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Vocational Training Report

On

STUDY OF TRANSPONDERS

TRAINING UNDERTAKEN

AT

DEFENCE ELECTRONICS APPLICATION

LABORATORY RAIPUR ROAD,

DEHRADUN-248001

PREPARED BY: - UNDER THE SUPERVISION OF:-

MOHIT KUMAR

B.Tech (ECE)

Univ. R.NO.-0981562805

NIEC, New Delhi

ASHOK KUMAR

Scientist ‘F’

Millimeter Wave Group

DEAL, Dehradun.



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Defence Electronics Application Laboratory

Raipur Road, Dehradun (UK)-248001

CERTIFICATE

This is to certify that the study project work entitled “Transponders” was carried outand 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



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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 greathelp 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 thesisin 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’), whoseconstant 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 partof 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 (TechnicalInformation 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



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Defence Research & Development Organization (DRDO) works under Departmentof Defence Research and Ministry of Defence. DRDO is dedicatedly working towards

enhancing self-reliance in Defence Systems and undertakes design & developmentleading 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 defensetechnologies 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 combataircrafts, radars, electronic warfare systems etc are on hand and significant achievements

have already been made in several such technologies.



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Defence Electronics Application Laboratory

The origin of Defence Electronics Applications Laboratory (DEAL) can betraced 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 propagationstudies, 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 theborder 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 inthe present location in 1976.

Shri RC Aggarwal has been appointed Director, Defence ElectronicsApplications Laboratory (DEAL), Dehradun , wef 01 December 2007.



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CONTENTS

1). A Brief Overview of Satellite Communication1.1). Abstract

1.2). Types of orbits

1.3). Basic terms in satellite communication.1.4). Components of a satellite

2) Satellite payloads2.1) Abstract

2.2) Basic operations at transmitting earth station.

3) Transponders3.1) Bent pipe3.2) On board processing

4) Case Study4.1 INTELSAT IV

5) Satellite Link budget5.1 Example of a link budget

5.2 Various terms in budget

6) Conclusion

7) Bibliography

1 - A Brief Overview of Satellite Communication



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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.



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1.2 Types of Orbits

Different orbits serve different purposes. Each has its own advantages and

disadvantages. There are several types of orbits:

1. Polar2. Sun Synchronous

3. Geosynchronous

Polar Orbits

The more correct term would be near polar orbits. These orbits have an inclination near90 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 atan 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 amountof 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 thesame 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 arelocated 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 theEarth. 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..



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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 onthe 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 6874mphto 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.



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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 describedin 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 coverthe earth's entire surface except for the polar caps. This makes it possible for one earthstation 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 to50% 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 canincrease the energy of the signal. The smaller the footprint, the stronger the signal, andthus 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-converterand 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



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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.



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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 kindwe are interested in here. The payloads on communications satellites are effectively justrepeaters. 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 adifferent 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



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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 differencefrequencies 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 theHPA 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 afew 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 smallheat sink.

6. The parabolic antenna is a high-gain reflector antenna. A typical parabolic antennaconsists 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 thereceive 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 assembly8. 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



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and easier. The 1st down converter mixes the signals with another frequency; the

result is both the sum and difference of the signals. By filtering out the original andthe 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.



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3. Transponders

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-piperepeaters 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 separatesthem 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 f 1, amplifies it and sends it back on a second frequency f 2. Figure shows atypical 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 elementsinclude receivers, mixers, oscillators, channel amplifiers, and RF switches. OBPtransponders 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



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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 flowingwater.

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 beingtransmitted 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 tominimize the crosstalk. In order to keep the amplifier in linear region a we use an

LNA X

LO

TWTAFILTER

UPLINK6 GHz in C-Band

14 GHz in Ku- band

DOWNLINK

4 GHz in C-Band

12 GHz in Ku- band

Network 1

Network 2



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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 thesatellite 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 2Network 1



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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 communicationssatellites with trade-offs in increased cost and complexity. The increased efficiency canbe 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: circuitswitched, 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 switchinghas become fairly common in recent years, the INTELSAT spacecraft being the common

example. On-boards ignal regeneration (demod-remod) is also now being used fairlyfrequently 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 isexpected to move forward rapidly.



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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 andcorrection, 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 antennasand is done to increase the capacity of the satellite. As we know the link capacity variesinversely 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.

S ot

interference



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This technique called frequency reuse allows satellites to communicate with anumber 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. Satelliteantennas have been designed to transmit several beams in different directions, using the

same reflector.

Adaptive antennasAn 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 usesmultiple 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 powerfulon 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 significantlyincreases 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 adigital switching subsystem that can route and reformat the streams and finally

remodulate them onto one or more downlinks.



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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

UPLINK6 GHz in C-Band

14 GHz in Ku- band

DOWNLINK

4 GHz in C-Band12 GHz in Ku- band



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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

redundancyGlobal

Rx

2

1

switch

TWTA

Odd channel

Even channel

Global transmitter

Spot transmitter

Input mux

assembly

output mux

assembly



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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 theother 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.



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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 assumption4 satellite EIRP 35.5 dBW 1-2+3

5 Free space loss 196 dB

6 Atmospheric absorption 0.1 dB Typical7 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

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System noise temperature (140

K) 21.5 dBK11 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/K13 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 dBi19 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

19-20-2123 Free space loss 200.4 dB

24 receive antenna gain 26.3 dBi

25 Receive waveguide loss 0.5 dB

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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 Hz31 Carrier to noise ratio 31.7 dB 28-29-30



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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 )R2f

2…………1

32 uplink C/N (31.7) 1479.1 ratio 31

33 N/C (uplink) 0.000676 ratio34 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 ratio38 Total thermal C/Nth 11.4 dB

39 Interference C/I 63.1 ratio assumption40 I/C 0.015848 ratio

41 Total noise (Nth+1)/C 0.0889 ratio 36+40

42 Total C/(Nth+1) 11.2 ratio43 Total C/(Nth+1) 10.5 dB

44 Required C/N 8 dB



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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 listingprovides 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 andthe 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 orfootprint which determines the gain of antenna, there being an inverse

relationship.

4. The EIRP specifies the maximum radiated power per transponder in the directionof a specific location on earth. This is the product of actual power given to



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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 P t 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)

WhereAo = 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 (p2D

2h/(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 smallvalue accounting for that loss.

9. Received carrier power is calculated directly by the power is calculated directlyby the power balance method. The computed value includes all the gains and losses inthe 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 constantT= equivalent noise temperature

B= bandwidth



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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 tosatellite.

17. An allocation of 2 dB is made to account for the loss between the HPA and the earthstation 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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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 practicaland 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. thatcome in role during the communication of two stations via a satellite.



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

study of transponders -deal

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