the basic idea
Satellites constantly transmit beacons along with the time-of-beacon and position (in predictable, corrected, and observed orbits)
Receivers listen for (phase-shifted) signals and compute distance based on propagation delay (assume magically synced clocks for now)
3 satellites gives you 2 points (in 3d); throw out the one in deep space
Compute position relative to satellites; use satellite position to get Earth coordinates