Rory 166 refers us back to AT1's post no.7124, now on page 357, where a telecoms engineer explains in striking detail how these satellites work and the accuracies involved.
This is dense stuff, but well written and I would like to add my thanks to AT1 for his work in putting that very helpful explanation together.
I've read all 7000+ posts here, but must have skipped this one!
The low data rate inmarsat services that work with omnidirectional antennas are not as sophisticated as the poster may imply. Inmarsat-C is a teletype era service.