An unmanned aerial vehicle, otherwise known as a UAV, remotely piloted vehicle, or unmanned aircraft system, is a flying vehicle that travels without a human pilot or crew present on the aircraft. UAVs are reusable; this is one of the major points that distinguishes them from other unmanned aerial objects such as missiles. This distinction is important because, like missiles, UAVs are most commonly used for military purposes, such as reconnaissance and targeted attack. An unmanned aerial vehicle may be used for military or nonmilitary purposes, though military UAV use is far more common than nonmilitary use. Nonmilitary uses include security, search and rescue, and firefighting.
An unmanned aerial vehicle is ideal for military use because there is no risk of loss of human life and because the lack of need for a human pilot opens up many previously impossible aircraft design possibilities. The most common use for military UAVs is remote sensing. A small, stealthy unmanned aerial vehicle can be fitted with a variety of different sensors, allowing it to gather a great deal of information about enemy positions, even late at night. UAVs can also be used for targeted missile strikes. This is particularly useful for precise strikes for purposes such as assassination and there is no risk of pilot death or capture.
A modern unmanned aerial vehicle usually combines some level of autonomy with direct remote control. Autonomy is very important, as a vehicle that can complete a mission and respond to unexpected events with minimum human intervention uses less manpower and is therefore more useful and valuable. There are many important aspects to this automation, ranging from trajectory planning to target acquisition. An unmanned aerial vehicle must also be able to control the basic aspects of flight, such as flight stabilization, speed, and response to changing air currents. Complete automation would involve the ability for prioritizing tasks, cooperative functioning, and independent action based on the synthesis of several different sources of information.
The lack of a human pilot in an unmanned aerial vehicle adds one other advantage: flight time. Without a pilot who needs to eat, drink, or sleep, an unmanned aerial vehicle can theoretically stay in the air for as long as its fuel supply will last. That fuel-mandated time constraint grows less important as alternate power sources, such as solar power, become more and more viable. In theory, advanced UAVs should be able to stay in the sky for years at a time if necessary.