Satellites are commonly classified, according to the purpose they serve, as scientific (or research), applications, or military satellites. These classifications overlap to some extent.
Scientific, or Research, Satellites
contain instruments designed to make observations and gather data for studies in geophysics, astronomy, and other scientific fields. Investigations of the earth's upper atmosphere, the Van Allen radiation belts, and levels of high-energy radiation in space were among the first investigations made by scientific satellites. Many kinds of astronomical observations that cannot be made from the surface of the earth can be made by satellites in orbit above the atmosphere. Of special importance have been satellites designed to detect and map celestial sources of various kinds of electromagnetic radiation, including ultraviolet radiation, infrared radiation, and X rays. Some satellites have been used to conduct biological experiments to determine how the conditions of space affect plants, insects, one-celled organisms, and other living things.
There have been several major series of American scientific satellites, including the Explorer, Orbiting Solar Observatory (OSO), Orbiting Geophysical Observatory (OGO), High Energy Astronomy Observatory (HEAO), and Great Observatories series.
are designed for such practical purposes as weather forecasting and telecommunications. Weather satellites obtain images of cloud patterns. They also obtain information on sea surface and air temperatures. Weather satellites are especially useful in tracking hurricanes and other large storms. Since the advent of weather satellites, the average number of people killed by hurricanes in the United States each year has declined greatly. Some American weather satellites are geostationary; others circle the earth in polar orbits.
Communications satellites serve as relays for long-range communications. Telephone messages, television programs, and various types of data are transmitted to a communications satellite as high-frequency radio signals. The satellite strengthens the signals and retransmits them to earth, where they can be received over a wide area. Many communications satellites can routinely handle signals for thousands of telephone circuits simultaneously.
A global network of Intelsat satellites is used for international communications. Some communications satellites are intended for communications services within a single country; some for specific users, such as ships at sea.
Many communications satellites are placed in geostationary orbits. In such an orbit, the satellite is available for uninterrupted service and fixed antennas can be used for transmission and reception.
Other applications satellites include navigation satellites and the Landsats. A network of navigation satellites, such as the U.S. Navy's Transit satellites, makes it possible for a ship's navigator to determine the ship's position very accurately anywhere in the world. Landsats carry various types of sensors that provide detailed images of the earth's surface for information about land use and crop production. The images are also useful in locating mineral resources and in determining the extent of snow cover or flood damage.
Military satellites have many uses. Some military satellites, such as military communications and weather satellites, have functions similar to those of civilian satellites Other military satellites are used for reconnaissance. They carry cameras that provide extremely detailed images of the earth's surface, showing even relatively small features and objects. These satellites are useful in monitoring the deployment of military forces and in checking for the construction of new military facilities. Yet other military satellites are designed for detecting nuclear test explosions or for detecting and tracking ballistic missiles.