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. Over 300 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.

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 will become operational in 2009. Also known as the Global Navigation Satellite System (GNSS), the European system is designed to be compatible with those of the United States and Russia, allowing receivers around the world to communicate with satellites in any of the three systems. The United States also negotiated an agreement with the EU that will allow it to scramble signals over a battlefield or military target without shutting down the entire system. The two systems will compete economically, however, for commercial applications.

C.

Weather Satellites

Weather satellites carry cameras and other instruments pointed toward Earth’s atmosphere. They can provide advance warning of severe weather and are a great aid to weather forecasting. NASA launched the first weather satellite, Television Infrared Observation Satellite (TIROS) 1, in 1960. TIROS 1 transmitted almost 23,000 photographs of Earth and its atmosphere. NASA operates the Geostationary Operational Environmental Satellite (GOES) series, which are in geostationary orbit. GOES provides information for weather forecasting, including the tracking of storms. GOES is augmented by Meteosat 3, a European weather satellite also in geostationary orbit. The National Oceanic and Atmospheric Administration (NOAA) operates three satellites that collect data for long-term weather forecasting. These three satellites are not in geostationary orbit; rather, their orbits carry them across the poles at a relatively low altitude.

D.

Military Satellites

Many military satellites are similar to commercial ones, but they send encrypted data that only a special receiver can decipher. Military surveillance satellites take pictures just as other earth-imaging satellites do, but cameras on military satellites usually have a higher resolution.

The U.S. military operates a variety of satellite systems. The Defense Satellite Communications System (DSCS) consists of five spacecraft in geostationary orbit that transmit voice, data, and television signals between military sites. The Defense Support Program (DSP) uses satellites that are intended to give early warning of missile launches. DSP was used during the Persian Gulf War (1991) to warn of Iraqi Scud missile launches.

Some military satellites provide data that is available to the public. For instance, the satellites of the Defense Meteorological Satellite Program (DMSP) collect and disseminate global weather information. The military also maintains the Global Positioning System (GPS), described earlier, which provides navigation information that anyone with a GPS receiver can use.

E.

Scientific Satellites

Earth-orbiting satellites can provide data to map Earth, determine the size and shape of Earth, and study the dynamics of the oceans and the atmosphere. Scientists also use satellites to observe the Sun, the Moon, other planets and their moons, comets, stars, and galaxies. The Hubble Space Telescope is a general-purpose observatory launched in 1990. Some scientific satellites orbit bodies other than Earth. The Mars Global Surveyor, for example, orbits the planet Mars.