Inmarsat Terminal

What defines this type

An Inmarsat terminal is a satellite communication unit certified for GMDSS use that links the ship to Inmarsat's geostationary satellite constellation for distress alerting, safety information reception and general voice or data communication. Its defining limitation is geometric: geostationary satellites sit over the equator, so coverage weakens and eventually disappears above roughly 70 to 76 degrees latitude depending on the specific satellite and antenna, which is why GMDSS regulations recognise Inmarsat as satisfying sea area A3 coverage but not the polar sea area A4, where an alternative such as an Iridium terminal or HF/MF radio is required instead. This distinguishes it from Iridium-based GMDSS terminals, which use a low-earth-orbit constellation with genuine pole-to-pole coverage but a different antenna and service architecture.

Main components

• Above-deck unit: a stabilised parabolic or phased-array antenna that tracks the satellite as the ship moves and rolls, housed in a radome
• Below-deck transceiver unit handling modulation, GMDSS distress button logic and interface to the bridge alarm panel
• Distress button, a dedicated, guarded, spring-loaded control that transmits a distress alert with the ship's identity and position directly, bypassing normal message routing
• Power supply from the ship's main and emergency source, since GMDSS equipment must function on emergency power
• Interface to the ship's GPS or GNSS receiver, since the distress alert must include current position

Selection and sizing

• GMDSS sea area the vessel actually trades in, which determines whether an Inmarsat terminal alone satisfies the carriage requirement or must be paired with another system for area A4
• Service type: FleetBroadband and similar terminals combine GMDSS distress functions with commercial voice and broadband data, while older, simpler terminals may provide GMDSS functions only
• Antenna size, since a larger stabilised antenna gives a more reliable link in heavy weather and higher data rates for commercial use, at greater installed cost and deck space
• Redundant power and cabling routing so the antenna and transceiver survive the same casualty scenario the distress function is meant to respond to

Regulations and class

SOLAS Chapter IV sets the GMDSS carriage requirements by sea area, specifying which combination of satellite and radio equipment a vessel must carry for the areas it operates in. IMO performance standards for Inmarsat ship earth stations set the technical requirements a type-approved terminal must meet, covering distress priority, automatic position inclusion and power backup. Class and flag administration surveys verify the terminal's GMDSS certification is current, that battery backup meets the required endurance, and that the distress button and alarm panel interface function correctly at each periodic survey.

Typical faults

What to look for in a supplier

• Current GMDSS type approval and confirmation of which sea areas the specific model satisfies on its own versus in combination with other equipment
• Service and airtime provider coverage matching the vessel's actual trading pattern, not just a headline global coverage claim
• Battery backup replacement and testing regime clearly documented for the surveyor
• Antenna stabilisation performance specification for the sea states the vessel actually encounters

Confirm which GMDSS sea area a specific Inmarsat terminal model is actually approved for before assuming it covers the vessel's full trading pattern, since equipment satisfying area A3 does not automatically extend to A4.