- Satellite imagery
Satellite imagery consists of photographs of
Earthor other planets made by means of artificial satellites.
All satellite images produced by
NASAare published by Earth Observatoryand are freely available to the public. Several other countries have satellite imaging programs, and a collaborative European effort launched the ERS and Envisatsatellites carrying various sensors. There are also private companies that provide commercial satellite imagery. 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.
Satellite images have many applications in
agriculture, geology, forestry, biodiversity conservation, regional planning, education, intelligence and warfare. Images can be in visible colours and in other spectra. There are also elevation maps, usually made by radar imaging. Interpretation and analysis of satellite imagery is conducted using software packages like ERDAS Imagineor ENVI.Some of the first image enhancement of satellite photos was conducted by the U.S. Government and its contractors. For example ESL Incorporateddeveloped some of the earliest two dimensional Fouriertransforms applied to digital image processingto address NASAphotos as well as national securityapplications.
Resolution and data
There are two different types of resolution when discussing satellite imagery; radiometric and geometric. Radiometric resolution refers to the effective bit-depth of the sensor (number of greyscale levels) and is typically expressed as 8-bit (0-255), 11-bit (0-2047), 12-bit (0-4095) or 16-bit (0-65,535). Geometric resolution refers to the satellite sensor's ability to effectively image a portion of the earth's surface in a single pixel and is typically expressed in terms of Ground Sample Distance, or GSD. GSD is a term containing the overall optical and systemic noise sources and is useful for comparing how well one sensor can "see" an object on the ground within a single pixel. For example, the GSD of Landsat is ~30m, which means the smallest unit that maps to a single pixel within an image is ~30m x 30m. Contrast this with the latest commercial satellite (GeoEye 1) with a GSD of 0.41m (effectively 0.5 due to govt restrictions). 3600 pixels from a GeoEye image of the same area of the earth will fit into a single Landsat pixel. The resolution of satellite images varies depending on the instrument used and the altitude of the satellite's orbit. For example, the
Landsatarchive offers repeated imagery at 30 meter resolution for the planet, but most of it has not been processed from the raw data. Landsat 7has an average return period of 16 days. For many smaller areas, images with resolution as high as 10 cm can be available.Fact|date=April 2008
Satellite imagery is sometimes supplemented with
aerial photography, which has higher resolution, but is more expensive per square meter. Satellite imagery can be combined with vector or raster data in a GIS provided that the imagery has been spatially rectified so that it will properly align with other data sets.
Before the launch of GeoEye-1, DigitalGlobe's
WorldView-1satellite provided the world's highest resolution commercial satellite imagery. [http://www.ballaerospace.com/page.jsp?page=81] The 50 cm resolution of WorldView-1's panchromaticimages allows the satellite to distinguish between objects on the ground that are at least 50 cm apart. Similarly DigitalGlobe's QuickBirdsatellite provides 2.4 meter resolution multispectralimages.
GeoEye's GeoEye-1 satellite was launched
September 6, 2008. [http://www.reuters.com/article/rbssTechMediaTelecomNews/idUSN0633403420080906?sp=true] The GeoEye-1 satellite will have the highest resolution of any commercial imaging system and be able to collect images with a ground resolution of 0.41 meters (16 inches) in the panchromatic or black and white mode. It will collect multispectral or color imagery at 1.65-meter resolution or about 64 inches, a factor of two better than existing commercial satellites with four-band multispectral imaging capabilities. While the satellite will be able to collect imagery at 0.41 meters, GeoEye's operating license from the U.S. Government requires re-sampling the imagery to 0.5 meters for all customers not explicitly granted a waiver by the U.S. Government.
Because the total area of the land on Earth is so large and because resolution is relatively high, satellite databases are huge and
image processing(creating useful images from the raw data) is time-consuming. Depending on the sensorused, weather conditions can affect image quality: for example, it is difficult to obtain images for areas of frequent cloud cover such as mountain-tops.
Commercial satellite companies do not place their imagery into the public domain and do not sell their imagery; instead, one must be licensed to use their imagery. Thus, the ability to legally make derivative products from commercial satellite imagery is minimized.
Privacyconcerns have been brought up by some who wish not to have their property shown from above. Google Maps responds to such concerns in their FAQwith the following statement: "We understand your privacy concerns... The images that Google Maps displays are no different from what can be seen by anyone who flies over or drives by a specific geographic location."
In 2005 the Australian company
Astrovision(ASX: HZG) announced plans to launch the first commercial geostationary satellite in the Asia-Pacific. It intended to 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 were going to provide this content to users of 3G mobile phones, over Pay TV as a weather channel, and to corporate and government users.
Unfortunately, the market response to the AstroVision concept fell into the classic chicken-egg problem: potential customers were excited by the possibilities offered, but they were unwilling (or, in government cases, generally unable) to sign contracts for a service that would not be delivered for 3-4 years (the length of time required to build and launch the satellite). AstroVision ran low on funds and was forced to shut down the program in 2006.
NASA World Wind
Earth observation satellite
* [http://esamultimedia.esa.int/images/EarthObservation/Envisat/tapisserie_100x55_H.jpgESA Envisat Meris - 300m] - the most detailed image of the entire Earth to date, made by the European Space Agency's Envisat Meris.
* [http://earthobservatory.nasa.gov/Newsroom/BlueMarble/ Blue Marble: Next Generation] - a detailed true-color image of the entire Earth.
* [http://worldwind.arc.nasa.gov/ World Wind] - an
open source3D Earth-viewing software developed by NASAthat accesses NASA JPL database
* [http://earth.Google.com/ Google Earth] -
* [http://maps.google.com/maps?spn=74.979492,123.240234&t=k&hl=en Google Maps satellite imagery]
* [http://terraserver.microsoft.com TerraServer-USA] - formerly "Microsoft TerraServer", a repository of free satellite images of the United States
* [http://www.china-profile.com/maps/map_overview_2.htm Satellite Images of China] - Images of Taklamakan desert, salt lakes, wind parks, infrastructure, cities.
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