How Your Phone Knows Where You Are
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Question
What is a GPS fix, mathematically?
Each GPS satellite broadcasts its position and a precise time stamp. Your receiver measures how long the signal took to arrive, and distance = speed of light x time. One satellite: you are somewhere on a sphere around it. Two: somewhere on the circle where two spheres intersect. Three: two possible points (one is usually on the other side of the Earth). Four: exactly one point - because the fourth satellite also corrects for the fact that your watch is not an atomic clock.
The math
d = c × Δt, with c = 299,792,458 m/s. A timing error of 1 nanosecond → 0.3 m of position error; 1 microsecond → 300 m. That is why the satellites carry atomic clocks (and why the +38 µs/day relativity correction matters).
Why it matters
The whole system is just distance = speed x time, done four times with some algebra. No dome, no drones - four satellites 20,000 km up, and a calculator in your pocket. The coordinates are Earth-centred, Earth-fixed (ECEF) - a spherical geoid model, which is why surveyors achieve centimetre-level accuracy with it. And the irony: flat-earthers use GPS to get to their meetups.