Earth Station Design

8/2/2019 Earth Station Design

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Earth station is a vital element in any satellitecommunication network.

The function of an earth station is to receiveinformation from or transmit information to,the satellite network in the most cost-effective and reliable manner while retaining

the desired signal quality.



The design of earth station configurationdepends upon many factors and its location.But it is fundamentally governed by itslocation which are listed below,

• In land• On a ship at sea• Onboard aircraft



The factors are• Type of services• Frequency bands used• Function of the transmitter

• Function of the receiver• Antenna characteristics



Any earth station consists of four majorsubsystems

• Transmitter

• Receiver• Antenna• Tracking equipment

Two other important subsystems are

• Terrestrial interface equipment• Power supply.



The earth station depends on the followingparameters• Transmitter power• Choice of frequency

• Gain of antenna• Antenna efficiency• Antenna pointing accuracy• Noise temperature

• Local conditions such as wind, weather etc,• Polarization• Propagation losses



Digital information in the form of binary digitsfrom terrestrial networks enters earth station andis then processed (filtered, multiplexed,formatted etc.) by the base band equipment.

The encoder performs error correction coding toreduce the error rate, by introducing extra digitsinto digital stream generated by the base bandequipment. The extra digits carry information.

The presence of noise and non-ideal nature of any communication channel produces error rateis established above which the receivedinformation is not stable.



The function of the modulator is to accept thesymbol stream from the encoder and use it tomodulate an intermediate frequency (I.F)carrier. In satellite communication, I.F carrier

frequency is chosen at 70 MHz forcommunication using a 36 MHz transponderbandwidth and at 140 MHz for a transponderbandwidth of 54 or 72 MHz. The I.F is neededbecause it is difficult to design a modulatorthat works at the uplink frequency of 6 GHz(or 14GHz) directly



The modulated I.F carrier is fed to the up-converter and frequency-translated to theuplink r-f frequency

This modulated R.F carrier is then amplifiedby the high power amplifier (HPA) to asuitable level for transmission and radiation

by the antenna to the satellite.



On the receive side, the earth station antennareceives the low-level modulated R.F carrierin the downlink frequency spectrum.

The low noise amplifier (LNA) is used toamplify the weak received signals andimprove the signal to Noise ratio (SNR). The

error rate requirements can be met moreeasily.



R.F is to be reconverted to I.F at 70 or 140MHz because it is easier design ademodulation to work at these frequenciesthan 4 or 12 GHz.

The demodulator estimate which of thepossible symbols was transmitted based onobservation of the received if carrier.



The decoder performs a function oppositethat of the encoder. Because the sequence of symbols recovered by the demodulator maycontain errors, the decoder must use the

uniqueness of the redundant digitsintroduced by the encoder to correct theerrors and recover information-bearingdigits.



The information stream is fed to the base-band equipment for processing for delivery tothe terrestrial network.

The tracking equipments track the satelliteand align the beam towards it to facilitatecommunication.



The antenna system options are

1. Large antenna: say, for INTELSAT earthstation typical diameter: 30M(cassegrain

geometry used)

2. Small antenna: say, for option o0f directbroad television (DBS – TV). For deep spacecommunication, the diameter of antenna maybe very large, say over 35m.



The primary feed system used in existing earthstations performs a number of functions.Depending on the type of earth station, thesefunctions may be:

• To illuminate the main reflector.• To separate the transmit and receive bands• To separate and combine polarizations in adual polarized system.

• To provide error signals for some types of satellite tracking system.



A horn antenna is commonly used as theprimary feed at microwave frequencies.

A horn antenna consists of an openwaveguide which is flared at the transmittingend so that the impedance of the free spacematches the impedance of the waveguide.

This ensures an efficient transfer of power.



The figure below shows the block diagram of an orthogonal polarization feed assembly.

A higher mode coupler (mode extractor)provides the error signal to the monopulsetracking system, if such a method is used.

The orthogonal mode junction (OMJ)assembly is used to separate the duallypolarized transmit and receive signal.



The orthogonal mode transducer (OMT)separates the two linear orthogonallypolarized signals into a composite linearorthogonally polarized signal on the transmit

side.

Because OMT operates on linearly polarized

signals, polarizer’s are used to convert acircular polarization to a linear.



Polarizer’s are therefore not required forlinearly polarized system.

Some earth stations have the capability to

compensate polarization variationsintroduced by atmospheric effects by meansof a feedback control system.

The polarization properties of an antenna aremainly affected by the characteristics of theprimary radiator and the polarizer.



Tracking is essential when the satellite drift, asseen by an earth station antenna is a significantfraction of an earth station’s antenna beamwidth.

An earth station’s tracking system is required toperform some of the functions such as

i)Satellite acquisitionii)Automatic trackingiii)Manual trackingiv)Program tracking.



Communication satellites transmit a beacon which is usedby earth stations for tracking.

The received beacon signal is fed into the auto-trackreceiver where tracking corrections or, in some auto-tracksystems estimated positions of the satellite, are derived. In

other auto-tack techniques the feed system provides therequired components of error signal.

The output of the auto-track receivers are processed andused to drive each axis of the antenna to the estimatedsatellite position.

In manual mode, an operator sets the desired angles foreach axis on a control console.



This position is compared with the actualantenna position, obtained through shaftencoders, and the difference signal is used todrive the antenna.

In the program track mode the desired antennaposition is obtained form a computer.

The difference in the actual and the desiredantenna positions constitutes the error and isused to drive the antenna.



There have been some interesting recentdevelopments in auto-track techniques whichcan potentially provide high accuracies at a lowcost .

In one proposed technique the sequential lobingtechnique has been I implemented by using rapidelectronic switching of a s single beam whicheffectively approximates simultaneous lobing.

The high rate of switching is achieved by the useof an electronically controlled feed.



This technique, sometimes referred to aselectronic beam squinting, requires a simplesingle channel receiver and has been reportedto achieve a tracking accuracy approaching

that of the auto-track technique

Earth Station Design

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