Editing Imagery intelligence (section)

==Satellite==
Though the resolution of satellite photographs, which must be taken from distances of hundreds of kilometers, is usually poorer than photographs taken by [[aerial photography|air]], satellites offer the possibility of coverage for much of the earth, including hostile territory, without exposing human pilots to the risk of being shot down.

[[File:TheGambitStoryPage154.png|thumb|right|150px|Ground-resolution distance achieved by KH-8]]
There have been hundreds of [[reconnaissance satellite]]s launched by dozens of nations since the first years of space exploration. Satellites for imaging intelligence were usually placed in high-inclination [[low Earth orbit]]s, sometimes in [[Sun-synchronous orbit]]s. Since the film-return missions were usually short, they could indulge in orbits with low [[perigee]]s, in the range of 100–200&nbsp;km, but the more recent CCD-based satellites have been launched into higher orbits, 250–300&nbsp;km perigee, allowing each to remain in orbit for several years. While the exact [[Optical resolution|resolution]] and other details of modern [[spy satellite]]s are classified, some idea of the trade-offs available can be made using simple physics. The formula for the highest possible resolution of an optical system with a circular aperture is given by the [[Angular resolution#The_Rayleigh_criterion|Rayleigh criterion]]:

:<math> \sin \theta = 1.22 \frac{\lambda}{D}.</math>

Using

:<math> \sin \theta =  \frac{\text{size}}{\text{distance}},</math>

we can get

:<math> \text{size} =  1.22\frac{\lambda}{D} \text{distance},</math>

where ''θ'' is the angular resolution, ''λ'' is the [[wavelength]] of light, and ''D'' is the diameter of the lens or mirror.  Were the [[Hubble Space Telescope]], with a 2.4 m telescope, designed for photographing Earth, it would be diffraction-limited to resolutions greater than 16&nbsp;cm (6&nbsp;inches) for green light (<math> \lambda \approx  550</math> nm) at its orbital altitude of 590&nbsp;km. This means that it would be impossible to take photographs showing objects smaller than 16&nbsp;cm with such a telescope at such an altitude.  Modern U.S. IMINT satellites are believed to have around 10&nbsp;cm resolution; contrary to references in popular culture, this is sufficient to detect any type of vehicle, but not to read the headlines of a newspaper.<ref>{{cite web|url=https://fas.org/irp/imint/resolve5.htm|publisher=Federation of American Scientists|title=Imint resolution comparison}}</ref>

The primary purpose of most spy satellites is to monitor visible ground activity. While [[Image resolution|resolution]] and clarity of images has improved greatly over the years, this role has remained essentially the same. Some other uses of satellite imaging have been to produce detailed 3D maps for use in operations and missile guidance systems, and to monitor normally invisible information such as the growth levels of a country's crops or the heat given off by certain facilities. Some of the multi-spectral sensors, such as thermal measurement, are more [[electro-optical MASINT]] than true IMINT platforms.

To counter the threat posed by these "eyes in the sky", the [[United States]], [[Soviet Union|USSR]]/[[Russia]], [[China]] and [[India]] have developed [[anti-satellite weapon|systems for destroying enemy spy satellites]] (either with the use of another 'killer satellite', or with some sort of Earth- or air-launched missile).

Since 1985, commercial vendors of [[satellite imagery]] have entered the market, beginning with the French [[SPOT (satellites)|SPOT]] satellites, which had resolutions between 5 and 20 metres. Recent high-resolution (4–0.5 metre) private imaging satellites include [[TerraSAR-X]], [[IKONOS]], [[Orbview]], [[QuickBird]] and [[Worldview-1]], allowing any country (or any business for that matter) to buy access to satellite images.