Whats behind GPS and satellite navigation?

The GPS (GLOBAL POSITIONING SYSTEM)  is the key to modern navigation. It is used for commercial, military and civil purposes. It's controlled by the US Government. For this reason Russia has it's own called GLONASS and China as well called COMPASS. Thankfully the Europeans are also developing their own navigation satellite called Galileo. The Galileo would be much more accurate then the other named satellites ( no more then 1 meter away).

What makes the satellites stay in their orbits?
The satellites are in a uniform circular motion where the the gravitational force is equal to the centripetal force acting from the satellite to the earth. This keeps the satellite above the earth at a fixed radius of the orbit.
 This is an example similar to the one stated above.                                                                                                                                                                                                                                                                                                                                                   Here is an article from BBC NEWS about the drawbacks using GPS and other navigational satellites:
http://www.bbc.co.uk/news/science-environment-29758872