Satellite Communication

5EC5-14

Unit #1

1.1 Introduction
1.2 Principles and Architecture of Satellite communication
1.3 Brief History of Satellite Communication
1.4 Advantages and Disadvantages
1.5 Applications
1.6 Frequency bands used for satellite communication

Course OutComes:

CO 1 Able to understand the dynamics and architecture of the

satellite.
CO 2 Solve numerical problems related to orbital motion.
CO 3 Examine the design of Earth station and tracking of the
satellites.
CO 4 Evaluate and design link power budget for the Satellites.
CO 5 Analyze the analog and digital technologies used for satellite
communication.

Text/Reference Books:

1. Timothy Pratt Charles W. Bostian, Jeremy E.Allnutt:

Satellite Communications: Wiley India. 2nd edition 2002.
2. Tri T. Ha: Digital Satellite Communications: Tata McGraw Hill, 2009
INTRODUCTION:

Satellites are specifically made for telecommunication purpose. They are used for mobile
applications such as communication to ships, vehicles, planes, hand-held terminals and for TV
and radio broadcasting.

They are responsible for providing these services to an assigned region (area) on the earth. The
power and bandwidth of these satellites depend upon the preferred size of the footprint,
complexity of the traffic control protocol schemes and the cost of ground stations.

A satellite works most efficiently when the transmissions are focused with a desired area. When
the area is focused, then the emissions don’t go outside that designated area and thus minimizing
the interference to the other systems. This leads more efficient spectrum usage.

Satellite’s antenna patterns play an important role and must be designed to best cover the
designated geographical area (which is generally irregular in shape). Satellites should be
designed by keeping in mind its usability for short and long term effects throughout its life time.

The earth station should be in a position to control the satellite if it drifts from its orbit it is
subjected to any kind of drag from the external forces.

PRINCIPLES OF SATELLITE COMMUNICATION:

Satellites orbit around the earth. Depending on the application, these orbits can be circular or
elliptical. Satellites in circular orbits always keep the same distance to the earth’s surface
following a simple law:

The attractive force Fg of the earth due to gravity equals

m·g (R/r) 2

The centrifugal force Fc trying to pull the satellite away equals

m·r·ω2

The variables have the following meaning: m is the mass of the satellite;

R is the radius of earth with R = 6,370 km;

ri s the distance of the satellite to the centre of the earth;

g is the acceleration of gravity with g = 9.81 m/s2;

ω is the angular velocity with ω = 2·π·f, f is the frequency of the rotation.

To keep the satellite in a stable circular orbit, the following equation must hold:

Fg = Fc, i.e., both forces must be equal. Looking at this equation the first thing to notice is that
the mass m of a satellite is irrelevant (it appears on both sides of the equation).
Solving the equation for the distance r of the satellite to the centre of the earth results in the
following equation. The distance r = (g·R2/(2·π·f)2)1/3

From the above equation it can be concluded that the distance of a satellite to the earth’s surface
depends on its rotation frequency.

Important parameters in satellite communication are the inclination and elevation angles. The
inclination angle δ (figure 1.1) is defined between the equatorial plane and the plane described
by the satellite orbit. An inclination angle of 0 degrees means that the satellite is exactly above
the equator. If the satellite does not have a circular orbit, the closest point to the earth is called
the perigee.

Figure 1.1: Angle of Inclination

The elevation angle ε in figure 1.2 is defined between the centre of the satellite beam and the
plane tangential to the earth’s surface. A so called footprint can be defined as the area on earth
where the signals of the satellite can be received.

Figure 1.2: Angle of Elevation

ARCHITECTURE OF SATELLITE SYSTEM:

A satellite communication system can be broadly divided into two segments, a ground segment
and a space-segment. The space system includes Satellite.

Space segment: The space segment consists of the satellite, which has three main systems: (a)
fuel system; (b) satellite and telemetry control system; and (c) transponders. The fuel system is
responsible for making the satellite run for years. It has solar panels, which generate the
necessary energy for the operation of the satellite. The satellite and telemetry control system is
used for sending commands to the satellite as well as for sending the status of onboard systems
to the ground stations. The transponder is the communication system, which acts as a relay in the
sky. The transponder receives the signals from the ground stations, amplifies them, and then
sends them back to the ground stations. The reception and transmission are done at two different
frequencies. The transponder needs to do the necessary frequency translation.

Figure 1.3: Architecture of Satellite Communication

Ground Segment: The ground segment consists of a number of Earth stations. In a star
configuration network, there will be a central station called the hub and a number of remote
stations. Each remote station will have a very small aperture terminal (VSAT), an antenna of
about 0.5 meter to 1.5 meters. Along with the antenna there will an outdoor unit (ODU), which
contains the radio hardware to receive the signal and amplify it. The radio signal is sent to an
indoor unit (IDU), which demodulates the signal and carries out the necessary baseband
processing. IDU is connected to end systems, such as a PC, LAN, or PBX.

The central station consists of a large antenna (4.5 meters to 11 meters) along with all associated
electronics to handle a large number of VSATs. The central station also will have a Network
Control Center (NCC) that does all the management functions, such as configuring the remote
stations, keeping a database of the remote stations, monitoring the health of the remotes, traffic
analysis, etc. The NCC's main responsibility is to assign the necessary channels to various
remotes based on the requirement.

The communication path from a ground station to the satellite is called the uplink. The
communication link from the satellite to the ground station is called the downlink. Separate
frequencies are used for uplink and downlink. When a remote transmits data using an uplink
frequency, the satellite transponder receives the signal, amplifies it, converts the signal to the
downlink frequency, and retransmits it. Because the signal has to travel nearly 36,000 km in each
direction, the signal received by the satellite as well as the remote is very weak. As soon as the
signal is received, it has to be amplified before further processing.
Satellite system consists of the following sub systems.

1. POWER SUPPLY:

The primary electrical power for operating electronic equipment is obtained from solar cells.
Individual cells can generate small amounts of power, and therefore array of cells in series-
parallel connection are required.

Cylindrical solar arrays are used with spinning satellites, (The gyroscopic effect of the spin is
used for mechanical orientation stability) Thus the array are only partially in sunshine at any
given time.

Another type of solar panel is the rectangular array or solar sail. Solar sail must be folded during
the launch phase and extended when in geo-stationary orbit. Since the full component of solar
cells are exposed to sun light ,and since the Sail rotate to track, the sun , they capable of greater
power output than cylindrical arrays having a comparable number of cells.

To maintain service during an eclipse, storage batteries must be provided.

2. ATTITUDE CONTROL:

The attitude of a satellite refers to its Orientation in space. Much of equipment carried abroad a
satellite is there for the purpose of controlling its attitude. Attitude control is necessary, for
example, to ensure that directional antennas point in the proper directions. In the case of earth
environmental satellites the earth-sensing instrument must cover the required regions of the
earth, which also requires attitude control. A number of forces, referred to as disturbance forces
can alter attitude, some examples being the gravitational forces of earth and moon, solar
radiation, and meteorite impacts.

3. STATION KEEPING:

A satellite that is normally in geo-stationary will also drift in latitude, the main perturbing forces
being the gravitational pull of the sun and the moon. The force cause the inclination to change at
the rate of about 0.85 deg./year. if left uncorrected, the drift would result in a cycle change in the
inclination going 0 to 14.67deg in 26.6 years and back to zero , when the cycle is repeated. To
prevent the shift in inclination from exceeding specified limits, jets may be pulled at the
appropriate time to return the inclination to zero. Counteracting jets must be pulsed when the
inclination is at zero to halt that change in inclination.

4. THERMAL CONTROL:

Satellites are subject to large thermal gradients, receiving the sun radiation on one side while the
other side faces into space. In addition, thermal radiation from the earth, and the earth's abedo,
which is the fraction on the radiation falling on the earth which is reflected can be sight for low
altitude, earth-orbiting satellites, although it is negligible for geo-stationary satellites. Equipment
in the satellite also generates heat which has to be removed. the most important consideration is
that the satellite's equipment should operate as near as possible in a stable temperature
environment. Various steps are taken to achieve this. Thermal blankets and shields may be used
to provide insulation. Radiation mirrors are often used to remove heat from communication
payload. These mirrored drums surrounded the communication equipment shelves in each case
and provide good radiation paths for the generated heat to escape in to surround space.

To maintain constant-temperature conditions, heaters may be switched on to make up for the

hearts may be switched on to make reduction that occurs when transponders are switched off.

5. TT & C SUBSYSTEM:
5.1 Telemetry system

The telemetry, tracking, and command (TT&C) subsystem performs several routine functions
abroad a spacecraft. The telemetry or "telemetering" function could be interpreted as
"measurement at a distance". Specifically, it refers to the overall operation of generating an
electrical signal proportional to the quantity being measured, and encoding and transmitting this
to a distant station, which for satellite is one of the earth stations, which for the satellite is one of
the earth stations. Data that are transmitted as telemetry signals include attribute information
such as obtained from sun earth sensors; environmental information such as magnetic field
intensity and direction; the frequency of meteorite impact and so on ;and spacecraft information
such as temperatures and power supply voltages, and stored fuel pressure.

5.2 Command Systems:

Command system receives instructions from ground system of satellite and decodes the
instruction and sends commands to other systems as per the instruction.

5.3 Tracking

Tracking of the satellite is accomplished by having the satellite is accomplished by having the
satellite transmit beacon signals which are received at the TT&C earth stations. Tracking is
obviously important during the transmitter and drift orbital phases of the satellite launch. When
on-station, a geo-stationary satellite will tend to shifted as a result of the various distributing
forces, as described previously. Therefore it is necessary to be able to track the satellites
movements and send correction signals as required. Satellite range is also required for time to
time. This can be determined by measurement of propagation delay of signals specially
transmitted for ranging purposes.
 Figure 1.4: Telemetry, Tracking & Control System of Satellite Communication

TRANSPONDERS:

A transponder is a wireless communications, monitoring, or control device that picks up the

signal from earth station and automatically responds to an incoming signal and send it to another
earth station. The term is a contraction of the words transmitter and responder. A transponder is
the series of interconnected units which forms a single communication channel between
receiving and transmitting antennas in a communication satellite. Some of the units utilized by a
transponder in a given channel may be common to a number of transponders. Thus, although
reference may be made to specific transponder, this must be thought of as an equipment channel
rather than single item of equipment.

Figure 1.5: Architecture of Transponder

Transponder consists of wideband receiving Antenna (Uplink Freq 6Ghz), input de-multiplexer,
power amplifier components and Transmitting Antenna (Down Link Freq 4 Ghz).
ANTENNA SUB SYSTEM:

The Antennas carried abroad a satellite provide the dual functions of receiving the uplink and
transmitting the down link signals. They range from dipole-type antennas, where omni
directional characteristics are required, to the highly directional antennas required for
telecommunications purposes and TV relay and broadcasting.

Brief History of Satellite Communication:

ADVANTAGES:

1. It is used for mobile and wireless communication applications independent of location.

2. It covers wide area of the earth hence entire country or region can be covered with just
one satellite.
3. It provides wider bandwidth based on SCPC or MCPC allocation types.
4. It co-exists with terrestrial microwave line of sight communication.
5. It is easy to install and manage the ground station sites.
6. It does not incur much of the costs per VSAT site.
7. It is used for voice, data, video and any other information transmission. Satellite system
can be interfaced with internet infrastructure to obtain internet service. It is also used for
GPS applications in various mobile devices for location determination.
8. It is easy to obtain service from one single provider and uniform service is available.
9. It has small fading margin on the order of about 3dB.
 10. It is used in wide variety of applications which include weather forecasting, radio/TV
signal broadcasting, gathering intelligence in military, navigation of ships and aircrafts,
global mobile communication, connecting remote areas etc.
11. LEO and MEO satellite types have lower propagation delay and lower losses compare to
GEO satellite. This will help them to be used for global coverage.\

DISADVANTAGES:

1. Satellite manufacturing requires more time. Moreover satellite design and development
requires higher cost.
2. Satellite once launched, requires to be monitored and controlled on regular periods so that
it remains in the orbit.
3. Satellite has life which is about 12-15 years. Due to this fact, another launch has to be
planned before it becomes un-operational.
4. Redundant components are used in the network design. This incurs more cost in the
installation phase.
5. In the case of LEO/MEO, large numbers of satellites are needed to cover radius of earth.
Moreover satellite visibility from earth is for very short duration which requires fast
satellite to satellite handover. This makes system very complex.

APPLICATIONS OF SATELLITE COMMUNICATION:

A.) Weather Forecasting

Certain satellites are specifically designed to monitor the climatic conditions of earth. They
continuously monitor the assigned areas of earth and predict the weather conditions of that
region. This is done by taking images of earth from the satellite. These images are transferred
using assigned radio frequency to the earth station. (Earth Station: it’s a radio station located on
the earth and used for relaying signals from satellites.) These satellites are exceptionally useful in
predicting disasters like hurricanes, and monitor the changes in the Earth's vegetation, sea state,
ocean color, and ice fields.

B.) Radio and TV Broadcast

These dedicated satellites are responsible for making 100s of channels across the globe available
for everyone. They are also responsible for broadcasting live matches, news, world-wide radio
services. These satellites require a 30-40 cm sized dish to make these channels available
globally.

C.) Military Satellites

These satellites are often used for gathering intelligence, as a communications satellite used for
military purposes, or as a military weapon. A satellite by itself is neither military nor civil. It is
the kind of payload it carries that enables one to arrive at a decision regarding its military or
civilian character.
D.) Navigation Satellites

The system allows for precise localization world-wide, and with some additional techniques, the
precision is in the range of some meters. Ships and aircraft rely on GPS as an addition to
traditional navigation systems. Many vehicles come with installed GPS receivers. This system is
also used, e.g., for fleet management of trucks or for vehicle localization in case of theft.

E.) Global Telephone Communication

One of the first applications of satellites for communication was the establishment of
international telephone backbones. Instead of using cables it was sometimes faster to launch a
new satellite. But, fiber optic cables are still replacing satellite communication across long
distance as in fiber optic cable, light is used instead of radio frequency, hence making the
communication much faster (and of course, reducing the delay caused due to the amount of
distance a signal needs to travel before reaching the destination.) Using satellites, to typically
reach a distance approximately 10,000 kms away, the signal needs to travel almost 72,000 kms,
that is, sending data from ground to satellite and (mostly) from satellite to another location on
earth. This cause‟s substantial amount of delay and this delay becomes more prominent for users
during voice calls.

F.) Connecting Remote Areas

Due to their geographical location many places all over the world do not have direct wired
connection to the telephone network or the internet (e.g., researchers on Antarctica) or because
of the current state of the infrastructure of a country. Here the satellite provides a complete
coverage and (generally) there is one satellite always present across a horizon.

G.) Global Mobile Communication

The basic purpose of satellites for mobile communication is to extend the area of coverage.
Cellular phone systems, such as AMPS and GSM (and their successors) do not cover all parts of
a country. Areas that are not covered usually have low population where it is too expensive to
install a base station. With the integration of satellite communication, however, the mobile phone
can switch to satellites offering world-wide connectivity to a customer. Satellites cover a certain
area on the earth. This area is termed as a „footprint‟ of that satellite. Within the footprint,
communication with that satellite is possible for mobile users. These users communicate using a
Mobile-User-Link (MUL). The base-stations communicate with satellites using a Gateway-Link
(GWL). Sometimes it becomes necessary for satellite to create a communication link between
users belonging to two different footprints. Here the satellites send signals to each other and this
is done using Inter-Satellite-Link (ISL).
ALLOCATION OF FREQUENCY BANDS USED FOR SATELLITE
COMMUNICATION

Allocation of frequencies to satellite services s 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

 Region2: North and South America and Greenland
 Region3: 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 system.

Meteorological satellite services: They are often used to perform Search and Rescue service.

Below are the frequencies allocated to these satellites:

Frequency Band (GHZ) Designations.

 VHF: 01-0.3
 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
 μm-band: 300-3000

Based on the satellite service, following are the frequencies allocated to the
satellites:
Frequency Band (GHZ) Designations:
 VHF:01-0.3 For Mobile & Navigational Satellite Services
 L-band: 1.0-2.0 For Mobile & Navigational Satellite
 Services
 C-band: 4.0-8.0 For Fixed Satellite Service
 Ku-band: 12.0-18.0 For Direct Broadcast Satellite Services
 Satellite Communication
5EC5-14
UNIT-2
Orbital Mechanics
2.1 Orbital equations
2.2 Kepler's laws
2.3 Apogee and Perigee for an elliptical orbit
2.4 Evaluation of velocity, orbital period, angular velocity of a satellite.
2.5 Concepts of Solar day and Sidereal day.

2.1 Orbital equation:

2.2.Kepler's laws:
2.2.1 Kepler’s First Law:
Keplers first law states that the path followed by a satellite around the primary will be an ellipse.
An ellipse has two focal points shown as F1 and F2 in Fig. The center of mass of the two-body
system is always centered on one of the foci

• Focus – one of two special points on

the major axis of an ellipse.

• Foci – plural of focus

F1+F2 is always
the same on
any point on
the ellipse

• The semimajor axis of the ellipse is

denoted by a,

and the semiminor axis, by b. The eccentricity e is given by

Eccentricity is the degree of flatness of satellite orbit e = c/a (e = 0 for circle)

c = center to focus
a = half of major axis/Semi-major axis

2.2.2 KepIer’s Second Law: (Law of equal Area)

 The line joining the planet to the sun sweeps out equal
areas in equal intervals of time.
 The area from one time to another time is equal to
another area with the same time interval.
 Facts:
 Planet moves faster when closer to the sun.
Force acting on the planet increases as distance decreases
and planet accelerates in its orbit
 Planet moves slower when farther from the sun.
 2.2.3 KepIer’s Third Law:

The square of the period of any planet is proportional to the cube of the semi-major
of its axis.
T² α a³
T = orbital period in years
a = semi-major axis in astronomical unit (AU)
It Can calculate how long it take (period) for planets to orbit if semimajor axis is
known.
Astronomical unit – AU
AU is the mean distance between Earth and the Sun
1 AU ≈ 1.5 x 108 km ≈ 9.3 x 107 miles

Examples of 3rd Law:

Calculating the orbital period of 1AU

T² = a³
T² = (1)³ = 1
T = 1 year
Calculating the orbital period of 4AU
T² = a³
T² = (4)³ = 64
T = 8 years
2.3 Apogee and Perigee for an elliptical orbit:

Apogee: The point of satellite farthest from earth.

Perigee: The point of satellite closest approach to earth.

Different Types of Orbit:

Difference between Geostationary, Geosynchronous and polar satellite:

(90 degree)

LOS Communication

2.4 Evaluation of velocity, orbital period, angular velocity of a satellite:

Satellites are made to revolve in an orbit at a height of few hundred kilometres. At this
altitude, the friction due to air is negligible. The satellite is carried by a rocket to the
desired height and released horizontally with a high velocity, so that it remains moving
in a nearly circular orbit. The horizontal velocity that has to be imparted to a satellite at
the determined height so that it makes a circular orbit around the planet is called
orbital velocity. Let us assume that a satellite of mass m moves around the Earth in a
circular orbit of radius r with uniform speed vo. Let the satellite be at a height h from
the surface of the Earth. Hence, r = R+h, where R is the radius of the Earth.
Time period of a satellite:
Time taken by the satellite to complete one revolution round the Earth is called time period.
 2.5 Concepts of Solar day and Sidereal day:
A solar day is the time it takes for the Earth to rotate about its axis so that
the Sun appears in the same position in the sky.

A sidereal day is the time it takes for the Earth to rotate about its axis so that the
distant stars appear in the same position in the sky. The sidereal day is ~4 minutes
shorter than the solar day.

The sidereal day is the time it takes for the Earth to complete one rotation about its axis
with respect to the ‘fixed’ stars. By fixed, we mean that we treat the stars as if they
were attached to an imaginary celestial sphere at a very large distance from the Earth.

a sidereal day lasts for 23 hours 56 minutes 4.091 seconds, which is slightly shorter
than the solar day measured from noon to noon. Our usual definition of an Earth day is
24 hours, so the sidereal day is 4 minutes faster. This means that a particular star will
rise 4 minutes earlier every night.
 Satellite Communication
5EC5-14

Unit # 3
Satellite Sub-Systems

3.1 Study of Architecture and Roles of various sub systems of a satellite system
such as Telemetry, tracking, command and monitoring (TTC & M).
3.2 Attitude and orbit control system (AOCS).
3.3 Communication sub-system, Power sub-systems etc.

3.1 Telemetry, Tracking, Command and Monitoring (TTC & M): Telemetry,
Tracking, Commanding and Monitoring (TTCM) subsystem is present in both satellite and earth
station. In general, satellite gets data through sensors. So, Telemetry subsystem present in the
satellite sends this data to earth station(s). Therefore, TTCM subsystem is very much necessary
for any communication satellite in order to operate it successfully.

It is the responsibility of satellite operator in order to control the satellite in its life time, after
placing it in the proper orbit. This can be done with the help of TTCM subsystem.

Telemetry and Monitoring Subsystem:

The word ‘Telemetry’ means measurement at a distance. Mainly, the following operations take
place in ‘Telemetry’.
 Generation of an electrical signal, which is proportional to the quantity to be measured.
 Encoding the electrical signal.
 Transmitting this code to a far distance.
3.1.1 Telemetry subsystem present in the satellite performs mainly two functions −

 Receiving data from sensors, and

 Transmitting that data to an earth station.
Satellites have quite a few sensors to monitor different parameters such as pressure,
temperature, status and etc., of various subsystems. In general, the telemetry data is transmitted
as FSK or PSK.
Telemetry subsystem is a remote controlled system. It sends monitoring data from satellite to
earth station. Generally, the telemetry signals carry the information related altitude,
environment and satellite.
 Figure 3.1: Telemetry, Tracking & Control System of Satellite Communication

3.1.2 Tracking Subsystem:

Tracking subsystem is useful to know the position of the satellite and its current orbit.
Satellite Control Center (SCC) monitors the working and status of space segment subsystems
with the help of telemetry downlink. And, it controls those subsystems using command uplink.
We know that the tracking subsystem is also present in an earth station. It mainly focuses on
range and look angles of satellite. Number of techniques that are using in order to track the
satellite. For example, change in the orbital position of satellite can be identified by using the
data obtained from velocity and acceleration sensors that are present on satellite.
The tracking subsystem that is present in an earth station keeps tracking of satellite, when it is
released from last stage of Launch vehicle. It performs the functions like, locating of satellite in
initial orbit and transfer orbit.

3.1.3 Commanding Subsystem:

Commanding subsystem is necessary in order to launch the satellite in an orbit and its working
in that orbit. This subsystem adjusts the altitude and orbit of satellite, whenever there is a
deviation in those values. It also controls the communication subsystem. This commanding
subsystem is responsible for turning ON / OFF of other subsystems present in the satellite
based on the data getting from telemetry and tracking subsystems.
In general, control codes are converted into command words. These command words are used to
send in the form of TDM frames. Initially, the validity of command words is checked in the
satellite. After this, these command words can be sent back to earth station. Here, these
command words are checked once again.
If the earth station also receives the same (correct) command word, then it sends an execute
instruction to satellite. So, it executes that command.
Functionality wise, the Telemetry subsystem and commanding subsystem are opposite to each
other. Since, the first one transmits the satellite’s information to earth station and second one
receives command signals from earth station.
3.1.4 Monitoring Subsystem:
 The monitoring system collects data from many sensors within the satellite & analyzes these
data to the controlling earth station.
 Monitoring parameters: pressure, temperature, and voltage, current.
 Evaluation of each component in the ground station is a very crucial process so as to
maintain optimal level in the performance of each of the components.
 Alarms can also be sounded if any vital parameter goes outside allowable limits.
 Attitude maintenance sighting devices are monitored via telemetry link.
 In failure case the satellite points in the wrong direction. The faulty unit must then be
disconnected and a spare brought in, via the command system, or some other means of
controlling attitude devised.
 These comparisons are done to take corrective or preventive action whenever required to
prevent failure or delays in the mission timelines.
 Parameters measured: AGC & BER
AGC: It is the plot between time and power. It helps us to determine the satellite anomalies.
BER: The figure of merit for a digital radio link is its Bit Error Rate

3.2 Attitude and orbit control system (AOCS):

We know that satellite may deviates from its orbit due to the gravitational forces from sun,
moon and other planets. These forces change cyclically over a 24-hour period, since the satellite
moves around the earth.
Altitude and Orbit Control (AOC) subsystem consists of rocket motors, which are capable of
placing the satellite into the right orbit, whenever it is deviated from the respective orbit. AOC
subsystem is helpful in order to make the antennas, which are of narrow beam type points
towards earth.
We can make this AOC subsystem into the following two parts.

 Altitude Control Subsystem

 Orbit Control Subsystem
Now, let us discuss about these two subsystems one by one.
Altitude Control Subsystem

Altitude control subsystem takes care of the orientation of satellite in its respective orbit.
Following are the two methods to make the satellite that is present in an orbit as stable.

 Spinning the satellite

 Three axes method

Spinning the Satellite:

In this method, the body of the satellite rotates around its spin axis. In general, it can be rotated
at 30 to 100 rpm in order to produce a force, which is of gyroscopic type. Due to this, the spin
axis gets stabilized and the satellite will point in the same direction. Satellites are of this type
are called as spinners.
Spinner contains a drum, which is of cylindrical shape. This drum is covered with solar cells.
Power systems and rockets are present in this drum.
Communication subsystem is placed on top of the drum. An electric motor drives this
communication system. The direction of this motor will be opposite to the rotation of satellite
body, so that the antennas point towards earth. The satellites, which perform this kind of
operation are called as de-spin.
During launching phase, the satellite spins when the small radial gas jets are operated. After
this, the de-spin system operates in order to make the TTCM subsystem antennas point towards
earth station.

Three Axis Method:

In this method, we can stabilize the satellite by using one or more momentum wheels. This
method is called as three-axis method. The advantage of this method is that the orientation of
the satellite in three axes will be controlled and no need of rotating satellite’s main body.
In this method, the following three axes are considered.
 Roll axis is considered in the direction in which the satellite moves in orbital plane.
 Yaw axis is considered in the direction towards earth.
 Pitch axis is considered in the direction, which is perpendicular to orbital plane.

Figure 3.2.1: Three Axis Method

These three axes are shown in below figure.
Let XR, YR and ZR are the roll axis, yaw axis and pitch axis respectively. These three axis are
defined by considering the satellite’s position as reference. These three axes define the altitude
of satellite.
Let X, Y and Z are another set of Cartesian axes. This set of three axis provides the information
about orientation of the satellite with respect to reference axes. If there is a change in altitude of
the satellite, then the angles between the respective axes will be changed.
In this method, each axis contains two gas jets. They will provide the rotation in both directions
of the three axes.
 The first gas jet will be operated for some period of time, when there is a requirement of
satellite’s motion in a particular axis direction.
 The second gas jet will be operated for same period of time, when the satellite reaches to
the desired position. So, the second gas jet will stop the motion of satellite in that axis
direction.

Orbit Control Subsystem:

Orbit control subsystem is useful in order to bring the satellite into its correct orbit, whenever
the satellite gets deviated from its orbit.
The TTCM subsystem present at earth station monitors the position of satellite. If there is any
change in satellite orbit, then it sends a signal regarding the correction to Orbit control
subsystem. Then, it will resolve that issue by bringing the satellite into the correct orbit.
In this way, the AOC subsystem takes care of the satellite position in the right orbit and at right
altitude during entire life span of the satellite in space.

3.3 Communication sub-system, Power sub-systems:

3.3.1 Communication Sub-System:
A satellite communication system can be broadly divided into two segments, a ground segment
and a space-segment. The space system includes Satellite.

Space segment: The space segment consists of the satellite, which has three main systems: (a)
fuel system; (b) satellite and telemetry control system; and (c) transponders. The fuel system is
responsible for making the satellite run for years. It has solar panels, which generate the
necessary energy for the operation of the satellite. The satellite and telemetry control system is
used for sending commands to the satellite as well as for sending the status of onboard systems
to the ground stations. The transponder is the communication system, which acts as a relay in the
sky. The transponder receives the signals from the ground stations, amplifies them, and then
sends them back to the ground stations. The reception and transmission are done at two different
frequencies. The transponder needs to do the necessary frequency translation.
 Figure 3.3.1: Architecture of Satellite Communication

Ground Segment: The ground segment consists of a number of Earth stations. In a star
configuration network, there will be a central station called the hub and a number of remote
stations. Each remote station will have a very small aperture terminal (VSAT), an antenna of
about 0.5 meter to 1.5 meters. Along with the antenna there will an outdoor unit (ODU), which
contains the radio hardware to receive the signal and amplify it. The radio signal is sent to an
indoor unit (IDU), which demodulates the signal and carries out the necessary baseband
processing. IDU is connected to end systems, such as a PC, LAN, or PBX.

The central station consists of a large antenna (4.5 meters to 11 meters) along with all associated
electronics to handle a large number of VSATs. The central station also will have a Network
Control Center (NCC) that does all the management functions, such as configuring the remote
stations, keeping a database of the remote stations, monitoring the health of the remotes, traffic
analysis, etc. The NCC's main responsibility is to assign the necessary channels to various
remotes based on the requirement.

The communication path from a ground station to the satellite is called the uplink. The
communication link from the satellite to the ground station is called the downlink. Separate
frequencies are used for uplink and downlink. When a remote transmits data using an uplink
frequency, the satellite transponder receives the signal, amplifies it, converts the signal to the
downlink frequency, and retransmits it. Because the signal has to travel nearly 36,000 km in each
direction, the signal received by the satellite as well as the remote is very weak. As soon as the
signal is received, it has to be amplified before further processing.

3.3.2 Power Sub Systems:

We know that the satellite present in an orbit should be operated continuously during its life
span. So, the satellite requires internal power in order to operate various electronic systems and
communications payload that are present in it.
Power system is a vital subsystem, which provides the power required for working of a
satellite. Mainly, the solar cells (or panels) and rechargeable batteries are used in these systems.
Solar Cells:
Basically, the solar cells produce electrical power (current) from incident sunlight. Therefore,
solar cells are used primarily in order to provide power to other subsystems of satellite.
We know that individual solar cells generate very less power. So, in order to generate more
power, group of cells that are present in an array form can be used.

Figure 3.3.2 Power Sub Systems of satellite

Solar Arrays:
There are two types of solar arrays that are used in satellites. Those are cylindrical solar arrays
and rectangular solar arrays or solar sail.
 Cylindrical solar arrays are used in spinning satellites. Only part of the cylindrical
array will be covered under sunshine at any given time. Due to this, electric power gets
generated from the partial solar array. This is the drawback of this type.
 The drawback of cylindrical solar arrays is overcome with solar sail. This one produces
more power because all solar cells of solar sail are exposed to sun light.

Rechargeable Batteries:
During eclipses time, it is difficult to get the power from sun light. So, in that situation the other
subsystems get the power from rechargeable batteries. These batteries produce power to other
subsystems during launching of satellite also.
In general, these batteries charge due to excess current, which is generated by solar cells in the
presence of sun light.
 Satellite Communication
5EC5-14
Unit # 4

Typical Phenomena in Satellite Communication:

Solar Eclipse on satellite, its effects, remedies for Eclipse, Sun Transit Outage phenomena, its effects and
remedies, Doppler frequency shift phenomena and expression for Doppler shift. Satellite link budget

4.1 Solar Eclipse on satellite:

• A satellite is said to be in eclipse when the earth or moon prevents sunlight from reaching it.
• If the earth’s equatorial plane coincides with the plane of earth’s orbit around sun, the
geostationary orbit will be eclipsed by the earth. This is called the earth eclipse of satellite.
• Solar eclipses are important as they affect the working of the satellite because during eclipse
satellite receives no power from its solar panels and it has to operate on its onboard standby
batteries which reduce satellite life.
• Satellite failure is more at such times when satellite enters into eclipse (sudden switch to no
solar power region) and when it moves out of eclipse (suddenly large amount of solar power
is bombarded on satellite) as this creates thermal stress on satellite.
• Eclipse caused by moon occurs when moon passes in front of sun but that is less important as
it takes place for short duration (twice in every 24 hours for an average of few minutes).
Way to avoid eclipse during satellite lifetime:
Satellite longitudes which are west rather than east of the earth station are most desirable.

• When satellite longitude is east of the earth station, the satellite enters eclipse during daylight
and early morning hours of the earth station. This can be undesirable if the satellite has to
operate on reduced battery power
• When satellite longitude is west of the earth station, eclipse does not occur until the earth
station is in darkness when usage is likely to be low.

Sun Transit Outage phenomena:

➢ Satellites are stationed over the equator. The distance between satellite and Earth is 36,000
Km. Now as we all know sun is full of energy and it emits the heated light of high intensity
generates the noise. This noise is called as the thermal noise. So when the sun with its
thermal noise aligns with satellite and receiving antenna on earth results loss of signal due to
interference is known as Sun outage.
➢ Sun transit outage is an interruption or distortion of geostationary satellite signals caused by
interference from solar radiation. Sun appears to be an extremely noisy source which
completely blanks out the signal from satellite. This effect lasts for 6 days around the
equinoxes. They occur for a maximum period of 10 minutes.
➢ Generally, sun outages occur in February, March, September and October. At these times, the
apparent path of the sun across the sky takes it directly behind the line of sight between an
earth station and a satellite. As the sun radiates strongly at the microwave frequencies used to
communicate with satellites (C-band, Ka band and Ku band) the sun swamps the signal from
the satellite.
Working of Sun Outage:

As Shown in the figure below there are three positions of sun. In the first position our signal
strength is good, but after sometime when sun reaches its second position our receiving signal
strength get low or even nil results freezing in services. Because in this position the sun is
directly aligned with satellite and receiving antenna result losses in signal due to interference of
sun emitted energy with satellite carried signal.
Again, when sun reaches at its third position then signal stand get normal our services started to
run properly.

Frequency is Downlink frequency (GHz) and Diameter of your Receiving Antenna.

Effects of sun transit outage:

The effects of a sun outage can include partial degradation, that is, an increase in the error rate,
or total destruction of the signal.

Effect on Indian stock exchanges:

The interference to satellites' signals has been shown to disturb smooth transmission of data of online
transactions so, for fairness, the share markets are closed for these short times each year. Trading is
normally extended the same day to compensate for the lost time.

Doppler frequency shift phenomena:

The frequency and wavelength of an electromagnetic field are affected by relative motion; This is known
as the Doppler effect. Only the radial component of motion produces this phenomenon. The Doppler
effect is significant in low-earth-orbit (LEO) satellite systems.
All LEO satellites are constantly moving relative to each other and to points on the surface. This causes
variations in the frequencies and wavelengths of received signals. In geostationary satellite systems,
Doppler effect is not a factor unless the end user is on board a spacecraft or high-speed aircraft.

Let the Transmitted frequency is fT and received frequency fR. The received frequency fR is higher then fT
when the transmitter is moving towards the receiver and lower then fT when transmitter is moving away
from the receiver.
The mathematically relationship between transmitted fT and received frequencies fR is

𝑓𝑅 −𝑓𝑇 ∆𝑓 𝑉𝑇
= =
𝑓𝑇 𝑓𝑇 𝑣𝑝

𝑉𝑇 𝑓𝑇 𝑉𝑇
∆𝑓 = =
𝑐 𝜆

Where VT = Transmitted Velocity

vp = C = Speed of Light
λ = wave length of transmitted signal
if transmitter is moving away from receiver then VT will be negative.

Satellite Link Budget:

Basic Transmission Theory:

• The calculation of power received by an earth station from a satellite is fundamental to
the understanding of satellite communication.
• Consider a transmitting source, in free space, radiating total power Pt watts uniformly in
all directions.
• Such source is called isotropic.
• At a distance R meters from isotropic source, flux density crossing the surface

F= Pt/ 4 πR2 (W/m2 )

The received power by Receiving Antenna will be:
Pr = F * A watts

Pr = Pt A/ 4 πR2 watts
—
For a transmitter with output Pt watts driving a lossless antenna with gain Gt , the flux density at
distance R meters is

F= PtGt / 4 πR2 (W/m2 )

The product Pt Gt is called effective isotropic radiated power or EIRP, it describes the
combination of transmitting power & antenna gain in terms of an equivalent isotropic source
with power Pt Gt watts.

A practical antenna with physical aperture area of A m2 will not deliver power as given in above
equation.
Some of the energy incident on aperture is reflected away from the antenna, some is absorbed by
lossy components. The effective aperture Ae is
Ae = ηA A

Where ηA aperture efficiency of the antenna.

For parabolic reflector ηA = 50 to 75%
For Horn antennas ηA = 90%

Thus the power received by real antenna with effective aperture area Ae m2 is

Pr = Pt Gt Ae / 4 πR2 (watts)…….. (A)

A fundamental relation in antenna theory is gain & area of an receiving antenna are related by

Gr = 4π Ae/ λ2

Substituting above equation in equation (A) gives

Pr = [PtGtGr/ (4 πR / λ )2 ]watts

This expression is known as link equation & essential in calculation of power received in any
radio link.

The term (4 πR / λ )2 is known as path loss Lp. Collecting various factors, we can write Power
received.

Pr = (EIRP * Receiving antenna gain / path loss) watts

In decibel, we have
Pr = EIRP + Gr – Lp …………………….. (B)

Where EIRP= 10log10 (Pt Gt ) dBW

Gr = 10log10 (4πAe/ λ2 ) dB
Lp = 10log10 (4πR / λ )2 dB

Numericals:
Eg.1

Now from Eg.1 the satellite is operate at a frequency of 11 GHz. The receiving antenna has a
gain of 52.3 dB, Find the received Power.
System Noise Temperature & G/T Ratio:
Noise Temperature:
Noise temperature provides a way of determining how much thermal noise is generated by active
and passive devices in the receiving system.
At microwave frequencies, a black body with physical temperature, Tp degrees kelvin, generate
electrical noise over a wide bandwidth.
The noise power is given by
Pn = kTn B
Where
k= Boltzmann’s constant= 1.38 * 10-23 J/K = -228.6 dBW/K/Hz
Tn = Noise temperature of source in K
B= noise bandwidth in which noise power is measured, in Hz.
System noise temperature Ts , is the noise temperature of noise source at the input of noiseless
receiver, which gives same noise power as the original receiver, measured at the output of
receiver.
Calculation of System Noise Temperature:
The noisy devices in the receiver are replaced by equivalent noiseless blocks with the same gain
and noise generators at the input to each block such that the block produce same noise at its
output as the device it replaces.

Equivalent Noise Sources

The thermal noise in its pre Amplifier is given by

Pn = GKTsB
The total noise power at the output of the IF amplifier of the receiver is given by
 The single source of noise shown in above figure with noise temperature Ts generates the same
noise power Pn at its output

From Above Equation:

Where Noise Temperature (Ts) is given by

Numerical:
Noise Figure & Noise Temperature:
Noise figure is used to specify the noise generated within a device.
The operational noise figure is
NF = (S/N)in /(S/N)out
Noise temperature is more useful in satellite communication systems, it is best to convert noise
figure to noise temperature, T
T = T0 (NF- 1)
Where:
NF is a linear ratio, not in decibels
T0 is the reference temperature (290 K)

G/T Ratio for earth stations:

The link equation can be rewritten in terms of (C/N) at the earth stations

Satellite Communication Link Design Procedure:

1. Determine the frequency band in which system must operate. Comparative designs may
be required to help make the selection.
2. Determine the communications parameters of the satellite. Estimate any values that are
not known.
3. Determine the parameters of the transmitting and receiving earth stations.
4. Start at the transmitting earth station. Establish an uplink budget and a transponder noise
power to find (C/N)up in the transponder.
5. Find the output power of the transponder based on transponder gain or output backoff.
6. Establish a downlink power and noise budget for the receiving earth station. Calculate
(C/N)dn and (C/N)o for a station at the edge of the coverage zone.
7. Calculate S/N or BER in the baseband channel. Find the link margin.
8. Evaluate the result and compare with the specification requirements. Change parameters
of the system as required to obtain acceptable (C/N)0 or S/N or BER values. This may
require several trial designs.
9. Determine the propagation conditions under which the link must operate. Calculate
outage times for the uplinks and downlinks.
10. Redesign the system by changing some parameters if the link margins are inadequate. Check
that all parameters are reasonable, and that the design can be implemented within the
expected budget.
▪ In satellite communication systems, there are two types of
power calculations. Those are transmitting power and
receiving power calculations. In general, these calculations
are called as Link budget calculations. The unit of power
is decibel.
▪ First, let us discuss the basic terminology used in Link
Budget and then we will move onto explain Link Budget
calculations.

Basic Terminology:
▪ An isotropic radiator (antenna) radiates equally in all
directions. But it doesn’t exist practically. It is just a
theoretical antenna. We can compare the performance of all
real (practical) antennas with respect to this antenna.
 Figure 5.1 Basic communications link

The basic parameters of the link are

pt = transmitted power (watts);
pr = received power (watts);
gt = transmit antenna gain;
gr = receive antenna gain;
r = path distance (meters).
 FIGURE 5.2 POWER FLUX DENSITY

5.1 Power Flux Density (pfd): The power flux density is the
ratio of power flow and unit area.
The power density, in watts/m2, at the distance r from the
transmit antenna with a gain gt , is defined as the power flux
density (pfd)r
 OR
IN TERMS OF EIRP (EFFECTIVE ISOTROPIC RADIATED
POWER)

The power flux density expressed in dB, will be

With r inmeters,
5.2 Calculation of System Noise Temperature for satellite receiver,
noise power calculation:

Already discussed in Unit 4

The power Receive Pr intercepted by the receiving antenna will be

Where Is the effective aperture

pt Transmitter power inwatts

gt Transmitter antennagain

Then
 𝐶
Where λ=
𝑓

Rearranging Equation in a slightly different form, We have Received Power.

Friis transmission Equation

inverse square loss

FREE SPACE PATH LOSS IS RECIPROCAL OF INVERSE SQUARE
LOSS:

4
Free space Path Loss

𝐶
Where λ=
𝑓
For the Range r in meters, and the frequency f in GHz
We now have all the elements necessary to define the basic link equation for
determining the received power at the receiver antenna terminals for a satellitelink.
Sample Calculation for Ku-Band Link

A=0.55
Where
5.5 Satellite link Budget & C/N ratio Calculation:
There are two types of link budget calculations since there are two links
namely, Uplink and Downlink.

Earth Station Uplink:

It is the process in which earth is transmitting the signal to the satellite and
satellite is receiving it. Its mathematical equation can be written as

Where:
is the carrier to noise density ratio

is the satellite receiver G/T ratio and units is dB/K

Here, Losses represent the satellite receiver feeder losses. The losses which depend
upon the frequency are all taken into the consideration.
The EIRP value should be as low as possible for effective UPLINK. And
this is possible when we get a clear sky condition.

Here we have used the (subscript) notation “U”, which represents the uplink
phenomena.

Satellite Downlink
In this process, satellite sends the signal, and the earth station
receives it. The equation is same as the satellite uplink with a
difference that we use the abbreviation “D” everywhere instead
of “U” to denote the downlink phenomena.
Its mathematical equation can be written as;

Where:
Link Budget:
If we are taking ground satellite into consideration, then the free
space spreading loss (FSP) should also be taken into consideration.

If antenna is not aligned properly then losses can occur. so we

take AML (Antenna misalignment losses) into account. Similarly,
when signal comes from the satellite towards earth it collides with
earth surface and some of them get absorbed. These are taken care
by Atmospheric Absorption loss given by “AA” and measured in
db.

Now, we can write the loss equation for free sky as

Where,
•RFL stands for received feeder loss and units are db.
•PL stands for polarization mismatch loss.
 Now the decibel equation for received power can be written as

Where,
•PR stands for the received power, which is measured in dBW.
•Gr is the receiver antenna gain.

The designing of downlink is more critical than the designing of

uplink. Because of limitations in power required for transmitting
and gain of the antenna.
 Multiple Access Multiple accesses in Satellite Communication

Multiple Access Multiple accesses is defined as the technique where in more than one pair of
earth stations can simultaneously use a satellite transponder. A multiple access scheme is a method
used to distinguish among different simultaneous transmissions in a cell. A radio resource can be
a different time interval, a frequency interval, or a code with a suitable power level.
If the different transmissions are differentiated for the frequency band, it will be defined as the
Frequency Division Multiple Access (FDMA).
Whereas, if transmissions are distinguished based on time, then it is considered as Time Division
Multiple Access (TDMA).
If a different code is adopted to separate simultaneous transmissions, it will be Code Division
Multiple Access (CDMA).

Frequency Division Multiple Access (FDMA): Frequency Division Multiple Access or FDMA
is a channel access method used in multiple- access protocols as a channelization protocol. FDMA
gives users an individual allocation of one or several frequency bands, or channels. It is particularly
commonplace in satellite communication.
• In FDMA all users share the satellite transponder or frequency channel simultaneously, but each
user transmits at single frequency.
• FDMA can be used with both analog and digital signal.
• FDMA requires high-performing filters in the radio hardware.
• FDMA is not vulnerable to the timing problems that TDMA has. Since a predetermined
frequency band is available for the entire period of communication, stream data (a continuous flow
of data that may not be packetized) can easily be used with FDMA.
• Each user transmits and receives at different frequencies as each user gets a unique frequency
slots.

Time Division Multiple Access (TDMA): Time division multiple access (TDMA) is a channel
access method for shared medium networks. It allows several users to share the same frequency
channel by dividing the signal into different time slots. This allows multiple stations to share the
same transmission medium (e.g. radio frequency channel) while using only a part of its channel
capacity.
• Shares single carrier frequency with multiple users.
• Slots can be assigned on demand in dynamic TDMA.
• Less stringent power control than CDMA due to reduced intra cell interference
• Higher synchronization overhead than CDMA
• Cell breathing (borrowing resources from adjacent cells) is more complicated than in CDMA.
• Frequency/slot allocation complexity.

Code Division Multiple Access (CDMA): Code division multiple access (CDMA) is a channel
access method used by various radio communication technologies. CDMA is an example of
multiple access, which is where several transmitters can send information simultaneously over a
single communication channel. CDMA uses the codes to identify connections. Their Signals are
encoded so that information from an individual transmitter can be detected and recovered only by
a properly synchronized receiving station, that knows the code being used.
• One of the early applications for code division multiplexing is in the Global Positioning System
(GPS). This predates and is distinct from its use in mobile phones.
Comparison between FDMA, TDMA & CDMA: