This work proposes an output feedback controller for stabilization of the quadrotor underactuated
system in the presence of time varying disturbances and model uncertainties. The proposed control is
an improvement to the sliding mode control (SMC). An extended high-gain observer (EHGO) when
combined with sliding mode control (SMC) able to give feasible performance beyond the performance
of the standard sliding mode. It is able to bring the state trajectories of the closed-loop system close to
the target system with a smaller ultimate bound of error and smaller control magnitude. The proposed
method is illustrated by simulation.