An Efficient Algorithm for Periodic Riccati Equation for Spacecraft   Attitude Control Using Magnetic Torques
  Spacecraft attitude control using only magnetic torques is a periodic time-varying system as the Earth magnetic field in the spacecraft body frame changes periodically while the spacecraft circles around the Earth. The optimal controller design therefore involves the solutions of the periodic Riccati differential or algebraic equations. This paper proposes an efficient algorithm for the periodic discrete-time Riccati equation arising from a linear periodic time-varying system $(\A,\B)$, which explores and utilizes the fact that $\A$ is time-invariant and only $\B$ is time-varying in the system, a special properties associated with the problem of spacecraft attitude control using only magnetic torques. 
Yaguang Yang
math.OC