EARTH STATION DESIGN
INTRODUCTION
#)Earth station is a vital element in any satellite communication network.
#)The function of an earth station is to receive information 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 configuration depends upon many factors and its location. But it is fundamentally governed by its location 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
EARTH STATION CONFIGURATION
#)Any earth station consists of four major subsystems
• Transmitter
• Receiver
• Antenna
• Tracking equipment
#)Two other important subsystems are
• Terrestrial interface equipment
• Power supply.
#)The earth station depends on the following parameters
• 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
#)The functional elements of a basic digital earth station are shown in the below figure
Fig. General configuration of an Earth Station
• Digital information in the form of binary digits from terrestrial networks enters earth station and is then processed (filtered, multiplexed, formatted etc.) by the base band equipment.
• The encoder performs error correction coding to reduce the error rate, by introducing extra digits into digital stream generated by the base band equipment. The extra digits carry information. The presence of noise and non-ideal nature of any communication channel produces error rate is established above which the received information is not stable.
• The function of the modulator is to accept the symbol stream from the encoder and use it to modulate an intermediate frequency (I.F) carrier. In satellite communication, I.F carrier frequency is chosen at 70 MHz for communication using a 36 MHz transponder bandwidth and at 140 MHz for a transponder bandwidth of 54 or 72 MHz. The I.F is needed because it is difficult to design a modulator that 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 the uplink r-f frequency.
• This modulated R.F carrier is then amplified by the high power amplifier (HPA) to a suitable level for transmission and radiation by the antenna to the satellite.
• On the receive side, the earth station antenna receives the low-level modulated R.F carrier in the downlink frequency spectrum.
• The low noise amplifier (LNA) is used to amplify the weak received signals and improve the signal to Noise ratio (SNR). The error rate requirements can be met more easily.
• R.F is to be reconverted to I.F at 70 or 140 MHz because it is easier design a demodulation to work at these frequencies than 4 or 12 GHz.
• The demodulator estimate which of the possible symbols was transmitted based on observation of the received if carrier.
• The decoder performs a function opposite that of the encoder. Because the sequence of symbols recovered by the demodulator may contain errors, the decoder must use the uniqueness of the redundant digits introduced by the encoder to correct the errors and recover information-bearing digits.
• The information stream is fed to the base-band equipment for processing for delivery to the terrestrial network.
• The tracking equipments track the satellite and align the beam towards it to facilitate communication.
ANTENNA SUBSYSTEM
#)The antenna system options are
1. Large antenna: say, for INTELSAT earth station typical diameter: 30M(cassegrain geometry used)
2. Small antenna: say, for option o0f direct broad television (DBS – TV). For deep space communication, the diameter of antenna may be very large, say over 35m.
FEED SYSTEM
#)The primary feed system used in existing earth stations performs a number of functions. Depending on the type of earth station, these functions may be:
• To illuminate the main reflector.
• To separate the transmit and receive bands
• To separate and combine polarizations in a dual polarized system.
• To provide error signals for some types of satellite tracking system.
#)A horn antenna is commonly used as the primary feed at microwave frequencies.
#)A horn antenna consists of an open waveguide which is flared at the transmitting end so that the impedance of the free space matches 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 monopulse tracking system, if such a method is used.
#)The orthogonal mode junction (OMJ) assembly is used to separate the dually polarized transmit and receive signal.
#)The orthogonal mode transducer (OMT) separates the two linear orthogonally polarized signals into a composite linear orthogonally polarized signal on the transmit side.
#)Because OMT operates on linearly polarized signals, polarizer’s are used to convert a circular polarization to a linear.
#)Polarizer’s are therefore not required for linearly polarized system.
#)Some earth stations have the capability to compensate polarization variations introduced by atmospheric effects by means of a feedback control system.
#)The polarization properties of an antenna are mainly affected by the characteristics of the primary radiator and the polarizer.
TRACKING SYSTEM
#)Tracking is essential when the satellite drift, as seen by an earth station antenna is a significant fraction of an earth station’s antenna beam width.
#)An earth station’s tracking system is required to perform some of the functions such as
i)Satellite acquisition
ii)Automatic tracking
iii)Manual tracking
iv)Program tracking.
#)The operation of the tracking system is explained by using its block diagram which show below.
#)Communication satellites transmit a beacon which is used by earth stations for tracking.
#)The received beacon signal is fed into the auto-track receiver where tracking corrections or, in some auto-track systems estimated positions of the satellite, are derived. In other auto-tack techniques the feed system provides the required components of error signal.
#)The output of the auto-track receivers are processed and used to drive each axis of the antenna to the estimataed satellite position.
#)In manual mode, an operator sets the desired angles for each axis on a control console.
#)This position is compared with the actual antenna position, obtained through shaft encoders, and the difference signal is used to drive the antenna.
#)In the program track mode the desired antenna position is obtained form a computer.
#)The difference in the actual and the desired antenna positions constitutes the error and is used to drive the antenna.
RecentTrackingTechniques:
#)There have been some interesting recent developments in auto-track techniques which can potentially provide high accuracies at a low cost .
#)In one proposed technique the sequential lobing technique has been I implemented by using rapid electronic switching of a s single beam which effectively approximates simultaneous lobing.
#)The high rate of switching is achieved by the use of an electronically controlled feed.
#)This technique, sometimes referred to as electronic beam squinting, requires a simple single channel receiver and has been reported to achieve a tracking accuracy approaching that of the auto-track technique
Fig: Block diagram of Tracking system
LOW NOISE AMPLIFIER
#)In the earliest earth stations, MASERs were used as the front-end amplifer.
#)These devices are relativelllllly narrow band(40-120MHz), require liquid helium temperatures and henceare expensive with difficult maintainenance requirements.
#)As a result, these were replaced by parametric amplifiers which could provide wide bandwidths, with the required low-noise temperatures at lower cost and complexity.
#)Several im kprovements have been made to param etric amplifiers over the years.
#)These have been made possible by the availability of im proved devices(e.g. varactors,Gunn oscillators) .and the use of thermoelectric cooling by the Peltier effect.
#)In recennnt years the advent of gallium arsenide field-effect transistors has greatly simplified the front-end amplifier design of earth stations.
#)These devices provide similar orders of noise temperature and bandwidths as those of parametric amplifiers but at a lower cost.
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