PERFORMANCE EVALUATION OF THE UNISAT ATTITUDE-CONTROL SYSTEM

The microsatellite concept involves obvious limitations in size, weigh t, and on-board available power. This undoubtedly affects the design o f 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 s ystem. UNISAT is a joint project of a group of Italian universities ai med at designing and developing a multi-mission microsatellite for sci entific 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 fluxgate magnetometer. Three-axis attitu de control is performed with three magnetic torquers. Control laws are derived separately for the attitude acquisition and the stationkeepin g phases using a linearized analytical model of the microsatellite att itude dynamics. Then, a numerical code, which simultaneously integrate s the microsatellite orbital and attitude dynamics and control, is use d to analyze the accuracy in the attitude angle determination from the sensor measurements and to test the effectiveness of the proposed con trol strategy for a range of sun-synchronous orbits and altitudes. Num erical results show that the proposed control techniques allow the sat ellite attitude acquisition to be performed in about one orbit and the attitude angles are controlled within +/-2.5 deg during stationkeepin g.