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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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