Thesis supervisor: Accociate Professor Mart Noorma, Tartu ülikooli füüsika instituut
Dr Linas Bukauskas (Vilniuse ülikool)
This research was carried out at the University of Tartu, Tartu Observatory, the Finnish Meteorological Institute and the Estonian Student Satellite Programme. This thesis presents the ESTCube-1 attitude determination system. The attitude is the satellite's orientation is space. ESTCube-1 is a satellite built according to the one-unit CubeSat standard (≈ 10 cm × 10 cm × 10 cm). The satellite was launched in May 2013 and operated until May 2015. The main scientific mission of ESTCube-1 was to perform the first in-orbit electric solar wind sail demonstration. The electric solar wind sail is a propellantless propulsion technology concept. The sail consists of long, thin, centrifugally stretched and positively charged tethers that deflect charged particles in the solar wind, hence generate spacecraft thrust.
The main requirement of the ESTCube-1 attitude determination system is to determine the attitude with an accuracy better than 2° for the following purposes: high rate spin control (hundreds of degrees per second) for centrifugal tether deployment; monitoring of tether deployment; to trigger the charging of the tether in synchronisation with the satellite spin; to measure angular velocity changes caused by the Coulomb drag interaction between the charged tether and the surrounding ionospheric plasma.
The attitude determination system has Sun sensors, magnetometers and gyroscopic sensors. A geomagnetic field model and a Sun position model were used to reference the respective sensor measurements. A Kalman filter was used to estimate the attitude. Before the launch, the system was characterised in the laboratory and by simulations. With in-orbit recalibration and validation, the system was significantly improved. For validation, an independent attitude determined from on-board images was used. By characterising and validating the system, it was shown that attitude determination accuracy is better than 1.75°, hence fulfils the requirement set by the electric solar wind sail experiment.