GPS model animation

A function to create an animation of the GPS constellation and working principle (pedagogic purpose).

For pedagogic / scientific diffusion purposes.

F5 to run and create the animation file named 'GPS_constellation &_working_principle_model.gif' that will appear in the same directory as you registered this .m file.

Dashed green lines links show visible spacecrafts (white dots) at a certain time on a certain place on the planet.

Theoritically minimum satellites vsible in the sky to give position and time is 4 (three for position, one for time)

Orbital angle values may be different from reality.

Old piece of code, still suboptimal cause not maximum factorized.