GPS runs off broadcasted signal from precisely positioned satellite. I'm curious, how were these satellite positioned to begin with and what keeps their positions correctly synched?
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2\$\begingroup\$ Your premise is wrong. They weren't "precisely placed" and their positions are not "synched". There is no need for that. Each satellite only needs to know its position, the time, and its orbital paremeters. \$\endgroup\$– Elliot AldersonCommented Aug 27, 2021 at 18:37
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1\$\begingroup\$ Their exact position is measured from the Earth and estimated for several hours in advance, the position efemeride as well the clock deviation is uploaded to the satellite. \$\endgroup\$– Marko BuršičCommented Aug 27, 2021 at 18:58
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They weren't precisely positioned. They are not in synchronous orbits, so from the ground they appear to be moving all the time.
The orbits, however, are known exactly.
There are ground stations that track the satellites. The orbits are corrected if needed, but most often they just update the orbit data so that the receivers can calculate the current positions of the satellites correctly.
I recommend you take a look at the Wikipedia GPS page. It is simplified, but still more detailed than what I can write here as an answer.
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\$\begingroup\$ mm accuracy of Chinese system wow \$\endgroup\$– D.A.S.Commented Aug 27, 2021 at 18:25
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\$\begingroup\$ Do you know how the satellite is precisely tracked? Is it optical? \$\endgroup\$ Commented Aug 27, 2021 at 18:29
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1\$\begingroup\$ @FourierFlux The US GPS system is managed by the Department of Defense. They aren't going to tell you how precisely they can track objects in space or what methods they use. \$\endgroup\$ Commented Aug 27, 2021 at 18:35