Attitude and orbit Control System(AOCS) Details
Attitude is the exact orientation of the satellite in space. The azimuth, elevation and rotational acceleration must be fixed in space so that its antenna always directs its beam towards the earth. This control is important because it enables the satellite to reassign and relay the signals from earth stations. The maximum limit of attitude is 3× 10-4 per second.
The effects of gravitational force of the sun, the moon, the earth and other planets, solar pressure on the antenna and body of the satellite, solar sails, thruster misalignments and earth’s magnetic field etc are the cause of change of satellite’s stable position.
The earth’s magnetic field generates eddy currents in the metallic structure of the satellite during the passage of satellite through the earth’s magnetic field. Solar pressure and earth’s magnetic field cause rotation of the body of the satellite; it is called nutation or wobble of the satellite. This wobbling is required to be damped out by the attitude control system.
The presence of gravitational fields from the sun and moon cause the orbit of a GEO satellite to change with time. At GEO orbit attitude, the gravitational force due to the moon is about twice that due to the sun. The moon’s orbit being inclined to the equatorial plane by an angle of about 5 deg. creates a force on the satellite. A component of this force normal to the satellite’s orbit. The plane of the earth’s rotation round the sun is inclined at an angle of 23 deg. To the earth’s equatorial plane. This resultant gravitational force tries to change the satellite’s orbit at the rate of 0.86 deg. per year.
The earth is not quite a perfect sphere. At the equator, there are bulges of about 65 m at longitude 162 deg. E and 348 deg. E, with the result that a satellite is accelerated towards one of two stable points in the GEO orbit at longitude 75 deg E and 252 deg E, as shown in fig.2. To maintain accurate station keeping, the satellite must be periodically accelerated in the opposite direction to the forces acting on it. This is done as a sequence of station-keeping maneuvers, using small rocket motors (sometimes called gas jets or thrusters) that can be controlled from the earth via the TTC&M systems.
