"A reconnaissance aircraft (colloquially, a spy plane) is a military surveillance aircraft designed or adapted to perform aerial reconnaissance with roles including collection of imagery intelligence ...
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(including using photography), signals intelligence, as well as measurement and signature intelligence. Modern technology has also enabled some aircraft and UAVs to carry out real-time surveillance in addition to general intelligence gathering.
In years prior to the development and common use of sophisticated electronic image recording devices and sensors such as radar, reconnaissance aircraft were also relied upon by military forces for distant visual observation and scouting of enemy movement. An example is the PBY Catalina maritime patrol flying boat used by the Allies in World War II: a flight of U.S. Navy Catalinas spotted part of the Japanese fleet approaching Midway Island, beginning the Battle of Midway." - (en.wikipedia.org 29.09.2019)