Modern satellites

The idea of launching artificial satellites that comes to life in 1957 by Soviet Union and followed by the United States has come along way. By the early 2000's, more than 40 countries owned satellites, and nearly 3,000 satellites were operating in orbit. Currently there are six main types of satellites:

Scientific research satellites

Scientific research satellites gather data for scientific analysis. These satellites are usually designed to gather information about the composition and effects of the space near Earth, to record changes in Earth and its atmosphere and to observe planets, stars, and other distant objects.

One of scientific research satellites Upper Atmosphere Research Satellite was launched in 1991. It measures ozone and chemical compounds found in the ozone layer which affect ozone chemistry and processes. UARS also measures winds and temperatures in the stratosphere as well as the energy input from the Sun.

Weather satellites

Weather satellites help scientists study weather patterns and forecast the weather. A weather satellite called the Geostationary Operational Environmental Satellite observes atmospheric conditions over a large area to help scientists study and forecast the weather.
On May 24, 2006 GOES-N was successfully launched into space. GOES-N is the first of a new series of weather satellites that provides forecasters with improved real-time information about atmospheric conditions throughout much of the Western Hemisphere.
The constant, real-time view provides essential information to forecasters and allows them to issue accurate advanced warnings of severe weather like hurricanes, tornadoes, hail storms, and heavy rain that can cause flash flooding. Other weather satellites are placed in high altitude. From these orbits, scientists can always observe weather activity over nearly half the surface of Earth at the same time.

Communications Satellites

Communications satellites serve as relay stations, receiving radio signals from one location and transmitting them to another. A communications satellite can relay several television programs or many thousands of telephone calls at once. Communications satellites are usually put in a high altitude, geosynchronous orbit over a ground station.
NASA launched series of three advanced Tracking and Data Relay Satellites, known as TDRS. These satellites provides high data-rate communication links with the Space Shuttle, International Space Station, Hubble Space Telescope and a host of other spacecraft, as well as tracking services for expendable launch vehicles.

Navigation satellites

Originally Navigation System with Timing And Ranging Global Positioning System, GPS was developed by the United States Department of Defense to provide all-weather round-the-clock navigation capabilities for military ground, sea, and air forces. A navigation satellite sends signals that operators of aircraft, ships, and land vehicles and people on foot can use to determine their location.

The satellites send out radio signals that are picked up by a computerized receiver carried on a vehicle or held in the hand. Navigation satellites operate in networks, and signals from a network can reach receivers anywhere on Earth.

Earth observing satellites

Earth observing satellites are used to map and monitor the planet's resources and ever-changing chemical life cycles. Under constant, consistent light from the sun, they take pictures in different colors of visible light and non-visible radiation then computers on Earth combine and analyze the pictures. Scientists use Earth observing satellites to locate mineral deposits, to determine the location and size of freshwater supplies, to identify sources of pollution and study its effects.

The satellite, Aqua, helps scientists study ocean evaporation and other aspects of the movement and distribution of Earth's water.


Military satellites

Military satellites include weather, communications, navigation, and Earth observing satellites used for military purposes. Some military satellites called spy satellites can detect the launch of missiles, the course of ships at sea, and the movement of military equipment on the ground.

http://www.nasa.gov/worldbook/artificial_satellites_worldbook.html
http://umpgal.gsfc.nasa.gov/
http://msl.jpl.nasa.gov/Programs/gps.html