launched by a country to provide intelligence information on the military activities of foreign countries. There are four major types. Early-warning satellites
detect enemy missile launchings. Nuclear-explosion detection satellites
are designed to detect and identify nuclear explosions in space. Photo-surveillance satellites
provide photographs of enemy military activities, e.g., the deployment of intercontinental ballistic missiles (ICBMs). There are two subtypes: close-look satellites provide high-resolution photographs that are returned to earth via a reentry capsule, whereas area-survey satellites provide lower-resolution photographs that are transmitted to earth via radio. Later satellites have combined these two functions. Other satellites use radar to provide images of enemy activity when there is cloud cover or it is dark. Electronic-reconnaissance (ferret) satellites
pickup and record radio and radar transmissions while passing over a foreign country. The United States, Russia (before 1991, the USSR), and other nations have launched numerous reconnaissance satellites since 1960.
See E. D. Conway, An Introduction to Satellite Image Interpretation (1997); P. Taubman, Secret Empire: Eisenhower, the CIA and the Hidden Story of America's Space Espionage (2003).
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.
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