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8/10/2019 Satellite Communication 140115005049 Phpapp01

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Working of satellite


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Satellite

The word satellite originated from the Latin word

Satellit- meaning an attendant, one who is constantly

covering around & attending to a master or big man

Earth Stations antenna systems on earth

Uplink transmission from an earth station to a satellite

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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Band Uplink (GHz) Downlink (GHz)

C 6 4

Ku 14 12

Ka 30 20

X 8.2 7.5

S 40 20

Q 44 21

L 1.525 to 1.559 1.626 to 1.660

Frequency Allocation for Satellite Communication

First satellite launched into space: Sputnik (1957).


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Frequency Bands Available for

Satellite Communications


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APPLICATIONS

1. Global Telecommunications : Land, Sea, Air

2. Broadcasting : Sound, TV, Multimedia, Cable TV, DTH, DTU, DBS, DVB

3. Navigation : Global Positioning System (GPS)

4. Remote Sensing Earth Observation

5. Weather

6. Emergency Communication Services Disasters

7. Mobile Communication Services

8. Corporate Communications

Virtual Private Network (VPN)- VSAT Technology

9. Military Communications


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Applications


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Communicationprocess


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Satellite Orbits

Polar Orbit

Low Earth Equatorial

Orbit

Geostationary Orbit

Molniya Orbit


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equator

celestial

sphere

south celestial pole

nor th celestial pole

south pole

north pole

equator

Major Axis


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Motion of Space Objects


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Three classes of Satellite orbits

1) Low Earth Orbit (LEO)

up to 2,000km altitude

remote sensing satellites, altimeter satellites, other

2) Medium Earth Orbit (MEO)

altitudes between 5,000km 20,000km

GPS satellites (12hr period twice a day)

3) Geostationary Earth Orbit (GEO)24hr period appears fixed

altitudes of 36,000km

communication satellites

Satellites orbits vary depending on

1) altitude 2) inclination 3) orbital period

An orbit is the path that a satellite follows as it revolves around Earth


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Satellite orbit altitudes


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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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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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Orbital location and footprint

The location of a geostationary satellite is referred toas its orbital location. International satellites are normallymeasured in terms of longitudinal degrees East ( E) from the

Prime Meridian of 0

The geographic area of the Earth's surface overwhich a satellite can transmit to, or receive from, is

called the satellite's "footprint."


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Motion of

Space Objects

1473 -1543 Copernicus

Heliocentric (sun in the center) Orbit

15461601 Tycho Brahe

Before telescope followed the planets (acquired quality data)

15711630 Johannes Kepler

Discovered orbital path to be elliptical around focus point

Keplers 3 laws of planetary motion

16421727 Sir Isaac Newton

Physical PrincipalsUniversal law of Gravitation


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Keplers 3 (empirical) laws of

Planetary Motion

First Law (elliptical orbit)

The orbital path of a planet takes the shape of an ellipse,

with the Sun located at one of its focal points.


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Keplers Second Law

The line from the sun to a planet sweeps out equal areas in

equal time intervals.

t1

t2

areaA

t3

t4

areaBareaA = areaB if t2-t1= t4-t3

perihelion

(fastest)

aphelion

(slowest)


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Keplers Third Law

The ratio of the square of the planets orbital

period and the cube of the mean distance from

the Sun is constant

(D1/D2)3= (P1/P2)2

Planets far from the sun take longer to orbit the

sun than do planets nearer the sun.


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Geometry of an Ellipse

Semi-major axis of the satellite orbit

Eccentricity of the satellite orbit (deviation from a circle)

A satellite is closest to the earth at a point called Perigee

A satellites farthest point from the earth is called apogee

GPS orbital period of 12 hours based on Keplers third lawcorresponds to a satellite altitude of about 20,000km above thesurface of the earth


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Inclination:

Angle between the orbital plane

and earths equatorial plane

Prograde orbit: Satellite moves in the same direction as the

earths rotation ( 0 to 90)

Retrograde orbit:Satellite moves in the counter direction to the

earths rotation

Mean anomaly: Average value of the angular position of the

satellite with reference to the perigee

True anomaly:Angle from perigee to the satellite position

measured at the earths center.


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S f O bit l


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Sources of Orbital

Perturbations

Several external forces cause perturbation

to spacecraft orbit

3rd body effects, e.g., sun, moon, otherplanets

Unsymmetrical central bodies

Space Environment: Solar Pressure, drag

from rarefied atmosphere


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Orbit perturbations

1) Keplers three laws of planetary motion

Apply to any orbiting object (Satellites)

2) GPS Satellites orbit the earth in an

elliptical path

3) Earth becomes the focal points


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Mean motion of the orbital

period is called as anomalisticperiod

P=2 /n sec

perturbation which must beaccounted for. Main effects:

Regression of nodes

Rotation of apsides

Orbital Perturbation Effects:


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Orbital Perturbation Effects:

Regression of Nodes

Regression of Nodes: Equatorial bulge causes component of gravityvector acting on SC to be slightly out of orbit plane

This out of orbit plane component

causes a slight precession of theorbit plane.

The resulting orbital rotation is called regression of node.

Note: Although regression rate is small for Geo., it must be accounted.

Orbital Perturbation:


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Orbital Perturbation:

Rotation of Apsides

Rotation of apsides caused by earth

oblateness is similar to regression of

nodes. The phenomenon is caused by

a higher acceleration near the equator

and a resulting overshoot at periapsis.

This only occurs in elliptical orbits.


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Satellites in geosynchronous orbit


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Uses of Geostationary Orbits

Geostationary orbits are primarily used for two functions: Weather monitoring

Telecommunications & Broadcasting

Commercial growth is focused on:

DTH TV (Direct To Home: Sky TV) Phone, Fax, Video, Data services

Mobile Communications

VSAT & USAT

Digital Radio

Three conditions are required for an


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qorbit to be geostationary:

The satellite must travel eastward atthe same rotational speed as theearth

The orbit must be circular

The inclination of the orbit must bezero


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How To File for a Geo Position

Only Allocated to National Governments

Go to National Government

Request Orbital Position (s)

US Companies (Non Governmental Entities)

work through FCC

UK through UK Radiocommunications Agency

Prepare ITU Paperwork

File & Coordinate

First Come, First Served = Priority!


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To determine the look angles for thegeostationary orbit we need

The earth station latitude

The earth station longitude

The longitude of the sub satellite

point

The average of radius R=6378 km

polar antenna

A single actuator is used which moves the antenna in a

circular arc ie known as polar mount antenna.


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Declination

The angle of tilt is often referred to as the declination which

must not be confused with the magnetic declination used in

correcting compass readings.

Limits of Visibility:

The limits will be set by the geographic coordinates of the

earth station and the antenna elevation.


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