Surveillance aircraft are military aircraft used for monitoring enemy activity, usually carrying no armament. This article concentrates on military aircraft used in this role, though a major civilian aviation activity is reconnaissance and ground surveillance for mapping, traffic monitoring, science, and geological survey. In addition, civilian aircraft are used in many countries for border surveillance, fishery patrols or the prevention of smuggling and illegal migration.
A surveillance aircraft does not necessarily require high-performance capability or stealth characteristics. It may in fact be a modified civilian aircraft which has been disguised in order to look harmless. Technically, anything which can fly and make observations (dynamically or via recording equipment/sensors) of visual information or electronic emissions qualifies as a surveillance aircraft.
Such efforts long predate the invention of heavier-than-air flight, with experiments using balloons to provide targeting information for artillery beginning in France in 1794. Continued attempts throughout the 19th Century proved militarily useless, but aerostat-based radar platforms are now in use.
Airborne reconnaissance goes back to the early era of ballooning. After the French Revolution, the new rulers became interested in using the balloon to observe enemy manoeuvres and appointed scientist Charles Coutelle to conduct studies using l'Entreprenant -- its name literally meaning "The Undertaking," which was the first reconnaissance aircraft. The balloon found its first use in the 1794 conflict with Austria, where in the Battle of Fleurus the French Aerostatic Corps gathered information and the demoralizing effect on the Austrian troops ensured victory for the French troops.
The first reconnaissance flights with winged aircraft in combat conditions took place during the Balkan wars, on 5 October 1912 by Greek and on 16 October 1912 by Bulgarian (Albatros) aircraft.
One of the first aircraft used for surveillance was the Rumpler Taube during World War I, when aviators like Fred Zinn evolved entirely new methods of reconnaissance and photography. The translucent wings of the plane made it very difficult for ground based observers to detect a Taube at an altitude above 400 m. The French also called this plane "the Invisible Aircraft", and it is sometimes also referred to as the "world's very first stealth plane". German Taube aircraft were able to detect the advancing Russian army during the Battle of Tannenberg (1914).
Before World War II the conventional wisdom was to use converted bomber types for airborne photo reconnaissance, since these were the only aircraft with the long range needed for the reconnaissance missions. These bombers retained their defensive armament, which was vital since they were unable to avoid interception.
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