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.

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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. An ideal feedback control system cancels out all errors, effectively mitigating the effects of any forces that may arise during operation and producing a response in the system that perfectly matches the user's wishes. The space applications this discipline encompass includes satellite attitude and orbit control, antennas or optical terminal fine pointing, and more generally guidance, navigation and control for space vehicles that have to accomplish specialised functions such as formation flying and orbital rendezvous, landing on asteroids and planetary bodies as well as re-entry through Earths atmosphere.

The general scope of a satellites trajectory is set by the launcher that hauls it skyward — selected by orbital dynamics experts long in advance of the satellite being built — after which smaller thrusters manoeuvre it into its operational orbit. After that the onboard closed-loop control is in charge of controlling the spacecrafts pointing direction — known as its attitude — as it proceeds along its orbital path.

The problem is that satellites have their attitude perturbed in various ways, whether by airdrag from the outermost layers of the atmosphere or Earths gravitational influence or solar radiation pressure exerted on large appendages, or interaction between Earth's magnetic field and satellite magnetic dipoles. A satellite attitude is also disturbed by its own contents which can set up undesirable vibrations liquid sloshing in a propellant tank and oscillations of large solar wings are classical examples.

The perturbing effects of such external and internal torques need then to be counteracted by the AOCS. This system incorporates sensors to identify the satellite's current attitude such as gyroscopes, startrackers, Sun sensors or magnetometers and actuators including thrusters, reaction wheels or magnetic torquers to trigger the desired corrective rotations around the satellites centre of mass. The set tasks of many spacecraft require them to maintain specific absolute and relative pointing within acceptable errors and this is achieved by the on-board AOCS.

Communication satellite antennas need to respect half-cone pointing errors for ground stations and users, while space astronomy observatories or Earth observing satellites have in addition to ensure high stability of the line-of-sight of optical or radio frequency instruments. Similarly, Guidance, Navigation and Control GNC systems ensure that planetary probes and transportation vehicles shall maintain not only their pointing orientation, but also their absolute or relative position in space in order to achieve a successful rendezvous with a target, or an autonomous and safe landing on a planetary body or to maintain the relative six degrees of freedom DOF positioning when it comes to a a formation flying mission.

Specialised sensors are required in this case, such as rendezvous sensors, Lidar or Optical Navigation cameras, radio frequency and optical metrology systems. Formation flying and planetary lander missions currently in the planning stages will need also high precision for relative navigation and positioning. You have already liked this page, you can only like it once!

What is the Control Systems domain? Why is Computer Systems important? Like Thank you for liking You have already liked this page, you can only like it once!


Attitude and Orbit Control Systems (AOCS)

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.


Control Systems

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.

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