What is the Attitude Determination & Control System?
The Attitude Determination and Control System (ADCS) is one of the most crucial subsystems of the spacecraft. It is responsible for maintaining a stable attitude and pointing accuracy of the satellite devices i.e. cameras, antennas, payloads, and other measuring instruments. Its essence is the process of controlling the orientation of the satellite with respect to an inertial frame of reference (usually ECI or ECEF), using either reaction wheels or magnetic coils (magnetorquers).
How does the ADCS work?
The first task that must be performed after spacecraft separation from the launcher, is to bring it to a condition with a sufficiently small angular momentum i.e. detumble the spacecraft. The detumbling process consists in bringing the spacecraft from an initial condition characterized by large angular momentum, to a final stable state, remaining essentially arbitrary in terms of the angular rate vector direction (for any reference frame). Furthermore, it must be compatible with the operation of subsequent ACS algorithms responsible for achieving nadir pointing or pointing at particular locations. This task (detumbling) is achieved by utilizing magnetorquers and MEMS magnetic field sensors (detecting effects explained by the Lorentz force) as angular rate sensors. The operation of magnetorquers is based on the interaction between the magnetic field generated by the coils and the Earth magnetic field, ultimately allowing to create proper actuating forces orienting the satellite to the desired attitude (pointing position).
In order to test the designed algorithm, at SatRevolution I have prepared a simulation program of phenomena occurring in the environment of the satellite on particular orbits.
Why designating algorithms are challenging?
The main obstacle associated with magnetic coils is the fact that magnetorquers are constrained to position itself (under the influence of the flowing current creating magnetic dipole) along the orthogonal plane to the local magnetic field vector. This leads to the complexity associated with the impossibility of control among the axis parallel to the Earth’s magnetic field so that the angular dynamics of a magnetically actuated spacecraft are completely controllable only in the time-varying sense for particular orbits. Reaction wheels (i.e. momentum wheels) are a type of flywheels operating in terms of rotation speed, in order to imbue the satellite with a given amount of stored angular momentum. This alters the spacecraft’s rotational dynamics allowing to reorient the satellite, having the final effect of tending a damped rotation in order to reach the desired attitude.
Employing unique features of a satellite designed for science, exploration, and space operations requires high precision of attitude determination. Particularly the satellite motion is governed by kinematic and dynamic equations. Developing an optimal path planning algorithm for the satellite maneuver operations allows us to increase the attitude precision, and maintain the satellite in a given attitude allowing to deliver proper measurements and high-quality images of particular regions of our planet.
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Special thanks to our engineer Bartosz Kopczyński for writing this article on the importance of ADCS at SatRevolution.