1The two things you need
A satellite knows "how far" — not "where"
Each satellite sends a beep with the exact time it left. Your phone times how long it took to arrive and turns that into a distance. But here's the twist…
1. How far away
The beep takes time to fly down. Longer time means farther away. So the satellite learns one number: how far you are. It still has no idea which way.
2. "This far" is a whole circle
One distance isn't one spot. Every point on this ring is exactly the same distance away. So "you're this far" really means "you're somewhere on this circle."
2The two situations
Still lost vs. pinned down
The "still-lost circle"
With one satellite you could be anywhere on a giant circle. With two, the circles cross at just two spots — better, but it's still a coin flip. Your phone can't pick.
The "one-and-only spot"
Add a third circle and it slices through only one of those two spots. Now every circle overlaps at a single place — and that place is you.
3The big question
Is one satellite enough? 🛰️
A single satellite has measured exactly how far away you are. That's a real fact about you. So — does your phone now know where you are?
Guess before you find out
With just ONE satellite measuring how far away you are, can your phone pin down where you are?
Now add them one at a time and watch the answer happen. The glowing spot is the only place left where you could be.
4So is it ever perfect?
More satellites help — but the dot still wobbles
Three pin you down; a fourth fixes your phone's tiny clock so the distances stay honest. The more satellites in view, the sharper your spot.
Signals slow down through the air and bounce off tall buildings, so the crossing is a fuzzy little zone, not a perfect dot.
A GPS satellite never sees you — it only knows how far away you are. Your phone finds you by overlapping the distance-circles from a few satellites until they pinch down to one spot.
Psst, 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 3-D each range is a sphere, and intersecting spheres give the position. Three ranges fix a 2-D point, but the receiver's own clock error is a fourth unknown, so you actually need four satellites to solve for position (x, y, z) and time together. This is trilateration, not triangulation — distances are measured, not angles. Accuracy depends on satellite geometry (dilution of precision) and errors from the ionosphere, troposphere, and multipath off buildings.