|
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. |
|
|
|
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. |
|
|
