Nonlinear Control Law Design For Satellite Fixed Ground Target Tracking

Abstract

Most of the tracking control algorithms introduced in the available literature care about pointing accuracy and ignore the time elapsed to reach these strict requirements. Actually there is an increasing demand from customers for a fast ground-target tracking even on the expense of pointing accuracy itself. When the target locations and the ground station are both within the satellite’s footprint, it could be more important to steer the main imager boresight towards the immediate required target areas before capturing the imaging data. The quick response allows data gathering and downloading in the same communication session for the military intelligence. In this paper, a nonlinear tracking control algorithm is modified and altered to be utilized with exchange momentum actuators (e.g. reaction wheels). The control law uses the commanded attitude rate and acceleration in addition to attitude error and gyroscopic compensation. Tracking error dynamics, equivalent to satellite closed-loop time-varying nonlinear dynamic system, is used alternatively to confirm that a global stability. The proposed controller is applied to fixed ground target tracking task. Generation of the needed target attitude and attitude rate are derived in details. The kinematics of the ground target relative to the satellite is analyzed and presented in the orbit reference frame. In this reference frame, the satellite dynamics are derived from first principles. The body z-axis of satellite is used as a pointing axis in the tracking scheme. Assuming ideality for attitude and orbit determination sensors and symmetric satellite inertia, the validity of proposed controller and target data generator is demonstrated using Matlab/Simulink. Robustness of the proposed control law is discussed against inertia matrix uncertainty. Simulations show that the proposed control law can be used onboard for fast tracking purposes