Abstract The main objective of this paper is to design and compare two altitude command tracker controllers for an UAV using H∞ and QFT techniques. Both methods require the use of specialized software tools. The different stages of QFT methodology have been done with help of software tool QFTIT (Quantitative Feedback Theory Interactive Tool). This is a free software tool that is characterized by its easy of use and interactive nature. QFTs standing as a viable control design method that can be applied to practical problems and produce implementable results. Tests with realistically large control inputs are use to validate and compare both designs. I. INTRODUCTION HERE is a considerable and great interest in using unmanned vehicles to perform a multitude of tasks [1]- [2]. UAVs already provide clear opportunity to reduce the risk of life threatening missions that might otherwise be performed by human-piloted craft. Nevertheless, the design of control systems for UAVs is clearly a complex task. The aircrafts response to control inputs depends heavily on the parameter uncertainties of the plant. For instance, the variations in the center of gravity or the time varying dependence of the mass affect to the control response. Hence, it is necessary the use of robust design methods with satisfactory performance over a specified range of plant parameter variations [3]-[5]. In [6] an autonomous flight control strategy is presented. Manuscript received October 13, 2006. This work was supported in part by the Spanish Ministry of Industry under project SISCANT (Reference number FIT-330101-2004-13) and SISCANT II (Reference number FIT- 330100-2005-129). J. Lpez is with Dynamic Systems Research Group, Universidad PolitØcnica de Madrid. R. Dormido is with Department of InformÆtica y AutomÆtica, Universidad Nacional de Educacin a Distancia (UNED) (corresponding author to provide phone: +34-91-3987192; fax: +34-91-3987690; e-mail: raquel@dia.uned.es) J. P. Gmez is head of Dynamic Systems Research Group, Universidad PolitØcnica de Madrid (e-mail: jgomez@euita.upm.es). S. Dormido is with Department of InformÆtica y AutomÆtica, Universidad Nacional de Educacin a Distancia (UNED) (e-mail: sdormido@dia.uned.es) J. M. Daz is with Department of InformÆtica y AutomÆtica, Universidad Nacional de Educacin a Distancia (UNED) (e-mail: josema@dia.uned.es) The control task of that strategy is divided into two parts, a robust inner-loop controller that is designed to achieve stability and robustness to expected parameter uncertainty; and an outer-loop for tracking reference performace. A controller designed with the H∞ technique for the inner loop was presented in that paper. In this paper two different altitude command tracker control strategies for the outer loop capable of high- performance tracking of a given flight trajectory in presence of parameter uncertainty have been compared. The first is an H ∞ approach, the second a QFT design. Advantages and disadvantages of both designs are described. In Section II of this paper the modeling and identification assumptions are outlined. Section III presents the statement of the control problem. Section IV and V detail the H∞ and QFT designs. Validation is presented in Section VI. Concluding remarks are made in Section VII. II. MODELING AND IDENTIFICATION The UAV is a 1/3 scaled down model of a Diamond Katana DA-20 shown in Fig. 1. The main characteristics of the aircraft are: --Span 3.9 m. --Wing surface 1.47 square meters. --Mean aerodynamic chord 0.39 m. Comparison of H ∞ with QFT applied to an Altitude Command Tracker for an UAV J. Lpez, R. Dormido, J. P. Gmez, S. Dormido, J. M. Daz T Fig. 1. KUAV scale model.