Satellite 2

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Satellite Communications


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Overview

Basics of Satellites

Types of Satellites

Capacity Allocation


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Satellite-Related Terms

Earth Stations antenna systems on earth

Uplink transmission from an earth station to asatellite

Downlink transmission from a satellite to an

earth station

Transponder electronics in the satellite that

convert uplink signals to downlink signals


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Ways to Categorize

Communications Satellites

Coverage area

Global, regional, national Service type

Fixed service satellite (FSS)

Broadcast service satellite (BSS)

Mobile service satellite (MSS) General usage

Commercial, military, amateur, experimental


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Classification of Satellite Orbits

Circular or elliptical orbit

Circular with center at earths center Elliptical with one foci at earths center

Orbit around earth in different planes

Equatorial orbit above earths equator

Polar orbit passes over both poles Other orbits referred to as inclined orbits

Altitude of satellites

Geostationary orbit (GEO)

Medium earth orbit (MEO)


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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

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 stations 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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16.8

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Orbits

FootprintThree Categories of Satellites

GEO Satellites

MEO Satellites

LEO Satellites

Topics discussed in this section:Topics discussed in this section:


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Classification of Satellite Orbits

Circular or elliptical orbit

Circular with center at earths center Elliptical with one foci at earths center

Orbit around earth in different planes

Equatorial orbit above earths equator

Polar orbit passes over both poles Other orbits referred to as inclined orbits

Altitude of satellites

Geostationary orbit (GEO)

Medium earth orbit (MEO)


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16.10

Figure 16.13 Satellite orbits


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Basics: How do Satellites Work

Two Stations on Earth want to communicate throughradio broadcast but are too far away to useconventional means.

The two stations can use a satellite as a relay stationfor their communication

One Earth Station sends a transmission to the

satellite. This is called a Uplink. The satellite Transponderconverts the signal and

sends it down to the second earth station. This iscalled a Downlink.


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Basics: Advantages of Satellites

The advantages of satellite communication

over terrestrial communication are: The coverage area of a satellite greatly exceeds

that of a terrestrial system.

Transmission cost of a satellite is independent ofthe distance from the center of the coverage area.

Satellite to Satellite communication is veryprecise.

Higher Bandwidths are available for use.


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Basics: Disadvantages of

Satellites

The disadvantages of satellite

communication: Launching satellites into orbit is costly.

Satellite bandwidth is gradually becoming used

up.

There is a larger propagation delay in satellitecommunication than in terrestrial communication.


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Basics: Factors in satellite

communication

Elevation Angle: The angle of the horizontal of the

earth surface to the center line of the satellitetransmission beam.

This effects the satellites coverage area. Ideally, you want

a elevation angle of0 degrees, so the transmission beam

reaches the horizon visible to the satellite in all directions.

However, because of environmental factors like objectsblocking the transmission, atmospheric attenuation, and the

earth electrical background noise, there is a minimum

elevation angle of earth stations.


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communication (cont.)

Coverage Angle: A measure of the portion of

the earth surface visible to a satellite takingthe minimum elevation angle into account.

R/(R+h) = sin(/2 - - )/sin( +/2)

= cos( + )/cos()

R = 6370 km (earths radius)h = satellite orbit height

= coverage angle

= minimum elevation angle


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communication (cont.)

Other impairments to satellite communication:

The distance between an earth station and a satellite (freespace loss).

Satellite Footprint: The satellite transmissions 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.


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Basics: How Satellites are used

Service Types

Fixed Service Satellites (FSS) Example: Point to Point Communication

Broadcast Service Satellites (BSS)

Example: Satellite Television/Radio

Also called Direct Broadcast Service (DBS). Mobile Service Satellites (MSS)

Example: Satellite Phones


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Satellite Network Configurations


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Figure 16.14 Satellite categories


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Types of Satellites

Satellite Orbits

GEO LEO

MEO

Molniya Orbit

HAPs

Frequency Bands


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GeostationaryEarth Orbit (GEO)

These satellites are in orbit 35,863 km above

the earths 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 ofearth.


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GEO (cont.)

Advantages

A GEO satellites distance from earth gives it alarge coverage area, almost a fourth of the earths

surface.

GEO satellites have a 24 hour view of a particular

area.

These factors make it ideal for satellite broadcast

and other multipoint applications.


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GEO (cont.)

Disadvantages

A GEO satellites distance also cause it to haveboth a comparatively weak signal and a time

delay in the signal, which is bad for point to point

communication.

GEO satellites, centered above the equator, have

difficulty broadcasting signals to near polar

regions


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Low Earth Orbit (LEO)

LEO satellites are much closer to the earth

than GEO satellites, ranging from 500 to1,500 km above the surface.

LEO satellites dont stay in fixed position

relative to the surface, and are only visible

for15 to 20 minutes each pass. A network of LEO satellites is necessary for

LEO satellites to be useful


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LEO (cont.)

Advantages

A LEO satellites proximity to earth compared to aGEO satellite gives it a better signal strength and

less of a time delay, which makes it better for

point to point communication.

A LEO satellites smaller area of coverage is less

of a waste of bandwidth.


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LEO (cont.)

Disadvantages

A network of LEO satellites is needed, which canbe costly

LEO satellites have to compensate for Doppler

shifts cause by their relative movement.

Atmospheric drag effects LEO satellites, causinggradual orbital deterioration.


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Medium Earth Orbit (MEO)

A MEO satellite is in orbit somewhere between 8,000

km and 18,000 km above the earths surface.

MEO satellites are similar to LEO satellites in

functionality.

MEO satellites are visible for much longer periods of

time than LEO satellites, usually between 2 to 8

hours.

MEO satellites have a larger coverage area than

LEO satellites.


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MEO (cont.)

Advantage

A MEO satellites longer duration of visibility andwider footprint means fewer satellites are needed

in a MEO network than a LEO network.

Disadvantage

A MEO satellites distance gives it a longer timedelay and weaker signal than a LEO satellite,

though not as bad as a GEO satellite.


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OtherOrbits

Molniya Orbit Satellites

Used by Russia for decades. Molniya Orbit is an elliptical orbit. The satellite

remains in a nearly fixed position relative to earth

for eight hours.

A series of three Molniya satellites can act like aGEO satellite.

Useful in near polar regions.


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OtherOrbits (cont.)

High Altitude Platform (HAP)

One of the newest ideas in satellitecommunication.

A blimp or plane around 20 km above the earthssurface is used as a satellite.

HAPs would have very small coverage area, but

would have a comparatively strong signal. Cheaper to put in position, but would require a lot

of them in a network.


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16.31

Figure 16.15 Satellite orbit altitudes


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Figure 16.16 Satellites in geostationary orbit


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Figure 16.17 Orbits for global positioning system (GPS) satellites


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Figure 16.19 LEO satellite system

UML: user mobile link

GWL: gateway link

ISL: intersatellite link


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16.35

Figure 16.20 Iridium constellation


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The Iridium system has 66 satellites in sixLEO orbits, each at an

altitude of750 km.

Note


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Iridium is designed to provide direct

worldwide voice and data communicationusing

handheld terminals, a service similar to

cellular telephony but on a global scale.

Note


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Figure 16.20 Teledesic


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16.39

Teledesic has 288 satellites in 12 LEO

orbits, each at an altitude of1350 km.

Note


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FrequencyBands

Different kinds of satellites use different frequency

bands. LBand: 1 to 2 GHz, used by MSS

S-Band: 2 to 4 GHz, used by MSS, NASA, deep space research

C-Band: 4 to 8 GHz, used by FSS

X-Band: 8 to 12.5 GHz, used by FSS and in terrestrial imaging, ex:

military and meteorological satellites

Ku-Band: 12.5 to 18 GHz: used by FSS and BSS (DBS)

K-Band: 18 to 26.5 GHz: used by FSS and BSS

Ka-Band: 26.5 to 40 GHz: used by FSS


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Capacity Allocation

FDMA

FAMA-FDMA DAMA-FDMA

TDMA

Advantages over FDMA


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FDMA

Satellite frequency is already broken into

bands, and is broken in to smaller channelsin Frequency Division Multiple Access

(FDMA).

Overall bandwidth within a frequency band is

increased due to frequency reuse (afrequency is used by two carriers with

orthogonal polarization).


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FDMA (cont.)

The number of sub-channels is limited by

three factors: Thermal noise (too weak a signal will be effected

by background noise).

Intermodulation noise (too strong a signal will

cause noise). Crosstalk (cause by excessive frequency

reusing).


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FDMA (cont.)

FDMA can be performed in two ways:

Fixed-assignment multiple access (FAMA):The sub-channel assignments are of a fixed

allotment. Ideal for broadcast satellite

communication.

Demand-assignment multiple access (DAMA):

The sub-channel allotment changes based on

demand. Ideal for point to point communication.


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TDMA

TDMA (Time Division Multiple Access)

breaks a transmission into multiple time slots,each one dedicated to a different transmitter.

TDMA is increasingly becoming more

widespread in satellite communication.

TDMA uses the same techniques (FAMA andDAMA) as FDMA does.


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TDMA (cont.)

Advantages of TDMA over FDMA.

Digital equipment used in time divisionmultiplexing is increasingly becoming cheaper.

There are advantages in digital transmission

techniques. Ex: error correction.

Lack of intermodulation noise means increasedefficiency.

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