Stereo Vision System for Guidance of Autonomous Aircraft

Unmanned aerial vehicles (UAVs) are increasingly replacing manned systems in situations that are either too dangerous, too remote, or too difficult formanned aircraft to access. Modern UAVs are capable of accurately controlling their position and orientation in space using systems such as the Global Positioning System (GPS) and the Attitude and Heading Reference System (AHRS). However, they are unable to perform crucial guidance tasks such as obstacle avoidance, low-altitude terrain or gorge following, or landing in an uncontrolled environment using these systems only. For such tasks, the aircraft must be able to continuously monitor its surroundings. Active sensors, such as laser range finders or radar can be bulky, low-bandwidth, and stealth-compromising. Therefore, there is considerable benefit to be gained by designing guidance systems for UAVs that utilise passive sensing, such as vision. Over the last two decades, a significant amount of research has shown that biological visual systems can inspire novel, vision-based solutions to some of the challenges facing autonomous aircraft guidance. A recent trend in biologically inspired vision systems has been to exploit optical flow information for collision avoidance, terrain and gorge following, and landing. However, systems that rely on optical flow for extracting range information need to discount the components of optical flow that are induced by rotations of the aircraft. Furthermore, altitude cannot be controlled in a precise manner using measurements of optical flow only, as optical flow also depends upon the aircraft’s velocity. Stereo vision, on the other hand, allows the aircraft’s altitude to be directly computed and controlled, irrespective of the attitude or ground speed of the aircraft, and independently of its rotations about the roll, pitch, and yaw axes. Additionally, we will show that a stereo vision system can also allow the computation and control of the orientation of the aircraft with respect to the ground. Stereo vision therefore provides an attractive approach to solving some of the problems of providing guidance for autonomous aircraft operating in low-altitude or cluttered environments. In this chapter, we will explore how stereo vision may be applied to facilitate the guidance of an autonomous aircraft. In particular, we will discuss a wide-angle stereo vision system that has been tailored for the specific needs of aircraft guidance, such as terrain following, obstacle avoidance, and landing. Finally, results from closed-loop flight tests conducted using this system will be presented to demonstrate how stereo vision can be successfully utilised to provide guidance for an autonomous aircraft performing real-world tasks. 16