We know that satellite may deviates from its orbit due to the gravitational forces from sun, moon and other planets. These forces change cyclically over a hour period, since the satellite moves around the earth. Altitude and Orbit Control AOC subsystem consists of rocket motors, which are capable of placing the satellite into the right orbit, whenever it is deviated from the respective orbit. AOC subsystem is helpful in order to make the antennas, which are of narrow beam type points towards earth. Altitude control subsystem takes care of the orientation of satellite in its respective orbit. Following are the two methods to make the satellite that is present in an orbit as stable.

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When discussing software engineering topics, it is only too easy and too tempting to remain on a purely theoretical plane and speculate at length about abstrusenesses of dubious practical relevance. To protect against this danger, this book concentrates on a concrete case study consisting in the design of an objectoriented framework for Attitude and Orbit Control System or AOCS of satellites. Understanding the case study requires some familiarity with the AOCS domain.

This chapter presents the necessary background. There is much variation across satellites in the way attitude and orbit control is performed. Those who are interested in a more comprehensive and more detailed view are directed to reference [] which is the standard reference in this field. Reference [62] is also relevant in that it covers the broader satellite mission context.

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Control Systems

This subsystem consists of rocket motors that are used to move the satellite back to the correct orbit when external forces cause it to drift off station and gas jets or inertial devices that control the attitude of the satellite. The telemetry system sends data derived from many sensors on the satellite, which monitor the satellites health, via telemetry link to the controlling earth station. The tracking system is located at this earth station and provides information on the range and the elevation and azimuth angles of the satellite. Based on telemetry data received from the satellite and orbital data obtained from the tracking system, the control system is used to correct the position and attitude of the satellite. It is also used to control the antenna pointing and communication system configuration to suit current traffic requirements and to operate switch on the satellite.


AOCS Requirements and Practical Limitations for High-Speed Communications on Small Satellites

In recent years, an increasing number of countries have shown a growing interest in developing their indigenous space capacity building through national small satellite programs. These satellites, which were initially focused on educational and training missions, currently are more scientific and operational-oriented. Thus, small satellite missions are being considered not only as educational tools but also as technological demonstrators or, even, mature enough for commercial and scientific missions, which might generate a huge amount of data to be transmitted to the ground segment. Therefore, an increasing demand on channel capacity will be needed for downloading the generated housekeeping and scientific data for missions based on small satellites.


Attitude and Orbit Control Systems (AOCS)

In order for satellites or space vehicles to accomplish their mission their orientation and position in space often require extremely precise management,performed by onboard control systems. Control engineering focuses on the modelling of dynamic systems and the design of closed-loop controllers that cause these systems to behave in the manner desired. In a closed-loop control system, a set of sensors monitors the output for example, the satellite pointing direction, or the space vehicle relative position and feeds the data to a computer which continuously adjusts the control input through actuators as necessary to keep the control error to a minimum that is, to maintain the desired pointing orientation or relative position. Feedback on how the system is performing allows the controller in the onboard computer to compensate dynamically for disturbances to the system.


Satellite Communication - AOC Subsystem


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