@SeenItAll
Differences in time of receipt of signal from satellites is how GPS works.
You are basically right with your explanation about time-measured signal location, but I know Inmarsat works on even more complex system architecture than this and the fundamentals of geo-location of transmissions within their system is somewhat more complex.
I will leave it to someone more qualified than me to explain, but I know that analysis of spot beam coverage and the transmission on particular spot beams comes into the reckoning.