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In 1972 the United States started the Landsat Program , the largest program for acquisition of imagery of Earth from space. Landsat 7 , the most recent Landsat satellite, was launched in 1999 . All satellite images produced by NASA are published by Earth Observatory and are freely available to the public. Other countries have their own satellite imaging programs. There are also private companies that provide commercial satellite imagery. Satellite Images have many applications in Agriculture , Geology , Forestry , Regional Planning , Education , Intelligence and Warfare . Images can be in visible colours and in other Spectra . There are also elevation maps, usually made with by radar imaging. Blue Marble was a project for mapping the visible color of the Earth with about 1- Km resolution. The beautiful resulting images have become very popular in the media and among the public. A satellite can only see a relatively small area at a time. For this reason mapping a large area (or the whole Earth) takes a lot of time and most satellite images are on average several years old. Because imaging satellites are in Low Earth Orbit s, real-time imaging of a certain area is complicated. However, Spy Satellite s can do this during wars and other military operations or to provide data for intelligence agencies. Still, the troops frequently find out that the Ground Truth is different from the images that were taken some time ago. Of the Image Resolution of satellite images varies. For most cities in the world photos with resolution of 10 meters or better are available. For areas outside cities the resolution can be 100-500 meters. Most of the Earth is imaged with resolution of 1 km or better. For many smaller areas images with resolution as high as 10 cm can be available. Satellite imagery is supplemented with aerial photography, which has higher resolution, but is more expensive per square meter. Because the total area of the land on Earth is so large and because resolution is relatively high, satellite databases are huge. Storing and serving satellite imagery is a big technical challenge. Satellite imagery can be combined with GIS data. For example, the EarthViewer can combine the images with maps and other information from various sources. In the early 21st Century satellite imagery became widely available when affordable, easy to use software with access to satellite imagery databases became offered by several companies and organizations. In 2005 the Australian company Astrovision (ASX: HZG) announced plans to launch the first ever commercial Geostationary satellite in the Asia-Pacific. It will provide true color, real-time live satellite feeds, with down to 250 metres resolution over the entire Asia-Pacific region, from India to Hawaii and Japan to Australia. They intend to provide this content to users of 3G mobile phones, over Pay TV as a weather channel, and to corporate and government users. One drawback of using satellite imagery is that images dissipate without the sun's presence (for instance, in weather). When viewing a satellite image, meteorologists often turn to other images such water vapour imagery or radars to check for precipitation and cloud formation. SEE ALSO
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