Communications Satellitessteffen/CPET 384/Papers06... · Web viewAlthough not the first communications satellite, Telstar is the best known of all and is probably considered by most

Communications SatellitesMathew Duguid

CPET 384 Research Paper

Index

Topic Page

Introduction 2

History of Communication Satellites 2

Radio 5

Television 6

Amateur Radio 7

Telephony 8

Conclusion 9

References 10

Page 1

Introduction

In 1957 the first satellite capable of transmitting radio signals was launched into space. In less than fifty years, communications satellites have become a vital part of modern day telecommunications.

History of Communications Satellites

The Soviet Union launched the first satellite, Sputnik 1, into an elliptical orbit on October 4, 1957. Sputnik measured temperature and pressure and transmitted the data in a series of radio beeps. The launching of Sputnik took the West by surprise and led to the organization of ARPA (Advanced Research Projects Agency) and initiated the great Space Race. After three weeks the batteries failed. (Graham, 2006)

The Space Race accelerated the development of satellites. America launched the Explorer 1, on January 31, 1958.“Instrumentation consisted of a cosmic-ray detection package, an internal temperature sensor, three external temperature sensors, a nose-cone temperature sensor, a micrometeorite impact microphone, and a ring of micrometeorite erosion gauges. Data from these instruments were transmitted to the ground by a 60-milliwatt transmitter operating on 108.03 megahertz and a 10 milliwatt transmitter operating on 108.00 MHz.”

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The first true Communications satellite, Project SCORE (Signal Communication by Orbiting Relay Equipment), was launched on December 18, 1958. Project SCORE tested the possibilities of a communication relay system in space. The payload consisted of a redundant pair of battery-powered vacuum tube-based UHF communication packages with a design life of only 21 days. The satellite was used to transmit a Christmas message from President Eisenhower.

“This is the President of The United States speaking. Through the marvels of scientific advance, my voice is coming to you from a satellite traveling in outer space. My message is a simple one: Through this unique means I convey to you and all mankind, America’s wish for peace on Earth and goodwill toward men everywhere.”(Project SCORE,2006)

After 13 days in orbit, the batteries failed.Project SCORE was proof that an active communications satellite could

transmit messages through the upper atmosphere from one ground station to one or more ground stations

NASA launched the Echo satellites in 1960. Each satellite acted as a passive reflector of microwave signals. The Echo satellites where used to redirect transcontinental and intercontinental telephone, radio, and television signals. The reflective surface of the Echo satellites made them visible to the unaided eye over most of the planet. “Brighter than most stars, it was probably seen by more people than any other man-made objects in space.” Echo 1A reentered the Earth’s atmosphere and burned up on May 24, 1968. (A World-Class Tinkerer, 2002)

Also launched in 1960 was the Courier 1B, built by Philco. The Courier 1B was the worlds first active repeater satellite. The Courier 1B was the first communications satellite to be powered by solar cells and rechargeable nickel cadmium storage batteries. Courier 1B demonstrated that high volume traffic, up to 100,000 words per minute, could be transmitted through space. After 17 days, the payload refused to respond to any commands from the ground. The cause of the failure is unknown however it is believed that the clock-based access codes got out of synchronization.

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The Telstar 1 was the first active direct relay communications satellite and the first privately sponsored satellite belonging to AT&T. The Telstar was designed to transmit telephone and high-speed data communications.

The Telstar 1 was equipped with solar cells capable of producing 14 Watts. Microwave signals were received from ground stations using a helical antenna. The signals were then amplified and re-broadcasted using a series of directional feed horns.

Two weeks after Telstar 1’s launch, the first broadcast of a live transatlantic television signal was scheduled. The broadcast was supposed to have been a speech by President Kennedy, but the signal was acquired before the president was ready, so the time was filled with a short segment of a major league baseball game. On the same day the first telephone call was transmitted through space.

The following quote by NASA’s Chief of Communications Satellite Programs sums up the importance of the Telstar satellite.” Although not the first communications satellite, Telstar is the best known of all and is probably considered by most observers to have ushered in the era of satellite communications. This impression was a result of the tremendous impact upon the public by the first transmission of live television across the Atlantic Ocean. (Jaffe, pg. 107)”

According to the US Space Object Registry, In November 2006 Telstar 1 was still in orbit above the earth.

The most important breakthrough in satellite technology was the discovery of geostationary orbit. The concept was first proposed by science fiction author Arthur C. Clarke in a paper in 1945. The first geostationary satellite Syncom 3, was used to broadcast the 1964 summer Olympics. This was the first television program to cross the Pacific Ocean. (Wikipedia,2006)

Geostationary Satellites were instrumental in building the television and radio industry. Geostationary satellites allow land-based satellite dishes to maintain a stationary position. (GEO, 2006)

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Satellite Radio

Satellite radio also called digital radio, offers uninterrupted, near CD-quality music that is broadcasted from direct broadcast satellites. In 1992 the United States Federal Communications Commission allocated a spectrum of space in the “S” band. Currently, only two companies that have the licenses are XM Radio and Sirius. Both companies use different satellite technologies. (Garth, 2006)

XM Radio was founded in 1992 and is based out of Washington, DC. In 1999 XM signed a 12-year “Distribution Agreement” with General Motors. The agreement specifies that GM must install a specified number of XM radio modules in their automobiles each year. XM launched services on November 12, 2001. As of December 31, 2005 XM had 5,932,957 subscribers.

Initially XM provided services using two high-powered satellites in geostationary orbit. The satellites are nicknamed “Rock” and “Roll” In addition to the satellites, XM also has a network of approximately 900 ground-based repeaters. The repeaters are needed to compensate for signal blockage by buildings, tunnels, and bridges. The combination of repeaters and satellites is designed to provide continuous coverage across Northern America.

After launching, a flaw was discovered in the satellites “Rock” and “Roll”. The solar panels were prone to fogging which significantly reduced the lifetime from 15 years to 6 years. To combat the issue, the satellite “Rhythm” was launched and moved into the place of “Rock”. “Rock” was moved to co-locate with “Roll”. This reduced the power required by “Rock” and “Roll” by 50 percent. XM will release the satellite “Blues” to replace “Rock” and “Roll” which will complete the satellite replacement.

Sirius was founded in 1990 and currently has about half as many viewers as XM.

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Sirius does not use geostationary satellites; instead three satellites that fly in a highly elliptical orbit provide complete coverage for the United States. Sirius also uses ground-based repeaters for signal blockage.

Satellite radio receivers include two parts, the antenna module and the receiver module. The antenna module picks up signals from the ground repeaters and satellites, amplifies the signal and filters out any interference. The signal is passed on to the receiver module that processes the signal.

Satellite Television

Satellite Television started in the 1970’s and used Fixed Service Satellite technology (FSS). In 1994, DirecTV was established as the first American company to use Direct Broadcast Satellites (DBS). DBS offered a much smaller dish size and lower equipment costs. FFS uses the C-band and the lower portions of the Ku-bands, while DBS operates in the Ku-Band. The early satellite television dishes where much larger than DirecTV’s dishes because the C band has a longer wavelength than the Ku-Band. Another factor that contributed to the larger sized dishes was the use of linear polarization instead of circular polarization. (Noeldeke, 2001)

FFS systems also need a movable dish, even though the satellites are geostationary, a single C-Band satellite cannot carry the entire listing of channels. On the other DBS satellite dishes do not require movement, all channels can be received using only one satellite.

With the arrival of DBS, FFS systems have become obsolete for personal satellite television.

Page 6

DBS systems require five major components, the programming source, the broadcast center, the satellite, the satellite dish, and the receiver.

The programming sources are the companies that provide programming for broadcast. The broadcast center is the central hub of the system. At the broadcast center, the television provider receives signals from various programming sources and converts all of this programming into a high-quality, uncompressed digital stream. At this point, the stream contains a vast quantity of data -- about 270 megabits per second for each channel. In order to transmit the signal from there, the broadcast center has to compress it. Otherwise, it would be too big for the satellite to handle. (Nice, 2006)

Currently DirecTV uses the MPEG 2 compressed video format. With MPEG-2 compression, the provider can reduce the 270-Mbps stream to about 5 or 10 Mbps depending on the type of programming. This is the crucial step that has made DBS service a success. With digital compression, a typical satellite can transmit about 200 channels. Without digital compression, it can transmit about 30 channels. (Nice, 2006)

Most DBS video satellite transmissions are in the process of converting from the standard MPEG-2 compression to MPEG-4. This is due to the large bandwidth required for high definition broadcasts. (Cobes, 2005/2006)

After the video is compressed, the provider encrypts the signal and rebroadcasts the signal to one of its satellites. The satellites receive the signals from the broadcast station and rebroadcast them to the ground. The viewer's dish picks up the signal from the satellite (or multiple satellites in the same part of the sky) and passes it on to the receiver in the viewer's house. The receiver decrypts and processes the signal. (Nice, 2006)

Amateur Radio

Ordinary people interested in amateur radio and communications satellites have constructed the OSCAR series of satellites. OSCAR 1 was launched in December of 1961, only four years after the launch of Sputnik. There are currently nineteen OSCAR satellites orbiting Earth with various communications capabilities and functions. Interestingly enough, the OSCAR series of satellites are actually ballasts for larger primary NASA payloads. It’s simpler and cheaper to ballast a rocket with dead weight rather than reducing the thrust. (AMSAT, 2006)

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Satellite Telephony

In 1990, Motorola announced Iridium; a large project intended to provide cellular voice, data, fax, and paging communications to any point on the earth’s surface. The name Iridium is derived from the original number of satellites and the atomic number of the element Iridium, both are 77. Iridium services went online on November 1, 1998. In less than a year the company had filed for bankruptcy. Its financial failure was largely due to the growing terrestrial cellular networks. (Collins,2005)

In 2001 The Company was purchased and services were re-established by a smaller privately owned company, Iridium Satellite LLC. The current Iridium system is being used extensively by the United States Department of Defense.

The Iridium system requires 66 active satellites in orbit to complete its constellation, with spare satellites in orbit in case of failure. Satellites are in low Earth orbit at a height of approximately 485 miles. Satellites communicate via intersatellite links. Each satellite can have four intersatellite links. The four intersatellite cross links on each satellite operate at 10 Mbit/s. The cross links were originally envisioned to be optical, however future satellites may be equipped with optical links. Such cross-links are unique in the satellite telephone industry as other providers such as Globalstar depend on local base stations and do not relay data between satellites. This in turn means that calls between satellite phones are cheaper, as many such calls never get passed through a ground-based repeater station.

The existing constellation of 66 satellites is expected to remain operational until at least 2014, with many satellites expected to remain in service until the 2020's. Iridium currently has about a dozen of the original satellites on the ground waiting to be launched but doesn't plan to launch them until at least 2010 and doesn't plan to produce any new satellites until at least 2020.

The satellites each contain seven Motorola PowerPC 603E processors running at roughly 200 MHz. Processors are connected by a custom backplane network. One processor is dedicated to each cross-link antenna and two processors are dedicated to satellite control with one being a spare. Late in the project an extra processor was added to perform resource management and phone call processing.

The original design envisioned a completely static 1960’s "dumb satellite" with a set of control messages and time-triggers for an entire orbit that would be uploaded as the satellite passed over the poles. It was found that this design did not have enough bandwidth in the space-based backhaul to upload each satellite quickly and reliably over the poles. Therefore, the design was scrapped in favor of a design that performed dynamic control of routing and channel selection late in the project, resulting in a one year delay in system delivery. Each satellite can support up to 1100 concurrent phone calls. (Wikipedia, 2006)

Page 8

Conclusion

Communications satellites have become part of almost every type of telecommunication. The communications satellites will continue to grow and develop with satellites becoming an even larger market. (Gregory, 2000)

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References

A World-Class Tinkerer. 2002. IEEE Spectrum, 39, 9 – 11

AMSAT, A Brief History of Amateur Satellites. (n.d.) Retreived December 2, 2006, from http://www.amsat.org/amsat/sats/n7hpr/history.html

Caceres, Marco. (2000), GEO commercial communications satellites grow bigger. Aerospace America, 38, 16

Collins, M. (2005). One World … One Telephone: Iridium, One Look at the Making of a Global Age. History and Technology, 21, 301-324

Cobes, S. (2005/2006) Satellite and next generation networks: Proposal for QOS architecture. Space Communications, 20, 101-119

Garth,Alper (2006). XM Reinvents Radio’s Future. Popular Music and Society, 29, 505-518

Graham, T. (2006). The Essentiality of Effetive Verification: From Sputnik to the Space Station. Problems of Post-Communism, 53, 17-29

Gregory, Bill. (2000), Evolving SATCOM Architecture. Armed Forces Journal International, 137, 22

Nice, K et all. (2006), How Satellite TV Works. Retreived on December 3, 2006 from http://www.howstuffworks.com/satellite-tv1.htm

Noeldeke, Christoph M. (2001), Air interface and payload architecture for GEO multimedia communications satellites. Space Communications, 17, 49.

Project SCORE. Retreived December 2, 2006, from http://www.narmc.amedd.army.mil/patterson/score.htm

Baig, M. (n.d.) Satellite Television and Its Future. Retreived on December 3, 2006 from http://ezinearticles.com/?Satellite-Television-and-Its-future&id=132238

Wikipedia. Retreived December 2, 2006, from http://en.wikipedia.org/wiki/Communications_satellite

Wikipedia. Retreived December 2, 2006, from http://en.wikipedia.org/wiki/Iridium_(satellite)

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