How the map knows where you are.
A 10-minute lesson in Orbit: the blue dot, the three distances, the fourth satellite setting the time and the relativity fine-tuning it.
How GPS locates you with three satellites and a corrected clock, and why it fails indoors.
The case
The map knows where you are within 5 meters. Your phone doesn't see you: it listens to satellites and calculates.
In 10 minutes you'll understand trilateration with no formulas: three distances, a corrected clock, and relativity working so you don't get lost.
What you will learn
- Explain trilateration with no formulas
- Tell what the fourth satellite is for
- Predict where GPS fails and why
The intersection
Your position is the point where three distances measured with perfect clocks cross. The fourth satellite sets the time.
Satellites
24+ perfect clocks flying at 20,000 km.
Distances
Your receiver measures how long each signal took.
Clock
Your cheap clock gets corrected by a fourth satellite.
Point
The intersection of the three spheres is you.
Frequently asked questions
Do satellites watch me?
No: satellites just shout the time. Your phone listens to those signals and calculates where it is. Receiving isn't transmitting.
Why are 4 satellites needed?
Three locate you in space; the fourth corrects your phone's cheap clock. Without exact time there's no exact fix.
Does GPS use data?
Positioning doesn't: it only listens to radio. The maps you see do download data, but the blue dot works without it.
What you take away
Explain how GPS locates you and why it fails indoors.