Some insight can be gained here:
Timing advance - Wikipedia, the free encyclopedia
GSM and Inmarsat C both use time division multiplex technique, the principle is the same.
Think of many a/c (transmissions) wanting to land at the same destination airport (satellite) with all a/c coming from different origination airports (airborne transceivers)
- Each a/c (transmission) has a time slot assigned when it's supposed to arrive.
- For obvious reasons those time slots may not overlap.
- Radio waves behave different than a/c : they can not fly holding patterns.
- So take off time of the radio transmissions (a/c) have to be timed, such that all transmissions (a/c) arrive at the satellite (destination airport) in their assigned time slots.
- ATC at destination (the satellite) has to measure the distance between origin and destination, so it can tell ATC at origin (airborn transceiver) when the a/c (transmission) has to take off.
- This distance measuring happens by measuring round trip times.