136660715-7064884-W-Tomasi-pp-pdf.pdf - II j~ I 7 ~ ~'~ f ~ ff!liVE.1 ~11i~1I M Communications I INTRODUCTION In the early 1960s the American Telephone

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INTRODUCTION ~ 7 ~ II ", ~'~ j~ I f ff' '!liVE ...1 ~11i~1I Communications I M In the early 1960s, the American Telephone and Telegraph Company (AT&T) released studies indicating that a few powerful satellites of advanced design could handle more traffic than the entire AT&T long-distance communications network. The cost of these satellites was estimated to be only a fraction of the cost of equivalent terrestrial microwave facilities. Unfortunately, because AT&T was a utility, government regulations prevented them from developing the satellite systems. Smaller and much less lucrative corporations were left to develop the satellite systems, and AT&T continued to invest bil- lions of dollars each year in conventional terrestrial microwave systems. Because of this, early developments in satellite technology were slow in coming. Throughout the years the prices of most goods and services have increased substan- tially; however, satellite communications services have become more affordable each year. In most instances, satellite systems offer more flexibility than submarine cables, buried underground cables, line-of-sight microwave radio, tropospheric scatter radio, or optical fiber systems. Essentially, a communications satellite is a radio repeater in the sky (transponder). A satellite system consists of a transponder, a ground-based station to control its opera- tion, and a user network of earth stations that provide the facilities for transmission and reception of communications traffic through the satellite system. Satellite transmissions are categorized as either bus or payload. The bus includes control mechanisms that sup- port the payload operation. The payload is the actual user information that is conveyed through the system. Although in recent years new data services and television broadcast- ing are more and more in demand, the transmission of conventional speech telephone sig- nals (in analog or digital form) is still the bulk of the satellite payload. 266
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HISTORY OF SATELLITES The simplest type of satellite is a passive reflector, a device that simply "bounces" a signal from one place to another. The moon is a natural satellite of the earth and, consequently, in the late 1940s and early 1950s,became the first passive satellite. In 1954, the U.S. Navy successfully transmitted the first messages over this earth-to-moon-to-earth relay. In 1956, a relay service was established between Washington, D.C. and Hawaii and, until 1962, offered reliable long-distance communications. Service was limited only by the availabil- ity of the moon. In 1957,Russia launched Sputnik I, the first active earth satellite. An active satellite is capable of receiving, amplifying, and retransmitting information to and from earth stations. Sputnik I tr$llsmittedtelemetry information for 21 days. Later in the same year, the United States launcned Explorer I, which transmitted telemetry information for nearly 5 months.
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