This document provides a summary of Mohit Kumar's vocational training project on transponders undertaken at the Defence Electronics Application Laboratory in Dehradun, India from June 2nd to July 2nd 2008. It includes a certificate signed by his mentor and the laboratory director certifying the successful completion of the project. The acknowledgements section expresses gratitude to various members of the laboratory for their support and guidance during the training period.

1. Vocational Training Report
On
STUDY OF TRANSPONDERS
TRAINING UNDERTAKEN
AT
DEFENCE ELECTRONICS APPLICATION
LABORATORY RAIPUR ROAD,
DEHRADUN-248001
PREPARED BY: - UNDER THE SUPERVISION OF:-
1
MOHIT KUMAR
B.Tech (ECE)
Univ. R.NO.-0981562805
NIEC, New Delhi
ASHOK KUMAR
Scientist ‘F’
Millimeter Wave Group
DEAL, Dehradun.

2. Defence Electronics Application Laboratory
Raipur Road, Dehradun (UK)-248001
CERTIFICATE
This is to certify that the study project work entitled “Transponders” was carried out
and successfully completed by Mohit Kumar, Roll.No-0981562805, a student of B.Tech
ECE from Northern India Engineering College, New Delhi (IP University) at MMW
Division, DEAL, Dehradun from 2nd June 2008 to 2nd July 2008.
Dated:
Ashok Kumar K.Sivakumar
Scientist ‘F’ Scientist ‘G’
MMW Systems Group Group Director
DEAL, Dehradun MMW Systems Group
DEAL, Dehradun
2

3. ACKNOWLEDGEMENT
I would like to express my gratitude to all those who gave me the possibility to
complete the project. The successful completion of this report is attributed from great
help and support I have received from various members of D.E.A.L family.
First of all, I want to thank Shri RC Aggarwal ,Director ,D.E.A.L and Mr.
Deshmukh ,Director ,H.R. Department ,D.E.A.L for kindly giving me his consent for
my practical training at D.E.A.L, Dehradun. I shall forever be indebted to them for
providing me with such a sterling opportunity.
I would like to extend my heartfelt gratitude to Mr.K.Sivakumar ,Scientist ‘G’,
Director, Millimeter Wave’s group for giving me permission to commence this thesis in
the first instance, to do the necessary research work and to use departmental data.
I am deeply indebted to my mentor Dr. Ashok Kumar (scientist ‘F’), whose
constant guidance, stimulating suggestions and encouragement helped me in all the time
of my training and successful completion of this project.
I have furthermore to thank Mr. Hoshiar Singh Kalsi, Technical Asst. ‘A”, Mrs.
Ranjana Thakur and Mr. Rajeev who helped and encouraged me to go ahead with my
project. This magnificent team has guided me through the most demanding part of my
engineering curriculum and I shall forever be indebted to them for providing me a strong
foundation to my career.
I would like to extend my cordial gratitude and regards to T.I.C (Technical
Information Center), D.EA.L. for providing standard text on the subject.
Last but not least, I would like to give my special thanks to all the members of
D.E.A.L family MMW group who have directly or indirectly helped me in the completion
of my project.
Mohit Kumar
3

4. Defence Research & Development Organization (DRDO) works under Department
of Defence Research and Ministry of Defence. DRDO is dedicatedly working towards
enhancing self-reliance in Defence Systems and undertakes design & development
leading to production of world class weapon systems and equipment in accordance with
the expressed needs and the qualitative requirements laid down by the three services.
DRDO is working in various areas of military technology which include aeronautics,
armaments, combat vehicles, electronics, instrumentation engineering systems, missiles,
materials, naval systems, advanced computing, simulation and life sciences.
DRDO was formed in 1958 from the amalgamation of the then already functioning
Technical Development Establishment (TDEs) of the Indian Army and the Directorate of
Technical Development & Production (DTDP) with the Defence Science Organization
(DSO). DRDO was then a small organization with 10 establishments or laboratories.
Over the years, it has grown multi-directionally in terms of the variety of subject
disciplines, number of laboratories, achievements and stature. Today, DRDO is a network
of more than 50 laboratories which are deeply engaged in developing defense
technologies covering various disciplines. Presently, the Organization is backed by over
5000 scientists and about 25,000 other scientific, technical and supporting personnel.
Several major projects for the development of missiles, armaments, light combat
aircrafts, radars, electronic warfare systems etc are on hand and significant achievements
have already been made in several such technologies.
4

5. Defence Electronics Application Laboratory
The origin of Defence Electronics Applications Laboratory (DEAL) can be
traced back to 1959 when the Defence Research Laboratory (DRL) was set up in the
barracks of British Military Hospital at Landour Cantt, Mussoorie as a small field unit of
the Defence Science Center (DSC), Delhi. DRL was engaged in radio wave propagation
studies, food preservation & packaging and study of problems at high attitudes. The
reorganization of DRDO in 1962 saw the consolidation of Propagation Studies in the
form of Propagation Field Research Station (PFRS), as a detachment of DLRL,
Hyderabad. PFRS became an independent entity as Himalayan Radio Propagation
Unit (HRPU) at Mussoorie with the strength of 84 persons on February 23, 1965.
HRPU was responsible for helping the Services to set up communication links in the
border areas and providing frequency prediction services using data collected from
propagation studies. HRPU moved to Dehradun in 1968 and was temporarily located in
the old barracks of Instruments Research & Developments Establishment (IRDE). It was
renamed as Defence Electronics Applications Laboratory (DEAL) and established in
the present location in 1976.
Shri RC Aggarwal has been appointed Director, Defence Electronics
Applications Laboratory (DEAL), Dehradun , wef 01 December 2007.
5

6. CONTENTS
1). A Brief Overview of Satellite Communication
1.1). Abstract
1.2). Types of orbits
1.3). Basic terms in satellite communication.
1.4). Components of a satellite
2) Satellite payloads
2.1) Abstract
2.2) Basic operations at transmitting earth station.
3) Transponders
3.1) Bent pipe
3.2) On board processing
4) Case Study
4.1 INTELSAT IV
5) Satellite Link budget
5.1 Example of a link budget
5.2 Various terms in budget
6) Conclusion
7) Bibliography
1 - A Brief Overview of Satellite Communication
6

7. 1.1 Abstract
Satellites have now become an integral part of the worldwide communication
systems. Although long–range and long distance communication took place much before
the introduction of satellite systems, they had a lot of disadvantages. Point – to – point
communication systems are very difficult in the case of remote & isolated locations,
which are surrounded by oceans, mountains and other obstacles created by nature.
The satellite is nothing more than a radio-relay station But, they have one
potential advantage- The capability of a direct line of sight path to 98% (excluding the
polar caps, which are in accessible to satellites) of the earth's surface.
One of the most important events in the history of satellite communication took
place when COMSAT or communication satellite corporation, launched four satellites
within 6 years that is between 1965 to 1979. The first of these series was the ‘Early
Bird’, which was launched in 1965. This was the first communication station to handle
worldwide commercial telephone traffic from a fixed position in space. The next series
INTELSAT was a group of satellites that served 150 stations in 80 countries.
Fig 1.1 Figure to show the basic components in satellite communication.
7

8. 1.2 Types of Orbits
Different orbits serve different purposes. Each has its own advantages and
disadvantages. There are several types of orbits:
1. Polar
2. Sun Synchronous
3. Geosynchronous
Polar Orbits
The more correct term would be near polar orbits. These orbits have an inclination near
90 degrees. This allows the satellite to see virtually every part of the Earth as the Earth
rotates underneath it. It takes approximately 90 minutes for the satellite to complete one
orbit. These satellites have many uses such as measuring ozone concentrations in the
stratosphere or measuring temperatures in the atmosphere.
Sun Synchronous Orbits
These orbits allow a satellite to pass over a section of the Earth at the same time of day.
Since there are 365 days in a year and 360 degrees in a circle, it means that the satellite
has to shift its orbit by approximately one degree per day. These satellites orbit at an
altitude between 700 to 800 km. These satellites use the fact since the Earth is not
perfectly round (the Earth bulges in the center, the bulge near the equator will cause
additional gravitational forces to act on the satellite. This causes the satellite's orbit to
either proceed or recede. These orbits are used for satellites that need a constant amount
of sunlight. Satellites that take pictures of the Earth would work best with bright sunlight,
while satellites that measure long wave radiation would work best in complete darkness.
Geosynchronous Orbits
Also known as geostationary orbits, satellites in these orbits circle the Earth at the
same rate as the Earth spins. The Earth actually takes 23 hours, 56 minutes, and 4.09
seconds to make one full revolution. So based on Kepler's Laws of Planetary Motion, this
would put the satellite at approximately 35,790 km above the Earth. The satellites are
located near the equator since at this latitude; there is a constant force of gravity from all
directions. At other latitudes, the bulge at the center of the Earth would pull on the
satellite.
Geosynchronous orbits allow the satellite to observe almost a full hemisphere of the
Earth. These satellites are used to study large scale phenomenon such as hurricanes, or
cyclones. These orbits are also used for communication satellites. The disadvantage of
this type of orbit is that since these satellites are very far away, they have poor resolution.
The other disadvantage is that these satellites have trouble monitoring activities near the
poles..
8

9. Fig 1.2 Figure to show the basic types of satellite orbits.
The communications satellites are placed in orbits called equatorial geostationary
orbit. The satellite placed in this orbit will appear stationery over a selected location on
the earth’s surface. So, communications satellites are placed in an orbit that is directly
over the equator, moving in a west to east direction at an altitude of 22,282 miles above
sea level (36,000 km appor. as explained earlier) and with a forward velocity of 6874mph
to complete one orbit in 24 hours. This orbit is called the Clarke orbit.
Fig 1.3 Figure to show final geostationary orbit
1.3 Basic terms in satellite communication.
9

10. Up-link and Down-link
All of the ground equipment along with the transmission path and receiving
antenna at the satellite are included in the up-link system. Basically, this includes
everything before the input terminals of the satellite receiver. The down-link is described
in terms of satellite transmitted output power, down link antenna gain and beam width
and the ground area that the transmitted signal will cover the foot print.
Cross –link
At the attitude of the Clarke-orbit, one satellite could command a footprint area of
42.2% of the earth's surface. The beam-width from the satellite for such coverage is 17.2
since such a satellite is not sufficient for global coverage; we need more than one to be
specific 3 satellites.
These three satellites are placed 120 apart in the Clarke orbit and would cover
the earth's entire surface except for the polar caps. This makes it possible for one earth
station to transmit to another station on the opposite side of the globe.
Satellite footprints
The footprint is the area on the earth covered by a satellite antenna. It may embrace up to
50% of the earth’s surface, or, by means of signal focusing, be restricted to small,
regional spots.
The higher the frequency of the signal emitted, the more it can be focused and the smaller
the footprint becomes. The focusing of the satellite signal on smaller footprints can
increase the energy of the signal. The smaller the footprint, the stronger the signal, and
thus the smaller the receiving antennae may be.
1.4 Components of a Satellite
There are 3 major components in a satellite, they are:
(i) Transponder and antenna system
The transponder is a high – frequency radio receiver, a frequency down-converter
and a power amplifier, which is used to transmit the downlink signal. The antenna system
contains the antennas and the mechanism to position them correctly. Once properly in
place, they will generally function trouble-free fro the life of the satellite.
(ii) Power Package
10

11. It is a power supply to the satellite. The satellite must be powered either from a
battery or a solar energy system. In case of communications satellites in the Clarke orbit,
a combination of battery power and solar energy is used. A solar cell system supplies the
power to run the electronics and change the batteries during the sunlight cycle and battery
furnishes the energy during the eclipse.
(iii) Control and information system & rocket thruster system
The control and information system and the rocket thruster system are called the
station keeping system. The function of the station keeping system is to keep the satellite
in the correct orbit with the antennas pointed in the exact direction desired.
11

12. 2 - Satellite Payloads
2.1 Abstract
A payload is the part of the satellite that performs the purpose it was put in space
for. There are many different types of satellite but communications satellites are the kind
we are interested in here. The payloads on communications satellites are effectively just
repeaters. They receive the signals that are transmitted to them and then retransmit them
at a different frequency back to earth.
Modern satellites do more than this. They receive the signals and then demodulate
them to access the data, the data can then be processed before being modulated and
retransmitted. The data can be stored for later retransmission or modulated using a
different method, even at a different data rate.
There is an uplink receiver chain and a downlink transmits chain. The central area
shown as ‘Processing’ is where the frequency is translated or any demodulation,
processing and modulation would take place.
Fig 2.1 Figure to show the basic steps in satellite communication.
2.2 The basic operations at transmitting earth station
12

13. The digital data input at the transmitting end is compressed.
1. The signal is then passed to the multiplexer as usually the bandwidth of channel is
much higher than the bandwidth of the original signal so many signals are combined
or multiplexed together to form a block of signals called channel.
2. The ordinary analogue data is converted to digital data and is modulated onto the
carrier
3. The technique used in the points 1, 2, 3 is usually termed as multiple access. The
basic multiple access techniques are FDM/FM/FDMA used in satellite telephony or
TDM/PSK/TDMA used in digital satellite communication.
4. The resulting baseband signal is then sent to the upconvertor.Usually more than one
upconvertors are used.The signals are sent to the up converters at around 70 MHz.
The 1st up converter mixes the signals with another frequency; the result is both the
sum and difference of the signals. By filtering out the original and the difference
frequencies the result is that the original frequencies are now the sum frequencies -
higher up in the frequency spectrum. An example would be the up conversion of 70
MHz to 1 GHz which is IF to L Band. The 2nd up converter then up converts the L
Band signals to a Radio Frequency (RF) of around 10 GHz. this is then ready for the
HPA to transmit through the antenna.
5. The HPA (High Power Amplifier), otherwise known as a TWTA (Traveling Wave Tube
Amplifier) or an SSHPA (Solid State High Power Amplifier),has one job. It amplifies a
specific band of frequencies by a large amount, sufficiently large to enable the
antenna to beam them up to the satellite. These can range in power from a few watts
up to over 1000 watts in power. The bigger the dish, usually the bigger the power
amplifiers. The largest have to be cooled using liquid nitrogen and resemble electron
microscopes. The smallest look more like a lump of metal bolted to a small heat sink.
6. The parabolic antenna is a high-gain reflector antenna. A typical parabolic antenna
consists of a parabolic reflector illuminated by a small antenna.
7. Diplexer or OMT- The circulator is used to make sure that the transmit signals go out
through the dish and not back into the receive chain. It also makes sure that the
receive signals come from the dish into the receive chain and not into the transmit
chain.This is often referred to as an Orthomode Transducer or OMT and is, these
days, built into the feed assembly
8. The down converters convert signals down in frequency. The signals arrive at the dish
at anything from 10 to 40 GHz and are then filtered and amplified, they now need to
be moved down the frequency spectrum so that the equipment can be made cheaper
and easier. The 1st down converter mixes the signals with another frequency; the
13

14. result is both the sum and difference of the signals. By filtering out the original and
the sum frequencies the result is that the original frequencies are now the difference
frequencies - lower down in the frequency spectrum. An example would be the down
conversion of 10 GHz to 1 GHz which is Ku band to L Band. The 2nd down
converter then down converts the L Band signals to an Intermediate Frequency (IF) of
around 70 MHz. this is then ready for the demodulator.
9. The LNA (Low Noise Amplifier), sometimes known as an LNB on receive only
terminals, is a very good amplifier which has the job of amplifying the small signals
picked up by the antenna without amplifying the noise.
3. Transponders
14

15. A transponder is a broadband RF channel used to amplify one or more carriers on
the downlink side of a geostationary communications satellite. It is part of the microwave
repeater and antenna system that is housed onboard the operating satellite
These satellites and most of their cohorts in the geostationary orbit have bent-pipe
repeaters using C and Ku bands; a bent pipe repeater is simply one that receives all
signals in the uplink beam, block translates them to the downlink band, and separates
them into individual transponders of a fixed bandwidth
The transponder itself is simply a repeater. It takes in the signal from the uplink at
a frequency f1, amplifies it and sends it back on a second frequency f2. Figure shows a
typical frequency plan with 24-channel transponder. The uplink frequency is at 6 GHz,
and the downlink frequency is at 4 GHz. The 24 channels are separated by 40 MHz and
have a 36 MHz useful bandwidth. The guard band of 4 MHz assures that the transponders
do not interact with each other.
Transponder complexity varies from the simple "bent pipe" approach to on-board
processing (OBP) and on-board switching (OBS) transponders. Common elements
include receivers, mixers, oscillators, channel amplifiers, and RF switches. OBP
transponders may include additional elements of demodulators, demultiplexers,
remodulators, and baseband switches.
The basic types of transponders are
 Bent Pipe
 On board processing.
3.1 Bent Pipe or conventional transponders
15

16. The bent pipe transponders are so called because it takes a band of signals and
bents it back to the earth just like a bent pipe which changes the direction of flowing
water.
Fig 3.1 – Fig to show the bent-pipe architecture of satellite
An onboard oscillator and mixer are used to translate the uplink band to a
different downlink band.
The translation is done in order to separate the uplink and downlink signals .This
is done in order to prevent the antenna receive the same signal that is being
transmitted by it. The uplink frequency is always greater than downlink frequency as
the antenna size at ground terminal can have larger size while the size of antenna on
the satellite is fixed as the gain is higher in upper frequencies.
Fig 3.2 Figure to show block diagram of a bent-pipe architecture .
Amplifier used may be linear or non- linear. Linear amplifiers are used to
minimize the crosstalk. In order to keep the amplifier in linear region a we use an
LNA X
LO
TWTA
FILTER
UPLINK
6 GHz in C-Band
14 GHz in Ku- band
DOWNLINK
4 GHz in C-Band
12 GHz in Ku- band
16
Network 1
Network 2

17. AGC or automatic gain control. If the amplifier operates in non-linear mode it
increases the cross talk caused due to intermodulation interference.
The major characteristics of Bent pipe architecture is
 Simpler satellites
 Complex ground stations
 Controlled by a ground station
 Longer propoagation delay
 Strong feeder links puts gound processor virtualy onboard.
 Limited means of sharing resources.
 Fixed interconnectivity
3.2 Regenerative / On board processing
In regenerative or onboard processing the signal is processed on the
satellite and then transmitted towards the destination. In this case the destination
may be a different network or any onter satellite. In this type of model inter-
satellite links or cross links is possible.
Fig 3.3 Figure to show the basic components of OBP satellite communication.
3.3 Classes of OBP
The three main classes of OBP are:
Network 2
Network 1
17

18. 1. Baseband processing and switching (routing) -- two subclasses: autonomous and
ground-controlled,
2. IF or RF switching (frequency or time domains), and
3. Support processing.
OBP can provide greatly increased efficiency and performance in communications
satellites with trade-offs in increased cost and complexity. The increased efficiency can
be used for significant mass reduction or for increased capacity. With the current trends
toward decreased launch costs/unit mass, the increased capacity appears to be the logical
benefit of choice.
ISL, Ku- or Ka-band receivers and transmitters, digital modulation and coding, and
multiple access techniques.
Satellite-switched networking can be implemented via two primary approaches:
(1) fully processed by the satellite, and
(2) support by terrestrial control.
However, response time and throughput efficiency are compromised.
Class 1: Baseband Processing and Switching.
Baseband processing and switching involves the demodulation and
demultiplexing of the received signal, performing error detection and correction,
removing routing and control information (if not transmitted in a common channel
signalling mode), routing the data, pointing directed antennas, buffering the data,
multiplexing the data, tra nsmitting the data. The data could be of three types: circuit
switched, message switched, or packet switched. Required technologies include multiple
beam antennas, signal processing, microprocessors, time and/or space switches, ISLs,
protocol processor s, and stored- program switches. LEO systems require sophisticated
position and pointing capabilities, satellite-to-satellite handover control, and beam-to-
beam handover control.
Class 2: RF or IF On-Board Switching.
On-board RF/IF switchi ng involves electronically controlled RF/IF switches which can
be reconfigured on a near-real- time basis via ground control. OBP for carrier switching
has become fairly common in recent years, the INTELSAT spacecraft being the common
example. On-boards ignal regeneration (demod-remod) is also now being used fairly
frequently to gain the signal to noise (thus low BERs). Baseband processing with
message and packet switching is much less common and is generally used for special-
purpose spacecraft only. H owever, with the rapidly increasing speed, power, and
reliability of microprocessors, the more significant baseband processing and switching is
expected to move forward rapidly.
18

19. Class 3: Support Function On-Board Switching.
On-board support processi ng encompasses several functional areas. They include
control of waveguide switching parameters, ephemeris calculations for small beamwidth,
electronically scanned antennas, communications network protocol processing, special
processing for such function s as handover for LEO spacecraft, error detection and
correction, and elastic buffering and control.
The major techniques used in On board processing are
 Antenna beam switching
 Adaptive antennas
 Demodulation-Remodulation
Antenna beam switching
This type of On board processing is applied in case of use of multiple antennas
and is done to increase the capacity of the satellite. As we know the link capacity varies
inversely with the square of diameter of the beam on earth so we use small spot beams
which are pencil thin to cover a smaller area on the earth.This technique is reffered to as
Spot- beams technique and is quite effective.
Fig 3.4 Figure to show spot beams.
19
Spot beams
interference

20. This technique called frequency reuse allows satellites to communicate with a
number of ground stations using the same frequency, by transmitting in narrow beams
pointed toward each of the stations. Beam widths can be adjusted to cover areas –
footprints – as large as the entire country or as small as a island. Two stations far enough
apart can receive different messages transmitted on the same frequency. Satellite antennas
have been designed to transmit several beams in different directions, using the same
reflector.
Adaptive antennas
An adaptive antenna is type of smart antenna. It's "smart" because it improves on
the traditional antenna by adjusting for traffic patterns at a given time to increase signal
strength and quality. To adjust for frequency and channel use, the adaptive antenna uses
multiple antennas and an algorithm in order to maximize the strength of the signals being
sent and received while eliminating, or at least reducing, interference.
Demodulation-Remodulation:
The technique called demodulation and remodulation is one of the most powerful
on board processing techniques
In order to convert the satellite signals back into digital signals for transport
across the onboard processor, the transponder must demodulate the signal and then
remodulate it before sending back down to earth. This remodulation significantly
increases the power of the signal, allowing satellite terminals to be similar in cost to
normal VSAT terminals while providing significantly increased performance. In this
scheme uplink is demodulated into a bit stream.This bit stream is then processed by a
digital switching subsystem that can route and reformat the streams and finally
remodulate them onto one or more downlinks.
20

21. Fig 3.5 Figure to show the basic components in OBP demodulaton - remodulation.
The basic characteristics of OBP are:
 Users in different antenna beams can be interconnected
 Uplink and downlink signals can be independently optimized
 It has a complex satellite but a relatively simpler and fewer ground station
 Works well with crosslinks
 Power sharing advantage
 Permits normalization of downlink power sharing
 Amount of power devoted to each downlink can be adjusted
 Downlink power is thus not wasted
DEMODULATOR
PROCESSING REMODULATOR
FILTER
LNA
TWTA
LO
UPLINK
6 GHz in C-Band
14 GHz in Ku- band
DOWNLINK
4 GHz in C-Band
12 GHz in Ku- band
21

22. 4) CASE STUDY
4.1) INTELSAT IV TRANSPONDER
Abstract-INTELSAT –IV is intended for broadband and multi-carrier operation.
Fig 4.1 Figure to show the block diagram of transponder in INTELSAT - IV.
Receiver 1
Receiver 2
Receiver 3
Receiver 4
3 dB
Hybrid
redundancy
Global
Rx
2
1
switch
TWTA
Odd channel
Even channel
Global transmitter
Spot transmitter
Input mux
assembly
output mux
assembly
22

23. Basic elements of transponder
Wideband front end receiver 6 GHz antenna and a receiver section which
translates the frequency to 4 GHz.the two sets of receiver is called redundancy as in case
of some failure in the first set of receiver the second sets automatically takes control of
the operation.The 3 dB hybrid circuit divides the input into two channels even and
odd.The first set of channel is polarized in one form either horizontal or vertical and the
other channel i.e the odd channel is polarized into the other form.This is done to achieve
frequency reuse in oder to efficiently utilize the channel bandwidth.
23

24. 5.1 Example of a link budget
Item Link Parameters Value unit computations
Link budget analysis for the downlink at 4 GHz(C-Band)
1 Transmit power 10 dBW assumption
2 Transmit waveguide losses 1.5 dB assumption
3 Transmit antenna gain 27 dBi assumption
4 satellite EIRP 35.5 dBW 1-2+3
5 Free space loss 196 dB
6 Atmospheric absorption 0.1 dB Typical
7 Receive antenna gain 40.2 dBi
8 Receive waveguide loss 0.5 dB
9 Received carrier power -121.7 dBW 4-5-6+7-8
10
System noise temperature (140
K) 21.5 dBK
11 Earth station G/T 18.2 dB/K 7-(8+10)
Link budget analysis for the uplink at frequency 6 GHz (C- Band)
12 Boltzmann's Constant -228.6 dBW/Hz/K
13 Bandwidth (25 MHz) 74 dB Hz
14 Noise Power -133.1 dBW 10+12+13
15 Carrier to noise ratio 11.4 dB (9-14)
16 Transmit power 29.3 dBW
17 Transmit waveguide losses 2 dB
18 Transmit antenna gain (7 m) 50.6 dBi
19 uplink EIRP 77.9 dBW 16-17+18
20 spreading loss 162.2 dB(m2
)
21 Atmospheric attenuation 0.1 dB
22 flux density at spacecraft -84.4 dBW/m2
19-20-21
23 Free space loss 200.4 dB
24 receive antenna gain 26.3 dBi
25 Receive waveguide loss 0.5 dB
26
System noise temperature (450
K) 26.5 dB(K)
27 spacecraft G/T -0.7 dB/K 24-25-26
Combining the uplink and downlink to estimate the overall link performance
28 Received G/T -122.9 dBW/K 19-23-21+27
29 Boltzmann's Constant -228.6 dBW/Hz/K
30 Bandwidth (25 MHz) 74 dB Hz
31 Carrier to noise ratio 31.7 dB 28-29-30
24

25. 32 uplink C/N (31.7) 1479.1 ratio 31
33 N/C (uplink) 0.000676 ratio
34 downlink C/N (11.4) 13.8 ratio 15
35 N/C (downlink) 0.0724 ratio
36 Total thermal noise (Nth/C) 0.0731 ratio 33+35
37 Total thermal C/Nth 13.7 ratio
38 Total thermal C/Nth 11.4 dB
39 Interference C/I 63.1 ratio assumption
40 I/C 0.015848 ratio
41 Total noise (Nth+1)/C 0.0889 ratio 36+40
42 Total C/(Nth+1) 11.2 ratio
43 Total C/(Nth+1) 10.5 dB
44 Required C/N 8 dB
The above table demonstrates the example of link budget.
5.2 Link budgets and their interpretation
The link between the satellite and earth is governed by the basic microwave radio link
equation given by
Pr =PtGtGrC2
/ (4 )R 2
f2
…………1
25

26. Where,
Pr power received by the receiving antenna
Pt is the power transmitted by the transmitting antenna
Gt is the gain of the transmitting antenna (true ration)
Gr is the gain of receiving antenna (true ratio)
C is the speed of light in m/s
f is the frequency in hertz
The same formulae when converted into decibels have the form of a power balance.
pr=pt + gt+ gr + - 20 log (f.R) +147.6 ……….(2)
the received power is in the form of dBW.
The last two terms represent the free space path loss.
We can correct the path loss for other frequencies and path lengths using the formula:
Ao = 183.5 + 20log(f) + 20log(R/35788) ………….(3)
where Ao is the free space path loss in decibels, f is the frequency in GHz and D is the
path length in Km.
The link power balance in the above equation considers only the free space path loss and
ignores the effects of the different layers of earth's atmosphere. The following listing
provides the dominant contributors that introduce additional path loss, which can vary
with time. Some are due to air and water content of the troposphere, while others result
from charged particles in the ionosphere.
Terms in the link budget.
1. The transponder onboard the satellite has a power output of 10 W equivalents to
10 dBW.
2. The microwave transmission line between the satellite power amplifier output and
the spacecraft antenna absorbs about 40% of the output radiated as heat. The loss
is represented as positive number and then subtracted.
3. The satellite is made to cover a particular area of earth called the coverage area or
footprint which determines the gain of antenna, there being an inverse
relationship.
4. The EIRP specifies the maximum radiated power per transponder in the direction
of a specific location on earth. This is the product of actual power given to
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27. transmitting antenna and antenna power gain of transmitting antenna.
That the equivalent isotropic power (EIRP) may be defined as
EIRP=PTGT …………(4)
EIRP is often expressed in decibels relative to 1 watt, or dBW. Let Pt be in Watts then
EIRP = [PT]+[GT] dBW …...(5)
5. Free space loss is the primary loss in the satellite link, amounting to 183 to 213dB
for frequencies between 1 to 30 GHz for a GEO satellite.
6. The atmospheric path loss is given by
Ao = 92.5+20 log(fD) ………(6)
Where
Ao = free space loss
f= frequency in GHz
D= distance in Km
7. The receiving antenna has the diameter of 3.2 m. The antenna gain is given by
GT=10 log (p2
D2
h/(3/f)2
) ………(7)
8. Waveguide or cable loss between the antenna feed and low noise amplifier
reduces the received signal and increases link noise. Thus we have assumed a small
value accounting for that loss.
9. Received carrier power is calculated directly by the power is calculated directly
by the power balance method. The computed value includes all the gains and losses in
the link.
10. The noise that exists in all the receiving systems is the main cause of degradation.
11. The earth station G/T is the difference in decibels between the net antenna gain
and the system noise temperature converted to decibels i.e.
G/T (dB/K) = Receiver Antenna gain – 10 log(system noise temp) …(8)
12.-14 –The noise power that reaches the receiver is equal to the product
N= kTB ….(9)
Where
K= Boltzmann’s constant
T= equivalent noise temperature
B= bandwidth
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28. 15.The difference in decibels between received carrier power and the noise power is the
carrier to noise a ratio.
C/No(dB Hz) = EIRP - path losses + G/T +228.6. ………(10)
16. The earth station high power amplifier provides the power to transmit the signal to
satellite.
17. An allocation of 2 dB is made to account for the loss between the HPA and the earth
station antenna feed.
18.A 7 m earth station antenna provides a 50.6 dBi gain.
19. Uplink EIRP must be sufficient to saturate the satellite transponder.
20. The spreading loss allows us to convert from earth station EIRP to the corresponding
value of flux density at the face of the satellite receiver.
21. assumed value same as in downlink.
22. The SFD causes the transponder to transmit the maximum EIRP in the downlink.
Uplink EIRP = spreading loss + atmospheric loss –SFD ………(11)
23. The atmospheric path loss is given by (6).
24.The space craft antenna is designed to cover a specific area.
25.An allocation of 0.5 dB for loss between the spacecraft antenna and receiver.
26.The typical C- band satellite system has a noise temperature of 450 K.
27.The receiving system figure of merit given by G/T.
28.The value of c/t is given by
C/T = EIRP-Ao+ G/T. ………(12)
29.-31 These values are considered in the same manner as in 12.
32-43 The C/N is calculated as in eq 9 and the calculation is done to calculate whole
C/N.
44. The required value of C/N is specified for receiver digital modulator.
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29. CONCLUSION
I have completed my project with the brief study of the transponders like Bent
pipe or regenerative and components in the transponders. I have gained both the practical
and theoretical knowledge of my project titled .Moreover I have also learned to calculate
the satellite link and various parameters such as free space path loss and G/T etc. that
come in role during the communication of two stations via a satellite.
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30. BIBLIOGRAPHY
1) Satellite communication systems by: - G. Maral ,M.Bousquet
2) A handbook on satellite communication- compiled by K .Miya
3) Communication systems encyclopaedia – John Proakis
4) wikipedia.com
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