Download - Lecture 11: Satellite Communcation
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Lecture 11: Satellite Communcation
Anders Västberg
08-790 44 55
Slides are a selection from the slides from chapter 9 from:http://williamstallings.com/Wireless/Wireless2e.html
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History
• 1945 Arthur C. Clarke: Extra Terrestrial Relays• 1957 Sputnik• 1960 ECHO• 1963 First Geostationary Satellite: SYNCOM• 1965 First commercial geostationary communication
satellite: INTELSAT 1• Followed by INTELSAT 2 (1967) INTELSAT 3 (1969)• 1976 MARISAT • 1982 INMARSAT-A• 1988 INMARSAT-C (Satellite Mobile Phones)• 1993 INMARSAT-M• Today: Iridium and Globalstar
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Applications
• Weather forecasting• Radio and TV broadcast satellites• Military satellites• Satellites for navigation• Global telephone backbones• Connections for remote or developing
areas• Global mobile communications
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Classification of Satellite Orbits
• Circular or elliptical orbit– Circular with center at earth’s center – Elliptical with one foci at earth’s center
• Orbit around earth in different planes– Equatorial orbit above earth’s equator– Polar orbit passes over both poles– Other orbits referred to as inclined orbits
• Altitude of satellites– Geostationary orbit (GEO)– Medium earth orbit (MEO)– Low earth orbit (LEO)– High elliptical orbit (HEO)
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Geometry Terms
• Elevation angle - the angle from the horizontal to the point on the center of the main beam of the antenna when the antenna is pointed directly at the satellite
• Minimum elevation angle
• Coverage angle - the measure of the portion of the earth's surface visible to the satellite
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Minimum Elevation Angle
• Reasons affecting minimum elevation angle of earth station’s antenna (>0o)– Buildings, trees, and other terrestrial objects block
the line of sight– Atmospheric attenuation is greater at low elevation
angles– Electrical noise generated by the earth's heat near
its surface adversely affects reception
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GEO Orbit
• Advantages of the the GEO orbit – No problem with frequency changes (Doppler
shift)– Tracking of the satellite is simplified– High coverage area
• Disadvantages of the GEO orbit– Weak signal after traveling over 35,000 km– Polar regions are poorly served– Signal sending delay is substantial
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LEO Satellite Characteristics
• Circular/slightly elliptical orbit under 2000 km• Orbit period ranges from 1.5 to 2 hours• Diameter of coverage is about 8000 km• Round-trip signal propagation delay less than 20 ms• Maximum satellite visible time up to 20 min• System must cope with large Doppler shifts• Atmospheric drag results in orbital deterioration
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LEO Categories
• Little LEOs– Frequencies below 1 GHz – 5MHz of bandwidth – Data rates up to 10 kbps– Aimed at paging, tracking, and low-rate messaging
• Big LEOs– Frequencies above 1 GHz – Support data rates up to a few megabits per sec– Offer same services as little LEOs in addition to voice and
positioning services
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MEO Satellite Characteristics
• Circular orbit at an altitude in the range of 5000 to 12,000 km
• Orbit period of 6 hours• Diameter of coverage is 10,000 to 15,000 km• Round trip signal propagation delay less than 50 ms• Maximum satellite visible time is a few hours
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Frequency Bands Available for Satellite Communications
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Satellite Link Performance Factors
• Distance between earth station antenna and satellite antenna
• For downlink, terrestrial distance between earth station antenna and “aim point” of satellite– Displayed as a satellite footprint
– Atmospheric attenuation
– Affected by oxygen, water, angle of elevation, and higher frequencies
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Signal Attenuation due to Atmospheric Absorption
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Satellite Network Configurations