PRESENTATION

ON

SATELLITE COMMUNICATIONS

BY
HUNACHAPPA
Assistant Professor
ECE, AVNIET, HYDERABAD

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UNIT-I
COMMUNICATION SATELLITE

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Overview of present and future

trends of satellite communications
introduction to satellite systems

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Need for satellite communications:
 Satellite communication is a wireless communication which covers very
large area. In this introduction we will try to see why do you need it in such
a difficult type of communication and then we will go back to history and
cover a few terms in this process.
 There is a need of space communication; what is the need, it is a wireless
communication; terrestrial microwave links are not suitable to meet a large
cover large geographical area particularly for radio, television networking
even cellular telephony large geographical area.
 The basic requirement is earth is not flat, earth surface is not flat and micro
wave communication; it goes straight line just like light. So, therefore, if
there are two towers, the second tower unless it is in the radio visibility it
will be not able to receive the signal from the transmitting tower. So,
therefore, only a short distance can be covered and you can see that our
mobile towers can cover a smaller distance, radio and television earlier days
the large towers some of you might have seen in big cities they cover only
the city. 4

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Classification of Satellite Orbits
Circular or elliptical orbit
Circular with center at earth‘s center
Elliptical with one foci at earth‘s center Orbit around earth
in different planes
Equatorial orbit above earth‘s equator
Polar orbit passes over both poles
Other orbits referred to as inclined orbits Altitude of satellites
Geostationary orbit (GEO)
Medium earth orbit (MEO)
Low earth orbit (LEO)

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Satellite Orbits are divided into 3 types
Equatorial
Inclined
Polar
Gravity depends on the mass of the earth, the mass of the satellite, and
the distance between the center of the earth and the satellite.For a satellite
traveling in a circle, the speed of the satellite and the radius of the circle
determine the force (of gravity) needed to maintain the orbitThe radius of the
orbit is also the distance from the center of the earth.
For each orbit the amount of gravity available is therefore fixed That in
turn means that the speed at which the satellite travels is determined by the orbit
From what we have deduced so far, there has to be an equation that relates the
orbit and the speed of the satellite:
r3 R^3=mu/n^2
T  2
4 10 1 4 N=2pi/T
T is the time for one full revolution around the orbit, in seconds
r is the radius of the orbit, in meters, including the radius of the earth (6.38x106m).

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CLASSICAL
SATELLITE SYSTEMS Satellites in circular orbits
attractive force F = m g g
(R/r)²
centrifugal force Fc = m r ²
m: mass of the satellite
R: radius of the earth (R =
6370 km)
r: distance to the center of the
earth
g: acceleration of gravity (g
= 9.81 m/s²)
: angular velocity ( = 2 
f, f: rotation frequency)
Stable orbit
Fg = Fc 2
gR
r3
(2 f ) 2
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satellite
period [h] 24 Velocity
Km/sec
12
velocity [ x1000 km/h]
20 10

16 8

12 6

8 4

4 2
synchronous distance
35,786 km

10 20 30 40 x106 m
radius
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elliptical or circular orbits

complete rotation time depends on distance satellite-earth
inclination: angle between orbit and equator
elevation: angle between satellite and horizon
LOS (Line of Sight) to the satellite necessary for connection
 high elevation needed, less absorption due to e.g. buildings
Uplink: connection base station - satellite
Downlink: connection satellite - base station
typically separated frequencies for uplink and downlink
transponder used for sending/receiving and shifting of frequencies
transparent transponder: only shift of frequencies
regenerative transponder: additionally signal regeneration

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Inclination

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Elevation

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Orbits I
Four different types of satellite orbits can be identified depending on the shape and
diameter of the orbit:
GEO: geostationary orbit, ca. 36000 km above earth surface
LEO (Low Earth Orbit): ca. 500 - 1500 km
MEO (Medium Earth Orbit) or ICO (Intermediate Circular Orbit): ca. 6000 - 20000
km
HEO (Highly Elliptical Orbit) elliptical orbits

Orbits II
Van-Allen-Belts:
ionized particles
2000 - 6000 km and
15000 - 30000 km
above earth surface

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Geostationary satellites
Orbit 35,786 km distance to earth surface, orbit in equatorial plane
(inclination 0°) complete rotation exactly one day, satellite is synchronous to earth
rotation fix antenna positions, no adjusting necessary
satellites typically have a large footprint (up to 34% of earth surface!),
therefore difficult to reuse frequencies bad elevations in areas with latitude above
60° due to fixed position above the equator high transmit power needed high latency
due to long distance (ca. 275 ms) not useful for global coverage for small mobile
phones and data transmission, typically used for radio and TV transmission

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LEO systems
Orbit ca. 500 - 1500 km above earth surface radio
visibility of a satellite ca. 10 - 40 minutes global
coverage possible

latency comparable with terrestrial long distance

connections, ca. 5 - 10 ms smaller footprints, better
frequency reuse but now handover necessary from one
satellite to another many satellites necessary for global
coverage more complex systems due to moving satellites

Examples:
Iridium (start 1998, 66 satellites)
Bankruptcy in 2000, deal with US DoD (free use,
saving from ―deorbiting‖)
Globalstar (start 1999, 48 satellites)
Not many customers (2001: 44000), low stand-by times for mobiles

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LEO systems
 Orbit ca. 500 - 1500 km above earth surface visibility
of a satellite ca. 10 - 40 minutes global radio coverage
possible
 latency comparable with terrestrial long distance
connections, ca. 5 - 10 ms smaller footprints, better
frequency reuse but now handover necessary from one
satellite to another many satellites necessary for global
coverage
more complex systems due to moving satellites

Examples:
Iridium (start 1998, 66 satellites)
Bankruptcy in 2000, deal with US DoD (free use,
saving from ―deorbiting‖)
Globalstar (start 1999, 48 satellites)
Not many customers (2001: 44000), low stand-by times for mobiles

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MEO systems
• Orbit ca. 5000 - 12000 km above earth surface
• comparison with LEO systems:
• slower moving satellites
• less satellites needed
• simpler system design
• for many connections no hand-over needed
• higher latency, ca. 70 - 80 ms
• higher sending power needed
• special antennas for small footprints needed

• Example:
• ICO (Intermediate Circular Orbit, Inmarsat) start ca. 2000
– Bankruptcy, planned joint ventures with Teledesic, Ellipso – cancelled again, start planned
for 2003

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Overview of LEO/MEO systems
Iridium Globalstar ICO Teledesic
# satellites 66 + 6 48 + 4 10 + 2 288
altitude 780 1414 10390 ca. 700
(km)
coverage global 70° latitude global global
min. 8° 20° 20° 40°
elevation
frequencies 1.6 MS 1.6 MS  2 MS  19 
[GHz 29.2  2.5 MS  2.2 MS  28.8 
(circa)] 19.5  5.1  5.2  62 ISL
23.3 ISL 6.9  7
access FDMA/TDMA CDMA FDMA/TDMA FDMA/TDMA
method
ISL yes no no yes
bit rate 2.4 kbit/s 9.6 kbit/s 4.8 kbit/s 64 Mbit/s 
2/64 Mbit/s 
# channels 4000 2700 4500 2500
Lifetime 5-8 7.5 12 10
[years]
cost 4.4 B$ 2.9 B$ 4.5 B$ 9 B$
estimation

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KEPLER‘S FIRST LAW
The path followed by a satellite around the primary will be an ellipse.
An ellipse has two focal points shown as F1 and F2.
The center of mass of the two-body system, termed the barycenter, is always centered on
one of the foci.
In our specific case, because of the enormous difference between the masses of the earth
and the satellite, the center of mass coincides with the center of the earth, which is
therefore always at one of the foci.
The semimajor axis of the ellipse is denoted by a, and the semiminor axis, by b. The
eccentricity e is given by

a b
e
a b

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Kepler‘s Second Law
For equal time intervals, a satellite will sweep out equal areas in its orbital plane,
focused at the barycenter.

Kepler‘s Third Law

The square of the periodic time of orbit is proportional to the cube of the mean distance
between the two bodies.
The mean distance is equal to the semimajor axis a. For the satellites orbiting the earth,
Kepler‘s third law can be written in the form

where n is the mean motion of the satellite in radians per second and is the earth‘s
geocentric gravitational constant. With a in meters, its value is

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GEO systemsA geosynchronous orbit is sometimes called a geostationary orbit.
Satellites in geosynchronous orbits make one full trip around the Earth in the same
amount of time it takes the Earth to make one full rotation. These satellites are always
above the same location on Earth‘s surface. By positioning a satellite in one place at the
same time, we are able to monitor Earth‘s weather and provide satellite television to
homes – that‘s why satellite dishes are always pointed the same way in the sky.
Orbit Description Example
Low Earth Orbit High enough that it misses mountain peaks ISS, Hubble Space
(LEO) but doesn’t gravitate back toward Earth Telescope
Medium Earth Orbits between Low Earth orbits and
GPS’ 24 satellites
Orbit (MEO) Geosynchronous orbits
Geosynchronous Always above one spot on Earth; great Communication
Orbit (GSO) distance from Earth satellites
Polar Orbit Travel north-south from pole to pole Weather satellites
Orbital path is an ellipse (oval-shaped). Speed
Elliptical – shape
of the satellite increases when it is closer to GPS’ 24 satellites
of some orbits
the object it orbits.
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A simulation of a geosynchronous orbit.

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ORBITAL MECHANICS
Apogee The point farthest from earth. Apogee height is
shown as ha in Fig
Perigee The point of closest approach to earth. The perigee
height is shown as hp
Line of apsides The line joining the perigee and apogee
through the center of the earth.
Ascending node The point where the orbit crosses the
equatorial plane going from south to north. Descending
node The point where the orbit crosses the equatorial plane
going from north to south.
Line of nodes The line joining the ascending and descending
nodes through the center of the earth.
Inclination The angle between the orbital plane and the
earth‘s equatorial plane. It is measured at the ascending node
from the equator to the orbit, going from east to north. The
inclination is shown as i in Fig.
Mean anomaly M gives an average value of the angular
position of the satellite with reference to the perigee.
True anomaly is the angle from perigee to the satellite
position, measured at the earth‘s center. This gives the true
angular position of the satellite in the orbit as a function of
time.

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Prograde orbit An orbit in which the satellite moves in the
same direction as the earth‘s rotation. The inclination of a
prograde orbit always lies between 0 and 90°.
Retrograde orbit An orbit in which the satellite moves in a
direction counter to the earth‘s rotation. The inclination of a
retrograde orbit always lies between 90 and 180°.
Argument of perigee The angle from ascending node to
perigee, measured in the orbital plane at the earth‘s center,
in the direction of satellite motion.
Right ascension of the ascending node To define
completely the position of the orbit in space, the position of
the ascending node is specified. However, because the earth
spins, while the orbital plane remains stationary the
longitude of the ascending node is not fixed, and it cannot
be used as an absolute reference. For the practical
determination of an orbit, the longitude and time of crossing
of the ascending node are frequently used. However, for an
absolute measurement, a fixed reference in space is
required. The reference chosen is the first point of Aries,
otherwise known as the vernal, or spring, equinox. The
vernal equinox occurs when the sun crosses the equator
going from south to north, and an imaginary line drawn
from this equatorial crossing through the center of the sun
points to the first point of Aries (symbol ). This is the line of
Aries.
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Six Orbital Elements
Earth-orbiting artificial satellites are defined by six orbital elements referred
to as the keplerian element set.
The semimajor axis a.
The eccentricity e
give the shape of the ellipse.
A third, the mean anomaly M, gives the position of the satellite in its orbit at
a reference time known as the epoch.
A fourth, the argument of perigee  , gives the rotation of the orbit‘s perigee
point relative to the orbit‘s line of nodes in the earth‘s equatorial plane.
The inclination I
The right ascension of the ascending node 
Relate the orbital plane‘s position to the earth.

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Forces acting on a satellite in a stable orbit around the earth
Gravitational force is inversely proportional to the square of the distance between the centers
of gravity of the satellite and the planet the satellite is orbiting, in this case the earth. The
gravitational force inward (FIN, the centripetal force) is directed toward the center of gravity
of the earth.
The kinetic energy of the satellite (FOUT, the centrifugal force) is directed opposite to the
gravitational force. Kinetic energy is proportional to the square of the velocity of the satellite.
When these inward and outward forces are balanced, the satellite moves around the earth in a
―free fall‖ trajectory: the satellite‘s orbit.

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Locating the satellite with respect to the earth:
A fixed rectangular coordinate system called the geocentric equatorial coordinate system
whose origin is the center of the earth. The rotational axis of earth is the axis, which is
through the geographic North Pole. The axis is from the center of the earth toward a fixed
location in space called the first point of Aries. This coordinate system moves through
space, it translates as the earth moves in its orbit around the sun, but it does not rotate as
the earth rotates. The direction is always the same, whatever the earth‘s position around the
sun and is in the direction of the first point of Aries. The plane contains the earth‘s equator
and is called the equatorial plane.
Coverage angle & Slant range:
Communication with a satellite is possible if the earth station is in the footprint of the
satellite. In other words, the earth-satellite link is established only when the earth station
falls in the beam width of the satellite antenna. This would be a function of time and the
satellite is to be tracked in case of a non-geostationary satellite. But for a geostationary
satellite once the link is established, the link is available throughout the lifetime of the
satellite without any tracking. To have the communication between the earth station-
satellite-earth stations, both the antennas of the transmitting and receiving earth station are
to be pointed towards the antenna of the spacecraft. With the help of look angle
determination, this can be established. To locate the earth station in the footprint of the
satellite, the information of slant range and coverage area/angle is required.
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Inclined orbits:
Satellites that can no longer be maintained in a fully geostationary orbit, but are
still used for communications services, are referred to as inclined orbit satellites, there are
advantages and disadvantages to inclined orbits, depending on the mission goals and the
data recovery requirements. The greater the inclination of the orbit is, the larger the surface
area of the earth that the satellite will pass over at some time in its flight.
The inclined orbit will take the space craft at one time or another, over the earth‘s
entire surface that lies approximately between the latitudes given by ± the orbital
inclination. The superior coverage of the earth with an inclined orbit satellite is counter
balanced by the disadvantage that the master control station (MCS) will not be able to
communicate directly with the satellite on every orbit as with an equatorial orbit satellite.
A LEO satellite orbits the earth with a period of 90 to 100 min and for an inclined
orbit satellite; the earth will have rotated the master control station out of the path of the
satellite on the next pass over the same side of the earth.

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Orbital perturbations due to earth‘s oblateness
Perturbations
Most of the studies for identifying potential relative motion orbits for flying formations have assumed
a spherical Earth. The relative motion orbits identified from this assumption result primarily from
small changes in the eccentricity and the inclination. This is satisfactory for identifying the potential
relative motion orbits, but unsatisfactory for determining long term motion, fuel budgets and the best
formations. Assuming that all satellites in the formation are nearly identical, the primary perturbation
is the differential gravitational perturbation due to the Earth‘s oblateness. Since the differential gravity
perturbations are a function of (a,e,i) the small changes in these elements result in different drift rates
for each satellite and the negation of these drifts result in different fuel requirements for each satellite.
Since some satellites running out of fuel before others will degrade the system performance it would
be advantageous to have the satellites have equal fuel consumption. Satellites do not describe perfect
elliptical orbits around its central body (this case is Earth).

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Effects of sun and moon:
Gravitational attractions by the sun and moon cause the orbital inclination of a
geostationary satellite to change with time. If not countered by north-south station keeping,
these forces would increase the orbital inclination from an initial 0° to 14.67° 26.6 years
later. Since, no satellite has such a long lifetime, the problem is not acute.

Eclipse of GEO satellite:

A geostationary satellite utilizes solar energy to generate the required DC power to
operate all the subsystems of the spacecraft. Solar energy is not available for a geostationary
satellite when eclipse occurs. This occurs when the earth comes in between the sun and
satellite in line and blocks the solar energy from reaching the solar panels of the satellite.

Sun transit outage:

The overall receiver noise will rise significantly to effect the communications when the
sun passes through the beam of an earth station antenna. This effect is predictable and can
cause outage for as much as 10 min a day for several days and for about 0.02% an average
year. The receiving earth station has to wait until the sun moves out of the main lobe of the
antenna. This occurs during the daytime, where the traffic is at its peak and forces the
operator to hire some other alternative channels for uninterrupted communication link.

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UNIT-II
SPACE SEGMENT

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Placement of satellite into geostationary orbit:
There is a considerable amount of expertise and technology used to ensure that satellites enter their
orbits in the most energy efficient ways possible. This ensures that the amount of fuel required is kept to
a minimum; an important factor on its own because the fuel itself has to be transported until it is used.
If too much fuel has to be used then this increases the size of the launch rocket and in turn this greatly
increases the costs.
Many satellites are placed into geostationary orbit, and one common method of achieving this is based
on the Hohmann transfer principle. This is the method use when the Shuttle launches satellites into
orbit. Using this system the satellite is placed into a low earth orbit with an altitude of around 180
miles. Once in the correct position in this orbit rockets are fired to put the satellite into an elliptical orbit
with the perigee at the low earth orbit and the apogee at the geostationary orbit as shown. When the
satellite reaches the final altitude the rocket or booster is again fired to retain it in the geostationary
orbit with the correct velocity.
Alternatively when launch vehicles like Ariane are
used the satellite is launched directly into the
elliptical transfer orbit. Again when the satellite is at
the required altitude the rockets are fired to transfer it
into the required orbit with the correct velocity.

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Satellite Subsystems
An operating communications satellite system consists of several elements or
segments, ranging from an orbital configuration of space components to ground
based components and network elements.
The particular application of the satellite system, (for example fixed satellite service,
mobile service, or broadcast service,) will determine the specific elements of the
system. The basic system consists of a satellite (or satellites)
in space, relaying information between two or more
users through ground terminals and the satellite.
*The information relayed may be voice, data, video,
or a combination of the three.
*The user information may require transmission
via terrestrial means to connect with the ground
terminal.
* The satellite is controlled from the ground
through a satellite control facility, often called the
master control center (MCC), which provides
tracking, telemetry, command, and monitoring
functions for the system
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The space segment equipment carried aboard the satellite can be classified under two
functional areas: the bus and the payload

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Bus :The bus refers to the basic satellite structure itself and the subsystems that support the
satellite.The bus subsystems are:
* the physical structure,
* power subsystem,
* attitude and orbital control subsystem,
* thermal control subsystem,
*command and telemetry subsystem.
Payload :The payload on a satellite is the equipment that provides the service or services intended for
the satellite.A communications satellite payload consists of:
*The communications equipment that provides the relay link between the up- and downlinks from the
ground.
*The communications payload can be further divided into:
*The transponder
*The antenna subsystems
Satellite Bus
Physical Structure
The basic shape of the structure depends of the method of stabilization employed to keep the satellite
stable and pointing in the desired direction, usually to keep the antennas properly oriented toward
earth.
Physical Structure: Two methods are commonly employed:
*spin stabilization
*Three-axis or body stabilization.
Both methods are used for GSO satellite.
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Spin Stabilization
A spin stabilized satellite is usually cylindrical in shape, because the satellite is required to
be mechanically balanced about an axis, so that it can be maintained in orbit by spinning on
its axis. For GSO satellites, the spin axis is maintained parallel to
the spin axis of the earth, with spin rates in the range of 30 to 100 revolutions per minute
Three-axis Stabilization
A three-axis stabilized satellite is maintained in space with stabilizing elements for each of
The three axes, referred to as roll,pitch, and yaw, in conformance with the definitions first
used in the aircraft industry. The entire body of the spacecraft remains fixed in space,
relative to the earth, which is why the three axis stabilized satellite is also referred to as a
body-stabilized satellite.

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Power Subsystem
The electrical power for operating equipment on a communications satellite is obtained
primarily from solar cells. *The radiation on a satellite from the sun has an intensity
averaging about 1.4 kW/m2. *Solar cells operate at an efficiency of 20–25% at beginning of
life (BOL), and can degrade to 5–10% at end of life (EOL), usually considered to be 15
years .All satellite and spacecraft must also carry storage batteries to provide power during
launch and during eclipse periods when sun blockage occurs.
A power conditioning unit is also included in the power subsystem, for the control of battery
charging and for power regulation and monitoring.

Attitude Control
Attitude control is necessary so that the antennas, are pointed correctly towards earth
Several forces can interact to affect the attitude of the spacecraft:
*Gravitational forces from the sun, moon, and planets;
*Solar pressures acting on the spacecraft body, antennas or solar panels;
*Earth‘s magnetic field
The attitude of a satellite refers to its orientation in space with respect to earth

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Orbital control
It is often called station keeping, is the process required to maintain a satellite in its
proper orbit location.
*GSO satellites will undergo forces that would cause the satellite to drift in the east-west
(longitude) and north-south (latitude) directions, *Orbital control is usually maintained
with the same thruster system as is attitude control
Thermal Control
Several techniques are employed to provide thermal control in a satellite.
Thermal blankets and thermal shields are placed at critical locations to provide
insulation. Radiation mirrors are placed around electronic subsystems, particularly for
spin-stabilized satellites, to protect critical equipment.
The satellite thermal control system is designed to control the large thermal gradients
generate in the satellite
Heat pumps are used to relocate heat from power devices such as traveling wave power
amplifiers to outer walls or heat sinks to provide a more effective thermal path for heat to
escape.
Thermal heaters may also be used to maintain adequate temperature conditions for some
components, such as propulsion( ) ‫عفد‬lines or thrusters,

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Tracking, Telemetry, Command, and Monitoring
The tracking, telemetry, command, and monitoring (TTC&M) subsystem 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 communications
system links.
TTC&M links may operate in the same frequency bands or in other bands

shows the typical TTC&M

functional elements for the
satellite and ground facility for a
communications satellite
application

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The satellite TTC&M subsystems comprise
*The antenna
*Command receiver
*Tracking and telemetry transmitter
*Tracking sensors

The elements on the ground include the TTC&M antenna, telemetry

receiver, command transmitter, tracking subsystem, and associated processing
and analysis functions.
Satellite control and monitoring is accomplished through monitors and
keyboard

INTERFACE
Telemetry data are received from the other subsystems of the spacecraft, such as the
payload, power, attitude control, and thermal control.
Command data are relayed from the command receiver to other subsystems to
control such parameters as antenna pointing, transponder modes of operation,
battery and solar cell changes, etc

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1Tracking: refers to the determination of the current orbit, position, and movement of the spacecraft.
The tracking function is accomplished by a number of techniques, usually involving satellite beacon
signals, which are received at the satellite TTC&M earth station
The Doppler shift of the beacon (or the telemetry carrier) is monitored to determine the rate at which
the range is changing (the range rate). Angular measurements from one or more earth terminals can
be used to determine spacecraft location. The range can be determined by observing the time delay
of a pulse or sequence of pulses transmitted from the satellite. . Acceleration and velocity sensors on
the satellite can be used to monitor orbital location and changes in orbital location.
2Telemetry:Its function involves the collection of data from sensors on-board the spacecraft and the
relay of this information to the ground. The telemetered data include following parameters -Voltage
and current conditions in the power subsystem, -Temperature of critical subsystems, -Status of
switches and relays in the communications and antenna subsystems, -Fuel tank pressures, and
attitude control sensor status.
3Command: is the complementary function to telemetry. The command system relays specific
control and operations information from the ground to the spacecraft. Parameters involved in typical
command links include changes and corrections in attitude control and orbital control; antenna
pointing and control; transponder mode of operation; battery voltage control.
The command system is used during launch to control the firing of the boost motor, deploy
appendages such as solar panels and antenna reflectors, and ‗spin-up‘ a spin-stabilized spacecraft
body. Security is an important factor in the command system for a communications satellite.
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Satellite Payload
Transponder The transponder is the series of components that provides the communications channel,
or link, between the uplink signal received at the uplink antenna, and the downlink signal transmitted
by the downlink antenna The key elements of the payload portion of the space segment, is the
transponder and antenna subsystems. There two types of transponder:
1. The frequency translation transponder
2. The on-board processing transponder
1. Frequency Translation Transponder: The frequency translation transponder, also referred to as a non-
regenerative repeater, or bent pipe, *It receives the uplink signal and, after amplification, retransmits it
with only a translation in carrier frequency, Frequency Translation Transponder, also called Repeater
Non-Regenerative Satellite ‗Bent Pipe‘ .Uplinks and downlinks are codependent From the Fig. its
clear that uplink radio frequency, fup, is converted to an intermediate lower frequency, fif , amplified,
and then converted back up to the downlink RF frequency, fdwn, for transmission to earth,Frequency
translation transponders are used for in both GSO and NGSO orbits,The uplinks and downlinks are
codependent, meaning that any degradation introduced on the uplink will be transferred to the
downlink, affecting the total communications link

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On-board P rocessing Transponder: The on-board processing transponder, called a regenerative

repeater demod/ remod transponder, or smart satellite. *The uplink signal at fup is demodulated to
baseband, f base band . *The baseband signal is available for processing on-board, including
reformatting and error-correction. The baseband information is then remodulated to the downlink
carrier at fdwn, after final amplification, transmitted to the ground.
*The demodulation/remodulation process removes uplink noise and interference from the
downlink,
while allowing additional on-board processing to be accomplished.
*Thus the uplinks and downlinks are independent with
respect to evaluation of overall link performance, unlike the frequency translation transponder

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 SATELLITE COMMUNICATIONS
Antennas
The antenna systems on the spacecraft are used for transmitting and receiving the RF
signals that comprise the space links of the communications channels
The most important parameters that define the performance of an antenna are:
 Antenna gain,
 Antenna beam width,
 Antenna side lobes
Most satellite communications applications require an antenna to be highly directional
 High gain,
 narrow beam width
 negligibly small side lobes
The common types of antennas used in satellite systems are
 The linear dipole
 The horn antenna
 The parabolic reflector
 The array antenna

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SATELLITE COMMUNICATION
Satellite frequency bands and allocations:

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 SATELLITE COMMUNICATIONS
Satellite Link Design
 Low earth orbit (LEO) & medium earth orbit (MEO) satellite systems are closer and
produces stronger signals but earth terminals need omni directional antennas
 The design of any satellite communication is based on
– Meeting of minimum C/N ratio for a specific percentage of time
– Carrying the maximum revenue earning traffic at minimum cost

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 SATELLITE
Link Budgets
COMMUNICATIONS
 C/N ratio calculation is simplified by the use of link budgets
 Evaluation the received power and noise power in radio link
 The link budget must be calculated for individual transponder and for each link
 When a bent pipe transponder is used the uplink and down link C/N rations must
be combined to give an overall C/N
Satellite Link Design – Downlink received Power
 The calculation of carrier to noise ratio in a satellite link is based on equations for
received signal power Pr and receiver noise power:
Pr = EIRP + Gr – Lp – La – Lta – Lra dBW,
Where:
EIRP = 10log1o (PtGt)
Gr = 10log10 (4Ae /  2)dB
PathLoss LP = 10log10 [(4 Ae / ) 2] = 20log1o (4R/ )dB
La = Attenuation in the atmosphere
Lta = Losses assosiated with transmitting antenna
Lra = Losses associated with receiving antenna

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 SATELLITE COMMUNICATIONS
Satellite Link Design – Downlink Noise Power
 A receiving terminal with a system noise temperature
TsK and a noise bandwidth Bn Hz has a noise power Pn referred to the output terminals of the antenna
where
Pn = kTsBn watts
 The receiving system noise power is usually written in decibel units as:
N = k + Ts + Bn dBW,
k where: is Boltzmann‘s constant )-228.6 Dbw/K/Hz)
Ts is the system noise temperature in DBK
Bn is the noise Bandwidth of the receiver in dBHz
Satellite Link Design – Uplink
 Uplink design is easier than the down link in many cases
– Earth station could use higher power transmitters
 Earth station transmitter power is set by the power level required at the input to the transporter, either
– A specific flux density is required at the satellite
– A specific power level is required at the input to the transporter
 Analysis of the uplink requires calculation of the power level at the input to the transponder so that uplink C/N
ratio can be found
 With small-diameter earth stations, a higher power earth station transmitter is required to achieve a similar
satellite EIRP.
– Interference to other satellites rises due to wider beam of small antenna
 Uplink power control can be used to against uplink rain attenuation

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 SATELLITE COMMUNICATIONS
Propagation Effects & their Impact
 Many phenomena causes lead signal loss on through the earths atmosphere:
– Atmospheric Absorption (gaseous effects)
– Cloud Attenuation (aerosolic and ice particles
– Tropospheric Scintillation (refractive effects)
– Faraday Rotation (an ionospheric effect)
– Ionospheric Scintillation (a second ionospheric effect)
– Rain attenuation
– Rain and Ice Crystal Depolarization
 The rain attenuation is the most important for frequencies above 10 GHz
– Rain models are used to estimate the amount of degradation (or fading) of the signal when
passing through rain.
– Rain attenuation models: Crane 1982 & 1985; CCIR 1983; ITU-R p,618-5(7&8)

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 SATELLITE COMMUNICATIONS
System noise temperature ,C/N and G/T
ratio Thermal noise in its pre amplifier
• Pn=ktsb
• System noise temperature is also called effective input noise temperature of the receiver
•It is defined as the noise temperature of a noise source located at the input of a noise
source located at the input of a noiseless receiver which will produce then same
contribution to the receiver output same contribution to the receiver output noise
As the internal noise of the actual system itself
Ts is located at the input to the receiver.
• RF amplifier
• IF amplifier
• Demodulator
• Over all gain at the receiver G
• Narrowest bandwidth is B
• Noise power at the demodulator input is

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SATELLITE COMMUNICATIONS

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SATELLITE COMMUNICATIONS

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SATELLITE COMMUNICATIONS

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SATELLITE COMMUNICATIONS

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SATELLITE COMMUNICATIONS

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SATELLITE COMMUNICATIONS

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SATELLITE COMMUNICATIONS

UNIT-III
PROPAGATION EFFECT AND
MULTIPLE ACCESS

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 SATELLITE COMMUNICATIONS

Multiple Access System

Applications employ multiple-access systems to allow two or more Earth stations to
simultaneously share the resources of the same transponder or frequency channel. These
include the three familiar methods: FDMA, TDMA, and CDMA.
1.Another multiple access system called space division multiple access (SDMA) has been
suggested in the past. In practice, SDMA is not really a multiple access method but rather a
technique to reuse frequency spectrum through multiple spot beams on the satellite.
2.Because every satellite provides some form of frequency reuse (cross-polarization being
included), SDMA is an inherent feature in all applications.

TDMA and FDMA require a degree of coordination among users:

 FDMA users cannot transmit on the same frequency and
 TDMA users can transmit on the same frequency but not at the same time.
Capacity in either case can be calculated based on the total bandwidth and power available
within the transponder or slice of a transponder.
 CDMA is unique in that multiple users transmit on the same frequency at the same time
(and in the same beam or polarization).
 This is allowed because the transmissions use a different code either in terms of high-
speed spreading sequence or frequency hopping sequence.
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SATELLITE COMMUNICATIONS
Taxonomy of multiple-access protocols

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 SATELLITE COMMUNICATIONS

The capacity of a CDMA network is not unlimited, however, because at some point the channel becomes
overloaded by self-interference from the multiple users who occupy it.
 Furthermore, power level control is critical because a given CDMA carrier that is elevated in power
will raise the noise level for all others carriers by a like amount.
Multiple access is always required in networks that involve two-way communications among multiple
Earth stations.
The selection of the particular method depends heavily on the specific communication requirements,
the types of Earth stations employed, and the experience base of the provider of the technology.
 All three methods are now used for digital communications because this is the basis of a majority of
satellite networks.
 The digital form of a signal is easier to transmit and is less susceptible to the degrading effects of the
noise, distortion from amplifiers and filters, and interference.
Once in digital form, the information can be compressed to reduce the bit rate, and FEC is usually
provided to reduce the required carrier power even further.
The specific details of multiple access, modulation, and coding are often preselected as part of the
application system and the equipment available on a commercial off-the-shelf (COTS) basis.
 The only significant analog application at this time is the transmission of cable TV and broadcast TV.
These networks are undergoing a slow conversion to digital as well, which may in fact be complete
within a few years.

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SATELLITE COMMUNICATIONS
Frequency-division multiple access (FDMA)

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SATELLITE COMMUNICATIONS
Time-division multiple access (TDMA)

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 SATELLITE COMMUNICATIONS

Time Division Multiple Access and ALOHA

TDMA is a truly digital technology, requiring that all information be converted into bit streams or data
packets before transmission to the satellite. (An analog form of TDMA is technically feasible but never reached the
market due to the rapid acceptance of the digital form.) Contrary to most other communication technologies,
TDMA started out as a high-speed system for large Earth stations. Systems that provided a total throughput of 60
to 250 Mbps were developed and fielded over the past 25 years. However, it is the low-rate TDMA systems,
operating at less than 10 Mbps, which provide the foundation of most VSAT networks. Lower speed means that
less power and bandwidth need to be acquired (e.g., a fraction of a transponder will suffice) with the following
benefits:
The uplink power from small terminals is reduced, saving on the cost of transmitters. The network
capacity and quantity of equipment can grow incrementally, as demand grows. TDMA signals are restricted to
assigned time slots and therefore must be transmitted in bursts. The time frame is periodic, allowing stations to
transfer a continuous stream of information on average. Reference timing for start-of-frame is needed to
synchronize the network and provide control and coordination information. This can be provided either as an
initial burst transmitted by a reference Earth station, or on a continuous basis from a central hub. The Earth station
equipment takes one or more continuous streams of data, stores them in a buffer memory, and then transfers the
output toward the satellite in a burst at a higher compression speed.
At the receiving Earth station, bursts from Earth stations are received in sequence, selected for
recovery if addressed for this station, and then spread back out in time in an output expansion buffer. It is vital that
all bursts be synchronized to prevent overlap at the satellite; this is accomplished either with the synchronization
burst (as shown) or externally using a separate carrier. Individual time slots may be pre-assigned to particular
stations or provided as a reservation, with both actions under control by a master station. For traffic that requires
consistent or constant timing (e.g., voice and TV), the time slots repeat at a constant rate.

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SATELLITE COMMUNICATIONS

Code Division Multiple Access

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 SATELLITE COMMUNICATIONS
Code Division Multiple Access
CDMA, also called spread spectrum communication, differs from FDMA and TDMA
because it allows users to literally transmit on top of each other. This feature has allowed CDMA to
gain attention in commercial satellite communication. It was originally developed for use in military
satellite communication where its inherent anti-jam and security features are highly desirable. CDMA
was adopted in cellular mobile telephone as an interference-tolerant communication technology that
increases capacity above analog systems. It has not been proven that CDMA is universally superior as
this depends on the specific requirements. For example, an effective CDMA system requires
contiguous bandwidth equal to at least the spread bandwidth. Two forms of CDMA are applied in
practice:
(1) direct sequence spread spectrum (DSSS) and
(2) frequency hopping spread spectrum (FHSS).
FHSS has been used by the Omni Tracs and Eutel-Tracs mobile messaging systems for more
than 10 years now, and only recently has it been applied in the consumer‘s commercial world in the
form of the Bluetooth wireless LAN standard. However, most CDMA applications over commercial
satellites employ DSSS (as do the cellular networks developed by Qualcomm).Consider the following
summary of the features of spread spectrum technology (whether DSSS or FHSS):Simplified multiple
access: no requirement for coordination among users; Selective addressing capability if each station
has a unique chip code sequence—provides authentication.

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 SATELLITE COMMUNICATIONS

A typical CDMA receiver must carry out the following functions in order to acquire the signal,
maintain
synchronization, and reliably recover the data:
Synchronization with the incoming code through the technique of correlation detection;
De-spreading of the carrier;
Tracking the spreading signal to maintain
synchronization; Demodulation of the basic data
stream;

Timing and bit detection;

Forward error correction to reduce the effective error rate;
The first three functions are needed to extract the signal from the clutter of noise and other signals.
The processes of demodulation, bit timing and detection, and FEC are standard for a digital
receiver, regardless of the multiple access method.
The bottom line in multiple access is that there is no single system that provides a universal
answer. FDMA, TDMA, and CDMA will each continue to have a place in building the
applications of the future. They can all be applied to digital communications and satellite links.
When a specific application is considered, it is recommended to perform the comparison to make
the most intelligent selection.

71
 UNIT-IV
EARTH STATION

72
 EARTH STATION

• The base band signal from the terrestrial network enters the
earth station at the transmitter after having processed
(buffered, multiplexed, formatted, etc.,) by the base band
equipment.
• After the encoder and modulator have acted upon the base
band signal, it is converted to the uplink frequency.
• Then it is amplified and directed to the appropriate
polarization port of the antenna feed.
• The signal received from the satellite is amplified in an LNA
first and is then down converted from the down link
frequency.
• It is then demodulated and decoded and then the original
base band signal is obtained.
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 EARTH STATION
• Critical components will often be installed redundantly with
automatic switch over in the event of failure so that
uninterrupted operation is maintained. The isolation of low
noise receiver from the high power transmitter is of much
concern in the design considerations of earth station

74
 TRANSMITTERS
• Here the signal to be transmitted is converted to the uplink frequency, with
proper encoding and modulation.
• It is then amplified and directed to the appropriate
polarization port of the antenna feed.
• In a large earth station there will be many transmitters as well as receivers
multiplexed together into one antenna to provide channelize communication
through satellite transponders.
• Transmitters are very much expensive part of the earth
station because of the tight specifications on out of band

emission, frequency stability and power control that are

necessary to avoid interference with other channels and
satellites.
• Further as transmitters are not manufactured in large scale so there cost is
high.

75
 TRANSMITTERS
• The cost increases with the increase in transmitted power
which may vary from tens to thousands of watts.
• Since earth stations require the transmission of microwave
power, they use high power amplifiers (HPAs) such as
travelling tubes and multi cavity klystrons.
• In fact compared to klystrons, TWTAs allow high power over a
wide bandwidth. These tubes require quite a good amount of
cooling that is provided by water circulation using a close
refrigeration system.

76
 RECEIVER
• Receiver of an earth station employs mainly low noise
amplifier (LNA), down converter, demodulator, decoder and
base band signal treatment equipments.
• In fact in the receive chain of the earth station the weak
signals from the satellite are accepted by the same feed that
carries the transmitter output.
• These two signals which differ in power by several orders of
magnitude or kept separate in the frequency domain as they
are assigned to the uplink and down link bands, and in
addition by means of orthogonal polarization, diplexers are
used to enhance the separation in the frequency domain.

77
 ANTENNAS
• The antenna systems consist of
• 1.Feed System
• 2.Antenna Reflector
• 3.Mount
• 4.Antenna tracking System.

Feed System: The feed along with the reflector is the

radiating/receiving element of electromagnetic waves.
• The reciprocity property of the feed element makes the earth
station antenna system suitable for transmission and
reception of electromagnetic waves.

78
The way the waves coming in and going out is called feed
configuration Earth Station feed systems most commonly used
in satellite communication are:
• i)Axi-Symmetric Configuration
• ii)Asymmetric Configuration

79
 ANTENNA REFLECTOR
• Mostly parabolic reflectors are used as the main antenna for
the earth stations because of the high gain available from the
reflector and the ability of focusing a parallel beam into a point
at the focus where the feed, i.e., the receiving/radiating
element is located .
• For large antenna system more than one reflector surfaces may
be used in as in the cassegrain antenna system. Earth stations
are also classified on the basis of services for example:
• 1. Two way TV, Telephony and data
• 2. Two way TV
• 3. TV receive only and two way telephony and data
• 4. Two way data

80
 ANTENNA REFLECTOR
For mechanical design of parabolic reflector the
following parameters are required to be considered:
• Size of the reflector
• Focal Length /diameter ratio
• RMS error of main and sub reflector
• Pointing and tracking accuracies
• Speed and acceleration
• Type of mount
• Coverage Requirement

81
 ANTENNA TRACKING SYSTEM
• Tracking is essential when the satellite drift, as seen by an
earth station antenna is a significant fraction of an earth
station’s antenna beam width.
• An earth station’s tracking system is required to perform some
of the functions such as
• i)Satellite acquisition
• ii)Automatic tracking
• iii)Manual tracking
• iv)Program tracking.

82
 ANTENNA TRACKING SYSTEM
• Recent Tracking Techniques: There have been some
interesting recent developments in auto-track techniques which
can potentially provide high accuracies at a low cost. In one
proposed technique the sequential lobbing technique has been
implemented by using rapid electronic switching of as single
beam which effectively approximates simultaneous lobbing.

83
 TERRESTRIAL INTERFACE

• The function of an earth station is to receive information from or transmit

information to, the satellite network in the most cost-effective and reliable
manner while retaining the desired signal quality.
• The design of earth station configuration depends upon many
factors and its location.
• Location are listed below,
1. In land
2. on a ship at sea
3. Onboard aircraft
• The factors are

1. Type of services 3. Function of the transmitter

2. Frequency bands 4. Function of the receiver

5.Antenna characteristics.
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 POWER TEST METHODS

Noise power ratio (NPR):

 Noise power ratio (NPR), the traditional measure of inter
modulation noise for FDM systems in the communication
field.
 The principle of NPR measurement involves loading the entire
base band spectrum, save for the one voice- frequency channel
slot, with noise, simulating in total the loading of the system
by actual voice traffic in all but that channel.
 Noise appearing in the unloaded slot is manifestation of inter
modulation.
 The ratio of that noise power to the per- channel loading
noise power is the NPR.

85
 THE SYSTEM CAN BE BETWEEN ANY TWO POINTS OF
INTEREST THE NOISE GENERATOR BAND IS LIMITED BY FILTERS
TO THE BASE BAND, AND THE NOISE GENERATOR LEVEL IS SET
TO SIMULATE FULL LOAD ACCORDING TO THE CCIR FORMULAS
P = -15 + 10 LOG N DBMO, N ≥ 240
BWR = 10 log
P = -1 + 4 log N dBmO, N < 240

= dBmO of loading calculation.

NLR = 10 log
86
 THE MEASUREMENT OF G/T:

 antenna gain, and as the antennas get larger, this

characteristic is not so easy to get.
• The gain of smaller antennas, say less than 7 or 8m,
can be found from pattern measurements on a range or
by comparison to gain standard, but these measures
are cumbersome and may be impartial for larger
antennas.
• Large earth stations, with antenna sizes up from 10m,
can sometimes use a carefully calibrated satellite
signal to measure.

87
• In effect, is calculated from the link equation, knowing the
other variables. This method is often used with intermediate
sized antennas (from 5 to 15m).
• An engineer’s method has been developed for the
measurement of for large antennas using the known radio
noise characteristics of stellar sources, usually called radio
stars.
• These characteristics, particularly S, the flux density of the
source in.Hz, have been accurately measured by radio
astronomers.

88
 LOWER ORBIT CONSIDERATIONS

• In order that a satellite can be used for communications

purposes the ground station must be able to follow it in order
to receive its signal, and transmit back to it.
• Communications will naturally only be possible when it is
visible, and dependent upon the orbit it may only be visible for
a short period of time.
• To ensure that communication is possible for the maximum
amount of time there are a number of options that can be
employed:
• The first is to use an elliptical orbit where the apogee is above
the planned Earth station so that the satellite remains visible
for the maximum amount of time.

89
• Another option is to launch a number of satellites with the
same orbit so that when one disappears from view, and
communications are lost, another one appears.
• Generally three satellites are required to maintain almost
uninterrupted communication.
• However the handover from one satellite to the next introduces
additional complexity into the system, as well as having a
requirement for at least three satellites.

90
 VSAT (VERY SMALL APERTURE TERMINAL) SYSTEMS
• VSAT (Very Small Aperture Terminal) describes a small
terminal that can be used for two-way communications via
satellite.
• VSAT networks offer value-added satellite-based services
capable of supporting the Internet, data, video, LAN, voice/fax
communications, and can provide powerful private and public
network communication solutions.
• They are becoming increasingly popular, as VSATs are a single,
flexible communications platform that can be installed quickly
and cost efficiently to provide telecoms solutions for
consumers, governments and corporations.

91
 VSAT NETWORK ARCHITECTURES
• Any telecommunication services there are three basic
implementations services: one-way, split-two-way (referred to
as split-IP sometimes, when referring to internet traffic) and
two-way implementation. Further division of two-way
implementation is star and mesh network architectures.
• There are two Architectures:

92
 STAR
• In Star network architecture, all traffic is routed via the
main hub station.
• If a VSAT want to communicate with another VSAT, they
have to go through the hub station.
• This makes double hop link via the satellite. Star is the
most common VSAT configuration of the TDM/TDMA.
•These have a high bit rate outbound carrier (Time Division
Multiplexed) from the hub to the remote earth stations, and
one or more low or medium bit rate (Time Division Multiple
Access) inbound carriers.
•In a typical VSAT network, remote users have a number of
personal computers or dumb terminals

93
 MESH

• Meshed VSAT networks provide a way to set up a switched

point to point data network that can have the capability for
high data rates of up to 2Mb/s.
• Links are set up directly between remote terminals usually on
a call by call basis. These networks are usually configured to
operate without a large central earth station and carry a mix of
data traffic and telephony traffic or only data traffic.
• These networks generally will have a network control station,
which controls the allocation of resources across the network.
• This control centre is only involved in the signaling for the call
setup/teardown and in monitoring the operation of the
network.

94
 ACCESS CONTROL

• In general, multiple access schemes suitable for use in VSAT

networks are packet-oriented.
• Loosely speaking, they may be classified into two broad
categories; namely, contention or random access schemes and
reservation schemes.
• The main contention schemes, suitable for use in VSAT
systems, are based on the ALOHA concept, of which there are
three variations; namely,
• Pure ALOHA,
• Slotted ALOHA
• Reservation ALOHA

95
 MULTIPLE ACCESS SELECTION

• One satellite can simultaneously support thousands of Pico

terminal accesses.
• This means that the number of users in a Pico terminal
network can be a multiple of this, resulting in a
communication network with an enormous size.
• To control such a number of terminals, the multiple access
schemes for Pico terminals may be a combination of frequency
division multiple access (FDMA) and code division multiple
access (CDMA).
• The CDMA spread-spectrum technique normally used for
satellite communications is direct sequence spread spectrum.

96
• The use of spread spectrum techniques in Pico terminal
networks has several advantages.
• It is advantageous as multiple access schemes, because
(asynchronous) SSMA does not need network control and
synchronization.
• A second advantage is the inherent interference protection of
the system. This is important for Pico terminals which will be
more or less sensitive to interference from unwanted directions
due to their small antennas.
• A third advantage is that Pico terminals can transmit with low
power densities giving less interference problems.
• Finally SSMA gives some kind of message privacy through the
encryption with a code word

97
 NGSO CONSTELLATION DESIGN: ORBITS
• NGSOs are classified in the following three types as per the
inclinations of the orbital plane
Polar Orbit
Equatorial Orbit
Inclined Orbit
• In polar orbit the satellite moves from pole to pole and the
inclination is equal to 90 degrees.
• In equatorial orbit the orbital plane lies in the equatorial plane
of the earth and the inclination is zero or very small.
• All orbits other than polar orbit and equatorial orbit are called
inclined orbit. A satellite orbit with inclination of less than 90
degrees is called a pro grade orbit.

98
• The satellite in pro grade orbit moves in the same direction
as the rotation of the earth on its axis.
• Satellite orbit with inclination of more than 90 degrees is
called retrograde orbit when the satellite moves in a
direction opposite to the rotational motion of the earth.
• Orbits of almost all communication satellites are pro grade
orbits, as it takes less propellant to achieve the final
velocit of the satellite in pro grade orbit by taking
yAdvantage of the earth’s rotational speed.

99
 COVERAGE
• The designer of a satellite system has few degrees of freedom
in designing a pay load to provide optimum coverage.
• This occurs in some missions where a shared space craft has
to accommodate a no. of payloads.
• A GEO can be selected or a constellation of NGSO satellites
can be designed to provide the necessary coverage overlap
between successive satellites.
• The determination of coverage area, while initially an exercise
in simple geometry, is eventually heavily influenced by the
available technology both on the ground and in space, and
other aspects such as the radiation environment.
• First consider the geometrical aspects of determining an
optimum coverage. The elevation angle to the satellite is θ.
using the sine rule:
This yield to
100
 FREQUENCY BANDS
• Low earth orbit satellite systems providing data and voice
service to mobile users tend to use the lowest available RF
frequency.
• The EIRP required by the satellite transponder to establish a
given C/N ratio in the mobile receiver is proportional to the
square of the RF frequency of the downlink.
• The power that must be transmitted by a mobile transmitter is
also proportional to RF frequency squared when the mobile
uses an Omni directional antenna.
• Since the cost of satellites increases as the EIRP of the
transponders increases, a lower RF frequency yields a lower
cost system.
• This is one reason why L-band is allocated for mobile satellite
services.
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 DELAY AND THROUGHPUT

• Delay in a communications link is not normally a problem

unless the interactions between the users are very rapid – a few
milli seconds apart in response time.
• Long delays, such as those associated with manned missions
to the moon. For most commercial satellite links that are over
long distances, particularly those with satellites in
geostationary orbit, the main problem was not delay, but
echo.
• A mismatched transmission line will always have a reflected
signal. If the mismatch is large, a strong echo will return.
• Over a GEO satellite link, the echo arrives back in the
telephone head set about half a second after the speaker has
spoken, and usually while the speaker is still speaking.

102
NGSO (NON GEOSTATIONARY ORBIT ) CONSTELLATION DESIGN

Constellation design:
• Basic formation
• Station keeping
• Collision avoidance
Constellation: set of satellites distributed over space intended to
work together to achieve a common objective. Satellites that are
in close proximity are called clusters or formations.

103
 Principal factors to be defined during constellation design
Factor Effect Selection criteria

NO. Of satellites Principal cost & Minimize number

coverage driver consistent with meeting
other criteria.
Constellation pattern Determines coverage Vs Select for best coverage
latitude

Minimum elevation Principal determinant of Minimum value

angle single satellite coverage consistent with
constellation pattern
Altitude Coverage, environment System level trade of
launch and transfer cost cost Vs performance

NO. Of orbit planes Determines coverage Minimize consistent

plateaus, growth and with coverage needs.
degradation
Collision avoidance Key to preventing Maximize the inter
parameters constellation destruction satellite distances at
plane crossings

104
 UNIT V
SATELLITE PACKET
COMMUNICATION

105
 MESSAGE TRANSMISSION BY FDMA

•With Frequency Division Multiple Access (FDMA) the entire

available frequency channel is divided into bands and each
band serves a single station.
•Every station is therefore equipped with a transmitter for a
given frequency band, and a receiver for each band.
•To evaluate the performance of the FDMA protocol, we
assume that the entire channel can sustain a rate of R bits/sec
which is equally divided among M stations i.e. R/M bits/sec for
each.
•The individual bands do not overlap as such there is no
interference among transmitting stations. This allows for
viewing the system as M mutually independent queues.
•If the packet length is a random variable P, then the service
time afforded to every packet is the random variable.
106
 M/G/I QUEUE

• Consider a queuing system, in which arrivals occur according

to a Poisson process with parameter and in which x is the
service rendered to the customers, is distributed according to a
distribution B (t).
• In such a queuing system, an outside observer sees the number
of customers in the system as equal to that seen by an arriving
customer, which equals that seen by a departing customer.
• The following holds for an M/G/I queuing system:
•D = x + W = x +
Where:
• D= Average delay time; p = λx= Load factor; W= Queuing
time.

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• Therefore, for an FDMA system, considering a typical user that
generates packets according to a Poisson process with rate λ
packets/sec and its buffering capabilities are not limited, the
time required for the transmission of a packet.
• Each node can therefore be viewed as an M/G/1 queue since
each packet size is not constant. Thus, using the known
system delay time formula for M/G/1 queuing systems we get
that the expected delay of a packet is:
• D= T +
• And the delay distribution is given by,
• Where (S) is the Laplace transform of the transmission time

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 MESSAGE TRANSMISSION BY TDMA

• In the time division multiple access (TDMA) scheme the

entire time frame is divided into time slots, pre-assigned to
the different stations.
• Centralized control is absent in a contention-based system,
as such when a node needs to transmit data, it contends for
control of the transmission medium.
• The major advantage of contention techniques is simplicity,
as they are easily implementable in individual nodes. The
contention techniques are efficient under light to moderate
network load, but performance rapidly degrades with
increase in load level.
• Message transmission by TDMA can be done using the
ALOHA protocol, packet reservation and tree algorithm.

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 PACKET RESERVATION

• Dynamic channel allocation protocols are designed to

overcome the drawback faced by static conflict-free protocols,
which involves (inefficient) under utilization of the shared
channel, especially when the system is lightly loaded or when
the loads of different users are asymmetric.
• The static and fixed assignment in these protocols, cause the
channel (or part of it) to be idle even though some users have
data to transmit.

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 TREE ALGORITHM
• This is a collision resolution protocol (CRP). As opposed to the
instability of the ALOHA protocol, the efforts of CRP are
concentrated on resolving collisions as soon as they occur.
• Here, the fixed-length packets involved in collision participate
in a systematic partitioning procedure for collision resolution,
during which time new messages are not allowed to access the
channel.
• The stability of the system is ensured provided that the arrival
rate of new packets to the system is lower than its collision
resolution rate.
• The tree-type protocols have excellent channel capacity
capabilities, but are vulnerable to deadlocks due to incorrect
channel observation

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Examples of improved binary-tree protocols are
• 1. The Modified binary-tree protocol: Its operation requires
ternary feedback, i.e., the users have to be able to distinguish
between idle and successful slots.
• 2. The Epoch Mechanism: Its operation models the system in
such a way that the CRI starts with the transmission of exactly
one packet (yields a throughput of 1) by determining when
packets are transmitted for the first time.
• 3. The Clipped binary-tree protocol: This improved on the
Epoch mechanism by adopting the rule that whenever a
collision is followed by two successive successful
transmissions, the packets that arrived in.

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 ERROR CONTROL FOR DIGITAL SATELLITE LINKS:
ERROR CONTROL CODING:

• The primary function of an error control encoder-decoder pair

is to enhance the reliability of message
• An error control code can also ease the design process of a
digital transmission system in multiple ways such as the
following:
a) the transmission power requirement of a digital transmission
scheme can be reduced by the use of an error control codec.
b) Even the size of a transmitting or receiving antenna can be
reduced by the use of an error control codec while
maintaining the same level of end-to-end performance.
c) Access of more users to same radio frequency in a multi-
access communication system can be ensured by the use of
error control technique.
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The major categories of activities on error control coding can
broadly be identified as the following:
a) to find codes with good structural properties and good
asymptotic error performance,
b)to devise efficient encoding and decoding strategies for the
codes and
c) to explore the applicability of good coding schemes in various
digital transmission and storage systems and to evaluate their
performance.

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115
 BLOCK CODES
The encoder of a block code operates on a group of bits at a
time. A group or ‘block’ of ‘k’ information bits (or symbols) are
coded using some procedure to generate a larger block of ‘n’ bits
(or symbols). Such a block code is referred as an (n, k) code.
Convolution codes:
• Convolutional codes, which are used in a variety of systems
including today’s popular wireless standards (such as 802.11)
and in satellite communications.
• Convolutional codes are beautiful because they are intuitive,
one can understand them in many different ways, and there is a
way to decode them so as to recover the mathematically most
likely message from among the set of all possible transmitted
messages.

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 IMPLEMENTATION OF ERROR DETECTION ON
SATELLITE LINKS
 The following three basic techniques can be used, which are
based on the type of the link used for retransmission request:
 In a one way simplex link, the ACK and NAK signal must
travel on the same path as the data, so the transmitter must
stop transmission after each block and wait for the receiver to
send back a NAK or ACK before it retransmits the last data
block or sends the next one.
 The data rate is very slow and thus useful for links in which
data are generated slowly.
 In a stop and wait system, the transmitting end sends a block
data and waits for the acknowledgement to arrive on the
return channel. Though the implementation is simple but the
amount of delay is the same as the simplex case.
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 In a continuous transmission system using the go- back – N
technique, data are sent in the form of a block continuously
and held in a buffer at the receiver of the end of the link.
 When the data block arrives, it is checked for error and the
appropriate ACK or NAK is send back to the transmitting end
with block number specified.
 When a NAK (N) is received, the transmit end goes back to
block N and retransmits all subsequent blocks.

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 DATA RELAY COMMUNICATION SATELLITES
• Data Relay Satellite System (DRSS) is primarily meant for
providing continuous/real time communication of Low-Earth-
Orbit (LEO) satellites/human space mission to the ground
station.
• A data relay satellite in the Geo-stationary Orbit (GEO) can see
a low altitude spacecraft for approximately half an orbit.
• Two such relay satellites, spaced apart in GEO, could
theoretically provide continuous contact for any spacecraft in
LEO.

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 ARCHITECTURE OF A GENERIC DRSS SATELLITE
a) DRSS Space Segment:
The DRSS space segment primarily consists of high altitude
satellite system of GEO or Molniya class, defined in a modular
way providing several payloads satisfying the data relay service
requirements at different orbital positions with on-board
state-of-the-art technologies.
b) DRSS Ground Segment:
DRSS Network Control Centre, whose task will include
managing and operating the end to end data relay links and to
provide data relay customer interface for mission request,
mission planning, scheduling and mission execution.
c) DRSS User Segment:
The users of DRSS services can be broadly categorized as
Institutional users (e.g. Space agency), Commercial users (e.g.
Other Launcher/Satellite Agencies). 120
 SATELLITE MOBILE SERVICES
• Mobile satellite service (MSS) is the term used to describe
telecommunication services delivered to or from the mobile
users by using the satellites.
• MSS can be used in remote areas lacking wired networks.
• Limitations of MSS are availability of line of sight
requirement
and emerging technologies.
• The basic Mobile satellite service (MSS) System comprises of
these three segments:
 SPACE SEGMENT
 USER SEGMENT
 CONTROL SEGMENT

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• Space segment: Space segment is equipped with satellite pay-
load equipment. The Pay load is used to enable the ability of
the satellite for users in space communication.
• User segment: The user segment consists of equipment that
transmits and receives the signals from the satellite.
• Control segment: The control segment controls the satellite
and operations of all internet connections to maintain the
bandwidth and adjust power supply and antennas.

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The mobile satellite services are classified into the following five
types:
• Maritime mobile satellite service (MMSS).
• Land mobile satellite service (LMSS):
• Aeronautical mobile satellite service (AMSS):
• Personal mobiles satellite service (PMSS):
• Broadcast mobile satellite service (BCMSS):

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 APPLICATIONS OF SATELLITES
• Satellites that are launched in to the orbit by using the rockets
are called man-made satellites or artificial satellites.
• Artificial satellites revolve around the earth because of the
gravitational force of attraction between the earth and satellites.
• Unlike the natural satellites (moon), artificial satellites are used
in various applications.

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The various applications of artificial satellites include:
1. Weather Forecasting
2. Navigation
3. Astronomy
4. Satellite phone
5. Satellite television
6. Military satellite
7. Satellite Internet
8. Satellite Radio.

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