This paper presents practical aspects of guidance and control design for UAV and its flight test results. The paper focuses on the lateral-directional control and guidance aspects. An introduction to the mission and guidance problem is given first. Waypoints for straight and turning flight paths are defined. Computation of various flight path parameters is discussed, including formulae for real-time calculation of down-range (distance travelled along the desired track), cross-track deviation, and heading error of the vehicle; these are then used in the lateral guidance algorithm. The same section also describes how to make various mission-related decisions online during flight, such as when to start turning and when a waypoint is achieved. The lateral guidance law is then presented, followed by the design of a robust multivariable controller for roll control and stability augmentation. The controller uses the ailerons and rudder for control of roll angle and stabilization of yaw rate of the vehicle. The reference roll angle is generated by the nonlinear guidance law. The sensors available on-board the vehicle do not measure yaw rate; hence, a practical method of its estimation is proposed. The entire guidance and control scheme is implemented on the flight control computer of the actual aerial vehicle and taken to flight. Flight test results for different mission profiles are presented and discussed. 1. Introduction Successful control system design for high performance UAVs requires efficient and effective techniques for the design of guidance and control algorithms that ensure satisfactory operation in the face of system uncertainties. In this paper, we develop and present one such technique. The UAV under consideration is shown in Figure 1. The cruising speed of said UAV is about 45？m/sec. The main tasks of the guidance and control system are to: (i)fly the vehicle on the desired mission path, with minimum cross-track deviation, (ii)make decisions about various flight events, such as to start/stop turning or achievement of waypoints,(iii)provide robust stabilization and control during flight. Figure 1: A photograph of the test vehicle. A lateral track control law for small UAVs has been discussed in [1]. This is based on a pure geometrical concept. The idea is to make the ratio of lateral deviation to lateral velocity equal to the ratio of longitudinal distance to longitudinal velocity. Simulation results indicate that the yaw rate command generated by the guidance law exhibits oscillations in the vehicle roll channel, this could be a problem for
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