Performance Evaluation of the UNISAT Attitude Control System

The microsatellite concept involves obvious limitations in size, weight, and on-board available power. This undoubtedly affects the design of the microsatellite subsystems, whose complexity has to be reduced to limit development times and costs. This paper deals with the analysis of the performance of the UNISAT attitude determination and control system. UNISAT is a joint project of a group of Italian universities aimed at designing and developing a multi-mission microsatellite for scientific and educational purposes. The attitude determination hardware consists of a miniature solid-state horizon sensor, a digital two-axis sun sensor, and a three-axis fiuxgate magnetometer. Three-axis attitude control is performed with three magnetic torquers. Control laws are derived separately for the attitude acquisition and the stationkeeping phases using a linearized analytical model of the microsatellite attitude dynamics. Then, a numerical code, which simultaneously integrates the microsatellite orbital and attitude dynamics and control, is used to analyze the accuracy in the attitude angle determination from the sensor measurements and to test the effectiveness of the proposed control strategy for a range of sun-synchronous orbits and altitudes. Numerical results show that the proposed control techniques allow the satellite attitude acquisition to be performed in about one orbit and the attitude angles are controlled within ±2.5 deg during stationkeeping.