, two-way messaging and polling. Fleet/FleetBroadband terminals provide voice and data but are not a substitute for the dedicated GMDSS functions unless type-approved and configured. Distress alerts go via a Land Earth Station (LES) to a Rescue Coordination Centre; the alert includes ship identity, position, course/speed, nature of distress and time.
Operational points
The terminal needs a current position input, correct LES access code and ocean region. Inmarsat-C can receive EGC messages for a selected NAVAREA/METAREA; if the receiver is not logged in or the area is wrong, MSI is missed. Inmarsat-C is not a voice service and cannot be used for on-scene voice coordination. A GMDSS installation has a dedicated distress button and battery backup.
Worked example
An A3 ship has FleetBroadband for business and an Inmarsat-C for GMDSS. The crew propose removing Inmarsat-C because FleetBroadband has voice. Reasoning: FleetBroadband is not automatically a GMDSS distress alerting unit; the Inmarsat-C is the type-approved GMDSS terminal providing distress alerting and SafetyNET. Removing it would break the GMDSS fit. The correct action is to keep and test the Inmarsat-C.
Traps
Do not assume satellite coverage is global; A4 is not covered. Do not use Inmarsat-C for voice. Do not leave the position input manual or stale. Do not test the distress function by sending a real alert; use the manufacturer's test or a designated LES test.