[alexcz]

It gets even more interesting if you take into account how the satellites know where they are. Around the world there are fundamental stations: https://en.wikipedia.org/wiki/Fundamental_station

I had the opportunity to visit one. Basically they measure their own position in relation to each other. They do that with Very-long-baseline interferometry, basically what is the time difference of quasar radio signals hitting their Radio telescopes. The things they account for is wild like local gravity field a couple of super prices atomic clocks etc. they then laser range find Satellites (all not only gps) which is a „fun“ summer student job at least at the one that I visited.

[sorz]

So gound stations need keep measuring their own positions due to continental drift! Never thought that before. Thanks.

[lxgr]

And this precision data can then in turn be used to map localized terrain movements due to volcanic activity, mining etc. using high precision GPS. I think some of these can detect (in the long term) movements of as little as a few millimeters!

[alfanick]

It's quite fun area, I used to work in related technologies. Some of the countries maintain their own networks, but there are also commercial networks - so called reference stations that provide RTK/Network-RTK, basically your GPS can be augmented with a model based on those reference stations (which are ground GPS/GNSS receivers with very well known position) and improve accuracy to even 1cm (based on differences from your GPS position vs positions in network compared to well known positions).