L1 adaptive control is a recent scheme elaborated in order to ensure a decoupling between robustness and fast adaptation. This controller was validated in simulations and experimental results. Research is currently being developped in order to improve its architecture and design challenges. Our study focuses on the inherent time lag observed in presence of a varying reference trajectory. The solution proposed in this regard is the extended L1 controller in which we suggest an augmentation of the original one with a PID aiming to reduce the tracking error. The stability analysis of this new scheme is shown in this paper. Experimental results on an underwater vehicle subjected to a varying trajectory under several disturbances are then displayed to illustrate the e ciency of the method.