How does GPS actually work?

A constellation of satellites orbits about 12,550 miles up, each carrying atomic clocks and broadcasting a signal containing the exact time it was sent and the satellite's position. Your receiver picks up several of these signals, and because radio waves travel at a known speed, the delay between transmission and reception tells it how far away each satellite is. One distance places you on a sphere, two narrows it to a circle, three to two points, and a fourth resolves the ambiguity while also correcting your device's own imprecise clock. That last function is the reason four satellites are needed rather than three: consumer receivers cannot afford atomic clocks, so the fourth measurement solves for the clock error along with the position.

Relativity is not a footnote here but a working engineering correction. The satellites move fast enough that special relativity slows their clocks slightly relative to ours, and they sit high enough in a weaker gravitational field that general relativity speeds their clocks up by a larger amount. The net effect runs about 38 microseconds per day, which sounds trivial and would introduce position errors accumulating to roughly 10 kilometers per day if uncorrected. The system is designed with those offsets built in, which makes GPS one of the most widely used practical confirmations of Einstein's predictions.

Accuracy in practice depends on conditions. Open sky with many visible satellites gives a few meters; urban canyons degrade it because buildings block signals and reflect others into multipath errors; indoors it typically fails entirely, which is why phones blend GPS with Wi-Fi positioning, cell tower triangulation, and inertial sensors. GPS is the American system, and modern receivers also use Europe's Galileo, Russia's GLONASS, and China's BeiDou simultaneously, which is why positioning has improved noticeably in recent years. One important asymmetry: the satellites only broadcast and never receive, so using GPS does not transmit your location anywhere. Location sharing happens through the apps and networks on your device, which is a separate question from the positioning itself.