**2. Quadrotor configuration**

The quadrotor UAV is a highly non-linear, 6 DoF, Multi-Input-Multi-Output (MIMO) and under-actuated system [8]. One can describe the vehicle as having four propellers in cross configuration as shown in **Figure 1**. Quadrotor motion is controlled by varying the speed of the four rotors. A quadrotor has two sets of clockwise and two sets of counter-clockwise rotating propellers to neutralize the effective aerodynamic drag. Vertical movement of the quadrotor system is controlled by simultaneously increasing or decreasing the thrust of all rotors. Yawing motion is created by proportionally varying the speeds of counter-clockwise rotating propellers and the rolling and pitching motions are created by applying differential thrust forces on opposite rotors of the quadrotor [9].

**Figure 1.** *Quadrotor UAV configuration.*
