Here's how GPS works in five logical steps:

1. The basis of GPS is "triangulation" from satellites.

2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals.

3. To measure travel time, GPS needs very accurate timing which it achieves with some tricks.

4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret.

5. Finally you must take care of any reasonable sources of error.