Passive magnetic attitude stabilization of the UNISAT-4 microsatellite

UNISAT-4 is the fourth educational microsatellite, completely designed and built by students and professors of the research group GAUSS (Gruppo di Astrodinamica dell’Università degli Studi “la Sapienza”) at the Scuola di Ingegneria Aerospaziale of University of Rome “La Sapienza”. The spacecraft is stabilized using a passive magnetic attitude stabilization system, based on a permanent magnet and an energy dissipation system, which consists of magnetic hysteresis rods. The main features of passive magnetic stabilization are simplicity and reliability. However, sizing the system parameters, predicting the in-orbit performance and obtainable accuracy of passive magnetic stabilization systems is not trivial. The main problem in the system design is accurate modeling of the hysteresis rods magnetization and the evaluation of the rods magnetic parameters, such as apparent permeability, remanence and coercitive force, which are considerably affected by the rods’ manufacturing technological process. In this paper the design and ground test of the UNISAT-4 magnetic attitude stabilization system is described. A method to experimentally determine the hysteresis rod parameters is described and an accurate model of the satellite dynamics is obtained, based on the results of the measurements. One of the main design parameters is the number of hysteresis rods necessary to obtain satellite stabilization. Numerical simulations for two hysteresis rods per axis and eight hysteresis rods per axis are discussed, showing that the satellite stabilizes in about 14 days, with a residual oscillation amplitude of less than 10°, if eight rods are used.