ESA GNC Conference Papers Repository

The main purpose of this work is to design a robust and reliable algorithm to control a satellite attitude exploiting thrusters? off-modulation logic. Low Earth Orbit (LEO) satellites are often required to perform frequent manoeuvres for orbit maintenance. Depending on the altitude, the duration of these manoeuvres can last up to 300 seconds. During the firing, displacement of the centre of gravity, uncertainties in the thrusters? accuracy and misalignments will generate parasitic torques. In addition, high precision requirements on pointing do not allow the employment of magnetic torquers, therefore the attitude is generally undertaken with reaction wheels only. To overcome this problem, part of the angular momentum for the attitude control can be provided by the off-modulated thrusters. This strategy consists of generating angular momentum by means of an appropriate thrusters switching off during the firing phase. An innovative feedback-based approach is developed in the paper and, afterwards, it is tested on a high-fidelity simulator (the MatLab SimuLink ? OHB Attitude and Orbit Control System (AOCS) simulator) to prove its robustness and efficiency. Thanks to the presence of two adjustable thresholds, the algorithm can be easily tailored to whichever mission requirement. Results are shown in the last chapter, where a Monte Carlo analysis is carried out to validate the algorithm in almost all the possible scenarios. Eventually, it will be shown that this new off-modulation approach brings remarkable improvements characterizing itself as one of the best solutions to be adopted by the AOCS system for the attitude control during thrusters firing.