Attitude stabilization of an underactuated satellite using two wheels

Failure of mechanical controllers onboard a satellite is a well-known phenomenon that has already occurred during several space missions. Particularly disastrous for attitude control is the loss of thrusters in one or more axes, or in the case studied here, the loss of a reaction wheel. Of course it has been standard practice to employ redundant actuators (e.g. 4 wheels) so that in the event of losing one wheel, full 3-axis control may still be possible. As an interesting alternative to this expensive solution, we present here a theory, which shows how full 3- axis control can still be achieved, despite losing one of the reaction wheels from a standard orthogonal 3-wheel configuration (or even two wheels failures ftom the expensive solution using a redundant 4-wheel configuration). Using a novel nonlinear time invariant and discontinuous approach, we show that the attitude is precisely and rapidly restored, without transient oscillations, to the required earth pointing. © 2003 IEEE.