I. Introduction
Over the last decade, the potential of unmanned aerial vehicles (UAVs) has been demonstrated in D-cube missions that are identified as dangerous, dirty, or dull. Recently, considerable interest has arisen in small-scale lightweight low-cost unpiloted aircraft (known as small UAVs) designed to fly at low altitudes with limited payloads and power constraints. Aside from military deployment, small UAVs are widely applicable in civilian scenarios, because they are easy to build, carry, and launch. When equipped with vision sensors, small UAVs can execute numerous monitoring missions, including remote sensing, traffic monitoring, forest fire surveillance, accident reconnaissance, mapping, search, and rescue. Vision sensors are indispensable for carrying out these missions. In addition, a robust, reliable, and inexpensive autopilot system is necessary, and attitude determination and control are the key to achieving autonomy [1], [2].