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Satellite
communication
Module 2 a: Satellite subsystem
1. Power supply subsystem
2. Attitude and Orbit control
3. Tracking, Telemetry and command subsystem
4. Payload
1. Types of earth station
2. Architecture
3. Design considerations
4. Testing,
5. Earth station Hardware
6. Satellite tracking.
Module 2 b: Earth Station
Satellite Subsystems
1. Mechanical structure
2. Propulsion subsystem
3. Thermal control subsystem
4. Antenna subsystem
5. Power supply subsystem
6. Telemetry, tracking and command (TT&C) subsystem
7. Attitude and orbit control subsystem
8. Payload subsystem
SATELLITE
SUBSYSTEMS
Mechanical structure
provides the framework for mounting other subsystems
of the satellite and also an interface between the
satellite and the launch vehicle
Propulsion subsystem
provide the thrusts required to impart the
necessary velocity changes to execute all the
maneuver's during the lifetime of the satellite
Thermal control subsystem
maintain the satellite platform within its
operating temperature limits for the type of
equipment on board the satellite.
Attitude and orbit control subsystem
It controls the orbital path, It also provides attitude
control, which is essential to prevent the satellite
from tumbling in space
Telemetry, tracking and command
(TT&C) subsystem
monitors and controls the satellite right from
the lift-off stage to the end of its operational
life in space.
Power supply subsystem
to collect the solar energy, transform it to electrical
power with the help of arrays of solar cells and
distribute electrical power to other components and
subsystems of the satellite.
Payload subsystem
that carries the desired instrumentation required
for performing its intended function
Antenna subsystem
used for both receiving signals from ground
stations as well as for transmitting signals
towards them
The power supply subsystem
The power requirement can vary from a few hundreds of watts to tens of kilowatts.
generates, stores, controls and distributes electrical power to other subsystems on board the satellite platform.
Types of Power sources
solar energy chemical energy nuclear energy
the solar energy driven power systems are
undoubtedly the favorite and are the most
commonly used ones
Batteries store electricity in the form of
chemical energy
The batteries used here are rechargeable
when solar radiation is falling on the
satellite.
During the periods of eclipse the batteries
supply electrical power to the
satellite.
The advantage of nuclear on
satellites is that it is practically
limitless
The disadvantage is the danger
of radioactive spread over Earth
in the event of the rocket used
to launch the satellite exploding
before it escapes the Earth’s
atmosphere.
Basic block schematic arrangement of a regulated bus power supply system
During the sunlight condition, the voltage of the solar generator and also the bus is maintained at a constant amplitude
with the voltage regulator connected across the solar generator
The battery is decoupled from the bus during this time by means of a battery discharge regulator (BDR) and is also
charged using the battery charge regulator (BCR).
During the eclipse periods, the battery provides power to the bus and the voltage is maintained constant by means of
the BDR.
Solar Panels
The solar panel is nothing but a series and parallel connection of a large number of solar cells.
In the case of three-axis stabilized satellites, the flat
solar panels can be rotated to intercept maximum solar
energy to produce maximum electric power
15 foot long solar panels on Intelsat-V series satellites
produce in excess of 1.2 kW of power. However, as the
solar panels always face the sun, they operate at
relatively higher temperatures and thus reduced
efficiency
in the case of spin-stabilized satellites, such as
Intelsat-VI series satellites, only one-third of the
solar cells face the sun at a time.
Principle of Operation of a Solar Cell
The operational principle of the basic solar cell is based on the photovoltaic effect
According to the photovoltaic effect, there is generation of an open circuit voltage across a P–N junction when it is
exposed to light, which is the solar radiation in the case of a solar cell, this circuit voltage leads to flow of electric
current through a load resistance connected across it,
impinging photon energy leads to the
generation of electron–hole pairs.
The electron–hole pairs either recombine and
vanish or start to drift in the opposite
directions, with electrons moving towards the
N-layer and holes moving towards the P-
layer.
This accumulation of positive and negative
charge carriers constitutes the open circuit
voltage.
Current–voltage and power–voltage characteristics of a solar cell
the solar cell generates its maximum power at a
certain voltage.
The power–voltage curve has a point of maximum
power, called the maximum power point (MPP).
The cell voltage and the corresponding current at the
maximum power point are less than the open circuit
voltage (Voc) and the short circuit current (Isc)
respectively.
Silicon, Gallium arsenide used as semiconductor
material for making solar cells
Batteries
Batteries are used on board the satellite to meet the power requirements when the same cannot be provided by
solar panels, as is the case during eclipse periods.
The choice of the right battery technology for a given satellite mission is governed by various factors.
These include the frequency of use, magnitude of load and depth of discharge
LEO satellites have an orbital period of the order
of 100 min and the eclipse period is 30–40 min
per orbit.
For GEO satellites, the orbital period is 24 hours
and the eclipse duration varies from 0 to a
maximum of 72 min during equinoxes
Batteries on LEO satellites are therefore subjected to a
lower depth of discharge (DoD of 40% ).
On the other hand, batteries on geostationary satellites are
subjected to a greater depth of discharge (DoD of 80%).
1. Nickel–cadmium (NiCd)
2. Nickel metal hydride (NiMH)
3. Nickel–hydrogen (NiH2)
4. Lithium Ion Battery
Commonly used batteries onboard satellites
Nickel–cadmium (NiCd) Nickel metal hydride (NiMH) Nickel–hydrogen (NiH2) Lithium Ion Battery
The nickel–cadmium battery is the most
commonly used rechargeable battery
The nickel metal hydride batteries are
cadmium-free
The nickel–hydrogen battery combines
the technologies of batteries and fuel
cells
Li-ion batteries use an intercalated
lithium compound as the material at the
positive electrode and typically graphite
at the negative electrode.
The basic galvanic cell in a nickel–cadmium
battery uses a cadmium anode, a nickel
hydroxide cathode and an alkaline electrolyte
The anode of the battery is made of a hydrogen
storage metal alloy and the cathode is made of
nickel oxide.
The battery uses nickel hydroxide as the
cathode as in the nickel–cadmium cell,
hydrogen as the active element in the
anode
A lithium-ion battery or Li-ion battery is a
type of rechargeable battery , lithium ions
move from the negative electrode through
an electrolyte to the positive electrode
during discharge, and back when charging
They can offer high currents at a constant
voltage of 1.2V
The basic cell produces a voltage of 1.2V Its resistance to repeated deep discharge
and tolerance for overcharge
Voltage ranges 3.6 / 3.7 / 3.8 / 3.85 V
they are highly prone to what is called the
‘memory effect’
Memory effect means that if a battery is only
partially discharged before recharging
repeatedly, it can forget that it can be further
discharged
these batteries are less affected
by the memory effect.
these batteries have problems at very high and
low temperatures and therefore are not
considered suitable for space applications.
Its disadvantages include its high cost
and low volumetric energy density.
These batteries do not suffer from the
memory
These batteries however, require special
handling as lithium ignites very easily
Telemetry, tracking and command (TT&C) subsystem
The tracking part of the subsystem determines the position of the
spacecraft and follows its travel using angle, range and velocity
information.
The telemetry part gathers information on the health of various
subsystems of the satellite. It encodes this information and then
transmits the same towards the Earth control centre.
The command element receives and executes remote control
commands from the control centre on Earth to effect changes to the
platform functions, configuration, position and velocity.
TTC functions
1. Continuously monitoring and reporting spacecraft health
2. Control orbit and attitude of the satellite
3. Monitoring command actions
4. Switch on and off the communication systems
5. Control of payload
Scientific ephemerides often contain further useful data about the moon,
planet, asteroid, or comet beyond the pure coordinates in the sky
Main parameters measured are Pressure,
temperature , voltage and current
These data are digitized and transmitted
in PSK low power telemetry carrier using
TDMA
Low data rate allows ES to have narrow
BW and hence high SNR
ES used to monitor, store and decode the
telemetry data and alarms can be sounded
if any parameter goes outside allowable
limits.
Telemetry
Velocity and acceleration sensors on the
satellite can be used to establish change in
orbit from last known position.
Active determination of range can be
achieved by transmitting a pulse or
sequence of pulses to the satellite and
observing delay before the pulse is
received again.
Ranging tones used for range
measurement.
Tracking Command
A command system is used to make changes in
attitude and corrections to the orbit and control
communication system.
Must possess safeguards unauthorized
attempts to make changes to satellite operation
and hence encryption is provided.
Control code is converted into command word
and sent in TDM frame to satellite.
After checking its validity in satellite, the word
is again sent back to control station via
telemetry link to check again.
If it is found correct, execute instruction sent
to satellite to execute command.
During launch phase and injection into geo
orbit, main TT&C in not operative ,hence
backup system used to control only important
sections of satellite.
Attitude and orbit control subsystem
Altitude and Orbit Control (AOC) subsystem capable of placing the satellite into the right orbit, whenever there is deviation
observed from the respective orbit.
AOC subsystem is very helpful in order to make the satellite antennas always pointing towards earth.
AOC subsystem consists of two parts.
•Attitude Control Subsystem
•Orbit Control Subsystem
Attitude 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.
Spin stabilization
Three axes method
the entire spacecraft rotates around its own vertical
axis, spinning like a top.
This keeps the spacecraft's orientation in space under
control.
The advantage of spin stabilization is that it is a very
simple way to keep the spacecraft pointed in a
certain direction.
In a spin-stabilized satellite, the satellite body is spun
at a rate between 30 and 100 rpm about an axis
perpendicular to the orbital plane
There are two types of spinning configurations
employed in spin-stabilized satellites.
These include the
simple spinner configuration
the dual spinner configuration.
In the simple spinner
configuration, the satellite
payload and other
subsystems are placed in
the spinning section, while
the antenna and the feed
are placed in the de-spun
platform.
In the dual spinner
configuration, the entire
payload along with the
antenna and the feed is
placed on the de-spun
platform and the other
subsystems are located
on the spinning body.
Intelsat-1 to
Intelsat-4,
Intelsat-6 and
TIROS-1
spin stabilization
Three-axis or Body Stabilization
In the case of three-axis stabilization, also known as body stabilization,
the stabilization is achieved by controlling the movement of the satellite
along the three axes, yaw, pitch and roll
The system uses reaction wheels or momentum wheels to
Correct orbit perturbation.
Intelsat-5, Intelsat-7, Intelsat-8, GOES-8, GOES-9,
TIROS-N and the INSAT series
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.
Orbit Control Subsystem
Orbit control subsystem is useful in order to bring the satellite into its correct orbit, whenever the satellite gets deviates from its orbit.
The TTC subsystem present at earth station monitors the position of satellite and 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.
Payload subsystem
Payload is the most important subsystem of any satellite.
Payload can be considered as the brain of the satellite that performs its intended function.
The payload carried by a satellite depends upon the mission requirements
The basic payload in the case of
a communication satellite, for
instance, is a transponder, which
acts as a
receiver/amplifier/transmitter
communication satellite payload weather forecasting satellites
the radiometer is the most
important payload has a set of
detectors sensitive to the
radiation in the visible, near-IR
and far-IR bands
Scientific satellites
payloads depending on
their mission
Earth observation satellite.
Depending upon the mode
of operation, radiometers
are classified as imagers
and sounders
High resolution visible (HRV)
cameras, multispectral scanners
and thematic mapper are the
main payloads
Satellites observing the stars
carry telescopes to collect
light from stars and
spectrographs operating over
a wide range of ultraviolet
(UV) wavelengths from 120
to 320 nm
L (2 GHz/1 GHz)
S (4 GHz/2 GHz)
C (6 GHz/4 GHz)
Ku (12–18 GHz)
Ka (27–40 GHz)
Communication satellite payload Weather forecasting satellites Scientific satellites Earth observation satellite.
Satellites for planetary
exploration have varied
equipment, like the plasma
detector to study solar winds
and radiation belts,
the magnetometer to
investigate the possible
magnetic field around the
planet,
the gamma spectrometer to
determine the radioactivity of
surface rocks, the neutral mass
spectroscope, the ion mass
spectroscope, etc
Light and heat reflected and
emitted from land and oceans,
which contain specific information
of the various living and nonliving
things, are picked up by these
sensors.
The images produced are then
digitized and transmitted to the
Earth stations, where they are
processed to give the required
information
Visible images show the
amount of sunlight being
reflected from Earth or
clouds whereas the IR
images provide information
on the temperature of the
cloud cover or the Earth’s
surface
The C band is the most popular
band and is being used for
providing domestic and
international telephone services.
Ku (12–18 GHz) Ka (27–40
GHz) high frequency bands
have advantages of higher
bandwidth and reduced antenna
size.
Transponder
The basic payload in the case of a communication
satellite, is a transponder, which acts as a
receiver/amplifier/transmitter.
Communication satellite payload
Thus, a transponder is a combination of elements
like sensitive high gain antennas for transmit–
receive functions
Satellites employ the L, S, C, X, Ku and Ka
microwave frequency bands for communication
purposes
L (2 GHz/1 GHz)
S (4 GHz/2 GHz)
C (6 GHz/4 GHz)
Ku (12–18 GHz)
Ka (27–40 GHz)
popular band and is being used for providing domestic and international telephone services
Module 2 a :Question Bank
Q no Question Marks
1 Discuss the functions of various subsystems of a typical satellite
2 With neat block diagram , explain basic block schematic arrangement of a regulated
bus power supply system in satellite.
3 Compare the types of sources of energies available to generate power in satellite.
4 With neat diagram, explain Principle of Operation of a Solar Cell
5 Compare commonly used batteries in satellites
6 With neat block diagram explain Telemetry, tracking and command (TT&C) subsystem
7 Explain Attitude and orbit control subsystem in satellites
8 Discuss Payload subsystem in used in satellites.
9 Explain typical payload of a Communication satellite with neat diagram.
1. Types of earth station
2. Architecture
3. Design considerations
4. Testing of Earth station
5. Earth station Hardware
6. Satellite tracking.
Module 2 b: Earth Station
Module 2 b: Earth Station
An Earth station is a terrestrial terminal station mainly located on the Earth’s surface
Earth stations are generally categorized on the basis of type of services or functions provided by them
types based on service provided by the Earth station
1. Fixed Satellite Service (FSS) Earth Stations
2. Broadcast Satellite Service (BSS) Earth Stations
3. Mobile Satellite Service (MSS) Earth Stations
Earth stations types based on their usage
1. Single function stations
2. Gateway stations
3. Teleports
Fixed Satellite Service (FSS) Earth
Station
Comparison of Earth stations
• large Earth stations (G/T∼=40 dB/K)
• medium Earth stations (G/T∼=30 dB/K),
• small Earth stations (G/T∼=25 dB/K),
• very small terminals with
transmit/receive functions (G/T∼=20
dB/K)
• Very small terminals with receive only
functions (G/T∼=12 dB/K)
• The service involves telephony, data
communications and radio and television
broadcast feeds.
• FSS satellites operate in either the C
band or the Ku band
• FSS operates at lower power levels
require a much larger dish
• FSS satellite transponders use linear
polarization
• large Earth stations (G/T∼=15 dB/K)
• small Earth stations (G/T∼=8 dB/K)
• large Earth stations used for
community reception
• small Earth stations used for
individual reception.(DBS)
Broadcast Satellite Service (BSS) Earth
Stations
Mobile Satellite Service (MSS) Earth
Stations
• large Earth stations (G/T∼=−4 dB/K),
• medium Earth stations (G/T∼=−12 dB/K)
• small Earth stations (G/T∼=−24 dB/K).
• Satellite phone is the most commonly
used mobile satellite service
Single Function Stations
• Single function stations are
characterized by a single type of link to
a satellite or a satellite constellation.
These stations may be
• transmit-only,
• receive-only or both.
Some common examples are-
• television receive-only (TVRO)
terminals used for TV reception by an
individual
• satellite radio terminals
• receive-only terminals used at a
television broadcast
• two-way VSAT terminals
• Handheld satellite telephone terminals
Gateway Stations Teleports
• Gateway stations serve as an interface
between the satellites and the terrestrial
networks and also serve as transit points
between satellites.
• These stations are connected to terrestrial
networks by various transmission
technologies, both wired such as coaxial
cables, optical fibers etc. and wireless such
as microwave towers.
• In gateway stations, signal processing is
the major activity as gateway station
receives a large variety of terrestrial signals
at any given time.
• These include telephone signals, television
signals, and data streams and so on and to
be converted into standard format before
sending to intended satellite.
• Teleport is a type of gateway station
operated by firms that are usually not a
part of a specific satellite system
• Teleports helps when line of sight is
missing due to the close proximity of
another tall building or some other
obstacle or site is located in crowded
place .
• Teleports are usually located on the
outskirts of the city and the
connectivity from the subscriber
company to the teleport station is
usually provided through a hub (hub in
turn is connected to the teleport
through a fiber-optic or a microwave
link)
Earth Station Architecture
The major components of an Earth station include
• the RF section,
• the baseband equipment and
• the terrestrial interface.
• support facilities
The RF section as mainly consists of
in the up-link channel (from ES to satellite)
• antenna subsystem,
• high power amplifier (HPA)
• the up-converter
in the down-link channel(from satellite to ES)
• antenna subsystem,
• low noise amplifier (LNA)
• down-converter
The job of up-converter in the up-link channel is to
up-convert the baseband signal to the desired
frequency.
The upconverted signal is then amplified to the
desired level before it is fed to the feed system for
subsequent transmission to the intended satellite
Similarly, a low noise amplifier amplifies the weak
signals received by the antenna.
The amplified signal is then down converted to the
intermediate frequency level before it is fed to the
modem in the baseband section
The antenna feed system provides the necessary
aperture illumination, introduces the desired
polarization and also provides isolation between the
transmitted and the received signals by connecting
HPA output and LNA input to the cross-polarized
ports of the feed.
The baseband section performs the
modulation/demodulation function with the specific
equipment required depending upon the modulation
technique and the multiple access method employed.
The baseband section input/output is connected to the
terrestrial network through a suitable interface known as
terrestrial interface.
The terrestrial network could be a fiber optic cable
link or a microwave link or even a combination of the
two.
Earth Station Design Considerations
Design of an Earth station is generally a two-step process.
The steps involved are
• Earth station requirement specifications
• identifying the most cost effective architecture
Earth station requirement specifications which in turn
• govern the choice of system parameters, Requirement
specifications affecting the design of an Earth station
include type of service offered (FSS,BSS or MSS)
• communication requirements (telephony, data,
television etc.)
• required base band quality at the destination
• system capacity and reliability.
• It is always advisable to minimize the overall
system costs including both development as well as
recurring costs of the Earth and space segments.
• cost incurred on the Earth station could be reduced
by having a more expensive space segment.
cost effective architecture
Key Performance Parameters
Key performance parameters governing Earth station design include
This is a transmitter parameter.
EIRP (Effective or Equivalent Isotropic Radiated Power) Figure-of-merit (G/T )
This is the indicative of receiver performance in terms of
sensitivity and the quality of the received signal
EIRP gives the combined performance of the high
power amplifier (HPA) and the transmitting antenna
gain
EIRP is defined for both Earth station transmitting
antenna as well as satellite transmitting
antenna.
It is important to note that EIRP is always measured at the
antenna
Receiver figure-of-merit is indicative of how the
receiving antenna performs together with the receiving
electronics to produce a useful signal.
Earth Station Design Optimization
the transmitter EIRP and receiver G/T together dictate the
performance of the communication system and therefore
one can be traded off against the other during the design
optimization process
Where C/No=carrier-to-total noise power spectral density,
EIRP = satellite’s effective isotropic radiated power,
Lp =path loss, Lm=link margin and k=Boltzmann constant
Environmental and Site Considerations
• External temperature and humidity
• rainfall and snow
• wind conditions
• likelihood of Earth quakes
• corrosive conditions of the atmosphere
Testing of Earth station
Having chosen the Earth station equipment,
it is important to ensure that the equipment would not only meet the specified requirements of the
intended Earth station;
it is also necessary to ensure that the Earth station would not cause any problems either to other users
of the satellite or to any adjacent satellites
Unit or component level testing is
usually done at the manufacturer’s
premises and the test data is made
available to the subsystem
designer making use of the
components
In subsystem or equipment level
testing, different subsystems are
comprehensively tested for their
electrical, mechanical and
environmental specifications.
System level testing is carried when the
complete system has been ordered on a
single supplier;
Mandatory Tests
(a) Transmit cross-polarization isolation
(b) Receiver figure-of-merit
Mandatory tests include measurements of
Schematic arrangement shown is used for
measuring transmit cross polarized isolation for
both linearly as well as circularly polarized antennas
Transmit cross-polarization isolation
this test is usually performed on-axis
The antenna under test (AUT) is initially driven to
transmit a relatively lower level carrier at the test
frequency.
Once the AUT has been bore sighted, the next step
is optimization of the polarization angle of the
antenna under test by observing the power level of
the cross-polarized component as the AUT rotates
the feed.
The on-axis transmit cross-polarization isolation is
then computed to determine whether or not the
measured value of cross-polarization isolation
meets the required on-axis transmit cross
polarization isolation specification
Receiver figure-of-merit
The antenna under test (AUT) measures the
received power level of either an unmodulated
beacon, or a test carrier.
The downlink EIRP of the unmodulated beacon or
the test carrier is also measured.
The receive system noise contribution is then
measured by steering the antenna off the spacecraft
and measuring the noise floor.
The difference between the two measurements is the
downlink (C + N)/N ratio.
From this the downlink, C/No is calculated by taking
into account corrections required for the effects of
the system thermal noise, spectrum analyzer
detection non-linearity and noise bandwidth.
G/T is then computed by solving the downlink
equation.
Earth station Hardware
Most Earth station hardware can be categorized into one of the three groups namely
1. RF equipment
2. IF and baseband equipment
3. Terrestrial interface equipment
The RF equipment comprises of up-
converters, high power amplifiers
(HPA) and the transmit antenna in the
transmit channel, and the receive
antenna, low noise amplifiers (LNA)
and down-converters in the receive
channel
RF subsection IF and baseband equipment
the modulation/ demodulation
scheme, multiple access method
Terrestrial interface equipment
Earth station that connects the Earth
station to the users.
Common interfaces needed in satellite
links and terrestrial networks include
telephone interface (voice), data
transmission interface (data) and
television interface (video).
RF subsection
The RF equipment comprises of up-converters, high power amplifiers (HPA) and the transmit antenna in the transmit
channel, and the receive antenna, low noise amplifiers (LNA) and down-converters in the receive channel.
Block schematic of the RF portion of the Earth station
Antenna
antennas of relevance to Earth stations is shown below.
The prime focus fed parabolic reflector antenna is used
for an antenna diameter of less than 4.5 m, more so for
receive only Earth stations.
An offset fed sectioned parabolic reflector antenna is
used for antenna diameters of less than 2 m.
Offset feed configuration eliminates the blockage of the
main beam due to feed and its mechanical support system
and thus improves antenna efficiency and reduces side
lobe levels.
prime focus fed parabolic reflector antenna
Offset fed sectioned parabolic reflector antenna
Offset fed Cassegrain antenna
Cassegrain antenna
Cassegrain antennas overcome most of the shortcomings of the
prime focus fed parabolic reflector antennas.
The Cassegrain antenna uses a hyperbolic reflector placed in front
of the main reflector, closer to the
This hyperbolic reflector receives the waves from the feed placed at
the centre of the main reflector and bounces them back towards the
main reflector.
Gregorian antenna (b) Offset fed Gregorian antenna
This configuration uses a concave secondary reflector just behind
the prime focus.
The purpose of this reflector is also to bounce the waves back
towards the dish.
The front end in this case is located between the secondary
reflector and the main reflector.
Offset feed configuration is also possible in case of Gregorian
antenna
High Power Amplifier
Different types of power amplifiers used in Earth stations include
(a) Traveling wave tube (TWT) amplifiers
(b) Klystron amplifiers
(c) Solid state power amplifiers (SSPA).
SSPA are used for relatively lower
power applications
Solid state power amplifiers are
comparatively cheaper and more
reliable though the power level
offered by them is limited
Klystrons are narrowband devices
providing a bandwidth of the
order of 40 to 80 MHz that is
tunable over a range of 500 MHz
or more.
Power levels offered are from
several hundred watts to few
kilowatts.
TWTA is a wideband amplifier offering a
bandwidth as large as 500 MHz or more
and a power level from a few watts to a
few kilowatts.
Traveling wave tube (TWT) amplifiers
Klystron amplifiers
Solid state power amplifiers (SSPA)
Up-converters/Down-converters
Up-converters and down-converters are frequency translators that convert the IF used in the modems and baseband
equipment to the operating RF frequency bands (C , Ku and Ka) and vice versa.
The up-converter translates the IF signal at 70 MHz
(or 140 MHz) from the modulator to the operating RF
frequency in C or Ku or Ka band
Up-converters Down-converters
The down-converter translates the received RF signal in C or
Ku or Ka band into IF signal, which is subsequently fed to the
demodulator
Low Noise Amplifier (LNA)
The low noise amplifier (LNA) is one of the key components deciding the system noise temperature and hence the figure-
of-merit G/T of the Earth station
Present day LNAs are configured around either Gallium Arsenide FET (GaAs FET) or High Electron Mobility Transistors
(HEMT).
In LNC, the amplifier can typically be tuned to
amplify over the entire bandwidth of a single
transponder, whatever that bandwidth may be, before
it down converts.
LNC (Low Noise Converter)
LNB uses a block converter and is capable of handling
block of frequencies from different transponders on the
satellite.
LNC uses the signal from a single
transponder.
Low Noise Block (LNB).
The output of LNB is a standard IF signal of around 1
GHz frequency. LNB is usually placed on the antenna
structure itself and is connected to the feed directly.
IF and Baseband Equipment
Important building blocks of IF and baseband equipment of the Earth station hardware include baseband processing circuits,
modulator/demodulator (MODEM), multiplexer/ demultiplexer etc.
The architecture of the IF and baseband section depends upon parameters like the modulation/ demodulation scheme,
multiple access method
Terrestrial Interface
Terrestrial interface is that part of the Earth station that connects the Earth station to the users.
Signals received from the terrestrial network therefore
need to be de-multiplexed and then changed from the
existing terrestrial formats for suitable for satellite
transmission.
Terrestrial interface – up-link
On the down-link side, the signals received from satellite/s
are processed in the down-link chain before they are sent to
standard converter. After reformatting, the signals are
multiplexed and put on the terrestrial network
Terrestrial interface – down-link
Satellite Tracking
Block schematic arrangement of satellite tracking system
Satellite Tracking System – Block Diagram
The Earth station antenna makes use of the beacon signal to
track itself to the desired positions
in both azimuth and elevation.
The auto track receiver derives the tracking correction data or
in some cases the estimated position of the satellite.
The estimated position is compared with the measured position
in the control subsystem whose output feeds the
servomechanism
Tracking Techniques
Commonly used tracking techniques include
1. Lobe switching
2. Sequential lobing
3. Conical scan
Lobe Switching
In the case of lobe switching tracking methodology,
the antenna beam is rapidly switched between two
positions around the antenna axis in a single plane
The amplitudes of the echo from the object to be
tracked are compared for the two lobe positions
4. Monopulse track
5. Step track
6. Intelligent tracking
The difference between the two amplitudes is
indicative of the location of the target with respect to
the antenna axis
Sequential Lobing
In sequential lobing, the beam axis is slightly shifted off the antenna axis.
This squinted beam is sequentially placed in discrete angular positions, usually four, around the antenna axis
The angular information about the object to be tracked is determined by processing several echo signals
Conical Scan
This is similar to sequential lobing except that in the case of conical scan, the squinted beam is scanned rapidly and
continuously in a circular path around the axis
If the object to be tracked is off the antenna axis, the amplitude of the echo signal varies with antenna’s scan
position.
The amplitude variation provides information on the amplitude of the angular error and the phase delay indicates
direction
Monopulse Tracking
Amplitude comparison monopulse tracking
the antenna uses four feeds placed symmetrically around the
focal point
As a consequence, in the case of satellite being on-axis, the
amount of energy falling on the four feeds representing four
quadrants (A, B, C and D) will be the same.
When the satellite is located off-axis, the amount of energy
falling on the four feeds will be different depending upon
which quadrant around the antenna axis, the satellite is
located b) satellite located above antenna axis
c)satellite located below antenna axis
d) satellite located towards right of antenna axis
e) satellite located towards left of antenna axis
a) satellite on-axis
phase comparison monopulse tracking
Monopulse Tracking
In the case of phase comparison monopulse tracking, it is the phase difference between the received signals in
different antenna elements that contains information on angular errors.
When the satellite is on axis the magnitude of phase
difference is zero
When the satellite is off axis the magnitude of phase
difference is not zero
Module 2 b :Question Bank
Q no Question Marks
1 Give the classification of Earth station based on service and usage.
2 Explain typical architecture of an Earth station.
3 Compare different Earth stations.
4 Mention important Earth Station Design Considerations
5 Discuss the important Key Performance Parameters related to Earth station.
6 Explain any TWO mandatory test to be conducted at Earth station.
7 Explain Any TWO hardware components of an Earth Station
8 Explain satellite tracking techniques.

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MOD 2 SVIT NOTES.pdf

  • 2. Module 2 a: Satellite subsystem 1. Power supply subsystem 2. Attitude and Orbit control 3. Tracking, Telemetry and command subsystem 4. Payload 1. Types of earth station 2. Architecture 3. Design considerations 4. Testing, 5. Earth station Hardware 6. Satellite tracking. Module 2 b: Earth Station
  • 3. Satellite Subsystems 1. Mechanical structure 2. Propulsion subsystem 3. Thermal control subsystem 4. Antenna subsystem 5. Power supply subsystem 6. Telemetry, tracking and command (TT&C) subsystem 7. Attitude and orbit control subsystem 8. Payload subsystem
  • 4. SATELLITE SUBSYSTEMS Mechanical structure provides the framework for mounting other subsystems of the satellite and also an interface between the satellite and the launch vehicle Propulsion subsystem provide the thrusts required to impart the necessary velocity changes to execute all the maneuver's during the lifetime of the satellite Thermal control subsystem maintain the satellite platform within its operating temperature limits for the type of equipment on board the satellite. Attitude and orbit control subsystem It controls the orbital path, It also provides attitude control, which is essential to prevent the satellite from tumbling in space Telemetry, tracking and command (TT&C) subsystem monitors and controls the satellite right from the lift-off stage to the end of its operational life in space. Power supply subsystem to collect the solar energy, transform it to electrical power with the help of arrays of solar cells and distribute electrical power to other components and subsystems of the satellite. Payload subsystem that carries the desired instrumentation required for performing its intended function Antenna subsystem used for both receiving signals from ground stations as well as for transmitting signals towards them
  • 5. The power supply subsystem The power requirement can vary from a few hundreds of watts to tens of kilowatts. generates, stores, controls and distributes electrical power to other subsystems on board the satellite platform. Types of Power sources solar energy chemical energy nuclear energy the solar energy driven power systems are undoubtedly the favorite and are the most commonly used ones Batteries store electricity in the form of chemical energy The batteries used here are rechargeable when solar radiation is falling on the satellite. During the periods of eclipse the batteries supply electrical power to the satellite. The advantage of nuclear on satellites is that it is practically limitless The disadvantage is the danger of radioactive spread over Earth in the event of the rocket used to launch the satellite exploding before it escapes the Earth’s atmosphere.
  • 6. Basic block schematic arrangement of a regulated bus power supply system During the sunlight condition, the voltage of the solar generator and also the bus is maintained at a constant amplitude with the voltage regulator connected across the solar generator The battery is decoupled from the bus during this time by means of a battery discharge regulator (BDR) and is also charged using the battery charge regulator (BCR). During the eclipse periods, the battery provides power to the bus and the voltage is maintained constant by means of the BDR.
  • 7. Solar Panels The solar panel is nothing but a series and parallel connection of a large number of solar cells. In the case of three-axis stabilized satellites, the flat solar panels can be rotated to intercept maximum solar energy to produce maximum electric power 15 foot long solar panels on Intelsat-V series satellites produce in excess of 1.2 kW of power. However, as the solar panels always face the sun, they operate at relatively higher temperatures and thus reduced efficiency in the case of spin-stabilized satellites, such as Intelsat-VI series satellites, only one-third of the solar cells face the sun at a time.
  • 8. Principle of Operation of a Solar Cell The operational principle of the basic solar cell is based on the photovoltaic effect According to the photovoltaic effect, there is generation of an open circuit voltage across a P–N junction when it is exposed to light, which is the solar radiation in the case of a solar cell, this circuit voltage leads to flow of electric current through a load resistance connected across it, impinging photon energy leads to the generation of electron–hole pairs. The electron–hole pairs either recombine and vanish or start to drift in the opposite directions, with electrons moving towards the N-layer and holes moving towards the P- layer. This accumulation of positive and negative charge carriers constitutes the open circuit voltage.
  • 9. Current–voltage and power–voltage characteristics of a solar cell the solar cell generates its maximum power at a certain voltage. The power–voltage curve has a point of maximum power, called the maximum power point (MPP). The cell voltage and the corresponding current at the maximum power point are less than the open circuit voltage (Voc) and the short circuit current (Isc) respectively. Silicon, Gallium arsenide used as semiconductor material for making solar cells
  • 10. Batteries Batteries are used on board the satellite to meet the power requirements when the same cannot be provided by solar panels, as is the case during eclipse periods. The choice of the right battery technology for a given satellite mission is governed by various factors. These include the frequency of use, magnitude of load and depth of discharge LEO satellites have an orbital period of the order of 100 min and the eclipse period is 30–40 min per orbit. For GEO satellites, the orbital period is 24 hours and the eclipse duration varies from 0 to a maximum of 72 min during equinoxes Batteries on LEO satellites are therefore subjected to a lower depth of discharge (DoD of 40% ). On the other hand, batteries on geostationary satellites are subjected to a greater depth of discharge (DoD of 80%).
  • 11. 1. Nickel–cadmium (NiCd) 2. Nickel metal hydride (NiMH) 3. Nickel–hydrogen (NiH2) 4. Lithium Ion Battery Commonly used batteries onboard satellites
  • 12. Nickel–cadmium (NiCd) Nickel metal hydride (NiMH) Nickel–hydrogen (NiH2) Lithium Ion Battery The nickel–cadmium battery is the most commonly used rechargeable battery The nickel metal hydride batteries are cadmium-free The nickel–hydrogen battery combines the technologies of batteries and fuel cells Li-ion batteries use an intercalated lithium compound as the material at the positive electrode and typically graphite at the negative electrode. The basic galvanic cell in a nickel–cadmium battery uses a cadmium anode, a nickel hydroxide cathode and an alkaline electrolyte The anode of the battery is made of a hydrogen storage metal alloy and the cathode is made of nickel oxide. The battery uses nickel hydroxide as the cathode as in the nickel–cadmium cell, hydrogen as the active element in the anode A lithium-ion battery or Li-ion battery is a type of rechargeable battery , lithium ions move from the negative electrode through an electrolyte to the positive electrode during discharge, and back when charging They can offer high currents at a constant voltage of 1.2V The basic cell produces a voltage of 1.2V Its resistance to repeated deep discharge and tolerance for overcharge Voltage ranges 3.6 / 3.7 / 3.8 / 3.85 V they are highly prone to what is called the ‘memory effect’ Memory effect means that if a battery is only partially discharged before recharging repeatedly, it can forget that it can be further discharged these batteries are less affected by the memory effect. these batteries have problems at very high and low temperatures and therefore are not considered suitable for space applications. Its disadvantages include its high cost and low volumetric energy density. These batteries do not suffer from the memory These batteries however, require special handling as lithium ignites very easily
  • 13. Telemetry, tracking and command (TT&C) subsystem The tracking part of the subsystem determines the position of the spacecraft and follows its travel using angle, range and velocity information. The telemetry part gathers information on the health of various subsystems of the satellite. It encodes this information and then transmits the same towards the Earth control centre. The command element receives and executes remote control commands from the control centre on Earth to effect changes to the platform functions, configuration, position and velocity. TTC functions 1. Continuously monitoring and reporting spacecraft health 2. Control orbit and attitude of the satellite 3. Monitoring command actions 4. Switch on and off the communication systems 5. Control of payload Scientific ephemerides often contain further useful data about the moon, planet, asteroid, or comet beyond the pure coordinates in the sky
  • 14. Main parameters measured are Pressure, temperature , voltage and current These data are digitized and transmitted in PSK low power telemetry carrier using TDMA Low data rate allows ES to have narrow BW and hence high SNR ES used to monitor, store and decode the telemetry data and alarms can be sounded if any parameter goes outside allowable limits. Telemetry Velocity and acceleration sensors on the satellite can be used to establish change in orbit from last known position. Active determination of range can be achieved by transmitting a pulse or sequence of pulses to the satellite and observing delay before the pulse is received again. Ranging tones used for range measurement. Tracking Command A command system is used to make changes in attitude and corrections to the orbit and control communication system. Must possess safeguards unauthorized attempts to make changes to satellite operation and hence encryption is provided. Control code is converted into command word and sent in TDM frame to satellite. After checking its validity in satellite, the word is again sent back to control station via telemetry link to check again. If it is found correct, execute instruction sent to satellite to execute command. During launch phase and injection into geo orbit, main TT&C in not operative ,hence backup system used to control only important sections of satellite.
  • 15. Attitude and orbit control subsystem Altitude and Orbit Control (AOC) subsystem capable of placing the satellite into the right orbit, whenever there is deviation observed from the respective orbit. AOC subsystem is very helpful in order to make the satellite antennas always pointing towards earth. AOC subsystem consists of two parts. •Attitude Control Subsystem •Orbit Control Subsystem Attitude 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. Spin stabilization Three axes method
  • 16. the entire spacecraft rotates around its own vertical axis, spinning like a top. This keeps the spacecraft's orientation in space under control. The advantage of spin stabilization is that it is a very simple way to keep the spacecraft pointed in a certain direction. In a spin-stabilized satellite, the satellite body is spun at a rate between 30 and 100 rpm about an axis perpendicular to the orbital plane There are two types of spinning configurations employed in spin-stabilized satellites. These include the simple spinner configuration the dual spinner configuration. In the simple spinner configuration, the satellite payload and other subsystems are placed in the spinning section, while the antenna and the feed are placed in the de-spun platform. In the dual spinner configuration, the entire payload along with the antenna and the feed is placed on the de-spun platform and the other subsystems are located on the spinning body. Intelsat-1 to Intelsat-4, Intelsat-6 and TIROS-1 spin stabilization
  • 17. Three-axis or Body Stabilization In the case of three-axis stabilization, also known as body stabilization, the stabilization is achieved by controlling the movement of the satellite along the three axes, yaw, pitch and roll The system uses reaction wheels or momentum wheels to Correct orbit perturbation. Intelsat-5, Intelsat-7, Intelsat-8, GOES-8, GOES-9, TIROS-N and the INSAT series 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. Orbit Control Subsystem Orbit control subsystem is useful in order to bring the satellite into its correct orbit, whenever the satellite gets deviates from its orbit. The TTC subsystem present at earth station monitors the position of satellite and 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.
  • 18. Payload subsystem Payload is the most important subsystem of any satellite. Payload can be considered as the brain of the satellite that performs its intended function. The payload carried by a satellite depends upon the mission requirements The basic payload in the case of a communication satellite, for instance, is a transponder, which acts as a receiver/amplifier/transmitter communication satellite payload weather forecasting satellites the radiometer is the most important payload has a set of detectors sensitive to the radiation in the visible, near-IR and far-IR bands Scientific satellites payloads depending on their mission Earth observation satellite. Depending upon the mode of operation, radiometers are classified as imagers and sounders High resolution visible (HRV) cameras, multispectral scanners and thematic mapper are the main payloads Satellites observing the stars carry telescopes to collect light from stars and spectrographs operating over a wide range of ultraviolet (UV) wavelengths from 120 to 320 nm L (2 GHz/1 GHz) S (4 GHz/2 GHz) C (6 GHz/4 GHz) Ku (12–18 GHz) Ka (27–40 GHz)
  • 19. Communication satellite payload Weather forecasting satellites Scientific satellites Earth observation satellite. Satellites for planetary exploration have varied equipment, like the plasma detector to study solar winds and radiation belts, the magnetometer to investigate the possible magnetic field around the planet, the gamma spectrometer to determine the radioactivity of surface rocks, the neutral mass spectroscope, the ion mass spectroscope, etc Light and heat reflected and emitted from land and oceans, which contain specific information of the various living and nonliving things, are picked up by these sensors. The images produced are then digitized and transmitted to the Earth stations, where they are processed to give the required information Visible images show the amount of sunlight being reflected from Earth or clouds whereas the IR images provide information on the temperature of the cloud cover or the Earth’s surface The C band is the most popular band and is being used for providing domestic and international telephone services. Ku (12–18 GHz) Ka (27–40 GHz) high frequency bands have advantages of higher bandwidth and reduced antenna size.
  • 20. Transponder The basic payload in the case of a communication satellite, is a transponder, which acts as a receiver/amplifier/transmitter. Communication satellite payload Thus, a transponder is a combination of elements like sensitive high gain antennas for transmit– receive functions Satellites employ the L, S, C, X, Ku and Ka microwave frequency bands for communication purposes L (2 GHz/1 GHz) S (4 GHz/2 GHz) C (6 GHz/4 GHz) Ku (12–18 GHz) Ka (27–40 GHz) popular band and is being used for providing domestic and international telephone services
  • 21. Module 2 a :Question Bank Q no Question Marks 1 Discuss the functions of various subsystems of a typical satellite 2 With neat block diagram , explain basic block schematic arrangement of a regulated bus power supply system in satellite. 3 Compare the types of sources of energies available to generate power in satellite. 4 With neat diagram, explain Principle of Operation of a Solar Cell 5 Compare commonly used batteries in satellites 6 With neat block diagram explain Telemetry, tracking and command (TT&C) subsystem 7 Explain Attitude and orbit control subsystem in satellites 8 Discuss Payload subsystem in used in satellites. 9 Explain typical payload of a Communication satellite with neat diagram.
  • 22. 1. Types of earth station 2. Architecture 3. Design considerations 4. Testing of Earth station 5. Earth station Hardware 6. Satellite tracking. Module 2 b: Earth Station
  • 23. Module 2 b: Earth Station An Earth station is a terrestrial terminal station mainly located on the Earth’s surface Earth stations are generally categorized on the basis of type of services or functions provided by them types based on service provided by the Earth station 1. Fixed Satellite Service (FSS) Earth Stations 2. Broadcast Satellite Service (BSS) Earth Stations 3. Mobile Satellite Service (MSS) Earth Stations Earth stations types based on their usage 1. Single function stations 2. Gateway stations 3. Teleports
  • 24. Fixed Satellite Service (FSS) Earth Station Comparison of Earth stations • large Earth stations (G/T∼=40 dB/K) • medium Earth stations (G/T∼=30 dB/K), • small Earth stations (G/T∼=25 dB/K), • very small terminals with transmit/receive functions (G/T∼=20 dB/K) • Very small terminals with receive only functions (G/T∼=12 dB/K) • The service involves telephony, data communications and radio and television broadcast feeds. • FSS satellites operate in either the C band or the Ku band • FSS operates at lower power levels require a much larger dish • FSS satellite transponders use linear polarization • large Earth stations (G/T∼=15 dB/K) • small Earth stations (G/T∼=8 dB/K) • large Earth stations used for community reception • small Earth stations used for individual reception.(DBS) Broadcast Satellite Service (BSS) Earth Stations Mobile Satellite Service (MSS) Earth Stations • large Earth stations (G/T∼=−4 dB/K), • medium Earth stations (G/T∼=−12 dB/K) • small Earth stations (G/T∼=−24 dB/K). • Satellite phone is the most commonly used mobile satellite service
  • 25. Single Function Stations • Single function stations are characterized by a single type of link to a satellite or a satellite constellation. These stations may be • transmit-only, • receive-only or both. Some common examples are- • television receive-only (TVRO) terminals used for TV reception by an individual • satellite radio terminals • receive-only terminals used at a television broadcast • two-way VSAT terminals • Handheld satellite telephone terminals Gateway Stations Teleports • Gateway stations serve as an interface between the satellites and the terrestrial networks and also serve as transit points between satellites. • These stations are connected to terrestrial networks by various transmission technologies, both wired such as coaxial cables, optical fibers etc. and wireless such as microwave towers. • In gateway stations, signal processing is the major activity as gateway station receives a large variety of terrestrial signals at any given time. • These include telephone signals, television signals, and data streams and so on and to be converted into standard format before sending to intended satellite. • Teleport is a type of gateway station operated by firms that are usually not a part of a specific satellite system • Teleports helps when line of sight is missing due to the close proximity of another tall building or some other obstacle or site is located in crowded place . • Teleports are usually located on the outskirts of the city and the connectivity from the subscriber company to the teleport station is usually provided through a hub (hub in turn is connected to the teleport through a fiber-optic or a microwave link)
  • 26. Earth Station Architecture The major components of an Earth station include • the RF section, • the baseband equipment and • the terrestrial interface. • support facilities The RF section as mainly consists of in the up-link channel (from ES to satellite) • antenna subsystem, • high power amplifier (HPA) • the up-converter in the down-link channel(from satellite to ES) • antenna subsystem, • low noise amplifier (LNA) • down-converter
  • 27. The job of up-converter in the up-link channel is to up-convert the baseband signal to the desired frequency. The upconverted signal is then amplified to the desired level before it is fed to the feed system for subsequent transmission to the intended satellite Similarly, a low noise amplifier amplifies the weak signals received by the antenna. The amplified signal is then down converted to the intermediate frequency level before it is fed to the modem in the baseband section The antenna feed system provides the necessary aperture illumination, introduces the desired polarization and also provides isolation between the transmitted and the received signals by connecting HPA output and LNA input to the cross-polarized ports of the feed.
  • 28. The baseband section performs the modulation/demodulation function with the specific equipment required depending upon the modulation technique and the multiple access method employed. The baseband section input/output is connected to the terrestrial network through a suitable interface known as terrestrial interface. The terrestrial network could be a fiber optic cable link or a microwave link or even a combination of the two.
  • 29. Earth Station Design Considerations Design of an Earth station is generally a two-step process. The steps involved are • Earth station requirement specifications • identifying the most cost effective architecture Earth station requirement specifications which in turn • govern the choice of system parameters, Requirement specifications affecting the design of an Earth station include type of service offered (FSS,BSS or MSS) • communication requirements (telephony, data, television etc.) • required base band quality at the destination • system capacity and reliability. • It is always advisable to minimize the overall system costs including both development as well as recurring costs of the Earth and space segments. • cost incurred on the Earth station could be reduced by having a more expensive space segment. cost effective architecture
  • 30. Key Performance Parameters Key performance parameters governing Earth station design include This is a transmitter parameter. EIRP (Effective or Equivalent Isotropic Radiated Power) Figure-of-merit (G/T ) This is the indicative of receiver performance in terms of sensitivity and the quality of the received signal EIRP gives the combined performance of the high power amplifier (HPA) and the transmitting antenna gain EIRP is defined for both Earth station transmitting antenna as well as satellite transmitting antenna. It is important to note that EIRP is always measured at the antenna Receiver figure-of-merit is indicative of how the receiving antenna performs together with the receiving electronics to produce a useful signal.
  • 31. Earth Station Design Optimization the transmitter EIRP and receiver G/T together dictate the performance of the communication system and therefore one can be traded off against the other during the design optimization process Where C/No=carrier-to-total noise power spectral density, EIRP = satellite’s effective isotropic radiated power, Lp =path loss, Lm=link margin and k=Boltzmann constant Environmental and Site Considerations • External temperature and humidity • rainfall and snow • wind conditions • likelihood of Earth quakes • corrosive conditions of the atmosphere
  • 32. Testing of Earth station Having chosen the Earth station equipment, it is important to ensure that the equipment would not only meet the specified requirements of the intended Earth station; it is also necessary to ensure that the Earth station would not cause any problems either to other users of the satellite or to any adjacent satellites Unit or component level testing is usually done at the manufacturer’s premises and the test data is made available to the subsystem designer making use of the components In subsystem or equipment level testing, different subsystems are comprehensively tested for their electrical, mechanical and environmental specifications. System level testing is carried when the complete system has been ordered on a single supplier;
  • 33. Mandatory Tests (a) Transmit cross-polarization isolation (b) Receiver figure-of-merit Mandatory tests include measurements of
  • 34. Schematic arrangement shown is used for measuring transmit cross polarized isolation for both linearly as well as circularly polarized antennas Transmit cross-polarization isolation this test is usually performed on-axis The antenna under test (AUT) is initially driven to transmit a relatively lower level carrier at the test frequency. Once the AUT has been bore sighted, the next step is optimization of the polarization angle of the antenna under test by observing the power level of the cross-polarized component as the AUT rotates the feed. The on-axis transmit cross-polarization isolation is then computed to determine whether or not the measured value of cross-polarization isolation meets the required on-axis transmit cross polarization isolation specification
  • 35. Receiver figure-of-merit The antenna under test (AUT) measures the received power level of either an unmodulated beacon, or a test carrier. The downlink EIRP of the unmodulated beacon or the test carrier is also measured. The receive system noise contribution is then measured by steering the antenna off the spacecraft and measuring the noise floor. The difference between the two measurements is the downlink (C + N)/N ratio. From this the downlink, C/No is calculated by taking into account corrections required for the effects of the system thermal noise, spectrum analyzer detection non-linearity and noise bandwidth. G/T is then computed by solving the downlink equation.
  • 36. Earth station Hardware Most Earth station hardware can be categorized into one of the three groups namely 1. RF equipment 2. IF and baseband equipment 3. Terrestrial interface equipment The RF equipment comprises of up- converters, high power amplifiers (HPA) and the transmit antenna in the transmit channel, and the receive antenna, low noise amplifiers (LNA) and down-converters in the receive channel RF subsection IF and baseband equipment the modulation/ demodulation scheme, multiple access method Terrestrial interface equipment Earth station that connects the Earth station to the users. Common interfaces needed in satellite links and terrestrial networks include telephone interface (voice), data transmission interface (data) and television interface (video).
  • 37. RF subsection The RF equipment comprises of up-converters, high power amplifiers (HPA) and the transmit antenna in the transmit channel, and the receive antenna, low noise amplifiers (LNA) and down-converters in the receive channel. Block schematic of the RF portion of the Earth station
  • 38. Antenna antennas of relevance to Earth stations is shown below. The prime focus fed parabolic reflector antenna is used for an antenna diameter of less than 4.5 m, more so for receive only Earth stations. An offset fed sectioned parabolic reflector antenna is used for antenna diameters of less than 2 m. Offset feed configuration eliminates the blockage of the main beam due to feed and its mechanical support system and thus improves antenna efficiency and reduces side lobe levels. prime focus fed parabolic reflector antenna Offset fed sectioned parabolic reflector antenna
  • 39. Offset fed Cassegrain antenna Cassegrain antenna Cassegrain antennas overcome most of the shortcomings of the prime focus fed parabolic reflector antennas. The Cassegrain antenna uses a hyperbolic reflector placed in front of the main reflector, closer to the This hyperbolic reflector receives the waves from the feed placed at the centre of the main reflector and bounces them back towards the main reflector.
  • 40. Gregorian antenna (b) Offset fed Gregorian antenna This configuration uses a concave secondary reflector just behind the prime focus. The purpose of this reflector is also to bounce the waves back towards the dish. The front end in this case is located between the secondary reflector and the main reflector. Offset feed configuration is also possible in case of Gregorian antenna
  • 41. High Power Amplifier Different types of power amplifiers used in Earth stations include (a) Traveling wave tube (TWT) amplifiers (b) Klystron amplifiers (c) Solid state power amplifiers (SSPA). SSPA are used for relatively lower power applications Solid state power amplifiers are comparatively cheaper and more reliable though the power level offered by them is limited Klystrons are narrowband devices providing a bandwidth of the order of 40 to 80 MHz that is tunable over a range of 500 MHz or more. Power levels offered are from several hundred watts to few kilowatts. TWTA is a wideband amplifier offering a bandwidth as large as 500 MHz or more and a power level from a few watts to a few kilowatts. Traveling wave tube (TWT) amplifiers Klystron amplifiers Solid state power amplifiers (SSPA)
  • 42. Up-converters/Down-converters Up-converters and down-converters are frequency translators that convert the IF used in the modems and baseband equipment to the operating RF frequency bands (C , Ku and Ka) and vice versa. The up-converter translates the IF signal at 70 MHz (or 140 MHz) from the modulator to the operating RF frequency in C or Ku or Ka band Up-converters Down-converters The down-converter translates the received RF signal in C or Ku or Ka band into IF signal, which is subsequently fed to the demodulator
  • 43. Low Noise Amplifier (LNA) The low noise amplifier (LNA) is one of the key components deciding the system noise temperature and hence the figure- of-merit G/T of the Earth station Present day LNAs are configured around either Gallium Arsenide FET (GaAs FET) or High Electron Mobility Transistors (HEMT). In LNC, the amplifier can typically be tuned to amplify over the entire bandwidth of a single transponder, whatever that bandwidth may be, before it down converts. LNC (Low Noise Converter) LNB uses a block converter and is capable of handling block of frequencies from different transponders on the satellite. LNC uses the signal from a single transponder. Low Noise Block (LNB). The output of LNB is a standard IF signal of around 1 GHz frequency. LNB is usually placed on the antenna structure itself and is connected to the feed directly.
  • 44. IF and Baseband Equipment Important building blocks of IF and baseband equipment of the Earth station hardware include baseband processing circuits, modulator/demodulator (MODEM), multiplexer/ demultiplexer etc. The architecture of the IF and baseband section depends upon parameters like the modulation/ demodulation scheme, multiple access method Terrestrial Interface Terrestrial interface is that part of the Earth station that connects the Earth station to the users. Signals received from the terrestrial network therefore need to be de-multiplexed and then changed from the existing terrestrial formats for suitable for satellite transmission. Terrestrial interface – up-link On the down-link side, the signals received from satellite/s are processed in the down-link chain before they are sent to standard converter. After reformatting, the signals are multiplexed and put on the terrestrial network Terrestrial interface – down-link
  • 45. Satellite Tracking Block schematic arrangement of satellite tracking system Satellite Tracking System – Block Diagram The Earth station antenna makes use of the beacon signal to track itself to the desired positions in both azimuth and elevation. The auto track receiver derives the tracking correction data or in some cases the estimated position of the satellite. The estimated position is compared with the measured position in the control subsystem whose output feeds the servomechanism
  • 46. Tracking Techniques Commonly used tracking techniques include 1. Lobe switching 2. Sequential lobing 3. Conical scan Lobe Switching In the case of lobe switching tracking methodology, the antenna beam is rapidly switched between two positions around the antenna axis in a single plane The amplitudes of the echo from the object to be tracked are compared for the two lobe positions 4. Monopulse track 5. Step track 6. Intelligent tracking The difference between the two amplitudes is indicative of the location of the target with respect to the antenna axis
  • 47. Sequential Lobing In sequential lobing, the beam axis is slightly shifted off the antenna axis. This squinted beam is sequentially placed in discrete angular positions, usually four, around the antenna axis The angular information about the object to be tracked is determined by processing several echo signals
  • 48. Conical Scan This is similar to sequential lobing except that in the case of conical scan, the squinted beam is scanned rapidly and continuously in a circular path around the axis If the object to be tracked is off the antenna axis, the amplitude of the echo signal varies with antenna’s scan position. The amplitude variation provides information on the amplitude of the angular error and the phase delay indicates direction
  • 49. Monopulse Tracking Amplitude comparison monopulse tracking the antenna uses four feeds placed symmetrically around the focal point As a consequence, in the case of satellite being on-axis, the amount of energy falling on the four feeds representing four quadrants (A, B, C and D) will be the same. When the satellite is located off-axis, the amount of energy falling on the four feeds will be different depending upon which quadrant around the antenna axis, the satellite is located b) satellite located above antenna axis c)satellite located below antenna axis d) satellite located towards right of antenna axis e) satellite located towards left of antenna axis a) satellite on-axis
  • 50. phase comparison monopulse tracking Monopulse Tracking In the case of phase comparison monopulse tracking, it is the phase difference between the received signals in different antenna elements that contains information on angular errors. When the satellite is on axis the magnitude of phase difference is zero When the satellite is off axis the magnitude of phase difference is not zero
  • 51. Module 2 b :Question Bank Q no Question Marks 1 Give the classification of Earth station based on service and usage. 2 Explain typical architecture of an Earth station. 3 Compare different Earth stations. 4 Mention important Earth Station Design Considerations 5 Discuss the important Key Performance Parameters related to Earth station. 6 Explain any TWO mandatory test to be conducted at Earth station. 7 Explain Any TWO hardware components of an Earth Station 8 Explain satellite tracking techniques.