Study and development of attitude control and determination simulation software and control algorithms for 3Cat-4 mission

The AOCS of a spacecraft is the responsible for determining the attitude of the satellite and controlling it to reach a specific pointing state, to alloww the execution of scientific experiments on board. In the case of 3Cat-4 mission, a magnetic control system is used to align the deployable helix antenna with the Nadir vector to perform the GNSS-R and L-band radiometry experiments for Earth Observation. This task is especially complex to perform in nanosatellites due to the limitations of mass, size, and power consumption. To analyze the performance of this subsystem, facilities (Helmholtz coils, TVAC, Sun simulator) are used that try to emulate the environment in which the satellite will operate, but this is not always possible and for this reason this project has been carried out. In this project, a simulation software programmed in MATLAB has been developed to analyze the performance of the AOCS subsystem. This tool has been used to validate the design and verify the compliance with the subsystem requirements, but also to support the design phase. In addition, the simulator has been used to analyze the AOCS of the 3Cat-4 mission developed by the UPC NanoSat Lab. This document explains the physics behind each of the blocks modeled to emulate the space environment, implementing the equations that govern the orbial dynamics and attitude of the satellite and including the disturbance forces that affect a satellite in orbit LEO . In addition, the development of the attitude determination and control system, both of the hardware part (i.e. sensors and actuators) and of the estimation and control algorithms, is presented. In particular, the Optimal-REQUEST algorithm has been implemented and the control laws for the Detumbling and Nadir Pointing Mode modes, corresponding to the 3Cat-4 mission, have been developed. The results of the analysis showthe correct operation of the simulator and the compliance of the subsystem requirements of the 3Cat-4 mission. The control modes meet the objectives defined, i.e. the detumbling mode is capable to stabilize the system and reduce the angular velocity of the CubeSat below 2°/s in less than 24 hours and the Nadir Pointing mode allows to reach the pointing towards Nadir, with a maximum angular deviation of 10° and angular velocity of 0.5°/s in the X and Y axes of the satellite. On the other hand, the results show the great negative impact of the aerodynamic drag, due to the physical characteristics of the CubeSat and the orbit in which it is located, causing the control system to have to be continuously active to compensate for the disturbances and reach the desired attitude state.