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Concept
Reconnaissance satellite
Summary
A reconnaissance satellite or intelligence satellite (commonly, although unofficially, referred to as a spy satellite) is an Earth observation satellite or communications satellite deployed for military or intelligence applications.
The first generation type (i.e., Corona and Zenit) took photographs, then ejected canisters of photographic film which would descend back down into Earth's atmosphere. Corona capsules were retrieved in mid-air as they floated down on parachutes. Later, spacecraft had digital imaging systems and downloaded the images via encrypted radio links.
In the United States, most information available about reconnaissance satellites is on programs that existed up to 1972, as this information has been declassified due to its age. Some information about programs before that time is still classified information, and a small amount of information is available on subsequent missions.
A few up-to-date reconnaissance satellite images have been declassified on occasion, or leaked, as in the case of KH-11 photographs which were sent to Jane's Defence Weekly in 1984.
On 16 March 1955, the United States Air Force officially ordered the development of an advanced reconnaissance satellite to provide continuous surveillance of "preselected areas of the Earth" in order "to determine the status of a potential enemy’s war-making capability".
There are several major types of reconnaissance satellite.
Missile early warning
Defense Support Program and Space-Based Infrared System
Provides warning of an attack by detecting ballistic missile launches. Earliest known are Missile Defense Alarm System.
Nuclear explosion detection
Detects nuclear detonation from space. Vela is the earliest known.
Electronic reconnaissance
Signals intelligence, intercepts stray radio waves. SOLRAD is the earliest known.
Optical imaging surveillance
Earth imaging satellites. can be a survey or close-look telephoto. Corona is the earliest known. Spectral imaging is commonplace.
Radar imaging surveillance
Most space-based radars use synthetic-aperture radar.
Official source
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