Calculating the distance of GPS satellite from the ground.
1. How far is it from the ground to a GPS satellite straight overhead?
2. How long does it take light (or radio waves) to go from a GPS satellite overhead to the ground?
3. Let's say the time measured in question 2 was measured by clocks on the ground. What would that time is if measured by clocks on the GPS satellite instead? (For example apply whatever speedup/slowdown factor you got above).
4. Multiply the time in question 3 by the speed of light to get a distance and evaluate it to the distance you had from question 1. That gives you an idea of how far off your GPS measurements would be if you didn't take into account special and general relativity.