Satellite Remote Sensing
a) Geo - Stationary satellite
b) Sun - Synchronous satellite
c) Spy - Satellite
Geo Stationary-Satellite: Geo stationary satellites are launched to collect information about a larger area of the earth surface at a regular time interval. These satellites are placed in space at an average altitude of 36,000 km above the equator. These satellites advance in the same direction and speed as the rotation of the earth. Hence, these satellites observe the same portion of the earth at all times. For example, the INSAT satellite which is placed above the Indian subcontinent collects data and monitors the weather conditions of India. We see such imageries as those received from INSAT while the Doordarshan (Indian Television) telecasts the news bulletins. The announcement of weather forecasting is based on the information collected from such imageries regarding land, water and cloud cover and also information collected from the meteorological stations. Since these satellite imageries show large portions of the earth's surface, minor elements of the earth cannot be seen.
Sun - Synchronous Satellite: These satellites are placed at an altitude of about 600 to 900 km in space. They move North South above the earth and collect information for use on the ground. These satellites take just about 50 minutes to traverse from north pole to south pole within. Since these satellites collect information with the help of sunlight and pass the equator at a specific day time, they are called sun synchronous satellites.
It was the USA that launched the first satellite in 1972. It was a sun synchronous satellite and was later known as LANDSAT. Later 6 satellites were launched, in a series and in collaboration with the European countries. The French Government launched SPOT 1 satellite in the year 1986. Later, SPOT 2 and SPOT 3 were also launched. As a parallel venture, the Indian Government launched IRS 1A in 1988 and later IRS 1B. Likewise, other satellites such as ERS (European Remote Sensing Satellite) and JRS (Japanese Remote Sensing Satellite) series currently revolve round the earth. Images from such satellites can give accurate information just as the topographic maps. These imageries give important information on land uses, water resources, roads and settlements.
Spy Satellites: To collect secret information such as the movement of army troops and locations of atomic power stations, the spy satellites are put to use, with sophisticated gadgetry. Manufacturing of these satellites are expensive and complex. Besides, these satellites operate only for a limited time period. Also, only a few countries in the world have such satellites. Israel is said to be one of those countries specialising in this remote sensing.
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