Minimum-error single-axis pointing for an underactuated spacecraft in the presence of a residual angular momentum

The paper investigates pointing strategies when the prescribed pointing cannot be reached exactly with zero residual angular rate. The minimum pointing error in the framework of admissible directions with zero residual angular speed is identified first. If such a strategy results into unacceptably large pointing errors, conditions for exact pointing with non-zero residual angular rate around the pointed axis are searched. For both cases, a command on wheel angular momentum is determined. The findings are analyzed for a representative operative scenario of a small-scale spacecraft.