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In this paper, the robust adaptive control scheme based on backstepping technique is presented that improves the trajectory tracking performance of the quadrotor unmanned aerial vehicles (UAVs), specially tasked for supply, rescue and combat missions. The proposed control scheme is designed to estimate all the system parameters that may posses uncertainties and effectively rejects the completely unknown time varying external disturbances. The adaptive laws, derived through Lyapunov stability theorem are robustified by merging with derivative-integral (DI) term, resulting in rapid and accurate adaptation. In addition, to avoid parametric drift phenomenon, we introduce the projection modification (PM) in the designed DI-adaptive laws that ensures the closed-loop system signals bounded. The trajectory tracking and parameter estimation performance of the UAV in the presence of external disturbances, the payload pick up/drop off effect on altitude and recoil effect on attitude is analyzed by means of numerical simulations. The results validate strict robustness with extended applicability of proposed control scheme.

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