2024. 7. 6. 10:17ㆍ카테고리 없음
Space visionaries in the early 20th century recognized that putting satellites into orbit could furnish direct and tangible benefits to people on Earth. For example, Arthur C. Clarke in 1945 described a way in which three satellites in orbit about 35,800 km (22,250 miles) above the Equator could relay communications around the globe. In this orbit, called a geostationary orbit, the satellites would have an orbital period equal to Earth’s rotational period and thus appear from the ground to be stationary in the sky. (For additional information on satellite orbits, see spaceflight: Earth orbit.) A report for the U.S. Army Air Forces in 1946 by Project RAND (the predecessor of the RAND Corporation) identified the benefits of being able to observe Earth from space, which included distinguishing the impact sites of bombs dropped by U.S. aircraft and improving weather forecasting.
Meteorology
Meteorologists initially thought that satellites would be used primarily to observe cloud patterns and thus provide warnings of impending storms. They did not expect space observations to be central to improved weather forecasting overall. Nevertheless, as the technology of space-based instrumentation became more sophisticated, satellites were called upon to provide three-dimensional profiles of additional variables in the atmosphere, including temperature, moisture content, and wind speed. These data have become critical to modern weather forecasting.
Meteorological satellites are placed in one of two different kinds of orbit. Satellites in geostationary orbit provide continuous images of cloud patterns over large areas of Earth’s surface. From changes in those patterns, meteorologists can d
Positioning, navigation, and timing
In 1957 scientists tracking the first satellite, Sputnik 1, found that they could plot the satellite’s orbit very precisely by analyzing the Doppler shift (see Doppler effect) in the frequency of its transmitted signal with respect to a fixed location on Earth. They understood that if this process could be reversed—i.e., if the orbits of several satellites were precisely known—it would be possible to identify one’s location on Earth by using information from those satellites.
educe wind speeds and locate developing storms
The United States regards GPS as a global utility to be offered free of charge to all users and has stated its intent to maintain and upgrade the system into the indefinite future. Concern has been expressed, however, that important worldwide civilian activities such as air traffic control should not depend on a system controlled by one country’s military forces. In response, Europe began in the late 1990s to develop its own navigation satellite system, called Galileo, to be operated under civilian control. Galileo became operational in 2016. In the early 21st century China began to develop its own global navigation system, called Beidou (“Compass”). Initially consisting of three satellites in geostationary orbit over China, Beidou began operation in 2000. A full global version of Beidou was completed in 2020. Japan and India developed systems for regional use, the Quasi-Zenith Satellite System (QZSS) and Navigation with Indian Constellation (NavIC), respectively, which both became operational in 2018
Military and national security uses of space
Those countries and organizations with armed forces deployed abroad were quick to recognize the great usefulness of space-based systems in military operations. The United States, Russia, the United Kingdom, France, China, the North Atlantic Treaty Organization (NATO), and, to a lesser degree, other European countries have deployed increasingly sophisticated space systems—including satellites for communications, meteorology, and positioning and navigation—that are dedicated to military uses. In addition, the United States and Russia have developed satellites to provide early warning of hostile missile launches. Many of these satellites have been designed to meet unique military requirements, such as the ability to operate in a wartime environment, when an opponent may try to interfere with their functioning.
Satellite telecommunications
Although some early space experiments explored the use of large orbiting satellites as passive reflectors of signals from point to point on Earth, most work in the late 1950s and early ’60s focused on the technology by which a signal sent from the ground would be received by satellite, electronically processed, and relayed to another ground station. American Telephone and Telegraph, recognizing the commercial potential of satellite communications, in 1962 paid NASA to launch its first Telstar satellite. Because that satellite, which operated in a fairly low orbit, was in range of any one receiving antenna for only a few minutes, a large network of such satellites would have been necessary for an operational system. Engineers from the American firm Hughes Aircraft, led by Harold Rosen, developed a design for a satellite that would operate in geostationary orbit. Aided by research support from NASA, the first successful geostationary satellite, Syncom 2, was launched in 1963; it demonstrated the feasibility of the Hughes concept prior to commercial use.
The United States also took the lead in creating the organizational framework for communications satellites. Establishment of the Communications Satellite Corporation (Comsat) was authorized in 1962 to operate American communications satellites, and two years later an international agency, the International Telecommunications Satellite Organization (Intelsat), was formed at the proposal of the United States to develop a global network. Comsat, the original manager of Intelsat, decided to base the Intelsat network on geostationary satellites. The first commercial communications satellite, Intelsat 1, also known as Early Bird, was launched in 1965. Intelsat completed its initial global network with the stationing of a satellite over the Indian Ocean in mid-1969, in time to televise the first Moon landing The original use of communications satellites was to relay voice, video, and data from one relatively large antenna to a second, distant one, from which the communication then would be distributed over terrestrial networks. This point-to-point application introduced international communications to many new areas of the world, and in the 1970s it also was employed domestically within a number of countries, especially the United States. As undersea fibre-optic cables improved in carrying capacity and signal quality, they became economically and technologically competitive with communications satellites, and the latter responded with comparable technological advances that allowed these space-based systems to meet the challenge. A number of companies in the United States and Europe manufacture communications satellites and vie for customers on a global basis. Other firms operate these satellites, often producing significant profits. around the world.