In this paper, I will try to explain how to keep geostationary communication satellites on orbit.

Communication satellites must stay stationary with respect to the earth. Only in this way we can watch TV via small and fixed antennas. To achieve this stationary position, satellite must have 24 hours turning period like earth. Period is given in this formula, means that it is only parameter of radius. Thus, radius must be R=42,164 km. to have 24 hours period. Earth radius is 6378 km. we can conclude as: satellite distance to the earth surface must be 35,768 km.

Figure - 1. Geo-stationary communication satellites are 35,768 Km far from earth surface

Other requirement for geostationary communication satellites is : orbit plane must lay on equatorial plane. Otherwise, satellite projection to the earth will be like number eight (8).

Figure - 2. A communication satellite with an orbit plane does not lay on equatorial plane

Figure - 3. ..and its projection to the earth. Looks like number eight (8).

Satellite orbit must be circular to stay stationary. If orbit is closer to an ellipse rather than circle, satellite projection to the earth surface will draw a zigzag (will swing back and fore) as shown on the below figures.

Figure - 4. Circular and elliptical orbit projections

These three requirements will be enough theoretically. But, Do we need any other requirement to have a stationary satellite? Will the satellite stay stationary? Unfortunately, NOT. Because, there are several factors causes to violate this ideal theoretical situation. Main of them are :

1. Earth is not perfect sphere.
2. Sun and Moon gravity forces.
3. Pressure affect of Sunlights.

If we look to the earth from North and South Pole direction, we will not see a circle. It looks like an egg. 75-degree east and 105-degree west is sharp points of this egg. Our example Turksat satellites are on 31 and 42 degree east. Thus, satellites will not stay at these points and will slide to Far East toward 75 degree.

Figure - 5. Earth's egg shape and Turksat satellite place.

As you will remember from high school years, there is a 23-degree angle between equatorial plane and earth orbit plane (turning around Sun). Similarly, Moon orbit plane does not coincide to the earth equatorial plane. Due to these angles Sun and Moon gravity forces tries to disturb coincidence of satellite and earth equatorial planes. This causes satellite projection to draw number eight instead of staying stationary.

Figure - 6. Sun and Moon gravity forces satellite orbit to make angle with earth equatorial plane.

Sun panels provide electrical energy required for satellite electronics and mechanical systems. Panels are 10-11 meter long and 1-2 meter wide. Panels are staying perpendicular to the Sun to get maximum energy. We can imagine satellite as a ship and panels as sail. Pressure of Sunlights to panels causes satellite orbit to become more elliptical.

Figure - 7. Sunlight pressure affect forces satellite orbit to become more elliptical.

These affects and some others all try to change satellite circular orbit. Thus, satellite will not stay stationary at a point but it will move. Satellite On orbit control (OOC) engineers keeps satellite within a 0.1-degree window by making manoeuvres around every 14 days.

Figure - 8. Keeping the satellite within this 0.1-degree window.

Thruster with a size of a matchstick performs these manoeuvres. Fuel spent during these manoeuvres defines satellite lifetime. Turksat generation satellites has around 900 Kg fuel and satellite itself is around 800 Kg. This fuel finishes within around 12-13 years. Before to finish the last remaining fuel, another special manoeuvre is performed to throw the satellite toward deep space (waste box) from this critical geostationary orbit (36,000 Km from earth surface).