We take for granted what it should be considered the magic of nowadays life: special and general relativity work to give us better GPS locations. You heard it. In order to track the exact position of an object you need to receive the signals from four of those 30 GPS satellites and then you will be able to “triangulate” your position. Remember, GPS satellites do not track you, they only send the signals.
These satellites carry atomic clocks and, since they apply so well corrections according to Einstein’s relativity they build up clock big as our entire Planet. Did you know that if you live on top of a sky-scrapper the time will pass a little faster when higher in the gravitational field? These and more can be found out from Minute Physics.