Comunicación Satelital

Satellite Communications
CSC 490: Wireless Networking Author: Michael Charles

Overview
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Basics of Satellites Types of Satellites Capacity Allocation

Basics: How do Satellites Work
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Two Stations on Earth want to communicate through radio broadcast but are too far away to use conventional means. The two stations can use a satellite as a relay station for theircommunication One Earth Station sends a transmission to the satellite. This is called a Uplink. The satellite Transponder converts the signal and sends it down to the second earth station. This is called a Downlink.

Basics: Advantages of Satellites
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The advantages of satellite communication over terrestrial communication are:
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The coverage area of a satellite greatly exceeds thatof a terrestrial system. Transmission cost of a satellite is independent of the distance from the center of the coverage area. Satellite to Satellite communication is very precise. Higher Bandwidths are available for use.

Basics: Disadvantages of Satellites
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The disadvantages of satellite communication:
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Launching satellites into orbit is costly. Satellite bandwidth is graduallybecoming used up. There is a larger propagation delay in satellite communication than in terrestrial communication.

Basics: Factors in satellite communication
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Elevation Angle: The angle of the horizontal of the earth surface to the center line of the satellite transmission beam.
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This effects the satellites coverage area. Ideally, you want a elevation angle of 0 degrees, sothe transmission beam reaches the horizon visible to the satellite in all directions. However, because of environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth electrical background noise, there is a minimum elevation angle of earth stations.

Basics: Factors in satellite communication (cont.)
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Coverage Angle: A measure of the portion ofthe earth surface visible to a satellite taking the minimum elevation angle into account. R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2) = cos(β + θ)/cos(θ)
R = 6370 km (earth’s radius) h = satellite orbit height β = coverage angle θ = minimum elevation angle

Basics: Factors in satellite communication (cont.)
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Other impairments to satellite communication:
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The distance between anearth station and a satellite (free space loss). Satellite Footprint: The satellite transmission’s strength is strongest in the center of the transmission, and decreases farther from the center as free space loss increases. Atmospheric Attenuation caused by air and water can impair the transmission. It is particularly bad during rain and fog.

Basics: How Satellites are used
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Service Types
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Fixed Service Satellites (FSS)
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Example: Point to Point Communication

Broadcast Service Satellites (BSS)
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Example: Satellite Television/Radio Also called Direct Broadcast Service (DBS).
Example: Satellite Phones

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Mobile Service Satellites (MSS)
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Types of Satellites
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Satellite Orbits
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GEO LEO MEO Molniya Orbit HAPs

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Frequency BandsGeostationary Earth Orbit (GEO)
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These satellites are in orbit 35,863 km above the earth’s surface along the equator. Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This means GEO satellites remain in the same position relative to the surface of earth.

GEO (cont.)
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Advantages
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A GEO satellite’s distance from earth gives ita large coverage area, almost a fourth of the earth’s surface. GEO satellites have a 24 hour view of a particular area. These factors make it ideal for satellite broadcast and other multipoint applications.

GEO (cont.)
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Disadvantages
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A GEO satellite’s distance also cause it to have both a comparatively weak signal and a time delay in the signal, which is bad for point to...