A robust velocity and altitude tracking control for an air-breathing hypersonic vehicle (AHSV) by means of output feedback is presented in this paper.The control scheme is performed on the assumption that only the states corresponding to velocity,altitude and pitch rate are measurable.Immersion and invariance based observers are first constructed to provide asymptotically stable estimates of the unmeasured states,flight-path angle and angle of attack.Then observer backstepping is applied to the new system,in which the unmeasured states have been replaced by their estimates.Stability analysis is presented and (L)2 stable of the closed-loop system with respect to the state estimation errors is achieved.Simulations performed under varying flight conditions illustrate the effectiveness and robustness of the proposed control scheme.