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COMMUNICATION SATELLITE

Lecture by- Kanchan Bakade

Assistant Prof:

Satellite Communication by Pratt,

Bostian and Dennis Roddy

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INTRODUCTION

As the cost for launching, monitoring andcontrolling the satellite is very high and thecost is recovered well within the expectedlifetime of the spacecraft.

The satellite is difficult to access after launchbecause of harsh enviornmental conditionsand these are

shocks and vibration during launch

Vaccum

large temperature variation

effect of small particles in space

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DESIGN CONSIDERATION w.r.t to communication

EIRP per carrier no. of carriers domestic fixed satellite Coverage area

No. of television channels direct broadcastsatellite

Coverage area w.r.t to environmental conditions

Zero gravity Difficulty of liquid fuel flow

Benefit: facilitates operation of deployment of antennas and solarpanels during launching Atmospheric pressure and temperature

atm. Pressure: Extremly low 10-7 Torr; gearing used for stablizationsystem

External pressure 330-350 K in presence of sunlight to 95-120 K ineclipse condition

Space Particles

Examples: Cosmic rays, protons, electrons, meteoroids and man-madespace debris

Bombardment by particles cause a degradation in solar cells and certainsolid state components within the satellite

Magnetic Fields Deflects charged particles which r trapped in the region surrounding it.

(Van allen belt)

The electric charges affect electronic components Special manufacturing technique used to harden the electronic

component against radiation.

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LIFE TIME AND RELIABITLY

Components high reliability in outer space

Strategy to allow some components to fail

Two approaches are used Space qualification

Redundancy SPACE QUALIFICATION

Each components is tested and that one is highly reliable inouter space.

Total system of spacecraft is tested to ensure its reliability

Three prototype models Mechanical

Thermal

Electrical model

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Mechanical Model Structural and mechanical parts Subject to vibration and shock testing G force to be encountered on launch

Thermal Model Electronic packages and other components maintained at

correct temperature Antennas to check for distortion of reflectors and

displacement or bending

Electrical Model Electronics parts corrected for electrical performance under

total vacuum and wide range of temperature Antennas to correct beamwidth, gain and polarization

properties.

Redundancy provided if one component fails, it can be switchedover to other by command from the ground.

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RELIABILITY Reliability theory to predict the future.

Reason for reliability calculation Know the probability of the system Provide redundant components

or subsystems

BATHTUB CURVE Gives probability of failure (pf)

For elex equipment: (pf) is higher at the beginning of life. Component testing under rigorous conditions, i.e. in vaccum

chamber and under radiant heat conditions. Semiconductors and ICs required to have high reliabitity are

subject to burn-in periods from 100 t0 1000 hours.

Burn-in

End of

life

time

Probab

-ility of

failure

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SATELLITE SUBSYSTEMS

Attitude and Orbit Control System (AOCS) Rocket Motors move the satellite back to correct orbit

Gas jets control the attitude of the satellite

Telemetry,Tracking, Command and Monitoring (TTC&M)

System partly on satellite and partly at the controlling earth

station (ES)

Telemetrysends data from many sensors on the satellite,monitors satellite health

Tracking System at ES; information on range, elevationand azimuth angles ; changes are detected

Control System correct the position and attitude of thesatellite

Control antenna pointing and communication systemconfiguration

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Power System

Electrical power from solar cells; used by transmitter and otherelectrical system; to support communication system

Communication Subsystem

Composed of one or more antennas, Rx and Tx over widebandwidths at microwave frequencies and set of transponders. Two types of transponder

Linear or bent pipe transducer Received and retransmits atdifferent (lower frequency)

Baseband processing transponder used with digital signals

Satellite Antennas Designed to operate in single frequency band (eg. C or Ku band)

Satellite which uses multiple frequency bands usually has four ormore antennas.

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ATTITUDE AND ORBIT

CONTROL SYSTEM

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satellite's antennas point toward the earth

several forces acting on an satellite that tend to change its attitudeand orbit.

The most important are the gravitational fields of the sun and themoon, irregularities in the earth gravitational field, solar pressurefrom sun and variations in the earth's magnetic field.

Solar pressure - tend to cause rotation of the satellite body.

Careful design - minimize these effects, but the orbital period of the

satellite makes many of the effects cyclic, which can cause nutation(a wobble) of the satellite.

The AOCS damp out nutation and counter any rotational torque ormovement.

gravitational fields from the sun and the moon cause the orbit of a

GEO satellite to change with time. The control system of the satellite must be able to move the satellite

back into the equatorial plane before the orbital inclination becomesexcessive.

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Earth is not perfect sphere.At the equator, there arebulges of about 65 m atlongitudes 15W and 165 E.

satellite is accelerated towardone of two stable points in theGEO orbit at longitude 105W and 75 E, as shown inFigure 3.2.

To maintain accurate stationkeeping, the satellite must beperiodically accelerated in theopposite direction to theforces acting on it.

station-keeping maneuvers,using small rocket motors thatcan be controlled from theearth via the TTC&M system.

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ATTITUDE CONTROL SYSTEM

two ways to make a satellite stable in orbit, when it is weightless.

the satellite can be rotated at a rate between 30 and 100 rpm, to create a gyroscopicforce that provides stability of the spin axis and keeps it pointing in the samedirection. Such satellites are known as spinners.

The satellite can be stabilized by one or more momentum wheels. This is called athree-axis stabilized satellite.

Increasing the speed of the momentum wheel causes the satellite to precess in theopposite direction, according the principle of conservation of angular momentum.

In the spinner design, The satellite consists of a cylindrical drum covered in solar cells

The communications system is mounted at the top of the drum and is drivenby an electric motor in the opposite direction to the rotation of the satellitebody to keep the antennas pointing toward the earth. Such satellites arecalled despun.

In this axis of rotation is usually Y axis, which is maintained close to YR axis,perpendicular to orbital plane.

Pitch correction is required only on the despun antenna system. Yaw and roll are controlled by pulsing radially mounted jets at the appropriate instant

as the body of the satellite rotates.

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Spinner satellite 3 axis stabilised satellite

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In a three-axis stabilized satellite, one pair of gas jets is needed for each axis to provide for rotation in

both directions of pitch, roll, and yaw. An additional set of controls, allowing only one jet on a given axis to be

operated, provides for velocity increments in the X, Y, and Z directions.

Let us define a set of reference Cartesian axes (XR, YR, ZR) with thesatellite at the origin, as shown in Figure 3.4.

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The ZRaxis is directed toward the center of the earth and is in theplane of the satellite orbit.

The XRaxis is tangent to the orbital plane and lies in the orbitalplane.

The YRaxis is perpendicular to the orbital plane. Rotation about the XR, YR, ZRaxes is defined as roll , pitchand yaw. The axes XR, YR, ZRare defined with respect to the location of the

satellite; and X, Y, Z, define the orientation of the satellite. Changes in a satellite's attitude cause the angles ,, to vary as

the X, Y, and Z axes move relative to the fixed reference axes XR,YR, ZR.

Attitude control of a three-axis stabilized satellite requires anincrease or a decrease in the speed of the inertia wheel.

When the upper or lower speed limit of the wheel is reached, it mustbe unloadedby operating a pair of gas jets and simultaneouslyreducing or increasing the wheel speed.

Closed-loop control of attitude is employed on the satellite tomaintain the correct attitude.

When large, narrow beam antennas are used, the whole satellitemay have to be stabilized within 0.01 on each axis.

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Orbit control system

The various forces - steadily pull it out of the correct orbit.

If the orbit is not circular, a velocity increase or decrease will have tobe made along the orbit, in the X-axis direction.

Altitude corrections are made by operating the Z-axis gas jets.

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The inclination of a satellite increases at an average rate of about0.85 per year.

Most GEO satellites are specified to remain within a box of O.05and so that fixed pointing antennas can be used at earth station.

Corrections are made every 2 to 4 weeks to keep the error small.

when the inclination ; reached 0 , an opposite gasjets must beoperated to stop the satellite at that position. This procedure isknown as N-S station keeping maneuver.

Correcting the inclination of a satellite orbit requires more fuel to beexpended than for any other orbital correction.

E-W station keepingis effected by use of the X-axis jets of thesatellite.

All of these satellite requires AOCS to enable them to point theirantennas or sensors correctly and all need to be able to maintain thecorrect orbit.

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Station Keeping: maintaining satellite in its correct orbital position. Gas jets are used along the 3 reference axes for velocity changes. These systems are used for making small corrections in orbits where low levels of

thrust are required.

North South Station Keeping: satellite inclination changes at a rate of 0.85 per year. Practically corrections are made every 2 to 4 weeks to keep the error small. To correct the inclination drift, a velocity at right angles to the orbital plane in YR

direction is applied. But when the inclination has reached zero degrees, an opposingjet is operated to stop the satellite at that position. This is known as north south

station keeping.

East West Station Keeping: Spacecraft located away from stable points at 75 E and 105 W, will try to drift

towards these points. X axis jets are used every 2 or 3 weeks to counter the drift and a small velocity

increment in the opposite direction

East-west station keeping is necessary in all geostationary satellites because thespacing between the satellites is 2 or 3 degrees

Excess drift is not tolerated because 6/4 GHz satellites are held within 0.1 of theirallotted position and 14/12 GHz are held with 0.05.

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TELEMETRY,TRACKING, COMMAND

AND MONITORING

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The TTC&M is part of the satellitemanagement task.

Main functions of satellitemanagement are: to control the orbit and

attitude of the satellite to monitor the status of all

sensors and subsystemson the satellite

To switch on or off sectionsof the communication system

Tracking is performed by theearth station.

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Monitoring system

collects data from many sensors within the satellite and sends these data to thecontrolling earth station.

Sensors located on the satellite to monitor: pressure in the fuel tanks voltage and current in the power conditioning unit

current drawn by each subsystem critical voltages and currents in the communications electronics. temperature

These data are reported back to the earth by the telemetry system. The sighting devices used to maintain attitude are also monitored via the telemetry

link: this is essential in case one should fail and cause the satellite to point in the

wrong direction. The faulty unit must then be disconnected and a spare broughtin, via the command system, or some other means of controlling attitude devised.

.

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Telemetry

data are usually digitized and transmitted as phase shift keying(PSK) of a low-power telemetry carrier using time divisiontechniques.

A low data rate is used, E S have a narrow bandwidth and thusmaintain a high carrier to noise ratio.

The entire TDM frame may contain 1000 of bits of data and takeseveral seconds to transmit.

At the controlling earth station a computer can be used to monitor,store, and decode the telemetry data so that the status of anysystem or sensor on the satellite can be determined immediately bythe controller on the earth.

Alarms can also be sounded if any vital parameter goes outsideallowable limits.

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Tracking

Used to determine the current orbit of a satellite.

The earth station controlling the satellite can observe:

the Doppler shift of the telemetry carrier

beacon transmitter carrier to determine the rate at which range

is changing.

range is used to determine the orbital element.

Active determination of range can be achieved by:

Transmitting a pulse or sequence of pulses to the satellite

Observing the time delay before the pulse is received again.

With precision equipment at the earth stations, the position of thesatellite can be determined within 10 m.

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Command

Used to launch the satellite and do the operation of satellite. Used to make changes in attitude and corrections to the orbit and to control

the communication system. During launch, it is used:

to control the firing of the apogee kick motor

to spin up a spinner or extend the solar sails and antennas of a three-axisstabilized satellite.

The command structure must possess safeguards against : unauthorized attempts to make changes to the satellite's operation

against inadvertent operation of a control due to error in a receivedcommand.

security by encryption of commands The control codeis converted into a command word, which is sent in a TDM

frame to the satellite. After checking for validity in the satellite, the word issent back to the control station via the telemetry link where it is checkedagain in the computer. If it is found to have been received correctly, anexecuteinstruction will be sent to the satellite so that the command isexecuted. The entire process may take 5 to 10 sec but minimizes the risk oferroneous commands causing a satellite malfunction.

During the launch phase and injection into geostationary orbit the

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During the launch phase and injection into geostationary orbit, themain TTC&M system may be inoperable because the satellite doesnot have the correct attitude or has not extended its solar sails. Abackup system is used at this time, which controls only the mostimportant sections of the satellite.

The backup system provides control of:

the apogee kick motor

the attitude control system

orbit control thrusters

the solar sail deployment mechanism

the power conditioning unit.

With these controls, the satellite turned to face the earth, andswitched to full electrical power so that hand over to the mainTTC&M system is possible.

In the event of failure of the main TTC&M system, the backupsystem can be used to keep the satellite on station.

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POWER SYSTEM

t llit bt i th i l t i l f l ll hi h t i id t li ht

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satellites obtain their electrical power from solar cells, which convert incident sunlightinto electrical energy.

The radiation intensity is of 1.39 kW/m2. Solar energy efficiency is typically 20 to 25% at beginning of life(BOL) but falls with

time because of aging of the cells and etching of the surface by micrometeor impacts.

At the end of life(EOL) of the satellite, about 15% extra area of solar cells is usuallyprovided as an allowance for aging.

A spin-stabilized satellite usually has a cylindrical body covered in solar cells and because of this half of

the cells are not illuminated at all, and at the edges of the illuminated half, thelow angle of incidence results in little electrical power being generated.

To obtain 10 kW from a spinner requires a very large body on which to place thesolar cells, which may then exceed the maximum payload dimensions of thelaunch vehicle.

More recently, large communications satellites for direct broadcast operationgenerate up to 6 kW from solar power.

A three-axis stabilized satellite

can make better use of its solar cell area, since the cells can be arranged on flatpanels that can be rotated to maintain normal incidence of the sunlight. Only one-third of the total area of solar cells is needed relative to a spinner, with

some saving in weight. by unfurling a folded solar array when the satellite reaches geostationary orbit,

power in excess of 10 kW can be generated with large arrays.

Solar sails rotated b electric motor once e er 24 ho r to keep cell in f ll

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Solar sails rotated by electric motor once every 24 hour to keep cell in fullsunlight.

A rotary joint is used to transfer current from the rotating sail to the body ofthe satellite.

The satellite must carry batteries to power the subsystems during launchand during eclipses.

The communication system load may be shut down during eclipse, but thistechnique is rarely used when telephony or data traffic is carried.

Batteries are usually of the nickel-hydrogen type which do not gas whencharging and have good reliability and long life, and can be safelydischarged to 70% of their capacity.

A powerconditioning unit controls the charging current and dumps excesscurrent from the solar cells into heaters or load resistors on the cold side of

the satellite.

Sensors on the batteries, power regulator, and solar cells monitortemperature, voltage, and current.

Typical battery voltages are 20 to 50 V with capacities of 20 to 100 ampere-

hours.

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COMMUNICATION

SUBSYSTEM

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The communication subsystem provides the receive and transmit coveragefor the satellite.

It consists of a communication antenna and a communication repeater.

The main function of the antenna is to provide shaped downlink and uplinkbeams for transmission and reception of communication signals in theoperating frequency bands.

The antenna may be used to provide a signal link for the satellite telemetry,

command, and ranging subsystem.

Th d i f

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The transponder consists of: Receiver/down converter. Input multiplexer. Travelling wave tube amplifiers (TWTAs). Output multiplexer.

The uplink signals are first filtered by a waveguide bandpass filter with about a 600 MHzbandwidth and then amplified by a parametric or GaAs FET low noise amplifier with a noise figureof 2 to 4 dB. The amplified signals are then down converted to 3.7 to 4.2 GHz for C band and 11.7to 12.2 GHz for Ku band. After down conversion, the signals are gain amplified by GaAs FETamplifier and passed through a ferrite isolator to the input multiplexer.

The input multiplexer separates the 500 MHz bandwidth into individual transponder channels. Theinput multiplexer consists of circulators, input filters, group delay equalizers, amplitude equalizersand output circulators.

The TWTAs amplify the low level downlink signals to a high level for transmission back to earth.

The output downlink signals from the channelized TWTAs are combined by the output multiplexerfor retransmission to earth.

The output multiplexer provides the required output outof-band attenuation, as well as theattenuation necessary to suppress signal harmonics and noise generated by the TWTAs.

Variable power dividers may be used to provide the necessary power split to select the desiredtransmit antenna coverage which can be selected by ground command.

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The attenuator can be controlled via the uplink command system to set thegain of the transponder and provides the necessary control to back off theoutput TWT.

The multiplexer is employed to separate the bandwidth into individual

transponder channels whose bandwidth depends on the satellite's mission.

Single conversion C-band transponder

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The amplification and filtering are performed at 1 GHz and a relativelyhigh-level carrier is translated back to 11 GHz for amplification by theHPA.

Phase variation across the pass band produces group delay distortion,which is troublesome with wide band FM signals and high speed phase

shift keyed data transmission.

Double conversion Ku-band transponder

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SATELLITE ANTENNA

Fo r main t pes of antennas are sed on satellites These are

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Four main types of antennas are used on satellites. These are Wire antennas:monopoles and dipoles. Horn antennas.

Reflector antennas.

Array antennas.

Wire antenna used at VHF (30-300MHz) and UHF(300-3000MHz) to provide communications for the TTC&M

systems.

They positioned carefully to provide omnidirectionalcoverage.

A satellite antenna used to provide coverage of a certain area of earth surface and have contoursof antenna gain which is EIRP of satellite antenna and transmitter.

Horn antenna used at microwave frequencies when relatively wide beams are required.

It is a good match between the waveguide impedance and free space.

used as feeds for reflectors, either singly or in clusters.

It is difficult to obtain gains much greater than 23 dB or beam widths narrower than about 10 withhorn antennas.

For higher gains or narrow beam widths a reflector antenna or array must be used.

Reflector antennas

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e ecto a te as illuminated by one or more horns and provide a larger aperture.

For maximum gain, a plane wave is generated in the aperture of thereflector.

This is achieved by Choosing a reflector profile that hasequal path lengths from the feed to the aperture.

Satellite antennas often use modified paraboloidal reflector profiles to tailorthe beam pattern to a particular coverage zone.

Array antennas Phased array antennas used to create multiple beams from a single

aperture, and have been used by Iridium and Globalstar to generate up to16 beams from a single aperture for their LEO mobile telephone systems II.

Some basic relationships that used to illustrate the selection of antenna forcommunication satellite are:

Antenna gain 3 dB beamwidth

Efficiency Radiation pattern

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It is of two types Spatial beam seperation

Orthogonal polarization

Spatial beam seperation

The same frequency bands are used for transmission and reception togeographically seperated regions of earth station.

By generating shaped beam using large clustered feeds, it is possible toreduce the side lobes from one beam to an acceptable level in the otherzone.

Interference between beams should be

-25 dB for FM transmitter -17 dB for digital tansmitter

Shaped zone beam, aim is to direct as much as power into its coveragezone.

By combining switched beam antenna with TDMA of the communicationchannel, much higher transmit EIRP can be achieved.

Frequency Reuse antennas

Orthogonal Polarization

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Orthogonal Polarization Each beam is generated in two polarization.

Reflector antenna tend to generate cross-polarized beam due to cross polarizedradiation from the feed system and curvature of reflector.

The requirement of narrow antenna beam with high gain over a small coverage zoneleads to large antenna structure on the satellite.

The antenna must be folded down during the launch phase. And once comes in orbit,the antennas can be deployed.

This method doubles the information capacity of the transmitter.

Diagram w.r.t spatial beam seperation

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E h S i fi i d

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Earth Station configuration and

characteristics Major RF component in an ES are the LNA of the Rxer and HPA of the

transmitter

Others are up and down convertors

Low Noise Amplifiers (LNA)

Large ES use parametric amplifier with liquid helium cooling at 4Kabove zero to achieve noise temperature of 20 to 40K at 4 GHz

Small ES use GaAs FET amplifiers to achieve noise temperature in therange of 50 to120K at 4GHz and 120 to 300K at 11GHz

1:1 redundancy configuration is used in large ES.

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Parametric Amplifier

Figure shows an equivalent circuit;

The two resonant circuits are coupled by a VVC provided by avaractor.

Capacitance ; pump frequency p= 1+2

Amplification is achieved because the amplifier operates as anegative resistance amplifier.

This provide very low noise temperature

GaAs FET Amplifier

Has very short gate length(0.5 m), yields very low noise temperature.

It is reliable and low cost amplifier

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High Power Amplifier (HPA)

HPA of 8.5kW of o/p power levels is use for large ES. Configured one HPA for each transponder In large ES , the o/ps of the up-converter are then

summed with hybrid couplers and a single broadbandFDM signal is applied to the HPA.

In small ES, it use solid-state amplifiers for theHPAs, by having one amplifier for each voicechannel.

The Ground Control Equipment Multiplexing , modulation- demodulation operations carried

out at bas band and IF. Up and down conversion interface between RF and IF of

TXer and RXer

Operations carried out on RF signals are amplificationsand filtering with minimal combining and splitting.

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TWTA is used as HPA.

1:1 Redundancy

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Uplink FDM/FM/FDMA Earth Station

FDM system transmit or receive many voice or data signals byallocating separate frequencies to each signal.

For an ES operates in FDM mode, terrestrial link is used.

FDM signal consist of 12 telephone channels or 1872 channels,is frequency modulated onto 70MHz IF carrier.

D li k FDM/FM/FDMA E th

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The bandwidth of the 70MHz IF stage is set between1.25 and 36MHz depending on the number ofchannels carried by the transformer.

FM demodulator used to achieve a low threshold in(C/N) for carriers upto 252 channels.

After demodulation, band limited to 3.1 to 3.4kHz.

Downlink FDM/FM/FDMA Earth

Station

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TDM Digital Earth Station

TDM system interrelate digital signals into frames which aretransmitted through separate transponders on satellite.

140MHz IF in ES when 120Mbps is sent by QPSK using 80MHzbandwidth

1.2 GHz IF in ES using Ku band and had 750MHz RF bandwidth One ES does not fill a complete frame in a TDMA system, it

transmits a burst of QPSK signal Quantity of synchronization and timing equipment is needed in

the transmit portion of GCE of a TDMA ES.

T itti /R i i E i t f

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Transmitting/Receiving Equipment for

TDM/TDMA Earth Station Bit error rate in QPSK demod

depends on accurate recovery ofthe carrier and on bit samplinginstant.

Achieve (C/N) of between 10 and25dB at the demodulator input.

Forward error correction(FEC) iscoding equipment:

Individual data signaldata encoded at anypoint between userand ES.

Decision to use FEC isleft to the user.

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Direct Broadcast Television (DBS)

DBS using high power satellite and regional coverageantennas.

DBS TV satellite provide a good quality service tolarge region of earth with the same powertransponder.

Receive terminals are smaller and cheaper becauseEIRP lies in 54 to 60dB range which is 10 to 20dBbetter.

Frequency band is vary from region to region and thisuses Ku band and has carrier frequency of 14/12GHz

Ku band: not used for terrestrial link, no intereference Smaller antenna size (high EIRP, narrow beamwidth) Parabolic reflectors 60cm in diameter compared to

3m for C band antennas