Single-Axis pointing of an underactuated spacecraft equipped with two reaction wheels

The note proposes a novel approach for single-axis pointing of an underactuated spacecraft by using only two reaction wheels (RW), based on a simple yet e ective wheel rate command. The control law can be used for aiming the line-of-sight of a sensor, a nozzle or an antenna towards a target direction, or solar panels towards the Sun, after failure of one wheel for a non-redundant control system hardware or in the case of multiple failures for redundant systems. Examples of this kind of situation are the Far Ultraviolet Spectroscopic Explorer (FUSE) and the Kepler space telescope. Both spacecraft su ered from failures that left only two wheels available for maneuvers. Failure of mechanical actuators is also expected to potentially a ect low-budget space missions based on small-size low-cost spacecraft (nano-, pico-, and cube-sat families). The control methodology here proposed represents the practical, dynamic implementation of a kinematic planning scheme under the assumptions of zero overall angular momentum and triaxial inertia tensor. The limited computational e ort necessary for implementing the control law makes it a practical solution also in the case of small-size satellites, where the computational budget is severely limited by the available CPU processing capabilities. The e ects of a non-zero residual angular momentum and control axes not aligned with the principal axes of inertia is also investigated, highlighting limitations on pointing precision and convergence performance.