Satellite Communication

Elective -IV
Dr. Mahadev S. Patil
Professor and Head
Kasegaon Education Society’s
RAJARAMBAPU INSTITUTE OF TECHNOLOGY,
Islampur, Dist. Sangli, Maharashtra, India - 415 414

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Look angle determination

Topic Learning Outcomes:

At the end of this lecture students will be able to

1. define look angles
2. describe azimuth and elevation angles

2
The definition of elevation (EI) and azimuth (Az).
The elevation angle is measured upward from the local
horizontal at the earth station and the azimuth angle is
measured from the true north in an eastward direction to the
projection of the satellite path onto the local horizontal plane.

NOTE: This is
True North
(not magnetic,
from compass)
Zenith and Nadir pointing directions
 Subsatellite point

The subsatellite point is the location on the surface of

the earth that lies directly between the satellite and
the center of the earth

The line joining the satellite and the center of the

earth, C, passes through the surface of the earth and
point Sub, the subsatellite point.
 Elevation angle calculations

Elevation Angle
Calculations

El =  - 90o
 = central angle
rs = radius to the satellite
re = radius of the earth

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 Ei

The elevation angle Ei is measured upward from the local

horizontal at the earth station and is mathematically given
by

sin  
cos (El)  1/ 2
 r  2
 re  
1     2  cos 
e

  rs   rs  

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Numerical

A satellite is in an elliptical orbit with a perigee

of 1000km and an apogee of 4000km. Using
a mean earth radius of 6378.14km find the
period of the orbit in hours, minutes and
seconds.

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 hp ha S
Perigee 2re Apogee
Earth
Orbital Elements

Topic Learning
Outcomes:

At the end of this lecture

students will be able to
1. list out orbital
elements
2. describe usefulness
of orbital elements

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Orbital elements

• A set of mathematical parameters that enables

us to accurately describe satellite motion

Purpose
• Discriminate one satellite from other satellites
• Predict where a satellite will be in the future or
has been in the past
• Determine amount and direction of maneuver
or perturbation
Orbital elements

Also called as the Six Keplerian Elements

• a Semi-major axis of the orbit ellipse -size of orbit

• e Eccentricity of the elliptical orbit -shape of orbit
• i Inclination of the orbit - position of satellite
•  Right Ascension of the Ascending Node - position
•  Argument of Perigee - position
• tp Time of Perigee - location of satellite

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Size/period (a)

• Size is how big or small your satellite’s orbit is….

defined by semi-major axis
• There are basically 4 sizes of orbits satellites use:
– Low Earth Orbit (LEO): approx 120 – 1200 miles above Earth
– Medium Earth Orbit (MEO) or Semi-synchronous Orbit:
approx 12,000 miles above Earth
– Highly Elliptical Orbit (HEO): altitude varies greatly! From
100 miles to sometimes several hundred thousand miles
– Geo-synchronous or Geo-stationary Orbit
(GEO): approx 22,300 miles from Earth
Shape (e)

Orbit shapes are either circular or not circular:

some sort of an Ellipse!!
How elliptical an orbit, is called Eccentricity
Inclination (i)

• Inclination is the tilt of your orbit

• At 0 degrees of inclination, you are orbiting the equator
• At 90 degrees of inclination, you are in a polar orbit

Equatorial Plane
Inclination:
Is this angle,
measured in
degrees

Inclination

Orbital Plane
Right Ascension ( )
• Right Ascension will determine where your satellite will
cross the Equator on the ascending pass
• It is measured in degrees
• Right Ascension is the swivel (spin) of your tilt, as measured
from a fixed point in space, called the First Point of Aries

Inclination

Right Ascension
is this angle,
measured in
degrees
Argument of Perigee ()

• Argument of Perigee is a measurement from a fixed point

in space to where perigee occurs in the orbit
• It is measured in degrees

Perigee

Argument of
Perigee: Is
this angle,
Inclination measured in

degrees 
Apogee 
True Anomaly (tp)

• True Anomaly is a measurement from a fixed point in space to

the actual satellite location in the orbit
• It is measured in degrees
True Anomaly:
Is this angle,
measured in

Direction of satellite
degrees tp
motion

Fixed point in
space
Orbital Perturbations

Topic Learning
Outcomes:

At the end of this

lecture students will be
able to
1. explain effects of
orbital perturbations on
satellite

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Perturbations

Definition
• A disturbance in the regular motion of a celestial
body
• External influences, interfering forces
• Orbital elements –vary with time
• Therefore orbit changes from true Keplerian orbit
• Satellite does not return to the same point in
space/revolution at perigee
Longitudinal changes (in plane changes)

• Earth is neither a perfect sphere nor a perfect ellipse

• Flattened at the poles
• Equatorial diameter is about 20 km more than the
polar diameter
• Equatorial radius is not constant
• Nonregular features of the earth (regions where
average density of earth is higher – mass
concentrations)- mascons
Therefore, nonuniform gravitational field exists and
hence force on orbiting satellite will vary. Therefore
orbital inclination occurs.
Gravitational Perturbations
• Earth’s asymmetrical mass causes a non-
central gravitational pull
• Ellipticity of the Earth causes gravity wells
and hills
• Stable points: 75E and 105W
-- Himalayas and Rocky Mountains
• Unstable points: 165E and 5W
-- Marshall Islands and Portugal
Drives the requirement for station keeping
Atmosphere drag

• Friction caused by impact of satellite with

particles in the Earth’s atmosphere
• Reduces satellite’s energy
• Changes the size (semi-major axis) and
shape (eccentricity)
• More impact on LEO satellite
Atmospheric Drag

Perigee remains same, Apogee decreases

Third body effects

• Gravitational pull of other massive bodies,

i.e. Sun, moon
• Mainly noticeable in deep space orbits
Solar wind/Radiation pressure

• Solar wind causes radiation pressure on the

satellite
• Effects similar to atmospheric drag
• Effects are more pronounced on satellites
with large surface areas
Electro-magnetic perturbations

• Interaction between the Earth’s magnetic

field and the satellite’s electro-magnetic
field results in magnetic drag
Inclination changes (out of plane changes)
Box (requirement for station keeping)

The net effect of acceleration forces induced by

the moon and the sun on a GEO satellite is to
change the plane of the orbit at an initial average
rate of change of 0.85 degrees per year from the
equatorial plane

This means that for GEOs inclination ellipticity

and longitudinal position are controlled so that the
satellite appears to stay within a “box” in the sky
that is bounded by ±0.05 degrees in latitude and
longitude over the subsatellite point.
Mechanics of launching a satellite
Topic Learning
Outcomes:

At the end of this lecture

students will be able to

1. list steps involved in

Launching

2. describe various ways

to put a satellite in
orbit

30
Launches and Launch vehicles
• Velocity vector and the Orbital height are
simultaneously correct.

• GEO satellite - 36000 km above the surface of the

earth with a velocity of 3074.7m/s tangential to the
earth in the plane of the orbit

• In any earth satellite launch, the largest fraction of

the energy expended by the rocket is used to
accelerate the vehicle from rest until it is about 20
miles above the earth.
Launch Vehicles

• Staging

• Expended Vehicle (ELV)

•
• Reusable Launch Vehicle (RLV)
Expendable Launch Vehicles
Launching and staging
Reusable Launch Vehicles
Launch vehicle selection factors
The decision on which particular rocket to use in a given
situation will depend on a variety of factors
Types of Launches
• Some of the launch vehicles deliver the spacecraft
directly to geostationary orbit (called a direct
insertion launch) while others inject the spacecraft
into a geostationary transfer orbit (GTO).
• The spacecraft launched into GTO must carry
additional rocket motors and/or propellant to
enable the vehicle to reach the geostationary orbit.
Placing satellite into GEO
Placing Satellite Into Geostationary Orbit
Geostationary transfer orbit and AKM
Slow orbit raising to GEO
Illustration of slow orbit raising to geostationary orbit
India’s GSAT 30 launch on 17th Jan 2020

GSAT-30 satellite was launched aboard Ariane-5 launch vehicle (VA251) from French Guiana on 02:35
IST, 17 January 2020. After three orbit raising burns with cumulative duration of 2 hours 29
minutes, GSAT-30 acquired station at 81°E on 25 January 2020. The satellite will act as a replacement
for the defunct INSAT-4A The satellite will provide advanced telecommunication services to the Indian
subcontinent. It will be used for VSAT networks, television uplinks, digital satellite news
gathering, DTH services and other communication systems. This is the 41st communication satellite
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launched by ISRO
Some Launch Vehicles
Launch Vehicles
Launch Vehicles
Launch Vehicles
Launch Vehicles
Solar Eclipse

• A satellite is said to be in eclipse when the satellite is in the

shadow of the earth
• As the satellite moves around the geostationary orbit, it will pass
through the shadow and undergo an eclipse period.
• Ground controllers perform battery conditioning routines prior to
ecllipse operations
• The batteries are deliberately discharged close to their max.
depth of discharge, and then fully recharged just before eclipse
season begins.
• The eclipse season is a design challenge for spacecraft builders.
Illustration
Sun transit outage
The sun is a hot microwave source (hot noise source) with an
equivalent temperature of about 6,000K to 10,000K depending on
the time at the frequencies used by communications satellites (4 to
50 GHz).

The earth station antenna will therefore receive not only the signals
from the satellite but also the noise temperature transmitted by the
sun.

The added noise temperature will cause the fade margin of the
receiver to be exceeded and an outage will occur.

These outages must be precisely predicted and traffic can be off-

loaded when the satellite is about to enter sun outages.
Schematic of sun outage conditions
Doppler effect

Orbital effects in communications systems performance

• To a stationary observer, the frequency of a

moving radio transmitter varies with the
transmitter’s velocity relative to the observer.
• If the true transmitter frequency is fT and the
received frequency is fR then
• fR is higher than fT when the transmitter is moving
towards the receiver and lower than fT when the
transmitter is moving away from the receiver.
Thank you