Attitude estimation and magnetic attitude control of a LEO satellite

Abstract

In this thesis, a complete low-cost ADCS is designed for a micro-satellite orbiting in LEO. The attitude determination system of the satellite is considered to be equipped with gyroscopes, TAM and Sun sensor which are the most common attitude sensors for small satellites. For satellite attitude control purposes, magnetorquers are selected as they are cheap, robust and easy to implement. Several static attitude determination algorithms which make use of the vector measurements are described and applied to the attitude determination problem by simulations. A comparison is made between different methods and QUEST algorithm is chosen considering the accuracy, space heritage and robustness. The fine attitude determination system is designed with the integrated scheme in which QUEST and MEKF are combined. In this scheme, QUEST outputs are used to update the filter. This structure provides flexibility with the number of attitude sensors and it can be interchangeably used with star trackers, when available onboard, without changing the filter structure, and also it has a robust structure and fast convergence characteristics. Besides, the multiplicative approach preserves the norm constraint of quaternion and so the filter does not encounter singularity issues by using the quaternion as the attitude parameter. The integrated QUEST/MEKF approach is implemented for the satellite attitude and angular rate estimation. Moreover, the integrated method is compared with the traditional MEKF in terms of convergence speed, accuracy and robustness. Before controlling the attitude, a satellite detumbling method is applied first to reduce the angular rate of the satellite. The Bdot control algorithm is selected as it is easy to apply and it needs only the magnetometer measurements. After detumbling the angular velocity, the attitude is controlled by using MPC approach. Although the magnetic control does not provide three-axis control instantly, the control can be achieved along the orbit in near-polar orbits as the magnetic field is changing over time. Since the magnetic field changes along the orbit, the MPC is chosen as it can predict the future behaviour of the system and adjust the control moment accordingly. Additionally, the magnetometers are fully-calibrated using the TWO-STEP algorithm. Thus, calibrated TAM measurements can be used in both the attitude determination and control algorithms. Eventually, the determination and the control parts of the ADCS are integrated to build a complete system for the satellite. The system is designed such that it first uses Bdot control and then MPC is applied once the spacecraft is detumbled. Integrated QUEST/MEKF outputs are fed to the MPC algorithm and the control moment is calculated by using the estimated states. The simulations are conducted in MATLAB environment for a sun-synchronous LEO micro-satellite.