Gravitational Fields

Gravitational field strength:
It's the force per unit mass on a small mass placed in the field. It can be represented by the equation g=F/m (F is the gravitational force acting on a small mass m) or g=GM/r²(r² is the distance from the centre of the large mass M).

Is the earths gravitational field radial or uniform?
It's radial because the earths gravitational field strength decreases as the distance from the centre of the earth increases.
                                          
Radial Field.                                                                     Uniform Field.

Newton's Law Of Gravitation: 
Sir Isaac Newton's law of gravitation tells us that there is a gravitational force acting between all matter. This law can be represented to it's general form: F=GMm/r²     (G is the universal gravitational constant, where G=6.67384 × 10-11 m3 kg-1 s-2)



Gravitational Potential:
The gravitational potential at a point in the gravitational field is the work done per unit mass to move a small object from infinity to that point. V=W/m (V "J/kg" is the gravitational potential and W is the work done "J").

Equipotentials are lines of constant potential.

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