How does your phone know exactly where you are?
Explored it? Here's the recap
How does your phone know exactly where you are?
The short answer
No satellite ever sees you. Each GPS satellite only knows how far away you are, measured from how long its signal took to reach your phone. Your phone finds you by overlapping the distance-circles from several satellites until they cross at a single spot.
How it works
Every GPS satellite far out in space broadcasts a carefully-timed signal and tells your phone where the satellite is. Your phone measures how long the signal took and turns that time into a distance: 'I am this far from that satellite.' But one distance isn't one place, it's a whole circle of places that are all that far away. A second satellite gives a second circle, and the two circles cross at just two spots. A third satellite's circle passes through only one of those spots. Where all the circles overlap is exactly where you are.
What people get wrong
Lots of people imagine a GPS satellite as a camera in the sky that looks down and sees you. It doesn't and can't. A satellite has no idea who or where you are, it only knows how far away you are from the time its signal took to arrive. Finding your spot is a distance puzzle your phone solves, not a photo.
The catch
More satellites in view give a sharper, more certain fix, and a fourth one fixes your phone's clock so the distances stay honest. But the distance-circles are never perfectly thin: signals bounce off tall buildings and slow down passing through the air, so the crossing point is a fuzzy little zone instead of a perfect dot. That is why your blue dot wobbles and why GPS struggles indoors and between skyscrapers.
Questions kids ask
Can a GPS satellite actually see me?
No. A satellite cannot see you at all. It only sends a timed signal, and your phone turns how long that signal took into a distance. Your location comes from overlapping those distances from several satellites, not from any picture.
Why do you need at least three satellites?
One satellite tells you only that you're somewhere on a big circle. Two satellites' circles cross at two spots, so it's still a guess. A third circle passes through only one of those spots, so three or more overlap at the single place that is you.
How does a satellite measure how far away I am?
It sends a signal stamped with the exact time it left. Your phone sees how long the signal took to arrive and multiplies that time by the speed of light. Time multiplied by speed gives distance, so a tiny delay becomes a real distance in miles.
Why does my blue dot sometimes jump around?
The distance-circles are never perfectly thin. Signals slow down passing through the air and bounce off tall buildings, so where the circles cross becomes a fuzzy zone instead of an exact dot. Indoors and between skyscrapers the signals get blocked, so the dot wobbles or drifts.
For grown-ups
Each satellite broadcasts its position and a precisely-timed signal; the receiver multiplies the signal's travel time by the speed of light to get a range. In three dimensions each range is a sphere, and intersecting spheres pin down the position. You'd need three ranges for a 2-D fix, but because the receiver's own clock error is a fourth unknown, you really need four satellites to solve for position (x, y, z) and time together. This is trilateration, not triangulation, because distances are measured, not angles. Accuracy depends on the satellites' geometry (dilution of precision) and on errors from the ionosphere, the troposphere, and signals bouncing off buildings (multipath).