Imagery Intelligence - Satellites

Satellites

Though the resolution of satellite photographs, which must be taken from distances of hundreds of kilometers, is usually poorer than photographs taken by 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.

There have been hundreds of reconnaissance satellites launched by dozens of nations since the first years of space exploration. While the information about the vast majority of such satellites are strictly classified, some information (such as that concerning the US Corona program) has been declassified with the end of the Cold War.

Early photographic reconnaissance satellites used photographic film, which was exposed on-orbit and returned to earth for developing. These satellites remained in orbit for days, weeks, or months before ejecting their film-return vehicles, called "buckets." Between 1959 and 1984 the U.S. launched around 200 such satellites under the codenames CORONA and GAMBIT, with photographic resolutions as high as 0.6- 1.2 meters (2–4 feet). The first successful mission concluded on 1960-08-19 with the mid-air recovery by a C-119 of film from the Corona mission code-named Discoverer 14. This was the first successful recovery of film from an orbiting satellite and the first aerial recovery of an object returning from Earth orbit. Because of a tradeoff between area covered and ground resolution, not all reconnaissance satellites have been designed for high resolution; the KH-5-ARGON program had a ground resolution of 140 meters and was intended for mapmaking.

Between 1961 and 1994 the USSR launched perhaps 500 Zenit film-return satellites, which returned both the film and the camera to earth in a pressurized capsule.

Satellites for imaging intelligence were usually placed in low-earth, high-inclination orbits, sometimes in sun-synchronous orbits. Since the film-return missions were usually short, they could indulge in orbits with low perigees, in the range of 100–200 km, but the more recent CCD-based satellites have been launched into higher orbits, 250–300 km perigee, allowing each to remain in orbit for several years. While the exact resolution and other details of modern spy satellites 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 Rayleigh criterion:

and

we can get

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 cm (6 inches) for green light ( nm) at its orbital altitude of 590 km. This means that it would be impossible to take photographs showing objects smaller than 16 cm with such a telescope at such an altitude. Modern U.S. IMINT satellites are believed to have around 10 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.

The U.S. KH-11 series of satellites, first launched in 1976, was made by Lockheed, the same contractor who built the Hubble Space Telescope. HST has a 2.4 metre telescope mirror and is believed to have had a similar appearance to the KH-11 satellites. These satellites used charge-coupled devices, predecessors to modern digital cameras, rather than film. Russian reconnaissance satellites with comparable capabilities are named Resurs DK and Persona.

The primary purpose of most spy satellites is to monitor visible ground activity. While 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, USSR/Russia, China and possibly others, have developed 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, 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.