I took a look at post 7124. I have to say that I don't buy this. It basically says that the antenna on the aircraft is tied to an extremely fast computer chip that is guaranteed to send a response to the "ping" from the satellite within a few nanoseconds of receipt. I don't have any documentation to back this, but I find this extremely improbable, especially for a 1980's system (classic aero). And, more generally, no one writes networking code like this, not even in perfectly controlled conditions, let alone for a noisy 36000 km long satellite link. The computer in charge of sending the response may have other things to do, it will reply eventually, but realtime response is not guaranteed.
This is exactly NOT what I was attempting to explain. Firstly, as a bit of context, one of the most powerful computers you will ever come close to is in your mobile phone. The digital signal processors in these cheap phones pack astonishing punch, and run "fast". They are pretty well dedicated hardware only good for running phone comms, so not as general purpose as an Intel i7 for example, but by some measures much more powerful. Don't underestimate the power in relatively cheap comms kit. Inmarsat C may not support high data rates, but remember it is serving many many planes and many many ships all at the same time.
The key point here is that I am NOT requiring a response in a nanosecond. The response could be half a second later - 500,000,000 nanoseconds later. The terminal just counts ticks while a far more pedestrian processor prepares the response and sets it up ready to be sent, and it is then sent at the right "tick". The satellite knows the defined protocol and so can eliminate the delay, giving it access to the actual "flight" time of the message.
But I also said these are communication systems, not navigation systems. The overall system is designed to maximise the data they can handle as it is data that makes money. They could at one extreme simply say we have no idea where each user station is, we will provide in the protocol a "window" wide enough to accommodate the user being anywhere over the 1/3 of the globe. That would mean the satellite had wasted potential fee earning capacity waiting for the full potential window for each and every communication exchange, when in fact it can reduce, though not eliminate the window.
My explanation was, as I had said, simplified. In practice there are a whole load of other complications, spreading data bits over many carrier cycles and error handling codes etc etc all of which will introduce more variability, but at the heart the system is operating at 1600 MHz, giving an inherent timing capability irrespective of GPS time stamps etc, with the potential of measuremment to quite high levels of accuracy. In practice the level will not approach this - but talk of "miliseconds" and hundreds of Km is not appropriate. The underlying principles may be designed in the 1980s - as was GPS - but we are not talking valves and clockwork here!
I say again, I can go to my hardware shop and buy for a few £/$/Eu a handheld laser tape measure that can fire and time a pulse of light sufficient to measure distance to a few mm. Give it a more stable clock - which the satellite will have, and using the same principle of counting ticks you can time to great accuracy over very long periods, eliminate the "processing" time at the receiver by specifying how many ticks later the reply is sent, and there you have a system with the potential of astonishing accuracy - just as GPS has.
But one last repeat - Inmarsat is a comms system designed to support comms. It is talked of as a bent pipe, but that is a great simplification. The content of the data being exchanged may appear to pass through a bent pipe, but there is a lot going on over and above passing the data to keep the pipe itself working.