Satellite communication involves transmitting information from one location to another using an artificial satellite orbiting Earth. A communication satellite receives signals from transmitting ground stations, amplifies and processes the signals, and transmits them back to receiving ground stations on Earth. The key components of satellite communication systems are the space segment, consisting of the satellite, and the ground segment, consisting of transmitting and receiving earth stations.

1. Satellite Communication
What is Satellite Communication?
Welcome to the Video Lecture of Satellite Communication, today’s topic is Introduction to Satellite
communication. This is a wireless communication which covers very large area; we will try to see
why you need it, such a difficult type of communication, what are the advantages, disadvantages and
applications.
So first we will discuss, what is satellite?
Satellite refers to an object that revolves around earth or any other body in space.
For example moon is a natural satellite, thousands of artificial or manmade satellite orbit earth.
Now the term Communication. What is Communication?
Communication refers to exchange of information between two or more entities through any medium
or channel.
Terrestrial microwave links are not suitable to cover large geographical. The basic requirement is
earth surface is not flat and microwave communication requires line of sight, it goes straight line just
like light. So, therefore, if there are two towers, only communicate if there is a radio visibility
between them unless it will be not able to receive the signal from the transmitting tower. So, mobile
towers can cover a smaller distance only. Radio and television in earlier days used large towers and
they cover only the city.
Satellite communication is the method of transporting information from one place to another using a
communication satellite in orbit around the Earth. A communication satellite is an artificial satellite
that transmits the signal via a transponder by creating a channel between the transmitter and the
receiver located at different locations on the Earth.
A communication satellite is an orbiting artificial earth satellite that receives a communications
signal from a transmitting ground station, amplifies and possibly processes it, then transmits it
back to the earth for reception by one or more receiving ground stations.
Communications information neither originates nor terminates at the satellite itself. The satellite
is an active transmission relay, similar in function to relay towers used in terrestrial microwave
communications.
Today’s communications satellites offer extensive capabilities in applications involving data,
voice, and video, with services provided to fixed, broadcast, mobile, personal communications,
and private networks users.
Need for Satellite Communication
We know that there are different ways to communicate and the propagation of these waves can take
place in different ways. Ground wave propagation and skywave propagation are the two ways in
which communication took place for a certain distance. The maximum distance covered by them is
1500 km and this was overcome by the introduction of satellite communication.

2. Now if we really want to cover a larger and larger area, we have to increase the height of the tower
and it can be seen that a very long height can cover one third of the earth. And on top of the tower we
can put a repeater.
Now here we are using a word repeater, which receives the microwave signal and transmits it back.
So, more earth coverage is possible at a higher altitude and that higher altitude, it is roughly
calculated as 36000 kilometres from the surface of the earth.
This repeater with its radio visibility will permit two or more users, any number of users that is
available within its radio visibility, to deliver and exchange the information in various forms.
Now let us understand, who has given this particular Idea
The idea of satellite communication was conceived by Arthur C. Clarke a scientist; later he became a
novelist, he wrote a seminal paper, he did some calculation but he did not use the word satellite, he
said a microwave repeater; if it can be floated and then, 3 such satellites can cover all the earth.
Arthur C. Clarke a scientist, actually pointed out that when a satellite is in a equatorial orbit with a
radius of approximately 42,000Km and if it has an angular velocity which matched with the earth then
it always remain above the same point on the ground and that particular unit could receive and relay
signals from most of the hemisphere.
How Satellite Communications Work?
A satellite is a body that moves around another body in a particular path. A communication
satellite is nothing but a microwave repeater station in space. It is helpful in
telecommunications, radio and television along with internet applications.
A repeater is a circuit, which increases the strength of the received signal and then
transmits it. But, this repeater works as a transponder. That means, it changes the
frequency band of the transmitted signal from the received one.
The frequency with which, the signal is sent into the space is called as Uplink frequency.
Similarly, the frequency with which, the signal is sent by the transponder is called
as Downlink frequency. The following figure illustrates this concept clearly.

3. The transmission of signal from first earth station to satellite through a channel is called
as uplink. Similarly, the transmission of signal from satellite to second earth station through
a channel is called as downlink.
Uplink frequency is the frequency at which, the first earth station is communicating with
satellite. The satellite transponder converts this signal into another frequency and sends it
down to the second earth station. This frequency is called as Downlink frequency. In
similar way, second earth station can also communicate with the first one.
The process of satellite communication begins at an earth station. Here, an installation is
designed to transmit and receive signals from a satellite in an orbit around the earth. Earth
stations send the information to satellites in the form of high powered, high frequency (GHz
range) signals.
The satellites receive and retransmit the signals back to earth where they are received by
other earth stations in the coverage area of the satellite. Satellite's footprint is the area
which receives a signal of useful strength from the satellite.
The communication satellites are similar to the space mirrors that help us in bouncing the signals such
as radio, internet data, and television from one side of the earth to another The working of satellite
communications involved three stages. These are:
 Uplink

4.  Transponders
 Downlink
Let’s consider an example of signals from a television. In the first stage, the signal from the television
broadcast on the other side of the earth is first beamed up to the satellite from the ground station on
the earth. This process is known as uplink.
The second stage involves transponders such as radio receivers, amplifiers, and transmitters. These
transponders are used for boosting the incoming signal and to change their frequency so that the
outgoing signals are not altered. Depending on the incoming signal sources, the transponders vary.
The final stage is downlink in which the data is sent to the other end of the receiver on the earth. It is
very important to understand that usually there is one uplink and multiple downlink
Satellite Communications in India
It’s interesting to know that the Indian National Satellite (INSAT) system is one of the largest
domestic communication systems that is placed in the geo-stational orbit. There are more than 200
transponders in the INSAT system and are used for various purposes such as telecommunications,
weather forecasting, television broadcasting, disaster warning, search and rescue operations, and
satellite newsgathering.
Below is the list of communication satellites along with their applications:
Satellite name Launch date Application
GSAT-30 Jan 17, 2020 Communication
GSAT-31 Feb 06, 2020 Communication
GSAT-15 Nov 11, 2015 Communication and navigation
GSAT-10 Sep 29, 2012 Communication and navigation
INSAT-3A Apr 10, 2003 Communication and climate and environment

5. KALPANA-1 Sep 12, 2002 Communication and climate and environment
Two major elements of Satellite Communications Systems are
The need for satellite communication becomes evident when we want to transmit the signal to far off
places, where the Earth’s curvature comes into play. This obstruction is overcome by putting
communication satellites in space to transmit the signals across the curvature. Satellite
communication uses two types of artificial satellites to transmit the signals:
The satellite communications portion is broken down into two areas or segments: the space
segment and the ground (or earth) segment.
General architecture of Satellite Communication

6. Space Segment:
The elements of the space segment of a communications satellite system are shown in Figure. The
space segment includes the satellite (or satellites) in orbit in the system, and the ground station that
provides the operational control of the satellite(s) in orbit.
The ground station is variously referred to as the Tracking, Telemetry, Command (TT&C) or the
Tracking, Telemetry, Command and Monitoring (TTC&M) station.
The TTC&M station provides essential spacecraft management and control functions to keep the
satellite operating safely in orbit. The TTC&M links between the spacecraft and the ground are
usually separate from the user communications links.
TTC&M links may operate in the same frequency bands or in other bands. TTC&M is most often
accomplished through a separate earth terminal facility specifically designed for the complex
operations required to maintain a spacecraft in orbit.
Ground segment:
The ground segment of the communications satellite system consists of the earth surface area based
terminals that utilize the communications capabilities of the Space Segment. TTC&M ground
stations are not included in the ground segment.
The ground segment terminals consist of three basic types:
• fixed (in-place) terminals;
• transportable terminals;
• mobile terminals.
Fixed terminals are designed to access the satellite while fixed in-place on the ground. They may
be providing different types of services, but they are defined by the fact that they are not moving
while communicating with the satellite. Examples of fixed terminals are small terminals used in

7. private networks (VSATs), or terminals mounted on residence buildings used to receive broadcast
satellite signals.
Transportable terminals are designed to be movable, but once on location remain fixed during
transmissions to the satellite. Examples of the transportable terminal are satellite news gathering
(SGN) trucks, which move to locations, stop in place, and then deploy an antenna to establish links
to the satellite.
Mobile terminals are designed to communicate with the satellite while in motion. They are further
defined as land mobile, aeronautical mobile, or maritime mobile, depending on their locations on or
near the earth surface.
 Passive Satellites: .
• A satellite that only reflects signals from one Earth station to another or from several
Earth stations to several others.
• It reflects the incident electromagnetic radiation without
any modification or amplification.
• It can't generate power, they simply reflect the incident power.
• The first artificial passive satellite Echo-I of NASA was launched in August 1960.

8. If you put a hydrogen balloon that has a metallic coating over it, up in the air, it technically becomes a
passive satellite. Such a balloon can reflect microwaves signals from one place to another. The
passive satellites in space are similar. These satellites just reflect the signal back towards the Earth
without amplification. Since the satellite orbit height can range from 2000 to 35786 km, attenuation
due to the atmosphere also comes into play and due to this; the received signal is often very weak
Disadvantages:
• Earth Stations required high power to transmit signals.
• Large Earth Stations with tracking facilities were expensive.
• A global system would have required a large number of passive satellites accessed
randomly by different users.
• Control of satellites not possible from ground.
• The large attenuation of the signal while traveling the large distance between the
transmitter and the receiver via the satellite was one of the most serious problems.
 Active Satellites:
Active Satellites:
• In active satellites, it amplifies or modifies and retransmits the signal received from the
earth.
• Satellites which can transmit power are called active satellite.
• Have several advantages over the passive satellites.
• Require lower power earth station.
• Not open to random use.
• Directly controlled by operators from ground.

9. Disadvantages:
• Requirement of larger and powerful rockets to launch heavier satellites in orbit.
• Requirement of on-board power supply.
• Interruption of service due to failure of electronics components.
Active Satellites, unlike passive satellites, amplify the transmitted signals before re-transmitting it
back to Earth, ensuring excellent signal strength. Passive satellites were the earliest
communication satellite but now almost all the new ones are active satellites.
To avoid mixing up and interference of signals, every user is allocated a specific frequency for
transmitting it. The International Telecommunication Union does this frequency
allocation. Geosynchronous satellites are of note here. Geostationary orbit is present at the height of
35786 km above Earth’s surface. If you can spot such a satellite with a telescope from Earth, it will
appear stationary to you. The orbital period of the satellite and the rotational rate of the Earth is in
sync.
Satellite Communication Services
There are two categories in which the satellite communication services can be classified:
 One-way satellite communication
 Two- way satellite communication
One-way satellite communication
In one-way satellite communication, the communication usually takes place between either one or
multiple earth stations through the help of a satellite.
The communication takes place between the transmitter on the first earth satellite to the receiver
which is the second earth satellite. The transmission of the signal is unidirectional. Some common
one-way satellite communication is:
 Position location services are provided by the radio
 Tracking is a part of space operations services
 Internet services take place with broadcasting satellites

10. Following is the figure which explains the one-way satellite communication:
Two-way Satellite Communication
In two-way satellite communication, the information is exchanged between any two earth stations. It
can be said that there is a point to point connectivity.
The signal is transmitted from the first earth station to the second earth station such that there are two
uplinks and two downlinks happening between the earth stations and the satellite.

11. Following is the figure of the two-way satellite communication:
Advantages of Satellite Communication
The following are the advantages of satellite communication:
 Universal: Satellite communications are available virtually everywhere.
 Versatile: Satellites can support all of today's communications needs.
 Reliable: Satellite is a proven medium for supporting a company's communications needs.
 Seamless: Satellite's inherent strength as a broadcast medium makes it perfect.
 Fast: Since satellite networks can be set up quickly, companies can be fast-to-market with
new services.
 Flexible
 Expandable
 High Quality
 Quick Provision of Services
 Mobile and Emergency Communication
 Suitable for both Digital and Analog Transmission
 Point to multipoint communication is possible
 Area of coverage is more than that of terrestrial systems
 Circuits for the satellite can be installed easily
 Resistance of these circuits is excellent
 With the help of satellite communication ,every corner of the earth can be covered
 Satellite cost is independent of the distance
 Sending and receiving information is independent of distance

12.  More bandwidth and broadcasting possibilities
 24hrs communication can be achieved
Disadvantages of Satellite Communication
The following are the disadvantages of satellite communication:
 Initial expenditure is expensive.
 There can be congestion of frequencies
 Propagation issues and interference may arise.
 Launching of Satellites into orbit is an expensive process
 Propagation delay of satellite systems is more than that of conventional terrestrial systems
 Time delay reduces the efficiency of satellite inn data transmission etc.
 Difficult to repair if any problem occurs in a satellite system.
 Transmitter and receiver used in satellite communication requires high power and large
diameter antennas
 Due to ageing effect the efficiency of satellite components decreases
 Free space lose is more
Applications of Satellite Communication
 Telephone
 Television
 Digital cinema
 Radio broadcasting
 Amateur radio
 Internet access
 Military
 Disaster Management
RelatedPhysics Links:
Earth’s Satellites
Energy Of An Orbiting Satellite
What is GPS (Global Positioning System)
Frequently Asked Questions – FAQs
What are the two main components of satellite communication?
The two main components of satellite communication are:
 The ground segment: This comprises either fixed or mobile transmission, reception, and
ancillary equipment.

13.  The space segment: The satellite itself is known as the space segment. There are three main
units, and they are the fuel system, the satellite along with telemetry controls, and the
transponder. The prime role of the space segment is to reflect electronic signals.
Name the countries having their own satellites.
There are total of 12 countries that are having their own satellites. Few of them are listed below:
 India – Rohini D1
 Japan – Ohsumi
 China – Dong Fang Hong I
What is a space station?
A space station is an artificial structure that is designed for humans to live and work in outer space.
What are the three laws of Kepler?
Following are the three laws of Kepler:
 Kepler’s first law states that every planet revolves around the sun in an elliptical orbit and sun
is one of the foci’s.
 Kepler’s second law states that for an equal interval of time, the area covered by the satellite is
equal with respect to the center of the earth.
 Kepler’s third law states that the square of the periodic time of the orbit is proportional to the
cube of the mean distance between the two bodies.
List the factors on which the carrier to noise ratio of a satellite depends on.
Following are the three factors on which the carrier to noise ratio depends on for a satellite:
 Bandwidth
 Free space for path losses
 Effective power radiated from the isotropic
Test your Knowl

14. BASIC CONCEPTS OF SATELLITE
COMMUNICATIONS
A communication satellite is an orbiting artificial earth satellite that receives a communications
signal from a transmitting ground station, amplifies and possibly processes it, then transmits it
back to the earth for reception by one or more receiving ground stations.
Communications information neither originates nor terminates at the satellite itself. The satellite
is an active transmission relay, similar in function to relay towers used in terrestrial microwave
communications.
Today’s communications satellites offer extensive capabilities in applications involving data,
voice, and video, with services provided to fixed, broadcast, mobile, personal communications,
and private networks users.
Passive Satellites:

15.  A satellite that only reflects signals from one Earth station to another or from several
Earth stations to several others.
 It reflects the incident electromagnetic radiation without any modification or
amplification.
 It can't generate power, they simply reflect the incident power.
 The first artificial passive satellite Echo-I of NASA was launched in August 1960.
Disadvantages:
 Earth Stations required high power to transmit signals.
 Large Earth Stations with tracking facilities were expensive.
 A global system would have required a large number of passive satellites accessed
randomly by different users.
 Control of satellites not possible from ground.
 The large attenuation of the signal while traveling the large distance between the transmitter
and the receiver via the satellite was one of the most serious problems.

16. Active Satellites:
 In active satellites, it amplifies or modifies and retransmits the signal received from the
earth.
 Satellites which can transmit power are called active satellite.
 Have several advantages over the passive satellites.
 Require lower power earth station.
 Not open to random use.
 Directly controlled by operators from ground.
Disadvantages:
 Requirement of larger and powerful rockets to launch heavier satellites in orbit.
 Requirement of on-board power supply.
 Interruption of service due to failure of electronics components.
Two major elements of Satellite
Communications Systems are:
The satellite communications portion is broken down into two areas or segments: the space
segment and the ground (or earth) segment.

17. General architecture of
Satellite
Communication

18. Space Segment:
The elements of the space segment of a communications satellite system are shown in Figure. The
space segment includes the satellite (or satellites) in orbit in the system, and the ground station that
provides the operational control of the satellite(s) in orbit.
The ground station is variously referred to as the Tracking, Telemetry, Command (TT&C) or the
Tracking, Telemetry, Command and Monitoring (TTC&M) station.
The TTC&M station provides essential spacecraft management and control functions to keep the
satellite operating safely in orbit. The TTC&M links between the spacecraft and the ground are
usually separate from the user communications links.
TTC&M links may operate in the same frequency bands or in other bands. TTC&M is most often
accomplished through a separate earth terminal facility specifically designed for the complex
operations required to maintain a spacecraft in orbit.
Ground segment:
The ground segment of the communications satellite system consists of the earth surface area based
terminals that utilize the communications capabilities of the Space Segment. TTC&M ground
stations are not included in the ground segment.
The ground segment terminals consist of three basic types:
• fixed (in-place) terminals;
• transportable terminals;
• mobile terminals.
Fixed terminals are designed to access the satellite while fixed in-place on the ground. They may
be providing different types of services, but they are defined by the fact that they are not moving
while communicating with the satellite. Examples of fixed terminals are small terminals used in

19. private networks (VSATs), or terminals mounted on residence buildings used to receive broadcast
satellite signals.
Transportable terminals are designed to be movable, but once on location remain fixed during
transmissions to the satellite. Examples of the transportable terminal are satellite news gathering
(SGN) trucks, which move to locations, stop in place, and then deploy an antenna to establish links
to the satellite.
Mobile terminals are designed to communicate with the satellite while in motion. They are further
defined as land mobile, aeronautical mobile, or maritime mobile, depending on their locations on or
near the earth surface.
Satellite Control Centre function:
 Tracking of the satellite
 Receiving data
 Eclipse management of satellite
 Commanding the Satellite for station keeping.
 Determining Orbital parameters from Tracking and Ranging data
 Switching ON/OFF of different subsystems as per the operational requirements

20. Advantages Of Satellite Communication
 Universal: Satellite communications are available virtually everywhere.
 Versatile: Satellites can support all of today's communications needs.
 Reliable: Satellite is a proven medium for supporting a company's communications needs.
 Seamless: Satellite's inherent strength as a broadcast medium makes it perfect.
 Fast: Since satellite networks can be set up quickly, companies can be fast-to-market with
new services.
 Flexible
 Expandable
 High Quality
 Quick Provision of Services
 Mobile and Emergency Communication
 Suitable for both Digital and Analog Transmission
 Point to multipoint communication is possible
 Area of coverage is more than that of terrestrial systems
 Circuits for the satellite can be installed easily
 Resistance of these circuits is excellent
 With the help of satellite communication ,every corner of the earth can be covered
 Satellite cost is independent of the distance
 Sending and receiving information is independent of distance
 More bandwidth and broadcasting possibilities
 24hrs communication can be achieved
Orbit: The path a Satellite follows around a planet is defined as an orbit.
Satellite orbits in terms of the orbital height:
According to distance from earth:
 Geosynchronous Earth Orbit (GEO)
 Medium Earth Orbit (MEO)
 Low Earth Orbit (LEO)
Geostationary or geosynchronous earth orbit
(GEO)

21. GEO satellites are synchronous with respect to earth. Looking from a fixed point from Earth, these
satellites appear to be stationary. These satellites are placed in the space in such a way that only
three satellites are sufficient to provide connection throughout the surface of the Earth (that is; their
footprint is covering almost 1/3rd of the Earth). The orbit of these satellites is circular. Lifetime
expectancy of these satellites is 15 years.
There are three conditions which lead to
geostationary satellites.
1) The satellite should be placed 35,786 kms (approximated to 36,000 kms) above the surface of the
earth.
2) These satellites must travel in the rotational speed of earth, and in the direction of motion of
earth, that is eastward.
3) The inclination of satellite with respect to earth must be 00

22. Geostationary satellite in practical is termed as geosynchronous as there are multiple factors which
make these satellites shift from the ideal geostationary condition.
1) Gravitational pull of sun and moon makes these satellites deviate from their orbit. Over the
period of time, they go through a drag. (Earth’s gravitational force has no effect on these satellites
due to their distance from the surface of the Earth.)
2) These satellites experience the centrifugal force due to the rotation of Earth, making them deviate
from their orbit.
3) The non-circular shape of the earth leads to continuous adjustment of speed of satellite from the
earth station.
These satellites are used for TV and radio broadcast, weather forecast and also, these satellites are
operating as backbones for the telephone networks.
Advantages Of GEO
 Minimal Doppler shift
 These factors make it ideal for satellite broadcast and other multipoint applications
 GEO satellites have a 24 hour view of a particular area.
 A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the
earth’s surface.
Disadvantages Of GEO
 The transmit power needed is relatively high which causes problems for battery powered
devices.
 These satellites cannot be used for small mobile phones.
 The biggest problem for voice and also data communication is the high latency as without
having any handovers.
 Transferring a GEO into orbit is very expensive.
Medium Earth Orbit (MEO) satellites:
MEOs can be positioned somewhere between LEOs and GEOs, both in terms of their orbit and due
to their advantages and disadvantages.
Using orbits around 20,000 km, the system only requires a dozen satellites which is more than a
GEO system, but much less than a LEO system. These satellites move more slowly relative to the
earth’s rotation allowing a simpler system design (satellite periods are about six hours). Depending

23. on the inclination, a MEO can cover larger populations, so requiring fewer handovers.

24. Advantages Of MEO
 A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are
needed in a MEO network than a LEO network.
Disadvantages Of MEO
 A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO
satellite, though not as bad as a GEO satellite.
Low Earth Orbit (LEO) satellites:
These satellites are placed 500-1500 kms above the surface of the earth. As LEOs circulate on a
lower orbit, hence they exhibit a much shorter period that is 95 to 120 minutes.
LEO systems try to ensure a high elevation for every spot on earth to provide a high quality
communication link. Each LEO satellite will only be visible from the earth for around ten minutes.
Using advanced compression schemes, transmission rates of about 2,400 bit/s can be enough for
voice communication. LEOs even provide this bandwidth for mobile terminals with Omni-
directional antennas using low transmit power in the range of 1W.
The delay for packets delivered via a LEO is relatively low (approx 10 ms). The delay is
comparable to long-distance wired connections (about 5–10 ms). Smaller footprints of LEOs allow
for better frequency reuse, similar to the concepts used for cellular networks. LEOs can provide a
much higher elevation in Polar Regions and so better global coverage.
These satellites are mainly used in remote sensing, providing mobile communication services (due
to lower latency).
Advantages Of LEO
 A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal
strength and less of a time delay, which makes it better for point to point communication.
 A LEO satellite’s smaller area of coverage, less waste of bandwidth.
Disadvantages Of LEO
 A network of LEO satellites is needed, which can be costly.

25.  LEO satellites have to compensate for Doppler shifts cause by their relative movement.
 Atmospheric drag effects to LEO satellites, causing gradual orbital deterioration.

26. FREQUENCY ALLOCATIONS FOR
SATELLITE SERVICES
Allocation of frequencies to satellite services is a complicated process which requires international
coordination and planning. This is done as per the International Telecommunication Union (ITU).
To implement this frequency planning, the world is divided into three regions:
 Region1: Europe, Africa and Mongolia
 Region 2: North and South America and Greenland
 Region 3: Asia (excluding region 1 areas), Australia and south-west Pacific.
Within these regions, the frequency bands are allocated to various satellite services. Some of them
are listed below.
 Fixed satellite service: Provides Links for existing Telephone Networks Used for
transmitting television signals to cable companies.
 Broadcasting satellite service: Provides Direct Broadcast to homes. E.g. Live Cricket
matches etc
 Mobile satellite services: This includes services for: Land Mobile, Maritime Mobile,
Aeronautical mobile
 Navigational satellite services: Include Global Positioning systems
 Meteorological satellite services: They are often used to perform Search and Rescue
service
Below are the frequencies allocated to these satellites:
Frequency Band Designations (GHz)
VHF 0.1-0.3
UHF 0.3-1.0
L-band 1.0-2.0
S-band 2.0-4.0
C-band 4.0-8.0
X-band 8.0-12.0
Ku-band 12.0-18.0 (Ku is Under K Band)
Ka-band 18.0-27.0 (Ka is Above K Band)
V-band 40.0-75.0
W-band 75-110
Mm-band 110-300

27. μm-band 300-3000

28. Uplink and Downlink frequency:
The frequency with which, the signal is sent into the space is called as Uplink frequency
Similarly, the frequency with which, the signal is sent by the transponder is called as Downlink
frequency
The following figure illustrates this concept clearly.
The transmission of signal from first earth station to satellite through a channel is called as Uplink.
Similarly, the transmission of signal from satellite to second earth station through a channel is
called as Downlink.
Uplink frequency is the frequency at which, the first earth station is communicating with satellite.
The satellite transponder converts this signal into another frequency and sends it down to the
second earth station. This frequency is called as Downlink frequency. In similar way, second earth
station can communicate with the first one.
Latitude and Longitude:-
Latitude of a given point is defined as the angle between (the line drawn from the point on the
surface of the earth to the geocenter) and (the line between the geocenter and the equator)
The 0° latitude is at the equator, and 90° latitude is at either the north or south pole (90N, 90S).

29. The Longitude (or Meridian) of a given point is the angle between (the line connecting the
geocenter of the earth to the point where the prime meridian and equator intersect) and the meridian
containing the given point of interest intersect.
 The 0 longitude is called the prime meridian (passing by Greenwich, England).
 The designation east or west is usually added to the longitude angle (10W, 20E)
Look Angles (Elevation and Azimuth Angle)
Knowing the location of the satellite is insufficient information for most earth stations that must
communicate with the satellite. The earth station need to know the azimuth and elevation settings of
its antenna to intercept the satellite.
Elevation angle is the vertical angle formed between the direction of travel of an electromagnetic
wave radiated from an earth station antenna pointing directly toward a satellite and the horizontal
plane.
 The smaller the angle of elevation, the greater the distance a propagated wave must pass
through Earth’s atmosphere.
 As distance increases, the signal quality deteriorates.
 Generally, 5° is considered as the minimum acceptable angle of elevation.

30. Azimuth angle is defined as the horizontal pointing angle of an earth station antenna. Azimuth
angle is generally referenced north (0°) or to south (180°) in clockwise.
For geostationary orbit, the look angles values does not change as the satellites are stationary with
respect to earth.
Angle of elevation and azimuth angle both depend on the latitude of the earth station and the
longitude of both the earth station and the orbiting satellite.
The procedure for determining angle of elevation and azimuth for geostationary satellites is as
follows:
1. From a good map, determine the longitude and latitude of the earth station.
2. From Table 1, determine the longitude of the satellite of interest.
3. Calculate the difference, in degrees (ΔL), between the longitude of the satellite and the longitude
of the earth station.
4. Then from Figure 12 determine the azimuth angle, and from Figure 13 determine the elevation
angle.

31. Footprint with Elevation Angle
The elevation angle between the satellite beam and the surface of the earth has an impact on the
illuminated area (known as the footprint)
The elevation angle θ of the satellite beam governs the distance of the satellite with respect to the
MS.
The satellite beam footprint (highlighted circle with 0 dB intensity) is considered to be an isoflux
region

32. The area of Earth covered by a satellite depends on:
1. The location of the satellite in its geosynchronous orbit,
2. The carrier frequency and
3. The gain of its antennas.
The radiation pattern from a satellite antenna may be categorized as either spot, zonal or earth.
1. Earth Coverage: The radiation patterns covers approximately one-third of Earth’s surface.
2. Zonal coverage: covers an area less than one-third of Earth’s surface.
3. Spot coverage: beams concentrate the radiated power in a very small geographic area.
Orbital Control
Orbital control, often called station keeping, is the process required to maintain a satellite in its
proper orbit location. It is similar to, although not functionally the same as, attitude control.
GSO satellites will undergo forces that would cause the satellite to drift in the east-west (longitude)
and north-south (latitude) directions, as well as in altitude, if not compensated for with active
orbital control jets. Orbital control is usually maintained with the thruster system.
The non-spherical (oblate) properties of the earth, primarily exhibited as an equatorial bulge, cause
the satellite to drift slowly in longitude along the equatorial plane. Control jets are pulsed to impart
an opposite velocity component to the satellite, which causes the satellite to drift back to its
nominal position. These corrections are referred to as east-west station keeping maneuvers, which
are accomplished periodically every two to three weeks.
Latitude drift will be induced primarily by gravitational forces from the sun and the moon. These
forces cause the satellite inclination to change about 0.075◦ per month if left uncorrected. Periodic
pulsing to compensate for these forces, called north-south station keeping maneuvers, must also
be accomplished periodically to maintain the nominal satellite orbit location.
Kepler’s laws:-
Kepler formulated three laws that changed the whole satellite communication theory and
observations. These are popularly known as Kepler’s laws. These are helpful to visualize the
motion through space.
Kepler’s First Law:-
Kepler’s first law states that the path followed by a satellite around its primary (the earth) will be an
ellipse. This ellipse has two focal points (foci) F1 and F2 as shown in the figure below. Center of

33. mass of the earth will always present at one of the two foci of the ellipse.

34. If the distance from the center of the object to a point on its elliptical path is considered, then the
farthest point of an ellipse from the center is called as apogee and the shortest point of an ellipse
from the center is called as perigee.
Apogee: A point for a satellite farthest from the Earth.
Perigee: A point for a satellite closest from the Earth.
Eccentricity "e" of this system can be written as −
Where, a & b are the lengths of semi major axis and semi minor axis of the ellipse respectively.
For an elliptical path the value of eccentricity (e) is always lie in between 0 and 1, i.e.0 < e < 1,
since a is greater than b. Suppose, if the value of eccentricity (e) is zero, then the path will be no
more in elliptical shape, rather it will be converted into a circular shape.
Kepler’s Second Law:-
Kepler’s second law states that for equal intervals of time, the area covered by the satellite will be
same with respect to center of mass of the earth. This can be understood by taking a look at the
following figure.

35. Assume, the satellite covers p1 and p2 distances in the same time interval. Then, the areas B1 and
B2 covered by the satellite at those two instances are equal.
Kepler’s Third Law
Kepler’s third law states that, the square of the periodic time of an elliptical orbit is proportional to
the cube of its semi major axis length. Mathematically, it can be written as follows −
Where, is the proportionality constant.
μ is Kepler’s constant and its value is equal to 3.986005 x1014 m3/sec2
ORBITAL PERTURBATIONS
 Theoretically, an orbit described by Kepler is ideal as Earth is considered to be a perfect
sphere and the force acting around the Earth is the centrifugal force. This force is supposed
to balance the gravitational pull of the earth.
 In reality, other forces also play an important role and affect the motion of the satellite.
These forces are the gravitational forces of Sun and Moon along with the atmospheric drag.
 Effect of Sun and Moon is more pronounced on geostationary earth satellites where as the
atmospheric drag effect is more pronounced for low earth orbit satellites.
 As the shape of Earth is not a perfect sphere, it causes some variations in the path followed
by the satellites around the primary. As the Earth is bulging from the equatorial belt, and
keeping in mind that an orbit is not a physical entity, and it is the forces resulting from an
oblate Earth which act on the satellite produce a change in the orbital parameters.
 This causes the satellite to drift as a result of regression of the nodes and the latitude of the
point of perigee (point closest to the Earth). This leads to rotation of the line of apsides. As
the orbit itself is moving with respect to the Earth, the resultant changes are seen in the
values of argument of perigee and right ascension of ascending node.

36.  Due to the non-spherical shape of Earth, one more effect called as the “Satellite
Graveyard” is seen. The non-spherical shape leads to the small value of
eccentricity at the equatorial plane. This causes a gravity gradient on GEO
satellite and makes them drift to one of the two stable points which coincide with
minor axis of the equatorial ellipse.
 Working satellites are made to drift back to their position but out-of-service
satellites are eventually drifted to these points, and making that point a Satellite
Graveyard.
Atmospheric Drag
For Low Earth orbiting satellites, the effect of atmospheric drag is more pronounces. The
impact of this drag is maximum at the point of perigee. Drag (pull towards the Earth) has
an effect on velocity of Satellite (velocity reduces).
This causes the satellite to not reach the apogee height successive revolutions. This leads
to a change in value of semi-major axis and eccentricity. Satellites in service are
maneuvered by the earth station back to their original orbital position.