Efficient Fast Open-Loop Attitude Control Strategy for Earth Imaging Nanospacecraft

Abstract

This paper proposes a computationally efficient attitude control strategy for nanospacecraft fast reorientation maneuvers. The paper considers a 3U CubeSat for visual Earth observation missions with deployable solar panels, equipped with three reaction wheels, three magnetorquers, and a miniature star imager, due to a 0.1° stringent targeting requirement of the payload. The star imager is very accurate, but operational only at very small angular rates. Hence it cannot be used for attitude measurement during fast slewing maneuvers. The proposed attitude control strategy overcomes this limitation by implementing a combination of open-loop and closed-loop control schemes based on the simplifying assumption of negligible gyroscopic torques. This leads to a straightforward onboard computation of the control actions required by the reaction wheels, within the limits imposed by saturation, without solving complex and computationally intensive time-optimal solutions onboard, which would not be compatible with CubeSats. The open-loop control phase is followed by an accurate closed-loop phase, accurately pointing the spacecraft toward the target orientation. The strategy is validated by numerical simulations, including robustness with respect to system uncertainties, by Monte Carlo analysis.