Reduced-Attitude Boresight Guidance and Control on Spacecraft for Pointing, Tracking, and Searching

A guidance and a control law are developed to maneuver the boresight axis of a spacecraft relative to a reference vector, using knowledge of the reference vector and the angular rates of the spacecraft. Therefore, this can be used in cases when the full attitude is not known or when the angle of the spacecraft around the boresight or reference vector does not need to be controlled. A Lyapunov-based analysis is performed to prove asymptotic stability of the system to a reference trajectory. Even in the presence of inertia uncertainty, it is shown that asymptotic stability to a stationary attitude can be guaranteed. To assist with practical gain selection, a linear analysis is performed. Finally, the boresight guidance and control laws are applied to four different scenarios to demonstrate their utility. The first two show examples of pointing in an inertially-fixed direction and tracking a time-varying target. The third and fourth demonstrate searching tasks. A novel example of searching for the Sun by reducing the search space from a full 4π-steradian sphere to a circle of a sphere is presented. If the full sphere must be searched, a spherical spiral maneuver is presented to perform this task.