Satellite communication - AOC subsystem
Satellite communication tutorial
Satellite Communication - AOC Subsystem
We know that the satellite can deviate from its orbit due to the gravitational forces of the sun, the moon and other planets. These forces change cyclically over a 24 hour period as the satellite moves around the Earth.
The Altitude and Orbit Control (AOC) subsystem consists of rocket motors, which are capable of placing the satellite in right orbit, whenever it deviates from the respective orbit. The AOC subsystem is useful so that the antennas, which are narrow beam type, point towards the earth.
We can make this AOC subsystem the following two parts .
- Altitude control subsystem
- Orbit control subsystem
Now let's talk about these two subsystems one by one.
Altitude control subsystem
The sThe altitude control subsystem supports the orientation of the satellite in its respective orbit. Here are the two methods to make the satellite present in an orbit stable.
- Rotate the satellite
- Three axis method
Rotate the satellite
In this method, the body of the satellite rotates around its axis of rotation . In general, it can be rotated from 30 to 100 rpm to produce force, which is gyroscopic type. For this reason, the axis of rotation stabilizes and the satellite will point in the same direction. Satellites of this type are called spinners.
Spinner contains a drum, which is cylindrical in shape. This drum is covered with solar cells. Power systems and rockets are present in this drum.
The communication subsystem is placed above the drum. An electric motor drives this systemcommunication. The direction of this motor will be opposite to the rotation of the body of the satellite, so that the antennas point towards the earth. Satellites, which perform this kind of operation are called de-spin .
During the launch phase, the satellite spins when small radial gas jets are activated. After that, the de-spin system works to point the antennas of the TTCM subsystem to the earth station.
In this method, we can stabilize the satellite using one or more motion wheels. This method is called a three axis method . The advantage of this method is that the orientation of the satellite on three axes will be controlled and no need to rotate the main body of the satellite.
In this method, the following three axes are considered.
The roll axis is considered inthe direction in which the satellite moves in the orbital plane.
The yaw axis is seen in the direction towards the earth.
The pitch axis is considered in the direction, which is perpendicular to the orbital plane.
These three axes are shown in the figure below.
Let X R , Y R and Z R are respectively the roll axis, the yaw axis and the yaw axis. 'pitch axis. These three axes are defined by considering the position of the satellite as reference . These three axes define the altitude of the satellite.
Let X, Y and Z be another set of Cartesian axes. This set of three axes provides information on the orientation of the satellite with respect to the reference axes. If there is a change in satellite altitude, thenthe angles between the respective axes will be changed.
In this method, each axis contains two jets of gas. They will ensure the rotation in both directions of the three axes.
The first gas jet will work for a certain period of time, when there is a requirement for movement of the satellite in a particular axis direction.
The second gas jet will operate for the same period of time, when the satellite reaches the desired position. Thus, the second gas jet will stop the movement of the satellite in this direction of axis.
Orbit control subsystem
Orbit control subsystem is useful for bringing the satellite into its orbit correct, whenever the satellite is deviated from its orbit.
The TTCM subsystem at the earth station monitors the position of the satellite. If there is a change in the orbit of the satellite, alors it sends a signal about the correction to the Orbit control subsystem. Then it will solve this problem by bringing the satellite into the correct orbit.
In this way, the AOC subsystem supports the position of the satellite in right orbit and at right altitude throughout the life of the satellite in the 'space.