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The Skylab Legacy

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One human generation (25 years) has passed since the United States' ambitious Skylab program. A major component of those missions was the acquisition of earth photography using a six camera, multispectral camera system. The first multispectral photography done from space was on the 1968 Apollo 9 mission. Four mounted Hasselblad cameras were aimed at the same target point and their shutters triggered simultaneously. A significant advance in Earth imaging technology resulted from subdividing spectral ranges of radiation into bands (intervals of continuous wavelengths), allowing acquisition of multispectral images. Especially important was the addition of infrared bands. Black and white infrared film was developed in Germany in the early 1900’s. The film emulsion differed from regular film in that it was sensitive to wavelengths of energy longer than red light and just beyond the range of the human eye. By 1930, black and white IR (infrared) films were being used for landform studies, and from 1930 to 1932 the National Geographic Society sponsored a series of IR photographs taken from hot air balloons. The infrared bands are often used to emphasize healthy vegetation in which light in the range of 700 - 1100 nm is strongly reflected from the internal cells of plants. A false color composite can demonstrate stressed or unhealthy vegetation and more clearly differentiate barren areas. The use of false color infrared imaging can be especially useful for monitoring change such as the deforestation of rain forests. The value of multispectral photography was not lost on scientists, such as geologists, hydrologists, agronomists, foresters, and those concerned with environmental monitoring and land use assessment. It quickly became a major scientific tool.
Skylab above the Amazon


The success of this experiment lead directly to the Skylab system as well as the Landsat series of satellites. By the late 1960s, the first unmanned satellite specifically dedicated to multispectral remote sensing entered the planning stages that led to the launch of ERTS-1 (Earth Resources Technology Satellite) in 1972. Renamed Landsat, this series of earth-observing satellites has continuously covered most of the Earth's surface since then, with Landsat-5 and Landsat-7 currently in operation.

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Three separate crews worked on Skylab in 1973-74 and participated in the Earth photography experiments. The purpose of the Earth Resource Experiment Package (EREP) was to determine what kind, and how much, photographic data could be acquired of the broad variety of Earth features witnessed on the mission's ground track. This activity was underwritten by intensive training before lift-off, real-time scientific mission planning and on-board procedural support. This focus elevated Earth Imaging from an opportunistic sideline to a major mission goal.
The Skylab spacecraft was launched on May 14, 1973 into a nearly circular orbit at an altitude of 435 km above the Earth. The launch azimuth inclined the orbital plane 50o with respect to the Equator and allowed observations of the Earth between latitudes 50oN and 50oS. It orbited the Earth every 93 minutes and repeated the ground track every five days.

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The photographic return from the Earth Resources Experiment Package (EREP) was as follows:

Skylab2 5/25/73-6/22/73 28 13 5,275
Skylab3 7/28/73-9/25/73 59 48 13,429
Skylab4 11/16/73-2/8/74 84 49 17,000
TOTAL 171 110 35,704


The EREP system included two major instruments:
The S190A Multispectral Camera System consisted of six bore-sighted 70mm cameras equipped with 152mm focal length lenses. Each photograph covered a square image area of 163 km on a side. Each camera was equipped with filters to cover a specific wavelength on a combination of color, black-and-white, color infrared and black-and-white infrared film. The cameras were mounted in a frame that provided programmable camera rotation to compensate for the forward motion of the spacecraft.

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S190B Earth Terrain Camera was a single 127mm camera equipped with a 457mm lens. Each photograph covered an area of 109 km square. Film could be selected from a choice of color, black-and-white or color infrared emulsions with an appropriate filter. An internal motion compensator corrected for the forward motion of the spacecraft.


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Click on this Image to Visit the Skylab EREP Album

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The Experiment

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