Also known as: Inmarsat, INMARSAT
Inmarsat is a network of geostationary satellites providing L-band mobile-satellite services — maritime, aeronautical, and land — including the store-and-forward STD-C data service and the Classic Aero aircraft link.1 Unlike the low-orbit Iridium and Globalstar constellations, Inmarsat parks a handful of large satellites over the equator so a fixed terminal simply points at a spot in the sky and stays connected.
Overview
Inmarsat began in 1979 as the intergovernmental International Maritime Satellite Organization, chartered to give ships reliable distress and safety communications; the first spacecraft entered service in 1982. It was privatised in 1999 and, after decades as a standalone operator, was acquired by Viasat in 2023. Its services span several generations of satellites and air interfaces, from the low-rate STD-C messaging terminal to the wideband FleetBroadband and BGAN IP services.1
Technical characteristics
| Property | Value |
|---|---|
| Orbit | Geostationary (equatorial, ~35 786 km) |
| Band | L-band, ~1.5 GHz downlink / ~1.6 GHz uplink |
| Access | FDMA channels with TDM/TDMA framing |
| Modulation | BPSK (STD-C), QPSK/OQPSK (Aero, BGAN) |
| STD-C rate | 1200 symbols/s, ~600 bps effective |
| Services | STD-C, SafetyNET/EGC, Classic Aero, FleetBroadband, BGAN |
STD-C is a text-and-data teleprinter service that also carries the maritime SafetyNET / Enhanced Group Call (EGC) broadcasts — navigational and meteorological warnings addressed to ships in a geographic area.2 Because these are open BPSK broadcasts on a fixed downlink, they are a well-known SDR reception target.
History
The organisation was created under an International Maritime Organization convention to answer the safety-of-life-at-sea need that HF and marine VHF could not fully meet. Each satellite generation added capacity and higher-rate services, moving from analogue voice through the digital Aero and Mini-M terminals to today’s IP-based FleetBroadband and Global Xpress Ka-band system.1
Deployment
Inmarsat underpins the Global Maritime Distress and Safety System (GMDSS): ships carry STD-C terminals to receive SafetyNET warnings and send distress alerts. Classic Aero and SwiftBroadband serve aviation datalink and cabin connectivity, and land terminals cover remote-area voice and data. The L-band downlinks are receivable at a fixed site with a small patch antenna and a low-noise amplifier; hobbyists routinely decode STD-C EGC traffic and Aero ACARS-style messages with free software.
Decoding it with GopherTrunk
GopherTrunk is a terrestrial land-mobile trunking scanner (P25, DMR, TETRA, and similar), so Inmarsat is out of scope for its decode chain — the framing, forward error correction, and message formats are entirely different, and the signals sit in L-band rather than the VHF/UHF land-mobile bands GopherTrunk tunes. Inmarsat is included here as a reference point: it is the geostationary counterpart to the LEO systems Iridium, Globalstar, and Orbcomm, and its open STD-C downlink is a good first satellite-decoding project with a dedicated tool.