How GPS Works
A satellite positioning system — the American GPS and its peers such as Europe's Galileo, Russia's GLONASS and China's BeiDou — is a constellation of satellites each broadcasting the exact time and its own position.
The idea in one line Your receiver listens to several satellites at once and measures how long each signal took to arrive. Each travel-time gives a distance; with distances to four or more satellites, only one point in space and time fits them all. This is called trilateration.
Because the signals travel at the speed of light, the satellites carry atomic clocks and the maths must account for tiny effects — including relativity. The result is a position usually accurate to a few metres, delivered anywhere on Earth, for free, to a receiver the size of a coin.
Limits and Backup
Satellite positioning feels infallible until it isn't. Signals are weak by the time they reach the ground, so they are easily blocked and degraded.
Where it struggles - Terrain and buildings: deep canyons, dense forest and city streets reflect or block signals, throwing the fix off by tens of metres. - Interference: signals can be jammed or spoofed, deliberately or accidentally. - Power: a dead battery or a dropped phone ends the service instantly.
This is why every serious expedition, ship and military unit still carries and trains on map and compass. Technology extends human skill; it does not replace it. The wise navigator treats the GPS as the fast tool and the map as the one that will still work when everything else has failed.