Introduction to Satellite Communications

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Introduction to Satellite

Communications

ENGR. MICHAEL V. SELDA ECE,Meng-CpE

SAN SEBASTIAN COLLEGE RECOLLETOS DE CAVITE


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Agenda

History

Overview and Basic concepts of SatelliteCommunications

Spectrum Allocation

Satellite Systems Applications System Elements

System Design Considerations

Current Developments and Future Trends


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Important Milestones (before 1950)Putting the concepts together

1600 Tycho Braches experimental observations on planetary motion.

1609-1619 Keplers laws on planetary motion

1926 First liquid propellant rocket lauched by R.H. Goddard in the US.

1927 First transatlantic radio link communication

1942 First successful launch of a V-2 rocket in Germany.

1945 Arthur Clarke publishes his ideas on geostationary satellites for

worldwide communications (GEO concept).


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Rocket motors produce thrust in a process which can be explained by Newton's third law (for every action

there is an equal but opposite reaction). In the case of rocket engines, the reactionary force is produced bythe combustion of fuel in a combustion chamber. This force then acts upon the rocket nozzle, causing the

reaction which propels the vehicle. Since rocket motors are designed to operate in space, they require an

oxidizer in order for combustion to take place. This oxidizer is, in many cases, liquid oxygen. There are three

different types of rocket engines:

1. Solid propelled rockets

2. Liquid propelled rockets

3. Nuclear rockets

The advantages and disadvantages of each type are shown below.

Solid Fueled RocketsIn solid fueled rockets, the fuel and oxidizer both in solid form and thoroughly mixed during manufacture. The

section where the fuel is stored is also the combustion chamber. One end of the chamber is closed (the

payload of the rocket would be attached to this end) and the other end of the chamber is a rocket nozzle.Advantages of solid fuel rockets include simplicity and reliability, since there are no moving parts and high

propellant density, which results in a smaller sized rocket. Among the disadvantages are these: once you turn

on a solid rocket motor, you can't shut it off. You have to wait for the fuel to run out. Also, the thrust of a solid

fuel rocket decreases greatly during its burn time.

Propulsion


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V2 Rocket


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Important Milestones (1950s)Putting the pieces together

1956 - Trans-Atlantic cable opened (about 12 telephone channels

operator).

1957 First man-made satellite launched by former USSR (Sputnik,

LEO).1958 First US satellite launched (SCORE). First voice communication

established via satellite (LEO, lasted 35 days in orbit after batteries

failed).


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Sputnik - I


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Explorer - I


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Important Milestones (1960s)

First satellite communications

1960 First passive communication satellite launched into space (Large

balloons, Echo I and II).

1962: First non-government active communication satellite launchedTelstar I (MEO).

1963: First satellite launched into geostationary orbit Syncom 1

(comms. failed).

1964: International Telecomm. Satellite Organization (INTELSAT)created.

1965 First communications satellite launched into geostationary orbit

for commercial use Early Bird (re-named INTELSAT 1).


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ECHO I


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Telstar I


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Intelsat I


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Important Milestones (1970s)GEO applications development

1972 First domestic satellite system operational (Canada).

INTERSPUTNIK founded.

1975 First successful direct broadcast experiment (one year duration;USA-India).

1977 A plan for direct-to-home satellite broadcasting assigned by the

ITU in regions 1 and 3 (most of the world except the Americas).

1979 International Mobile Satellite Organization (Inmarsat) established.


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Important Milestones (1980s)GEO applications expanded

1981 First reusable launch vehicle flight.

1982 International maritime communications made operational.

1983 ITU direct broadcast plan extended to region 2.

1984 First direct-to-home broadcast system operational (Japan).

1987 Successful trials of land-mobile communications (Inmarsat).

1989-90 Global mobile communication service extended to land mobile

and aeronautical use (Inmarsat)


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Important Milestones (1990s)

1990-95:

- Several organizations propose the use of non-geostationary (NGSO)satellite systems for mobile communications.

- Continuing growth of VSATs around the world.- Spectrum allocation for non-GEO systems.- Continuing growth of direct broadcast systems. DirectTV created.

1997:

- Launch of first batch of LEO for hand-held terminals (Iridium).

- Voice service telephone-sized desktop and paging service pocket sizemobile terminals launched (Inmarsat).1998: Iridium initiates services.1999: Globalstar Initiates Service.2000: ICO initiates Service. Iridium fails and system is sold to Boeing.


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Iridium


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Overview and Basic concepts ofSatellite Communications


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Main orbit types:

LEO 500 -1000 km

GEO 36,000 km

MEO 5,00015,000 km


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USEFUL ORBITS 1:

GEOSTATIONARY ORBIT

In the equatorial plane

Orbital Period = 23 h 56 min. 4.091 s

= one Sidereal Day(definedas one complete rotation relative to the fixedstars)

Satellite appears to be stationary over a

point on the equator to an observerRadius of orbit, r, = 42,164.57 km

NOTE: Radius = orbital height + radius of the earth

Average radius of earth = 6,378.14 km


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USEFUL ORBITS 2:

Low Earth Orbit (>250 km); T 92 minutes

Polar (Low Earth) Orbit; earth rotates about23o each orbit; useful for surveillance

Sun Synchronous Orbit(example, Tiros-N/NOAA satellites used for search and rescueoperations)

8-hour and 12-hour orbits

Molniya orbit (Highly Elliptical Orbit (HEO); T 11h 38 min; highly eccentric orbit;inclination 63.4 degrees


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MOLNIYA VIEW OF THE EARTH

(Apogee remains over the northern hemisphere)


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Molniya Variants (HEOs)

Tundra Orbit Lies entirely above the Van Allen

belts.The Russian Tundra system, which employstwo satellites in two 24-hour orbits separatedby 180 deg around the Earth, with an apogee

of 53,622 km and a perigee of 17,951 km.The Molniya orbit crosses the Van Allen belts twicefor each revolution, resulting in a reduction ofsatellite life due to impact on electronics

the Russian Molniya system employs threesatellites in three 12-hour orbits separated by120 deg around the Earth, with an apogee of39,354 km and a perigee of 1000 km.


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Molniya Variants (HEOs)

The LOOPUS orbit.The LOOPUSsystem employs three satellites inthree eight-hour orbits separatedby 120 deg around the Earth, with

an apogee of 39,117 km and aperigee of 1238 km.

The ELLIPSO orbit


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A Highly Elliptical Orbit (HEO)

A satellite in HEO typically has a perigee at about 500 km above thesurface of the Earth and an apogee as high as 50,000 km. The orbitis usually inclined at 63.4 deg to provide communications services tolocations at high northern latitudes. This inclination value is selectedto avoid rotation of the apses; thus, a line from the Earth's center tothe apogee always intersects the Earth's surface at a latitude of 63.4deg North. Orbit period varies from eight to 24 hours. Owing to thehigh eccentricity of the orbit, a satellite spends about two-thirds ofthe orbital period near apogee, during which time it appears to bealmost stationary to an observer on the Earth (a phenomenon known

as `apogee dwell').


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During the brief time the satellite is below the local

horizon, a hand-off to another satellite in the sameorbit is required in order to avoid loss ofcommunications. Free space loss and propagationdelay for this type of orbit are comparable to that of

geosynchronous satellites. However, due to thecomparatively great movement of a satellite in HEOrelative to an observer on the Earth, satellite systemsusing this type of orbit must cope with large Dopplershifts.


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A Medium-Earth Orbit (MEO)By setting the altitude parameters at 10,000 km, you generated amedium-Earth orbit (MEO). This one happens to be an IntermediateCircular Orbit (ICO), since the apogee and perigee are equal. Its orbitperiod measures about seven hours. The maximum time duringwhich a satellite in MEO orbit is above the local horizon for an

observer on the Earth is a few hours. A global communicationssystem using this type of orbit requires relatively few satellites in twoto three orbital planes to achieve global coverage. MEO systemsoperate similarly to LEO systems. In MEO systems, however, hand-over is less frequent, and propagation delay and free space loss are

greater. Examples of MEO (specifically ICO) systems are Inmarsat-P(10 satellites in 2 inclined planes at 10,355 km), and Odyssey (12satellites in 3 inclined planes, also at 10,355 km).


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A Low-Earth Orbit (LEO)By selecting a relatively short period (90 minutes), we have generated

a satellite in low-Earth orbit (LEO). A typical LEO is elliptical or, moreoften, circular, with a height of less than 2000 km above the surface ofthe Earth. The orbit period at those altitudes ranges between 90minutes and two hours. The radius of the footprint of acommunications satellite in LEO ranges between 3000 and 4000 km.The maximum time during which a satellite in LEO is above the localhorizon for an observer on the Earth is 20 minutes. A global

communications system using this type of orbit requires a largenumber of satellites, in a number of different orbital planes. When asatellite serving a particular user moves below the local horizon, it musthand over its duties to a succeeding one in the same orbit or in anadjacent one. Due to the comparatively great movement of a satellitein LEO relative to an observer on the Earth, satellite systems using thistype of orbit must cope with large Doppler shifts. Satellites in LEO arealso affected by atmospheric drag that causes the orbit to graduallydeteriorate.

Examples of major LEO systems are GlobalstarTM (48+8 satellites in 8orbital planes at 1400 km) and Iridium (66+6 satellites in 6 orbitalplanes at 780 km). There are also a number of small LEO systems,such as PoSat, built by SSTL in 1993 and launched into an 822 by 800

km orbit, inclined at 98.6 deg.


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Geosynchronous & Geostationary OrbitsA geosynchronous orbit is defined as an orbit with a period of one sidereal day (1436.1 minutes). A

geostationary orbit is a special case of a geosynchronous orbit with zero inclination and zeroeccentricity, i.e., an equatorial, circular orbit. A satellite in a geostationary orbit appears fixedabove a location on the surface of the Earth. In practice, a geosynchronous orbit typically has smallnon-zero values for inclination and eccentricity, causing the satellite to trace out a small figureeight in the sky. The footprint or service area of a geosynchronous satellite covers almost one-thirdof the Earth's surface (from about 75 deg South to about 75 deg North latitude), so that near-global coverage can be achieved with as few as three satellites in orbit. A disadvantage of ageosynchronous satellite in a voice communication system is the round-trip delay of approximately250 milliseconds.

A Polar OrbitThe plane of a polar orbit is inclined at about 90 deg to the equatorial plane, intersecting the Northand South poles. The orbit is fixed in space, and the Earth rotates underneath. Thus, in principle,the coverage of a single satellite in a polar orbit encompasses the entire globe, although there are

long periods during which the satellite is out of view of a particular ground station. This gap incoverage may be acceptable for a store-and-forward communications system. Accessibility can, ofcourse, be improved through the deployment of two or more satellites in different polar orbits.Most small LEO systems employ polar or near-polar orbits. An example is the COSPAS-SARSATMaritime Search and Rescue system, which uses eight satellites in near polar orbits: four SARSATsatellites moving in 860 km orbits inclined at 99 deg (which makes them Sun-synchronous) andfour COSPAS satellites moving in 1000 km orbits inclined at 82 deg.


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A Sun-Synchronous OrbitIn a Sun-synchronous or helio-synchronous orbit, theangle between the orbital plane and Sun remainsconstant, resulting in consistent light conditions forthe satellite. This can be achieved by careful

selection of orbital altitude, eccentricity andinclination, producing a precession of the orbit (noderotation) of approximately 1 deg eastward each day,equal to the apparent motion of the Sun. Thiscondition can be achieved only for a satellite in aretrograde orbit. A satellite in Sun-synchronous orbitcrosses the equator and each latitude at the sametime each day. This type of orbit is thereforeadvantageous for an Earth observation satellite, sinceit provides constant lighting conditions.

P t D t i i O bit


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Parameters Determining Orbit

Size and ShapeParameter Definition

SemimajorAxis

Half the distance between the two points in the orbit that are farthest apart

Apogee/Perigee Radius

Measured from the center of the Earth to the points of maximum andminimum radius in the orbit

Apogee/Perigee Altitude

Measured from the "surface" of the Earth (a theoretical sphere with a radiusequal to the equatorial radius of the Earth) to the points of maximum andminimum radius in the orbit

Period The duration of one orbit, based on assumed two-body motion

Mean Motion The number of orbits per solar day (86,400 sec/24 hour), based on assumedtwo-body motion

Eccentricity The shape of the ellipse comprising the orbit, ranging between a perfectcircle (eccentricity = 0) and a parabola (eccentricity = 1)


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Satellite Location parametersTo specify the satellite's location within its orbit at epoch.

Parameter Definition

TrueAnomaly

The angle from the eccentricity vector (points toward perigee) tothe satellite position vector, measured in the direction of satellitemotion and in the orbit plane.

MeanAnomaly

The angle from the eccentricity vector to a position vector wherethe satellite would be if it were always moving at its angular rate.

EccentricAnomaly

An angle measured with an origin at the center of an ellipse fromthe direction of perigee to a point on a circumscribing circle fromwhich a line perpendicular to the semimajor axis intersects theposition of the satellite on the ellipse.

Argumentof Latitude

The sum of the True Anomaly and the Argument of Perigee.

Time PastAscendingNode

The elapsed time since the last ascending node crossing.

Time PastPerigee

The elapsed time since last perigee passage.


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Parameters determining satellite position


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GSO AND NGSO FACTORSNGSO OPTIONS:

LEO

MEO

HEO

AVOID

RADIATION

BELTS IF

POSSIBLE


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LEO, MEO and GEO Orbit Periods

0.0

5.0

10.0

15.0

20.0

25.0

30.0

0 5000 10000 15000 20000 25000 30000 35000 40000

Altitude [km]

Hours


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F1(Gravitational

Force)

v (velocity)

Why do satellites stay moving

and in orbit?

F2(Inertial-Centrifugal

Force)


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Radio Frequencies (RF)RF Frequencies: Part of the electromagnetic spectrumranging between 300 MHz and 300 GHz. Interestingproperties:

Efficient generation of signal power

Radiates into free space

Efficient reception at a different point.

Differences depending on the RF frequency used:

- Signal Bandwidth

- Propagation effects (diffraction, noise, fading)

- Antenna Sizes


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I i h F S l i


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LEO satellites need lower RF frequencies:

Omni-directional antennas on handsets have low gain- typically G = 0 db = 1

Flux density F in W/m2at the earths surface in anybeam is independent of frequency

Received power is F x A watts , where A is effectivearea of antenna in square meters

For an omni-directional antenna A = G 2/ 4 =2/ 4

At 450 MHz, A = 353 cm2, at 20 GHz, A =0.18 cm2

Difference is 33 dB - so dont use 20 GHz with an

Insights on Frequency Selection:(Part 1: Lower frequencies, stronger links)


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GEO satellites need more RF frequencies

High speed data links on GEO satellites need about 0.8

Hz of RF bandwidth per bit/sec.

A 155 Mbps data link requires 125 MHz bandwidth

Available RF bandwidth:

C band 500 MHz (All GEO slots

occupied) Ku band 750 MHz (Most GEO

slots occupied) Ka band 2000 MHz

(proliferating)

Q/V band ?

Insights on Frequency Selection:

(Part 2: Higher frequencies, higher capacity)


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Initial application of GEO Satellites:

Telephony

1965 Early Bird 34 kg 240 telephonecircuits

1968 Intelsat III 152 kg 1500 circuits

1986 Intelsat VI 1,800 kg 33,000 circuits

2000 Large GEO 3000 kg 8 - 15 kW power1,200 kg payload


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Current GEO Satellite Applications:

Broadcasting - mainly TV at presentDirecTV, PrimeStar, etc.

Point to Multi-point communicationsVSAT, Video distribution for Cable TV

Mobile ServicesMotient (former American Mobile Satellite),INMARSAT, etc.

S t llit N i ti


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GPS is a medium earth orbit (MEO) satellite system

GPS satellites broadcast pulse trains with very

accurate time signalsA receiver able to see four GPS satellites cancalculate its position within 30 m anywhere in world

24 satellites in clusters of four, 12 hour orbital period

You never need be lost againEvery automobile and cellular phone will eventuallyhave a GPS location read-out

Satellite Navigation:

GPS and GLONASS


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Space Segment

Satellite Launching Phase Transfer Orbit Phase Deployment Operation

TT&C - Tracking Telemetry and Command Station:Establishes a control and monitoring link with satellite. Tracks orbitdistortions and allows correction planning. Distortions caused byirregular gravitational forces from non-spherical Earth and due tothe influence of Sun and Moon forces.SSC - Satellite Control Center, a.k.a.: OCC - Operations Control Center

SCF - Satellite Control FacilityProvides link signal monitoring for Link Maintenance andInterference monitoring.

Retirement Phase


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Types of Satellite StabilizationSpin Stabilization

Satellite is spun about the axis on which

the moment of inertia is maximum (ex., HS376, most purchased commercialcommunications satellite; first satelliteplaced in orbit by the Space Shuttle.)

Three-Axis StabilizationBias momentum type (ex., INTELSAT V)

Zero momentum type (ex., Yuri)


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Satellite SubsystemsCommunications

Antennas

TranspondersCommon Subsystem (Bus Subsystem)

Telemetry/Command (TT&C)

Satellite Control (antenna pointing,attitude)

PropulsionElectrical Power

Structure

Thermal Control


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Ground Segment

Earth Station = Satellite Communication Station (air, ground or sea, fixed or mobile).

FSSFixed Satellite Service MSSMobile Satellite Service

Collection of facilities, users and applications.


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System Design Considerations


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Basic Principles Satellite

Uplink

Earth

Station

Downlink

TxSource

Information RxOutput

Information

Earth

Station


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Separating SignalsUp and Down:

FDD: Frequency Division Duplexing.

f1 = Uplink

f2 = Downlink

TDD: Time Division Duplexing.

t1=Up, t2=Down, t3=Up, t4=Down,.

Polarization

V & H linear polarization

RH & LH circular polarizations

Separating Signals


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Separating Signals(so that many transmitters can use the same transponder simultaneously)

Between Users or Channels (Multiple Access):FDMA: Frequency Division Multiple Access; assignseach transmitter its own carrier frequency

f1 = User 1; f2 = User 2; f3 = User 3,

TDMA: Time Division Multiple Access; eachtransmitter is given its own time slot

t1=User_1, t2=User_2, t3=User_3, t4 = User_1, ...

CDMA: Code Division Multiple Access; eachtransmitter transmits simultaneously and at the samefrequency and each transmission is modulated by itsown pseudo randomly coded bit stream

Code 1 = User 1; Code 2 = User 2; Code 3 = User 3

Di i l C i i S


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Digital Communication System

RECEIVER

RF

Channel

Output

Data

Source

Decoding

Channel

Decoder

Demodulator

Source

Data

Source

Coding

Channel

Coding

Modulator

TRANSMITTER


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C t T d i S t llit


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Bigger, heavier, GEO satellites with multiple roles

More direct broadcast TV and Radio satellitesExpansion into Ka, Q, V bands (20/30, 40/50 GHz)

Massive growth in data services fueled by Internet

Mobile services:May be broadcast services rather than point to point

Make mobile services a successful business?

Current Trends in Satellite

Communications

Th F t f S t llit


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Growth requires new frequency bands

Propagation through rain and clouds becomes a problem

as RF frequency is increased

C-band (6/4 GHz) Rain has little impact

99.99% availability is possible

Ku-band (10-12 GHz) Link margin of 3 dB needed

for 99.8% availabilityKa-band (20 - 30 GHz) Link margin of 6 dB needed

for 99.6% availability

The Future for Satellite

Communications1



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Low cost phased array antennas for mobiles areneeded

Mobile systems are limited by use of omni-directional

antennas

A self-phasing, self-steering phased array antenna with

6 dB gain can quadruple the capacity of a system

Directional antennas allow frequency re-use


Communications - 2


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END OF PRESENTATION

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Introduction to Satellite Communications