bent 3163 chapter 5 satellite system

20/04/2012

fauzihjabdulwahab 2 2011/2012

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UNIVERSITI TEKNIKAL MALAYSIA MELAKAUNIVERSITI TEKNIKAL MALAYSIA MELAKA

KompetensiKompetensi TerasTeras KegemilanganKegemilangan

BENT 3163TELECOMMUNICATION SYSTEM ENGINEERING

CHAPTER 5

SATELLITE SYSTEM

FAUZI HJ ABDUL WAHABFAUZI HJ ABDUL WAHAB

2_2011/2012



Objective

� Students able to:

� explain satellite system.� explain satellite system.

� analyse satellite system.

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References

• Roddy D., Satellite Communications, 4th Edition, McGraw Hill, 2006

• Tomasi W., Electronic Communication Systems: Fundamental through Advanced, • Tomasi W., Electronic Communication Systems: Fundamental through Advanced,

5th Edition, Prentice Hall, 2004.

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Chapter Outline

1. Introduction to satellites

2. Brief history

3. Advantages of satellite system, Satellite limitations

4. Satellite Communication System

5. Kepler’s Laws

6. Satellite orbits

o LEO, MEO, GEO

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o Pattern

o GEO satellite

7. Satellite link system

o Uplink, transponder, downlink

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Introduction

• From everyday’s life, satellite communication seen as only for satellite televisions.

• What not known to public, it is an essential part of communication system.• What not known to public, it is an essential part of communication system.

o carrying large amount of data and telephone traffic.

• Satellite offers features that is not readily available with other communication

system.

• Wide coverage suitable to form star point communication system.

• Remote sensing?

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• Remote sensing?

o Water pollution detection, weather monitoring, etc.



Brief History

1957 Russian artificial satellite (Sputnik)

1958 US communication satellite (Score)

1960Regular satellite communication satellite and the moon is part of the

system

1964 IntelSAT organisation formed.

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1965High quality voice transmission. Used by the US military and as

stepping stone to Initial Defence Communication Satellite Program

2001 IntelSAT became private company (with 40 investing entities)

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Advantages of Satellite System

Advantages

• Capacity

o Capable of handling thousands of communication channels.

• Reliability

o Satellite frequencies not dependant on reflection or refraction(but slightly by

atmospheric phenomena).

o Limited only by equipment reliability and the O & M personnel.

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o Limited only by equipment reliability and the O & M personnel.

• Vulnerability

o Destructive of single communication satellite is difficult & expensive (although

destruction by enemy is possible)

o How to destroy all satellite vehicles? Comparing this with capacity & reliability.



Satellite System Limitations

• Determined by technical characteristics and the orbital parameters.

i.e. power, receiver sensitivity, availabilityi.e. power, receiver sensitivity, availability

• Power

o Amount of power limited by the weight imposed on the satellite.

o Early satellite carries inefficient solar cell (~50 watts) therefore weak RF

downlink signal transmitted.

Adding high–gain antennas and amplifier may solve this but impose

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o Adding high–gain antennas and amplifier may solve this but impose

complexity to the system.

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Receiver sensitivity

o Powerful transmitters with high directional antennas used at earth stations.o Powerful transmitters with high directional antennas used at earth stations.

o Satellite antennas receive a very weak signal.

o Critical for downlink signal as satellite impose lack of power to accommodate

this requirement.

o Therefore, high–gain and high–sensitivity antennas helped to solve the

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problem.




Availability

o Satellite act as relay stations between earth stations. o Satellite act as relay stations between earth stations.

o All satellites except synchronous orbit satellites, will be in view of any given

pair of earth stations at one time.

o The length of time for a non–synchronous satellite in a circular orbit will be in

the mutual visibility depends on height at which the satellite is circulating.

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Satellite Communication System

• Uses satellites to relay radio transmission between earth stations.

• It could be an active or passive system.• It could be an active or passive system.

• How passive system works?

• How active system works?

• Active system act as repeaters.

• What repeaters do?

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• A typical link involves active satellite and 2 (or more) earth stations.

• Uplink and down link frequencies?




Let’s look on frequency allocations.

• Complicated process & require proper planning (ITU)• Complicated process & require proper planning (ITU)

• Divided into 3 regions

o Region 1 (Europe, Africa, former Soviet Union, Mongolia)

o Region 2 (North & South America, Greenland)

o Region 3 (Asia, Australia, South–West Pacific)

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• And services provided by Satellites:

Band Other BandBand Other Band

Fixed satellite service (FSS) (telephone network)Ku Band

C Band

Broadcasting satellite services (BSS) (DBS) –

Mobile satellite services (mobile, maritime, aeronautical)

VHFL Band

Navigational satellite services (GPS)

Meteorological satellite services (search & rescue) –

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• Uplink ⇒ Higher frequency

• Downlink ⇒ Lower frequency

Why?



Kepler’s Laws

• Satellite orbiting earth same as motion of planets around the sun.

• Johannes Kepler (1571 ~ 1630) derived empirically 3 laws for planetary motion.• Johannes Kepler (1571 ~ 1630) derived empirically 3 laws for planetary motion.

• In 1665, Sir Isaac Newton (1642 ~ 1727) derived Kepler’s law for law of mechanics

& theory of gravitation.

• Generally, Kepler’s law equate gravitation interaction of 2 bodies in space.

o Primary (more massive), secondary (satellite)

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Kepler’s Laws

1st Law (Law of Orbits)

“the path followed by satellite around the primary will be an eclipse”“the path followed by satellite around the primary will be an eclipse”

• Ellipse has 2 focal points, F1 & F2

• Mass centre of two–body system (barycentre) always centred on one of the foci.

• Earth mass >>> satellite ⇒ correspond with earth centre

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Kepler’s Laws

Diagram courtesy of Dennis Roddy (Satellite Communications)

• Ellipse semi–major axis, a, semi – minor axis, b

• Eccentricity, e ba22 −

=

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• Eccentricity, e

(5.1)

0 < e < 1 ⇒ elliptical orbit

E = 0 ⇒ circular orbit

a

bae

−=

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Kepler’s Laws

2nd Law (Law of Areas)

“for equal time intervals, a satellite will sweep equal areas in its orbital plane, focused “for equal time intervals, a satellite will sweep equal areas in its orbital plane, focused

at the barycentre”

• Assume satellite travels distances S1 and S2 in 1s, then area of A1 is equal to A2.

• The average velocity in each case is S1 and S2 m/s.

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Kepler’s Laws

Diagram courtesy of Dennis Roddy (Satellite Communications)

• From the law, velocity at S2 is less than S1.

o Satellite takes longer to travel a given distance when its farther away from

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o Satellite takes longer to travel a given distance when its farther away from

earth.

o So this increase length of time a satellite “seen” from certain regions of earth.

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Kepler’s Laws

• 3rd Law (Law of Periods)

“the square of the periodic time of orbit is proportional to the cube of the mean “the square of the periodic time of orbit is proportional to the cube of the mean

distance between the two bodies”

• The mean distance is equal to the semi–major axis, a.

• For artificial satellites orbiting earth,

(5.2)2

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na

µ=

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n - mean motion of satellite (rad/s)

µ - earth’s geocentric gravitation constant (3.986005 X 1014 m3/s2)

n



Kepler’s Laws

• Equation (5.2) is for ideal satellite orbiting in perfectly spherical earth of uniform

mass, no disturbing forces (e.g. atmospheric drag)

• Now, the orbital period is given by:

(5.3)

• So from the third law shows

o Fixed relationship between period and semi–major axis.

• Geostationary orbit determined by rotational period of earth.

nP

π2=

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• Geostationary orbit determined by rotational period of earth.

• Let’s determine the approximate radius of geostationary orbit.

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Kepler’s Laws

• The orbit is circular, semi–major axis is also the radius.

• Let the ideal value of radius in exercise be a constant, A (ideal satellite, right?)• Let the ideal value of radius in exercise be a constant, A (ideal satellite, right?)

• From (5.2) the Kepler’s third law mathematical represented as

(5.4)32

APa =

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A - constant (ideal satellite semi–major axis or radius)

a - semi–major axis (kilometres)

P - mean solar earth day



Kepler’s Laws

• Now sidereal time is time measured to fixed stars.

• So, one complete earth rotation relative to the fixed stars but not a complete • So, one complete earth rotation relative to the fixed stars but not a complete

rotation to the sun (earth orbiting sun?)

o Earth rotation (stars) shorter than a solar day.

• Sidereal day is one complete rotation of earth relative to the fixed stars.

o 1 sidereal day ⇒ 24 sidereal hours

1 sidereal hour ⇒ 60 sidereal minutes

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o 1 sidereal hour ⇒ 60 sidereal minutes

o 1 sidereal minute ⇒ 60 sidereal seconds

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Kepler’s Laws

• From Bate et al. (1971)

• 1 mean solar day = 1.0027379093 mean sidereal days• 1 mean solar day = 1.0027379093 mean sidereal days

= 24h 3m 56.55536s sidereal time

= 86636.55536 mean sidereal seconds

And

• 1 mean sidereal day = 0.9972695664 mean solar days

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= 23h 56m 4.09054s mean solar time

= 86,164.09054 mean solar seconds



Kepler’s Laws

• So now let P be the ratio of time of 1 sidereal day, ts to the time of one revolution

of earth on its own axis, te.stP =

e

(5.5)

• Now, geosynchronous satellites can be analysed.

o Semi–major axis

o height above sea level (earth equatorial radius ≅ 6378 km)

circumference orbit

e

s

t

tP =

24

o circumference orbit

o velocity

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Satellite Orbits

• Kepler’s Laws apply in general to satellite motion around a primary body.

• Certain terms used to describe the orbit patterns and the position of the earth • Certain terms used to describe the orbit patterns and the position of the earth

orbit (with respect to earth).

o Sub–satellite path

• Path traced out on earth’s surface directly below satellite.

o Apogee

• Point farthest from earth.

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• Point farthest from earth.

o Perigee

• Point closest approach to earth.



Satellite Orbits

o Line of apside

• Line joining the perigee and apogee through the centre of earth.

o Ascending node

• Point where the orbit crosses the equatorial plane going from south to north.

o Descending node

• Point where the orbit crosses the equatorial plane going from north to south.

o Line of nodes

• Line joining the ascending and descending nodes through the centre of the

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• Line joining the ascending and descending nodes through the centre of the

earth.

o Inclination

• Angle between the orbital plane and earth’s equatorial plane. Measured at

ascending node from equator to orbit (east to north).

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Satellite Orbits

o Prograde orbit

• Satellite (orbit) moves in the same direction as earth. Also known as direct

orbit. Inclination between 0° ~ 90°.

o Retrograde orbit

• Satellite moves opposite to earth’s rotation. Inclined between 90° ~ 180°.

o Argument of perigee

• Angle from ascending node to perigee, measured in the orbital plane at

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• Angle from ascending node to perigee, measured in the orbital plane at

earth’s centre in direction to satellite motion.

o Right ascension of the ascending node

• Specified ascending node position of the orbit in space and practically

determined by the longitude and time of crossing.



Satellite Orbits

• Apogee and Perigee heights

o Although not specified as orbital elements, apogee and perigee heights are o Although not specified as orbital elements, apogee and perigee heights are

often required.

o Length of vectors at apogee and perigee calculated as:

(5.6)

(5.7)

r - apogee height

)1( eara +=

)1( earp −=

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ra - apogee height

rp - perigee height

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Satellite Orbits

Classifications:

• Low Earth Orbit• Low Earth Orbit

o Also known as LEOSATS.

o 1.0 ~ 2.5 GHz

o Orbits 500 km ~ 1500 km above earth’s surface.

o Path loss lower

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o Lower transmit power, smaller antenna, less weight

o Visibility 15 ~ 20 minutes (require network?)



Satellite Orbits

Classifications:

• Medium Earth Orbit• Medium Earth Orbit

o 1.2 ~ 1.6 GHz

o Orbits 8000 km ~ 20000 km above earth’s surface.

o Similar functions as LEOSATs

o Higher altitude means greater communications range

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o Longer pass time (2 ~ 8 Hours)

o Larger coverage than LEOSATs.

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Satellite Orbits

Classifications:

• Geosynchronous Earth Orbit• Geosynchronous Earth Orbit

o High–altitude earth orbit satellite (HEO)

o 2 ~ 18 GHz

o Orbits ~36000 km

o Large coverage area, almost a fourth of the earth’s surface.

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o 24 hour view (ideal for satellite broadcast and other multipoint applications).

o But distance causes comparatively weak signal and time delay (500 ~ 600 ms).

o Centred above equator making broadcasting difficult for near polar regions.



Satellite Orbits

• GEO (cont’d)

o Sophisticated and require heavy propulsion devices.o Sophisticated and require heavy propulsion devices.

o High transmit powers & greater sensitivity receiver.

o High–precision operators to maintain

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Look Angle

• Required for direct and correctly pointed (earth station ≅ satellite).

• Easier for GEOSATs (since no change of direction) but hassle for LEOSATs and • Easier for GEOSATs (since no change of direction) but hassle for LEOSATs and

MEOSATs (require tracking).

o Azimuth

• Horizontal angular distance from reference direction

o Elevation

• Vertical angle between direction of EM wave travelling from earth station

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• Vertical angle between direction of EM wave travelling from earth station

towards satellite and horizontal plane.

• Minimum 5° to avoid atmospheric absorption



Orbit Perturbations

• Kepler’s Laws assumes ideal cases (uniform spherical mass and only gravitational

pull of the earth).

• Other forces should taken into account

o Gravitational forces of sun and moon (mostly affects GEOSATs).

o Atmospheric drag (affects LEOSATs below 1000 km)

o Non–spherical earth

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Satellite Link System

Considering satellite as a sophisticated telecommunication equipment, the link

system can be segregated into 3 basic sections:

• Uplink

o Earth station transmitter is the primary element.

• IF modulator, IF–to–RF microwave up–converter, high power amplifier,

output bandpass filter.

o IF modulator converts input baseband signal to IF frequency.

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o IF modulator converts input baseband signal to IF frequency.

• FM or PSK or QAM

o Up–converter converts IF to RF carrier frequency.

o HPA for gain and output power




Uplink

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Diagram courtesy of Wayne Tomasi (Electronic Communication System)

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• Downlink

o Earth station receiver.o Earth station receiver.

o Bandpass filter, Low Noise Amplifier, RF–to–IF down–converter.

o Bandpass filter normalises noise signal power.

o LNA highly sensitive low noise device.

o RF – to – IF down – converter mixer/ bandpass filter combination converts

received RF signal to IF frequency.

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received RF signal to IF frequency.




Downlink

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• Transponder (or transmission–responder)

o The Satellite (RF–to–RF repeater).o The Satellite (RF–to–RF repeater).

o Input bandpass filter, input LNA, frequency translator, low–power amplifier,

output bandpass filter.

o input bandpass filter limits total noise input signal of LNA.

o LNA output fed to frequency translator (high–band uplink frequency to low–

band downlink.

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band downlink.

o Low–level amplifier amplifies RF signal for transmission (downlink).




Transponder

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bent 3163 chapter 5 satellite system

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