Satellite Communications (Inmarsat) and Maritime Safety Information
This domain tests the satellite element of A3: which service does what, how distress priority reaches an RCC, and how MSI is delivered by SafetyNET.
Inmarsat basics
Inmarsat is geostationary, so coverage is roughly 70°N–70°S. Ocean regions are AOR-E, AOR-W, IOR and POR; the terminal must be logged on to the region covering the ship. Inmarsat-C is a store-and-forward, data-only service (no voice) with distress priority to an RCC via a Land Earth Station, and it can receive Enhanced Group Calls. Fleet/FleetBroadband provides voice and data but is not a substitute for the Inmarsat-C MSI function unless the installation is approved for it. Iridium is now also a recognised GMDSS satellite provider.
SafetyNET and MSI
SafetyNET is the Inmarsat EGC service that broadcasts MSI: NAVAREA navigational warnings, METAREA forecasts and warnings, SAR information and piracy warnings. Messages can be addressed to all ships in an ocean region, to a circular or rectangular area, or to a group. FleetNET is the commercial equivalent and is not MSI.
MSI sources overall
NAVTEX (518 kHz), SafetyNET, HF NBDP, and coastal VHF in some areas. MSI is free to ships and is not a commercial subscription.
Worked example
A ship in the Indian Ocean is beyond NAVTEX range and has missed a NAVAREA warning. Check the Inmarsat-C terminal is logged on to IOR, the EGC receiver is enabled and the position input is current; retrieve the relevant NAVAREA messages via SafetyNET; then enter the warning in the radio log and plot it. Reasoning: NAVTEX is line-of-sight MF, so distance is the limiting factor, not receiver failure.
Traps
- Expecting voice calls from Inmarsat-C.
- Confusing SafetyNET (MSI, free) with FleetNET (commercial).
- Forgetting that EGC area addressing depends on an updated position.
- Assuming Inmarsat coverage extends into A4.