satellite sub systems

8/10/2019 Satellite Sub Systems

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BITSPilaniK K Birla Goa Campus

EEE C 472

Satellite Communication


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BITS Pilani, K K Birla Goa Campus


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Spin Stabilized Satellite


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Three Axes Stabilized Satellite


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1. Communications Subsystem,

2. Telemetry, command and ranging sub-system,

3. Attitude and Orbit Control Sub-system (AOCS),

4. Electrical Power Sub-system,

5. Reaction Control Sub-system,

6. Apogee Kick Motor,

Sub-systems ofCommunication Satellite


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Communications Sub-Systems



Transponders

Antennas

Explore recent advancement in SatelliteCommunication Technologies

Communications Subsystems



Communication Satellite in GEO for relaying voice, videoand data communications,

Small part of volume, weight and cost of entire system,but provides largest traffic capacity possible

Larger, heavier, costlier satellites have been putoperation, with cost per transmitted bit reducing

Distance of 36000 km; power of transmitters is low ; lowpower signals are received (low gain antennas), power

of signal must exceed noise power Thus, narrower receiver bandwidth and effective

modulation schemes must be used.

Context for CommunicationsSystem



1. The total channel capacity of a satellite can beincreased only if bandwidth can be increased or reused.

2. The trend in high-capacity satellites has been to reusethe available bands by employing several directional

beams at the same frequency (spatial frequency reuse)and orthogonal polarizations at the same frequency(polarization frequency reuse)

3. Different frequency is to be used for transmit path than

for receive path4. The spacing between GEO satellite is reduced to 2o

5. Trend in favor of use of Ka band frequencies

Direction of advancement



Illustration of Advancement ofIntelsat satellites



Satellites are provided with zone or beam antennas,LNAs, and redundancy

For both receive and transmit paths, allotted 500 MHzbandwidth is divided into channel of 36 MHz wide plus 2

MHz Guard Bands on either sides, (the channelallocation is therefore 40 MHz)

Each channel is handled by separate transponder,

A transponder consists of a band-pass filter,downconverter and output amplifier

Many transponder (including spares) are provided,switching of transponder can be effected from earthstation

Transponders



Transponder arrangement ofsatellites and frequency plan


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Switch matrix allows many possible interconnections

between uplink beams and downlink transmitters.



Bulk of traffic is carried by 6/4 GHz section, though 14/11GHz section is also provided,

Total of 2000 MHz bandwidth is available by frequencyreuse,

Switch matrix provides very large number of variations inconnecting receivers and transmitters, thus flexibility insetting up links through the satellite,

When more than one signal shares a transponder, the

power amplifier must be operated below its maximumoutput power to maintain linearity and reduceintermodulation products (output backoff)

SATCOM with Switch Matrix


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Transponders

The 500 MHz bandwidth is divided up into

channels, often 36 MHz wide which are

each handled by a separate transponder.

A transponder consists of a band passfilter to select particular channel.


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A transponder also consists of a down

converter to change the frequency from 6

GHz to the input to 4 GHz at the outputand an output amplifier.

12 to 44 active transponders are carried

by a high capacity satellite some of whichmay be spares. (Redundancy)

TWT [Travelling Wave Tube] gives the

best performance at microwavefrequencies as an amplifier.


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Simplified block diagram for single

conversion transponder is shown in fig.

This transponder is commonly used inmany satellites for the 6/4 GHz band.

When high output power is required

travelling wave tube power amplifier isused instead of solid state power amplifier.

The local oscillator is at 2225 MHz to

provide the appropriate shift in frequencyfrom 6 GHz to 4 GHz.


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The attenuator is used to control the gain

via uplink command system.

Providing a spare HPA in eachtransponder greatly increases the

probability of transponders still operational

after the satellite life time. Transponders for use in the 14/11 GHz

bands normally employ a double

frequency conversion scheme. In thesescheme 1 GHz carrier can be used as IF.


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Bent Pipe Transponder: single conversiontransponder for 6/4 GHz band

HPA can be SSPA or TWTA (for output in excess of 50

W).HPA is a least reliable component, and M for Nredundancy is provided


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A traveling-wave tube (TWT) is a specialized vacuumtube that is used in electronics to amplify radio frequency

(RF) signals to high power, usually as part of an

electronic assembly known as a traveling-wave tube

amplifier(TWTA). The bandwidth of a broadband TWT can be as high as

one octave, although tuned (narrowband) versions exist,

and operating frequencies range from 300 MHz to 50

GHz. The power gain of the tube is on the order of 70decibels.

Travelling-wave tube (TWT)


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1) Operates by interaction between an electron beam andradio wave,

2) Electron beam, generated by a cathode raised to a hightemperature, is focused and accelerated by a pair ofanode,

3) Wave propagates along a helix, the electron beam,whose focus is maintained by concentrically locatedmagnets, flows within the helix,

4) Axial velocity of wave is artificially reduced by the helix

to a value close to the velocity of the electrons. Theinteraction leads to a slowing of electrons which give uptheir K.E.

Travelling Wave TubeAmplifier (TWTA) ..1/4..


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1) Collector captures the electrons at the output of helix.2) Division of the collector into several stages at different

potentials permits better matching to the dispersion ofthe residual energy of the electrons and hence anincrease in the efficiency of the tube.

3) The residual energy is to be dissipated in the form ofheat.

4) The collector conducts the heat to be dissipated eitherby conduction (conduction-cooled) towards the satellite

radiative surfaces of the satellite or directly into spaceby a self-radiating system part of the TWT (radiation-cooled).

TWTA ..3/4..


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1) Radiation-cooling makes it possible to reduce thethermal load of the satellite and to decrease the overallplatform mass for a given RF performance.

2) The past six decades of helix TWT development have

resulted in a constant increase in the overall DC to RFconversion efficiency, up to 75% with potential toapproach 80% for commercial satellite communicationapplications.

TWTA. ..4/4..


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1. Power at saturation: from 20

250 W

2. Efficiency at saturation: 60 75%

3. Gain at saturation: Around 55 dB

4. (C/N)IM at saturation: 10-12 dB (two carriers ofequal amplitude)

5. AM/PM conversion coefficient Kp: around4.5o/dB (near saturation)

Characteristics of TWT


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1. Power: 20

40 W

2. Efficiency: 30 45%

3. Gain at saturation: 70 90 dB (depending on

number of stages)4. (C/N)IM at saturation: 14 18 dB (two carriers

of equal amplitude)

5. AM/PM conversion coefficient, Kp: around2o/dB (near saturation)

Solid State Power Amplifier(SSPA)- Characteristic values


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Characteristic TWTA SSPA

Operating Band C, Ku, Ka L, C

Saturated power o/p (W) 20 - 250 20 - 40

Gain at saturation (dB) ~ 50 70 -90

(C/N)IM3(dB) 10 - 12 14 - 18

AM/PM conversion (o/dB) 4.5 2

DC to RF efficiency (%) 50 - 65 60 - 45

Mass including EPC (kg) 1.52.2 0.8- 1.5

Failure in 109h (FIT) < 150 < 150

Comparison of TWTA andSSPA


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Double Frequency Conversion

IF 1100 MHz is used, as it is easy to makefilters, amplifiers and equalizers for thisfrequency

Stringent requirements for filters, as thesemust reject unwanted frequencies, IMproducts, and amplitude and phase ripples

Communication capacity can be increased by

using OBP with switched beam technology(narrow beams at Ka band (0.4o) with highgain can be generated)


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Multiple Beam, Onboard processing transponder


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Multiple beam antennas with baseband processingtransponders are used on GEO and LEO satellitesproviding service to mobile terminals and handheldtelephones.

The low gain of the omni-directional antenna of a mobileearth station must be compensated by a high gainantenna on the satellite, necessitating the use of multiplebeam antennas

The bandwidth can be conserved by using differentmodulation techniques on U/L and D/L and by providingbaseband processor on the satellite

Technological Advancement..


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Satellite Antennas


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Satellite antennas

Four main types of antennas are used onsatellites . These are:

1. Wire antennas,

2. Horn antennas,3. Reflector antennas,

4. Array antennas,

Wire antennas: these are used primarilyat VHF and UHF to providecommunications for the TTC&M systems.


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Horn antennas are used at microwave

frequencies when relatively wide beams

are required, as for global coverage. A horn is a flared section of waveguide

that provides an aperture several

wavelengths wide and good matchbetween the waveguide impedance and

free space.


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Reflector antennas are usually illuminated by

one or more horns and provide a larger

aperture that can be achieved with a hornalone.

Parabolic reflector is the most common type

of reflector antennas and is commonly usedfor earth stations.

Phased arrays are also used on satellites to

create multiple beams from a from a singleaperture. these are used by IRIDIUM &

GLOBALSTAR.

Typical satellite antenna patterns and coverage zones. The


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Typical satellite antenna patterns and coverage zones. The

antenna for the global beam is usually a waveguide horn.

Scanning beams and shaped beams require phased array

antennas or reflector antennas with phased array feeds.


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. .Contour plot of the spot beam of ESAs OTS satellite projected onto the

earth. The contours are in 1 dB steps, normalized to 0 dB at the center of

the beam.

Figure 3 16 (p 81)


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Figure 3.16 (p. 81)Typical coverage patterns for Intelsat satellites over the Atlantic

Ocean.

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