  We propose a tracking control law for the fully actuated rigid body system in
the presence of any unknown constant disturbance by employing quaternions with
the stable embedding technique and Lyapunov stability theory. The stable
embedding technique extends the attitude dynamics from the set of unit
quaternions to the set of quaternions, which is a Euclidean space, such that
the set of unit quaternions is an invariant set of the extended dynamics. Such
a stable extension of the system dynamics to a Euclidean space allows us to
employ well studied Lyapunov techniques in Euclidean spaces such as
LaSalle-Yoshizawa's theorem. A robust tracking control law is proposed for the
attitude dynamics subject to unknown constant disturbance and the convergence
properties of the tracking control law is rigorously proven. It is demonstrated
with the help of numerical simulations that the proposed control law has a
remarkable performance even in some challenging situations.
