  This paper proposes a novel unit-quaternion observer-based controller for
attitude tracking (attitude and angular velocity) with guaranteed transient and
steady-state performance. The proposed approach is computationally cheap and
can operate based on measurements provided, for instance by a typical low-cost
inertial measurement unit (IMU) or magnetic, angular rate, and gravity (MARG)
sensor without the knowledge of angular velocity. First, an observer evolved on
$\mathbb{S}^{3}\times\mathbb{R}^{3}$ is developed guaranteeing asymptotic
stability of the closed loop error signals starting from any initial condition.
Afterwards, the observer is combined with the proposed controller such that the
observer-based controller ensures asymptotic stability of the closed loop error
signals starting from any initial condition. Simulation performed in discrete
form at low sampling rate reveals the robustness and effectiveness of the
proposed approach. Keywords: Observer-based controller, attitude, estimation,
control, MARG, IMU, asymptotic stability.
