Reconnaissance is the military term for the active gathering information about an enemy, or other conditions, by physical observation. It is part of combat intelligence. Compare to counterintelligence.
It is often referred to as recce (British & Commonwealth) or recon (American). The associated verb is reconnoiter (reconnoitre in British English).
Examples of reconnaissance include patrolling by troops, ships, submarines, or aircraft, or by setting up covert observation posts. Reconnaissance may also be carried out by satellites or unmanned aircraft.
Reconnaissance seeks to collect a range of information about an enemy. This includes their locations, numbers, and intentions. A number of acronyms exist for the information to be gathered - mainly coined by the US - including salt (size, activity, location, and time), salute (size, activity, location, unit, time, and equipment), sam & doc (strength, armament, movement, deployment, organization, and communications)
Thus reconnaissance is a fundamental tactic which helps to build an intelligence picture.
Airborne photo reconnaissance
Before the Second World War the conventional wisdom was to use converted bomber types for airborne photo reconnaissance. These bombers retained their defensive armament, which was vital since they were unable to avoid interception.
In 1939 Flying Officer Maurice Longbottom was among the first to suggest that airborne reconnaissance may be a task better suited to fast, small aircraft which would use their speed and high service ceiling to avoid detection and interception. Although this seems obvious now, with modern reconnaissance tasks performed by fast, high flying aircraft, at the time it was radical thinking. He proposed the use of Spitfires with their armaments and radios removed and replaced with extra fuel and cameras. This led to the development of the Spitfire PR variants. Spitfires proved to be extremely successful in their reconnaissance role and there were many variants built specifically for that purpose.
After World War II, the onset of the Cold War necessitated the development of highly specialized and secretive strategic reconnaissance aircraft (or "spyplanes"), such as the Lockheed U-2 and its sucessor, the SR-71 Blackbird (both from the United States). Flying these aircraft became an exceptionally demanding task, as much because of the aircraft's extreme speed and altitude as it was because of the risk of being captured as spies. As a result, the crews of these aircraft were invariably specially selected and trained.