Satellite Technology

The world changed on October 4, 1957, when the Soviet Union launched the Earth's first artificial satellite (the Moon is a natural satellite). Sputnik, a Russian word meaning "fellow traveler," was an 83 kilogram (183-pound) satellite the size and shape of a basketball. It did little except orbit the Earth every 98 minutes and emit a simple radio signal, a recording of which can be downloaded from a National Aeronautics and Space Administration (NASA) web site (http://www.hq.nasa.gov/office/pao/History/sputnik/). Yet, this simple event started what was to be known as "The Space Race" that eventually led to the lunar landings as well as space shuttle missions, and weather and direct broadcast television satellites.

Although the satellite concept is theoretically simple—an object placed high enough above Earth's atmosphere to be moving at a speed of eight km/sec (17,280 miles/hour)—the successful launch, orbital insertion, and control of any satellite is extremely complex. This is evidenced by the many failures that occurred before Sputnik (and since). However, as of 2002 there are more than 2,670 artificial satellites orbiting Earth. In addition to these, the U.S. Space Command is also tracking 6,186 pieces of space debris. This demonstrates that humans have not only mastered satellite technology but also succeeded in extending well-developed littering capabilities into space.

A satellite's orbit is described in one or more of three dimensions: the perigee, its closest distance from the Earth; the apogee, its furthest distance from the Earth; and, its inclination, the angle the orbit makes with the equator. Satellites are put into particular types of orbit depending on their mission. In a geostationary orbit, the satellite speed is synchronized with the Earth's rotation so that the satellite stays in the same relative position. A polar orbit is characterized by its 90-degree inclination to Earth's equator. When a satellite is in low Earth orbit, the apogee and perigee are each only about 483 kilometers or 300 miles.

Types of Satellites

Most satellites today are in place for communication, environmental monitoring, or navigational purposes. There are both government and commercial satellites in space.

Communication.

Of all satellite technologies, communication technology has probably had the greatest impact on our world. It has been called one of the greatest forces for the "super-tribalization" of the human species. Communication satellites have also proven to be one of the most successful commercial applications of space technology. In 2002 there are about 200 communication satellites orbiting above Earth.

The first telephone communication satellites, ECHO and Telstar, were launched in 1960 and 1962, respectively. These and many subsequent satellites carried analog signals to all parts of the world. Because the computer resources in such satellites were minimal, in computer terms they could be called "dumb," since they were, for the most part, simple passive transceivers.

However, the demands of the Internet and personal communication devices such as pagers and wireless phones have resulted in radically changed communication satellite technologies. Present satellites not only utilize digital signals and processing but also, in some systems, provide satellite-to-satellite communications. Perhaps this was nowhere more evident than in the Iridium system, a constellation of 66 Low Earth Orbit (LEO) communication satellites that featured sophisticated computer resources and protocols for inter-satellite communication.

Television broadcast or, more precisely, relay, from satellite also began in 1962 with the launch of the Relay satellite. This technology has similarly improved, as is evidenced by the shrinking sizes of the ubiquitous home satellite "dishes" that, owing to greater satellite transmission power and other advances in technology, are now almost unnoticeable.

Environmental Monitoring.

The view from a geostationary environmental satellite is often shown during televised weather broadcasts. The Geostationary Operational Environmental Satellites (GOES) orbit simultaneously with the Earth's rotation at an altitude of just under 37,115 kilometers or 23,000 miles. Typically, there are two such satellites in orbit stationed to offer views of both the eastern and western parts of the United States. The GOES are true environmental satellites; in addition to images of the clouds, they also provide information on water vapor, land and sea temperatures, winds, and estimates of precipitation.

Landsat is also an environmental as well as a natural resource satellite. Over the last 30 years, seven Landsat satellites have been launched and two are currently in polar, sun-synchronous orbits. A sun-synchronous orbit is one in which the satellite passes over points on the ground at the same local time. Landsat's multi-spectral scanner (MSS) and thematic mapper imaging systems provide digital imagery over discrete sections of the visible and infrared portions of the electromagnetic spectrum. Such multi-spectral imagery and its analyses are routinely used in environmental studies such as deforestation, water pollution, tracking oil spills, and monitoring forest fires and droughts. It is also utilized in natural resource studies such as land use classification, vegetation and soil mapping, and geological, hydrological, and coastal resource studies.

Navigation.

Lines of latitude and longitude have been noted on maps since ancient times. Yet, the accurate determination of one's exact position on the Earth has always been a vexing problem. Perhaps at no time was this more evident than during the eighteenth century when scholars and inventors vied to solve the problem of accurately determining longitude at sea. Such a determination required a highly accurate (and stable) clock, since it was necessary to know simultaneously the time on the ship and at a land-based point of known longitude. It is interesting to note that the problem of determining accurate time was the basis of accurate position determination then, as it is also the basis of today's most accurate worldwide satellite navigation or position system. Indeed the navigation satellites of the twenty-first century have precise clocks that are accurate to within three nanoseconds, or three billionths of a second.

The navigational system known as the Global Positioning System (GPS) is a constellation of 24 NAVSTAR (NAVigation Satellite Timing And Ranging) satellites orbiting at 20,278 kilometers or 12,600 miles altitude in six orbital planes. Each orbital plane is at an inclination of 55 degrees with four satellites spaced so that a minimum of five satellites is visible from any location on the planet. Each satellite broadcasts time and orbit information. Receivers on the ground also contain internal clocks. The difference in time between when a signal was sent and when it was received from each observable satellite is used to solve a spherical trigonometry problem to determine the exact location of the observer. With certain receiver systems, this satellite technology means that someone's exact position on the surface of the Earth can be determined to within one centimeter or four-tenths of an inch!

see also Communication Devices; Telecommunications; Wireless Technology.

Robert D. Regan

Bibliography

Sobel, Dava. Longitude. New York: Walker and Company, 1995.

Internet Resources

"GOES Next at a Glance." USA Today. 6 January 1999. <http://www.usatoday.com/weather/wds10.htm>

Edelson, Burton I., Joseph N. Pelton, and Neil R. Helm. NASA/NSF Panel Report: Satellite Communication Systems and Technology (1993 Study). <http://itri.loyola.edu/ar93_94/scst.htm>