Spacecraft attitude and rate estimation using sliding mode observers

Different methods for spacecraft attitude and rate estimation using variable structure systems theory are presented. The simplest approach involves the study of the spacecraft system decoupled in all three axes, where three sliding observers are run in parallel using a linearized model of the system for the observer dynamics. The second approach investigates the possibility of extracting the angular rate data by low-pass filtering of the derivative of angular measurements. Outputs of the filter are then fed to the observer as measurements and the angular rate estimates are obtained using a sliding observer. The final approach regards the spacecraft axes coupled. The sliding observer is implemented on the nonlinear dynamic and kinematic equations of motion to obtain the angular rate and attitude estimates. It is shown on the CATSAT (Cooperative Astrophysics and Technology SATellite) simulation model that actual rates are successfully estimated regardless of the large quantities of uncertainty in the inertia matrix and disturbance in the input torques.