How does GPS actually work?
Your device does not transmit anything. It listens to signals from satellites and works out its own position from the tiny differences in how long each signal took to arrive — which makes it a receiver, not a tracker.
The mechanism. Each satellite continuously broadcasts its identity, its precise orbital position, and the exact time the signal left, according to an onboard atomic clock. Your receiver compares that timestamp with its own clock to derive the travel time, and multiplies by the speed of light to get a distance. One distance places you on a sphere; three intersecting spheres narrow it to a point.
Why you need four satellites, not three. Your device has a cheap quartz clock, not an atomic one, and even a microsecond of error becomes 300 metres of position error. Rather than fitting an atomic clock in every phone, the system solves for four unknowns — latitude, longitude, altitude and the receiver's clock error — using four satellites. Every GPS receiver is therefore also an extremely accurate clock, which is why the system underpins financial timestamping, telecoms and power grids.
Why relativity is not optional here. Satellite clocks run faster than ground clocks due to weaker gravity, and slower due to orbital velocity. The net effect is around 38 microseconds a day, which would accumulate as roughly 10km of positional error daily. The correction is built in — one of the most concrete everyday applications of general and special relativity.
What limits accuracy: the ionosphere and troposphere delaying signals unpredictably; multipath, where signals bounce off buildings and arrive late, which is why cities are worse; satellite geometry; and obstruction indoors and in tunnels.
How phones do better than raw GPS: assisted GPS, downloading satellite data over the network so a fix takes seconds rather than minutes; Wi-Fi and cell positioning; inertial sensors; and using several constellations at once — GPS, Galileo, GLONASS and BeiDou, which is what GNSS means.
Corrections like RTK deliver centimetre accuracy for surveying and agriculture.