One thing that shocked me was how accurately scientists were able to measure the movement of tectonic plates, down to distances on the order of tenths of a millimeter. The part that's even more impressive? It's done via GPS satellites, 20,000 kilometers above. I'm by no means an expert on GPS, but it seems like we'd have to know the speed and position of each GPS satellite down to absurd levels of precision, with respect to some non-moving origin (and considering tectonic plates are always moving, that's pretty hard). Even more crazy is the fact that the atmosphere between the GPS satellite and the Earth is always moving as well, distorting the apparent distance and requiring a detailed model of airflows, as well as clocks in both the satellite and ground station that are accurate down to the amount of time it takes light to travel 0.1 mm! I'm astonished it's possible to measure this accurately, and it's a testament to both how accurate our equipment is and how low we've gotten the uncertainty in these measurements to make that 0.1 mm meaningful.
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Now that you point it out, that is fascinating. I never thought about the fact that we must have a shockingly accurate position on each satellite with extremely accurate corrective measures on the instruments that view the tectonic plates. I wonder what kind of reference is used to determine plate movements and how we account for all of those distorting factors. I am also no expert on GPS, but I wonder if there is some referential point on Earth or in space that is assumed to be static or if the satellite images are taken are oriented in reference to other satellites.