DIGITAL SATELLITE COMMUNICATIONS
Prepared by :
Nisreen Bashar AL-Madanat
Under supervision of :
Principle of Satellite Communication
Basics of Satellites
Types of Satellite
Sources of Impairment
Satellite Network Configurations
Types of satellite categories
Application of satellite communication
Satellite Applications Overview
After completing this lecture the student will be able to :
Discuss the concept of digital satellite communications and its
Describe how does a satellite communication works.
Build the main block diagram of the earth station and
Discuss the advantages and disadvantage of satellite system.
Identify satellites main factors , parameters and impairments.
Identify the basic types of satellites .
Got awareness of the frequency bands and their applications .
Distinguish the satellite network configurations .
Discuss the capacity allocation .
Identify satellite categories and orbits.
Mention its applications and how satellites are used .
The use of satellite in communication systems has become very
common now - a- days. This is because the satellite can “see” a
very large area of the earth.
Hence satellites can form a star point of a communication net, to
link many users together, simultaneously. This will include users
widely separated geographically.
A communication satellite is a station in space that is used for
telecommunication, radio and television signals.
The construction and launch cost of a satellite are extremely high.
These costs are “distance insensitive”, that means the cost of a
short distance satellite link is approximately same as that of a
long distance link.
Therefore a satellite communication system is economical only
where the system is used continuously and a large number of
users use it.
PRINCIPLE OF SATELLITE COMMUNICATION
A geostationary communication satellite is basically a
relay station in space.
It receives signal from one earth station, amplifies it,
improves the signal quality and radiate the signal back to
other earth stations.
Such a relay system allows us to communicate with any
corner of the world.
Satellite System: The use of orbiting satellites to relay
transmissions from one satellite dish to another or multiple
BASICS :HOW DOES A SATELLITE
Two Stations on Earth want to communicate through radio
broadcast but are too far away to use conventional means.
An Earth Station sends message in GHz range. (Uplink)
Satellite Receive and retransmit signals back (Downlink)
Other Earth Stations receive message in useful strength
Main Earth Station is in Delhi, Kolkata, Chennai, Mumbai
Functioning of Transponder
TO /FROM EARTH STATION
EARTH STATION BLOCK DIAGRAM
BASICS: ADVANTAGES OF SATELLITES
The advantages of satellite communication
over terrestrial communication are:
The coverage area of a satellite greatly exceeds
that of a terrestrial system.
Transmission cost of a satellite is independent of
the distance from the center of the coverage area.
Satellite to Satellite communication is very
Higher Bandwidths are available for use.
BASICS: DISADVANTAGES OF SATELLITES
The disadvantages of satellite communication:
Launching satellites into orbit is costly.
Satellite bandwidth is gradually becoming used up.
There is a larger propagation delay in satellite
communication than in terrestrial communication.
BASICS: FACTORS IN SATELLITE
Elevation Angle: The angle of the horizontal of the earth
surface to the center line of the satellite transmission
This effects the satellites coverage area. Ideally, you want a
elevation angle of 0 degrees, so the transmission beam
reaches the horizon visible to the satellite in all directions.
However, because of environmental factors like objects
blocking the transmission, atmospheric attenuation, and
the earth electrical background noise, there is a minimum
elevation angle of earth stations.
BASICS: FACTORS IN SATELLITE
Coverage Angle: A measure of the portion of the earth
surface visible to a satellite taking the minimum elevation
angle into account.
R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2)
= cos(β + θ)/cos(θ)
R = 6370 km (earth’s radius)
h = satellite orbit height
β = coverage angle
θ = minimum elevation angle
BASICS: FACTORS IN SATELLITE
Other impairments to satellite communication:
The distance between an earth station and a satellite (free
Satellite Footprint: The satellite transmission’s strength is
strongest in the center of the transmission, and decreases
farther from the center as free space loss increases.
Atmospheric Attenuation caused by air and water can impair
the transmission. It is particularly bad during rain and fog.
BASICS: HOW SATELLITES ARE USED
Fixed Service Satellites (FSS)
•Example: Point to Point Communication
Broadcast Service Satellites (BSS)
•Example: Satellite Television/Radio
•Also called Direct Broadcast Service (DBS).
Mobile Service Satellites (MSS)
•Example: Satellite Phones
TYPES OF SATELLITES
In space, satellites move in
certain specific paths. These
paths are called as orbits.
A result stays in an orbit
because the two forces acting on
it namely the centripetal force
and the gravitational force are
The selection of a particular
orbit depends on the following
Transmission path loss.
Earth coverage area.
Time period for which the
satellite should be visible.
GEOSTATIONARY EARTH ORBIT (GEO)
These satellites are in orbit 35,863 km above the earth’s
surface along the equator.
Objects in Geostationary orbit revolve around the earth at
the same speed as the earth rotates. This means GEO
satellites remain in the same position relative to the
surface of earth.
A GEO satellite’s distance from earth gives it a large coverage area,
almost a fourth of the earth’s surface.
GEO satellites have a 24 hour view of a particular area.
These factors make it ideal for satellite broadcast and other multipoint
A GEO satellite’s distance also cause it to have both a comparatively
weak signal and a time delay in the signal, which is bad for point to
GEO satellites, centered above the equator, have difficulty broadcasting
signals to near polar regions
LOW EARTH ORBIT (LEO)
LEO satellites are much closer to the earth than GEO
satellites, ranging from 500 to 1,500 km above the surface.
LEO satellites don’t stay in fixed position relative to the
surface, and are only visible for 15 to 20 minutes each pass.
A network of LEO satellites is necessary for LEO satellites
to be useful
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 is less of a waste of
A network of LEO satellites is needed, which can be costly
LEO satellites have to compensate for Doppler shifts cause by
their relative movement.
Atmospheric drag effects LEO satellites, causing gradual orbital
MEDIUM EARTH ORBIT (MEO)
A MEO satellite is in orbit somewhere between 8,000 km
and 18,000 km above the earth’s surface.
MEO satellites are similar to LEO satellites in
MEO satellites are visible for much longer periods of time
than LEO satellites, usually between 2 to 8 hours.
MEO satellites have a larger coverage area than LEO
A MEO satellite’s longer duration of visibility and wider
footprint means fewer satellites are needed in a MEO
network than a LEO network.
A MEO satellite’s distance gives it a longer time delay and
weaker signal than a LEO satellite, though not as bad as a
Molniya Orbit Satellites
Used by Russia for decades.
Molniya Orbit is an elliptical orbit. The satellite remains in
a nearly fixed position relative to earth for eight hours.
A series of three Molniya satellites can act like a GEO
Useful in near polar regions.
OTHER ORBITS (CONT.)
High Altitude Platform (HAP)
One of the newest ideas in satellite communication.
A blimp or plane around 20 km above the earth’s surface is
used as a satellite.
HAPs would have very small coverage area, but would have
a comparatively strong signal.
Cheaper to put in position, but would require a lot of them
in a network.
SOURCES OF IMPAIRMENT
Distance between earth and satellite
Terrestrial Distance between the receiving antenna and
the aim point of the satellite (Satellite Footprint)
Distances and Free Space Loss
LdB 20 log( ) 20 log( d ) 21.98 dB
GEO: Losses at the equator
LdB 20 log( ) 173.07 dB
GEO: Losses at maximum distance (horizon) d=42711 Km)
L dB 20 log( ) 174.59 dB
DISTANCES AND FREE SPACE LOSS
1500 km - LEO
12000 km - MEO
Oxygen and Water – primary
Angle of Elevation
Angle ε between center of satellite
and surface of the earth.
Elevation needed to at least
communicate with the satellite.
The elevation angle between the
beam and the surface of earth has an
on the illuminated area (footprint)
Satellite Footprint: In geostationary orbit, communications satellites
have direct line-of sight to almost half the earth - a large "footprint"
which is a major advantage. A signal sent via satellite can be
transmitted simultaneously to every U.S. city. Multiple downlinks can be
aimed at one satellite and receive the same program; called point to
Different kinds of satellites use different frequency
1 to 2 GHz
Mobile satellite service (MSS)
2 to 4 GHz
MSS, NASA, deep space research
4 to 8 GHz
Fixed satellite service (FSS)
8 to 12.5 GHz
FSS military, terrestrial earth
12.5 to 18 GHz
FSS, broadcast satellite service
18 to 26.5 GHz
26.5 to 40 GHz
SATELLITE NETWORK CONFIGURATIONS
Point to Point
SATELLITE NETWORK CONFIGURATIONS
Sub-Type of Broadcast : VSAT (very
small aperture terminal)
Subscribers use low cost VSAT
Stations share a satellite
capacity for transmission to a hub
Hub can exchange messages with
the subscribers and relay messages
between the subscribers
Satellite frequency is already broken into bands, and is
broken in to smaller channels in Frequency Division
Multiple Access (FDMA).
Overall bandwidth within a frequency band is increased
due to frequency reuse (a frequency is used by two carriers
with orthogonal polarization).
The number of sub-channels is limited by three
Thermal noise (too weak a signal will be effected by
Intermodulation noise (too strong a signal will cause noise).
Crosstalk (cause by excessive frequency reusing).
FDMA can be performed in two ways:
Fixed-assignment multiple access (FAMA): The subchannel assignments are of a fixed allotment. Ideal for
broadcast satellite communication.
Demand-assignment multiple access (DAMA): The subchannel allotment changes based on demand. Ideal for
point to point communication .
FREQUENCY DIVISION MULTIPLEXING
Satellite squeezes 24 channels in 500 MHz using
TDMA (Time Division Multiple Access) breaks a
transmission into multiple time slots, each one dedicated to
a different transmitter.
TDMA is increasingly becoming more widespread in
TDMA uses the same techniques (FAMA and DAMA) as
Advantages of TDMA over FDMA.
Digital equipment used in time division multiplexing is
increasingly becoming cheaper.
There are advantages in digital transmission techniques.
Ex: error correction.
Lack of intermodulation noise means increased efficiency.
TYPES OF SATELLITE CATEGORIES
Satellites are divided in four major categories as
Geostationary satellites when used for the communication
application are called communication satellite.
They are used in applications such as point to point
communication radio broadcasting, TV transmission,
military application, Commercial application.
Communications satellites are classified based on the
coverage are as global, regional or domestic satellites.
A term “INTELSAT” is often used in satellite
communication. INTELSAT means International
These are communication satellites which are used for
communication of telephony, computer data, TV signals
etc… More than 140 nations are interconnected via the
These satellites are used for weather prediction, for taking
the photographs of clouds. TIROS-1 was the first weather
satellite. Other weather satellites are Nimbus and
India’s “INSAT” satellite is being used for three
application i.e. communications. TV transmission and
meteorology or weather prediction. Thus it is s
This type of satellites can be used to find out the
condition of crops , forests or minerals
underground , condition of soil etc. Indian
satellite IRS is a remote sensing satellite.
From the information obtained from the remote
sensing satellite detailed maps can be prepared.
These satellites are relatively simpler and of
short life span.
They are used to carry out various scientific
studies. India’s “Aryabhatta” was a scientific
APPLICATION OF SATELLITE COMMUNICATION
The main application of satellite communication is in the
field of communication. The communication of video signals
(TV), audio signals (telephones, satellites phones) and
computer data (internet).
To gain meteorological or weather information. The
photographs taken by the satellites are analyzed for
To monitor the status of earth’s resources such as land,
forests and oceans. We can get very important information
about crops, lakes, rivers, forests, fire etc…
To spot our mineral resources, polluted areas, sources of
SATELLITE APPLICATIONS OVERVIEW
Systems using geostationary satellites:
Inmarsat (International Maritime Satellite Organization)
used for voice, data, especially for ships worldwide except
MSAT (Mobile Satellite) used for voice data, mainly for
land mobile western hemisphere.
Systems using low-earth-orbit (LEO) satellite:
Iridium: major uses are voice, paging, low speed data.
Globalstar: major uses are voice paging, low speed date.
Teledesic: major uses high speed data, voice.
SATELLITE APPLICATIONS OVERVIEW
Systems using little LEO satellite:
ORBCOMM: major uses are paging, short messaging, e-mail,
LEO: one major uses are paging, short messaging, e-mail vehicle
E-sat: major uses are remote meter reading.
Systems using Medium earth orbit (MEO):
Ellipso: major uses are voice communication using portable and
ICO (Intermediate Circular Orbit): major uses are satellite to
Satellite Communications Systems: Systems, Techniques
and Technology (5th edition ) by Michel Bousquet
Satellite communication systems(3rd edition) by B.G.Evans
Fundamentals of Satellite Communications, by Howard
Housman/President-MITEQ, Inc. Hauppauge, NY
11788/May 29, 2008
Satellite Communications CSC 490:
Wireless Networking /Author: Michael Charles
from "A Technical Guide to Teleconferencing and Distance
Learning," 3rd edition