  This technical note focuses on the control of a quadrotor unmanned aerial
vehicle (UAV) tethered to the ground. The control objective is to stabilize the
UAV to the desired position while ensuring that the cable remains taut at all
times. A cascade control scheme is proposed. The inner loop controls the
attitude of the UAV. The outer loop gives the attitude reference to the inner
loop, and is designed so that (i) the gravity force is compensated, (ii) the
cable is taut at all times, and (iii) the trajectory of the UAV follows the
geodesic path. To prove asymptotic stability, small gain arguments are used.
The control scheme is augmented with a reference governor to enforce
constraints.
