Satellite Communications: Inmarsat and Other GMDSS Services
The role of satellites
Satellite services provide alerting and communications where terrestrial systems cannot reach, especially in Sea Area A3. Inmarsat geostationary satellites support voice, data and distress alerting through ship earth stations, and the system includes a dedicated distress function that routes an alert to a rescue coordination centre. Know that geostationary satellites do not cover the polar regions, which is why A4 exists.
Distress and safety services
Inmarsat services include distress priority calling, which pre-empts normal traffic, and enhanced group call (EGC) for MSI, including SafetyNET. A trap is confusing EGC/SafetyNET with NAVTEX: EGC is satellite-based and can address specific areas, while NAVTEX is a terrestrial MF service with limited range. Another trap is assuming a satellite terminal alone satisfies all GMDSS functions; it does not replace VHF or MF for on-scene communications.
Other satellite systems
Other GMDSS-recognised satellite services exist, and questions may test that the GMDSS is not tied to a single provider. The key concept is that a recognised satellite service must meet the functional requirements for alerting and communications. Know that EPIRBs use dedicated satellite systems (such as COSPAS-SARSAT) for alerting, which is separate from Inmarsat ship earth stations.
Operational points
Satellite terminals need a clear view of the sky; blockage by superstructure or mast can interrupt service. Distress priority must be used only for genuine distress. Testing should follow manufacturer and regulatory guidance and must not generate false alerts.
Worked example
A vessel in A3 with an Inmarsat terminal can send a distress alert via the terminal, then follow up by voice. If the terminal fails, the vessel must have an alternative means, such as HF DSC, depending on its equipment fit. A vessel proceeding to A4 cannot rely on Inmarsat and must use HF.