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One of the common technologies that are used to produce satellite
images is remote sensing. Remote sensing describes the instruments
and techniques used to observe the earth from a distance. These
include the satellites and their sensors, the computer systems used
to process images, and the techniques scientists and researchers
employ to interpret the images.
There is a wide variety of applications for remote sensing. Scientists
use satellites to track weather systems and forest fires. Military
services utilize remote sensing to define objectives and provide air
and ground forces with information. Commercial companies operate
satellites that provide high-resolution maps and geographic information.
With this advanced technology, it is now possible to observe the entire
globe, scan hemispheres, or examine minute details. Satellites such
as Terra and Landsat record large expanses. Others, including
QuickBird, IKONOS, and OrbView satellites, capture incredible detail
over small areas.
Satellite images use several types of light. Our eyes are sensitive to
only a small range of wavelengths in the electromagnetic spectrum,
called visible light. Remote sensing satellites use both visible and
infrared wavelengths to observe the land surface and atmosphere.
Ultraviolet wavelengths are used for some atmospheric observations,
and radar sensors use microwave wavelengths.
Manned missions circle the earth in equatorial orbits, which are
between 300 and 500 miles (482 and 804 kilometers) above the
surface. Many remote sensing satellites are placed into polar orbits,
traveling north and south over the poles. A sun-synchronous orbit is
similar but is inclined at an angle to allow a satellite to observe the
earth's surface with a constant sun angle. Weather satellites are
placed into geostationary orbits, 12,000 miles (19,312 kilometers)
above the earth's surface. At this distance, the satellites, "parked"
over one spot on the globe, travel around the earth once every
24 hours.
The satellite image processing is a complex process. Data from
remote sensing satellites is recorded first from radio transmissions.
The data is stored in computer systems, and then transformed into
images. Each number sent by the satellite represents one pixel
(picture element), visible as "boxes" or squares. Most remote
sensing satellites can detect many wavelengths simultaneously.
When every pixel is assigned a brightness value, the results are
many grayscale images, one for each wavelength.
Colors are assigned to each wavelength and then digitally
combined to create a full-color picture. The selection of colors is
arbitrary, but they are usually chosen to enhance specific details.
Red, for example, can be assigned to wavelengths that are
reflected by plant life.
Remote sensing satellites have solar panels to produce electricity,
antennae for transmitting data, and sensors for observing Earth's
surface. Depending on the mission, they carry several sensors or
just one. The majority of satellites are operated by government
space programs or military agencies. Other satellites have been
launched by private companies for mapping applications.
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