video for embedding at http://scitech.quickfound.net/environment/environment_news.html
Overview of Landsat studies of global pollution and damage to the biosphere.
Segment from the series "Landsat: Satellite for All Seasons"
below from "Mission to Earth: Landsat Views the World" 459MB pdf
http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770010588_1977010588.pdf
This satellite moves in an almost perfectly circular orbit at an altitude of 570 miles inclined at 81° relative to a plane passing through the Earth's Equator. This near polar orbit is also Sun synchronous, crossing the Equator on the day side of Earth 14 times every day at approximately 9:30 a.m. local time in each transit. Each successive orbit shifts westward about 1785 miles at the Equator.
On the following day the next 14 orbits parallel those of the previous day, but each one is offset westward by about 99 miles. Images obtained for any two adjacent orbits show about 15 percent sidelap at the Equator; this sidelap increases to about 85 percent near the poles. All parts of a large region, e.g., a continent, are imaged during the succession of shifted orbits in a cycle lasting 18 days. Thus, in principle, any area can be imaged every 18 days, but, in practice, cloud cover usually reduces the coverage...
Two imaging sensor systems operate on the Landsats. One, a television camera system referred to by the acronym RBV, was shut down early in the Landsat I operation owing to a switching malfunction. The second is a multispectral scanner (MSS), which produces a continuous image strip built up from successive scan lines extended perpendicular to the forward direction of the satellite's orbital motion.
Reflected light from the ground is transferred by an oscillating mirror in the MSS to a recording system after passing through filters that select different wavelength intervals of this light. Each of the four wavelength channels processes a predetermined spectral interval or band...
One principal use of this multispectral capability stems from a basic property of materials. Because various classes of features found on the surface reflect differing amounts of light at different wavelengths or wavelength intervals, they can be separated and identified by their own characteristic reflectance patterns, or spectral "signatures." For example, vegetation typically reflects more green light than red and is very reflective (bright) in the infrared...
The light reflectance data obtained by the MSS on board Landsat are first converted to electrical signals, which vary in proportion to the intensity measured for each band. These analog signals then are converted into a digital form and transmitted to one of the three receiving stations in the continental United States. (By agreement with other nations, foreign stations are constructed and operated by participating countries. Canada, Brazil, and Italy have each developed a station and Zaire, Iran, and Chile are proceeding with similar developments.) Data from the satellite can be collected by a receiving station over about a 2800-km (1700-mile) radius depending somewhat on antenna size, station elevation, and local terrain features. Wherever a region of the Earth lies beyond a receiving station, data obtained during a Landsat overpass are stored on an onboard tape recorder for playback later when the satellite is within range of one of the U.S. stations...
Each of the four black and white images represents a particular spectral band...
Color images are made from combinations of individual black and white images by projecting each given band through a particular filter. The usual combination consists of band 4 (green) projected through a blue filter, band 5 (red) projected through a green filter, and band 7 (infrared) projected through a red filter. In this rendition (called a false color image), which is equivalent to the standard false color infrared product of conventional color infrared photography, growing vegetation will appear in various shades of red, rocks and soils will normally show colors ranging from bluish through yellows and browns, water will stand out as blue to black depending on depth and amount of suspended sediment, and cultural features (towns and roads) will usually be recognized by bluish-black tones arranged in characteristic patterns.
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