GEO-STATIONARY ORBIT.

Travelling on the circle orbit of radius r the satellite is acted upon by the force of the earth's gravity g mM/r2, where g is the gravity constant, m is the mass of the satellite and M is the mass of the planet (earth in our case). According to second Newton law this force is equal to centripetal force mv2/r. From these two formulas we find the equation for the velocity of the satellite motion in the circle orbit:

v=(g M/r)1/2

The period of the satellite revolution around the earth Tsat is equal to the length of the orbit 2p r divided by the velocity of the satellite motion v:

Tsat=2p r/v=2p (r3/g M)1/2

If this orbit period Tsat is equal to the period of the globe spinning (about 24 hours), then the satellite will hang over the same equatorial region of the earth and such an orbit is called the geo-stationary orbit. The sky coordinates of the geo-stationary satellite are the same all the time and we can precisely orient/beam on it the parabolic antenna of our receiver.