Unmanned Aerial Vehicle (UAV) technology has recently experienced increasing development, leading to the creation of a wide variety of autonomous solutions. In this paper a guidance strategy for straight and orbital path following of fixed-wing small UAV is presented. The proposed guidance algorithm is based on a reference Vector Field as desired 16course for the UAV to follow. A Sliding Mode approach is implemented to improve robustness and effectiveness andasymptotic convergence of the aircraft to the desired trajectory in presence of constant wind disturbances is proved according to Lyapunov. The algorithm exploits the banking dynamics and generates reference signals for the inner loop ailerons control. A MATLAB&Simulink® simulation environment is used to verify performance and robustness of the compared guidance algorithms. This high-fidelity model considers the six Degrees-of-Freedom (DoF) whole flight dynamics of the UAV and it is based on experimental flight test data to implement the aerodynamic behavior.