Satellites, Types of
Satellites, Types of
Not long after the Soviet Union launched the first satellite in 1957, satellites began to play an increasingly important role in our lives. The first satellites were small because of the lack of powerful launch vehicles, and almost all had scientific missions. However, as larger rockets became available and engineers used new technologies to build more efficient payloads , the first prototypes of many of the satellites that were still in use in 2001 were launched and changed our world.
One of the earliest classes of satellites was designed to observe Earth from orbit. Among the first were military reconnaissance satellites, such as the American Corona and the Soviet Zenit, which took photographs with film that had to be returned to Earth to be developed. Over the years more sophisticated electronic imaging technology made it possible for spy satellites to obtain very high-resolution images and transmit them almost immediately to analysts. This technology has been useful in gauging a potential adversary's intentions, for verifying compliance with treaties, and in other important ways.
In 1960 early television surveillance technology was used for the first weather satellites. By 2001 those satellites (flying in polar orbits and geosynchronous orbits ) were equipped not only with cameras but with a range of sensors that employed the latest infrared technology. In addition to providing the weather pictures that people see every day on television, these satellites supply meteorologists with the highly detailed information they need to track storms and predict the weather. This application of satellite technology alone has saved countless lives.
Starting in the 1970s some of this technology was applied to remote sensing satellites such as Landsat. Instead of monitoring military targets at high resolution, these satellites monitor Earth's natural resources on a more moderate scale. These data provide the information needed to locate new sources of raw materials and determine the effects of natural disasters and pollution on the environment. Because this information is so valuable, many commercial remote sensing satellites, such as the French SPOT, have been launched and their data have been sold to a wide range of government and private users.
Recently declassified reconnaissance and cartographic photographs from American and former Soviet spy satellites are now available, giving researchers more varied long-term data on the environment. Radar mapping technology originally used by the military to make observations through clouds has found numerous civilian applications.
Voices in the Sky
By far the most common satellite type launched, and perhaps the one that has had the greatest impact on people's lives, is the communication satellite, or "comsat." Beginning in 1958 early experimental comsats operated as relays or repeaters in relatively low orbits, in part because of the lack of powerful launch vehicles and the crude nature of their electronics. Although such low orbiting comsats still have a place, a large number of them are required to provide continuous coverage around the globe.
As early as 1946 the space visionary and author Arthur C. Clarke recognized the value of placing comsats in geosynchronous orbits. From an altitude of 35,786 kilometers (22,300 miles) above the equator, satellites match Earth's spin and appear to hang motionless in the sky. From this great height over one-third of the planet's surface can be seen, allowing a satellite to relay signals over long distances. After several successful experiments the first commercial geosynchronous comsat, Early Bird, was launched in 1965. In the succeeding decades, improved rockets allowed larger comsats to be launched. Combined with major advances in microelectronics, each of the dozens of active comsats in orbit in 2001 had thousands of times the capacity of their earliest ancestors.
Although geosynchronous comsats are useful at low latitudes, they appear too close to the horizon at high or polar latitudes. To overcome this problem, since 1965 the Soviet Union (and later Russia) has launched Molniya satellites into highly elliptical 12-hour orbits inclined to the equator. This type of orbit allows them to be seen high above the horizon over most of Russia's territory for long periods. From this vantage point, Molniya satellites can relay television and telephone signals across that nation's vast expanses. During the Cold War, such orbits were used by some signal intelligence, or sigint, satellites to intercept radio signals. Sigint satellites also are used to track ships at sea, locate radar installations, and monitor other activities such as various types of radio transmissions.
A type of comsat known as a navigation satellite, or navsat, has become important to military and civilian users. Operating in precisely known orbits thousands of miles above Earth, these satellites broadcast a precise timing signal. Signals from three or more navsats can be used to determine a position on or above the Earth's surface within a few feet. The first experimental navsats were built by the U.S. Navy in the 1960s and were used by ships to determine their exact positions at sea. Today a constellation of satellites forming the Global Positioning System (GPS) allows military and civilian users to accurately determine their locations anywhere in the world.
Science in Space
Whereas a large number of satellites with practical applications have been launched, science satellites still provide important information about the space environment and the universe beyond. Satellites monitoring Earth's magnetosphere can provide warnings about communications blackouts and other effects of solar storms. Since the 1960s larger, more capable observatories employing increasingly advanced technologies have been launched to observe the Sun and the rest of the heavens over the entire electromagnetic spectrum . The Hubble Space Telescope is a well-known example. As a result of the data returned from these satellites, much more has been learned about everything from the Sun and how it affects Earth to the origins of the universe.
see also Clarke, Arthur C. (volume 1); Navigation from Space (volume 1); Reconnaissance (volume 1); Remote Sensing Systems (volume 1); Satellite Industry (volume 1); Satellites, Future Designs (volume 4); Small Satellite Technology (volume 1).
Andrew J. LePage
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