Artificial Satellite

I

Introduction

Artificial Satellite, any object purposely placed into orbit around Earth, other planets, or the Sun. Since the launching of the first artificial satellite in 1957, thousands of these “human-made moons” have been rocketed into Earth orbit. Today, artificial satellites play key roles in the communications industry, in military intelligence, and in the scientific study of both Earth and outer space. See also Space Exploration.

II

Types of Satellites

Engineers have developed many kinds of satellites, each designed to serve a specific purpose or mission. For instance the telecommunications and broadcasting industries use communications satellites to carry radio, television, and telephone signals over long distances without the need for cables or microwave relays. Navigational satellites pinpoint the location of objects on Earth, while weather satellites help meteorologists forecast the weather (see Meteorology). The United States government uses surveillance satellites to monitor military activities (see Remote Sensing). Scientific satellites serve as space-based platforms for observation of Earth, the other planets, the Sun, comets, and galaxies, and are useful in a wide variety of other applications.

A

Communications Satellites

Almost all of the earliest satellites included some communications equipment. The National Aeronautics and Space Administration (NASA) launched the first telephone and television satellite, AT&T’s Telstar 1, in 1962. The U.S. Department of Defense launched Syncom 3 in 1964. Syncom 3 was the first communication satellite to use a geostationary orbit—that is, an orbit that keeps the satellite over the same spot above Earth’s equator. Hundreds of communications satellites have been launched since 1957. Today satellites in geostationary orbit provide voice, data, and television communications, including the direct broadcast of television to homes around the world.

B

Navigation Satellites

Navigation satellites can help locate the position of ships, aircraft, and even automobiles that are equipped with special radio receivers. A navigation satellite sends continuous radio signals to Earth. These signals contain data that a special radio receiver on Earth translates into information about the satellite’s position. The receiver further analyzes the signal to find out how fast and in what direction the satellite is moving and how long the signal took to reach the receiver. From this data, the receiver can calculate its own location. Some navigation satellite systems use signals from several satellites at once to provide even more exact location information.

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The U.S. Navy launched the first navigation satellite, Transit 1B, in 1960. The United States ended its support of the Transit system in 1996. The U.S. Air Force operates a system, called the NAVSTAR Global Positioning System (GPS), that consists of 24 satellites. Depending on the type of receiver and the method used, GPS can provide position information with an accuracy from 100 m (about 300 ft) to less than 1 cm (less than about 0.4 in). The Global Orbiting Navigation Satellite System (GLONASS) of the Russian Federation consists of 24 satellites and provides accuracy similar to GPS. In December 2005 the European Union (EU) launched the first of 30 satellites that will make up a civilian satellite navigation system called Galileo. The system will have an accuracy of about 1 m (3.3 ft) and is set for full deployment by 2013.

The international Global Navigation Satellite System (GNSS) combines coverage from all of these satellite navigation systems and any that may be developed in the future. The GNSS is designed to allow receivers around the world to communicate with satellites in any of the three existing systems. The United States also negotiated an agreement with the EU that will allow it to scramble Galileo signals over a battlefield or military target without shutting down the entire system. The two systems will compete economically, however, for commercial applications.

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