Originally Posted by
hamster3null
What if they tracked the right plane, but they messed up their calculations?
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.
What I _could_ easily buy is the presumption that the satellite has a very precise clock, and the aircraft has a different clock, and the response to the ping has a timestamp that the satellite can compare against its own clock, thus estimating time of flight. We are still talking about extremely precise timing. The entire process could be rendered useless if there's an unpredictable source of lag on the order of as little as 1 millisecond between timestamping and sending/receiving, or if the clock that's attached to the Classic Aero antenna on the aircraft drifts off by 1 millisecond over the course of flight. Since this system was never designed for the purposes of tracking aircraft, there can be any number of potential unknown sources of error.
I think you will find that messages are timestamped probably synched to GPS clock.
The calculations will have been checked by a whole raft of international experts in SATCOM that have the raw data from INMARSAT. I don't think someone worked this out on the back of an envelope using iffy data and phoned it through.