Satellite Communications (Inmarsat) and Maritime Safety Information
What the exam tests here
Inmarsat is the satellite provider recognised for GMDSS distress alerting and MSI in Sea Area A3. Questions cover the constellation's limits, the services that count for GMDSS, and the way distress alerts reach an RCC.
The system
Inmarsat satellites are geostationary, about 35,786 km above the equator, arranged in four ocean regions (Atlantic Ocean Region East and West, Indian Ocean, Pacific Ocean). Coverage is 70°N to 70°S, which is exactly why Sea Area A3 stops there. Coverage ends because a geostationary satellite is below the horizon at high latitude, not because of weather.
GMDSS services
- Inmarsat-C - small, store-and-forward, about 600 bit/s, with a dedicated distress button. The alert carries the ship's identity, position (fed from GPS), course and speed, and time, and is routed through a Land Earth Station to an RCC. Press and hold the button for about five seconds.
- EGC receiver - receives SafetyNET MSI broadcasts, either built into the Inmarsat-C terminal or as a separate EGC-only receiver.
- Fleet 77 was the recognised GMDSS voice service; FleetBroadband was not originally a GMDSS service. Inmarsat has announced the eventual retirement of Inmarsat-C, and Fleet Safety is being introduced as its recognised successor, so know both the classic and transitional positions.
Practical points
The terminal must be logged in to the correct ocean region, must receive a GPS position feed, and must be left powered when the ship is alongside so that alerts and MSI continue to be received.
Worked example
A ship in the Indian Ocean sees "no satellite" on the Inmarsat-C display after departure. The usual causes are an antenna obstructed by a crane or mast, an incorrect ocean region selected, or a failed GPS input. Check the antenna's clear view first, then re-select the region, because a log-in to the wrong ocean region will not produce a distress path to the right RCC.
Traps
- Claiming worldwide Inmarsat coverage.
- Leaving Inmarsat-C switched off in port, which also stops MSI reception.
- Forgetting that a GPS feed is needed; a stale position in a distress alert is worse than none.