Modelling and control of a gravity gradient stabilised satellite

This paper describes attitude control, i.e., 3-axes stabilisation and point ing, of a proposed Norwegian small gravity gradient stabilized satellite to be launched into low earth orbit. Generally, a gravity gradient stabilised system has limited stability and pointing capabilities, and wheels and/or magnetic coils are added in order to improve the attitude control. The best attitude accuracy is achieved using wheels, which can give accuracies down to less than one degree, but wheels increase the complexity and cost of th e satellite. Magnetic coils allow cheaper satellites, and are an attractive solution to small, inexpensive satellites in low earth orbits and may prov ide an attitude control accuracy of a few degrees. Scientific measurements often require accurate attitude control in one or two axes only. Combining wheel and coil control may, in these cases, provide the best solutions. The simulation results are based on a linearised mathematical model of the sat ellite.