A GPS receiver determines its position without ever transmitting anything to the satellites above it. The system is a one way broadcast, the satellites continuously transmit and the receiver only listens, which is why it scales to an unlimited number of users, none of them ever contends for a channel or announces itself. The mechanism underneath is not fundamentally about position at all, it is about time. Each satellite carries an atomic clock and broadcasts, continuously, the precise current time together with its own orbital location, and the receiver's task is to turn those time signals into a position.
The receiver measures how long each signal took to arrive and multiplies that interval by the speed of light to obtain its distance from that satellite. One such distance places the receiver somewhere on the surface of a sphere, a second narrows it to the circle where two spheres intersect, and a third reduces that to a point. A fourth satellite is required for a reason that is easy to miss, the receiver's own clock is not accurate enough to measure these intervals directly. Light travels roughly thirty centimeters per nanosecond, so a timing error of a single microsecond corresponds to a position error of hundreds of meters, and no consumer device carries an atomic clock. The receiver therefore treats its own clock as an additional unknown and uses the fourth signal to solve for it, resolving four unknowns together, latitude, longitude, altitude, and the exact time.
The element of the system that most people never encounter is its dependence on relativity. The clocks aboard the satellites run measurably faster than identical clocks on the ground, by an amount that would render the system useless within a day if left uncorrected. Two effects predicted by Einstein act in opposition. Special relativity predicts that the satellites, moving at several kilometers per second relative to the surface, should tick slightly slower, while general relativity predicts that clocks higher in the weaker gravitational field of orbit should tick slightly faster, and the gravitational effect dominates. The net result is that the satellite clocks gain roughly thirty eight microseconds per day, which would introduce a positioning error on the order of ten kilometers within a single day, so the clocks are deliberately adjusted before launch to run at an offset that cancels the effect once they are in orbit.
The position shown on a screen is therefore the output of a timing calculation, a set of clock readings from orbit resolved into the one location on Earth consistent with them. GPS does not locate a device so much as broadcast time with extraordinary precision and let the device infer where it must be to have received those signals when it did. It stands as one of the few technologies in ordinary daily use whose correct operation depends directly on relativistic physics, a reminder that infrastructure most people treat as mundane can rest on some of the deepest results in modern science, and that the reliability we take for granted was engineered by taking those results seriously.