Each GPS satellite keeps transmitting "here I am and this is the time". From how late each signal arrives, the receiver works out its distance to at least four satellites — and from that its position and a very accurate time.
In practiceBesides navigation, GPS time keeps military radio nets, data links and frequency-hopping radios in step. If GPS drops out, communications can falter as well as navigation — one reason to carry accurate clocks of your own.
A GNSS receiver determines its position by measuring the travel time of the signal from several satellites. Each satellite continuously transmits its exact position and an extremely accurate timestamp (from an atomic clock on board). The receiver compares that timestamp with its own clock and calculates the distance from the time difference — just as radar derives range from travel time (Distance from a target to the radar), but one-way.
The measured distance is called a pseudorange: "pseudo", because the cheap clock in the receiver does not run in sync with the atomic clocks in the satellites. That clock error is a fourth unknown, alongside the three position coordinates (x, y, z). Four unknowns require at least four satellites:
A by-product of that fourth unknown is that every GNSS receiver also delivers an extremely accurate time. That is why telecom networks, power grids, stock trading and data links depend on GPS time — and why disrupting or falsifying GNSS time is just as harmful as disrupting position.
The satellites are in a MEO orbit (Medium Earth Orbit) at an altitude of ~20,200 km. Over that enormous distance the signal that reaches the Earth is minuscule — and that is exactly the weak spot worked out in The fundamental vulnerability.