Orbital Control System (OCS)
A GEO stationary satellite is subjected to various forces which tend to pull the satellite from its correct orbit. The orbit control system is used to return it to the correct orbit.
A geo-stationary satellite must lie in the equatorial plane. Its orbit must be circular and it should have the correct attitude. Gas jets, used along the three references axes of the satellite, are used for orbit control. Gas jets can impart velocity changes along the three axes. The inclination of the orbit of a GEO satellite increases at about 0.85 deg. per year. Most GEO satellites are specified to remain within ± 0.05 deg. inclination changes.
To maintain this, corrections called north-south station keeping maneuvers are made every 2-4 weeks.
At intervals of 2 weeks east-west corrections are made.
One of the gas jets is used to increase velocity normal to the orbital plane. This helps the satellite to move at right angle to the orbital plane and reduce the inclination.
As the inclination reaches zero deg. the other jet is fired to oppose the satellite motion in that direction until it stops the satellite in that position.
The correction of the inclination of a satellite orbit needs more fuel to be used than for any other orbital correction.
A 2000 kg satellite requires about 65-85 kg of fuel for orbital control. Thus a satellite that must maintain very accurate station keeping will have more weight due to requirement of more fuel for station keeping.
East-west station keeping requires only a modest amount of fuel. This is necessary on all GEO satellite to maintain the spacing between adjacent satellites. Orbital control is done after receiving command from the control centre on the earth.
