A new angular velocity observer for attitude tracking of spacecraft

This paper proposes a new angular velocity observer for attitude tracking of spacecraft based on contraction analysis. The observer is designed in the inertial reference frame via estimating the inertial angular momentum to avoid the square term of the angular velocity when the spacecraft dynamics is expressed in the body frame. It employs a continuous angular-velocity dependent innovation term generated by means of a simple first-order linear filter, instead of a discontinuous attitude-dependent innovation term commonly used in angular velocity observer designs, resulting in a smooth behavior. The global exponential convergence is achieved. Moreover, when combined with the exponentially convergent tracking controller devised in this paper, it gives an overall system with exponential stability relying on a separation property. Finally, a switching function with hysteresis is introduced to stabilize the closest equilibrium in the configuration space, achieving the global exponential stability. Numerical simulations are included to illustrate the performance of the proposed observer in the closed loop, comparison with similar results, and robustness verification under inertia parameter uncertainties and noisy measurements.