SATELLITE COMMUNICATION

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Satellite subsystem

4.1 Introduction

• The space segment will obviously include the satellites.
• In a communications satellite, the equipment which provides the connecting link between the satellite’s transmit and receive antennas is referred to as the transponder.

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• Irrespective of the intended application, be it a communications satellite or a weather forecasting satellite or even a remote sensing satellite, different subsystems comprising a typical satellite

4.2 Power supply subsystem

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• The primary function of the power supply subsystem is to collect the solar energy, transform it to electrical power with the help of arrays of solar cells and distribute electrical power to other components and subsystems of the satellite.
• The satellite also has batteries, which provide standby electrical power during eclipse periods, during other emergency situations and also during the launch phase of the satellite when the solar arrays are not yet functional.

• The power supply subsystem generates, stores, controls and distributes electrical power to other subsystems on board the satellite platform.
• The electrical power needs of a satellite depend upon the intended mission of the spacecraft and the payloads that it carries along with it in order to carry out the mission objectives.
• The power requirement can vary from a few hundreds of watts to tens of kilowatts.

• The power for operating the electronic equipment is obtained from solar cells.

The HS 601 can be designed to provide dc power from 2 to 6 kW.

Power supply subsystem

4.2 The Power Supply

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4.2 The Power Supply

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4.5.1 Types of Power System

The power systems for satellite applications have been developed based on the use of solar energy, chemical energy and nuclear energy, the solar energy driven power systems are undoubtedly the favourite and are the most commonly used ones.

• There are power systems known as heat generators that make use of heat energy in solar radiation to generate electricity.
• A parabolic dish of mirrors reflects heat energy of solar radiation through a boiler, which in turn feeds a generator, thus converting solar energy into electrical power.
• This mode of generating power is completely renewable and efficient if the satellite remains exposed to solar radiation.
• It can also be used in conjunction with rechargeable batteries.
• Heat generators, however, are very large and heavy and are thus appropriate only for large satellites.

• Batteries store electricity in the form of chemical energy and are invariably used together with solar energy driven electrical power generators to meet the uninterrupted electrical power requirements of the satellite.

• The batteries used here are rechargeable batteries that are charged during the period when solar radiation is falling on the satellite.
• During the periods of eclipse when solar radiation fails to reach the satellite, the batteries supply electrical power to the satellite.

• Nuclear fission is currently the commonly used technique of generating nuclear energy

• In nuclear fission, the heavy nucleus of an atom is made to split into two fragments of roughly equivalent masses, releasing large amounts of energy in the process.
• On satellites, nuclear power is generated in radio isotopic thermoelectric generators (RTGs).
• The advantage of nuclear power is its use on satellites is that it is practically limitless and will not run out before the satellite becomes useless for other reasons.
• The disadvantage is the danger of radioactive spread over Earth in the event of the rocket used to launch the satellite exploding before it escapes the Earth’s atmosphere.

4.5.2 Solar Energy Driven Power Systems

The major components of a solar power system are the solar panels (of which the solar cell is the basic element), rechargeable batteries, battery chargers with inbuilt controllers, regulators and inverters to generate various d.c and a.c voltages required by various subsystems.

• During the sunlight condition, the voltage of the solar generator and also the bus is maintained at a constant amplitude with the voltage regulator connected across the solar generator. The battery is decoupled from the

bus during this time by means of a battery discharge regulator (BDR) and is also charged using the battery charge regulator (BCR) as shown in the figure.

• During the eclipse periods, the battery provides power to the bus and the voltage is maintained constant by means of the BDR.

4.3 Attitude Control

• The attitude of a satellite refers to its orientation in space.
• A number of forces, referred to as disturbance torques, can alter the attitude.

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• The attitude and orbit control subsystem performs two primary functions.
• It controls the orbital path, which is required to ensure that the satellite is in the correct location in space to provide the intended services.
• It also provides attitude control, which is essential to prevent the satellite from tumbling in space and also to ensure that the antennae remain pointed at a fixed point on the Earth’s surface.

4.3 Attitude Control

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4.3 Attitude Control

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• The telemetry, tracking and command (TT&C) subsystem monitors and controls the satellite right from the lift-off stage to the end of its operational life in space.
• The tracking part of the subsystem determines the position of the spacecraft and follows its travel using angle, range and velocity information.
• The telemetry part gathers information on the health of various subsystems of the satellite, encodes this information and then transmits the same.
• The command element receives and executes remote control commands to effect changes to the platform functions, configuration, position and velocity.

Telemetry, tracking and command (TT&C)

Block schematic arrangement of the basic TT&C subsystem

• The payload subsystem is that part of the satellite that carries the desired instrumentation required for performing its intended function and is therefore the most important subsystem of any satellite.
• The nature of the payload on any satellite depends upon its mission. The basic payload in the case of a communication satellite is the transponder, which acts as a receiver, an amplifier and a transmitter.
• In the case of a weather forecasting satellite, a radiometer is the most important payload.
• High resolution cameras, multispectral scanners and thematic mappers are the main payloads on board a remote sensing satellite.
• Scientific satellites have a variety of payloads depending upon the mission. These include telescopes, spectrographs, plasma detectors, magnetometers, spectrometers and so on.

Payload subsystem

Earth Station

• Three essential elements of any satellite communication network or system include the Earth segment, the space segment and the up/down link between the space segment and the Earth segment.
• The space segment, mainly in terms of the different hardware components that constitute the satellite

After reading the chapter we will learn the following:

• The role of an Earth station in overall satellite communication set-up

• Types of Earth station with reference to size and complexity and type of service

• Earth station architecture

• Design considerations for an Earth station

• Earth station subsystems and function of each subsystem

• Earth station figure-of-merit

• Satellite tracking methodologies

• Services offered by some of the major Earth stations around the world

Earth Station

Earth Station

• An Earth station is a terrestrial terminal station mainly located on the Earth’s surface.
• It could even be airborne or maritime.
• Those located on the Earth’s surface could either be fixed or mobile. The Earth station is intended for communication with one or more manned or unmanned space stations as shown in Figure

Earth station communicating with satellites

Earth station communicating with another Earth station

Earth Station

• In most of the applications related to communication satellites, Earth stations transmit to and receive from satellites.
• In some special applications, the Earth stations only transmit to or receive from satellites.
• Receive-only Earth station terminals are mainly of relevance in the case of broadcast transmissions.
• Transmit-only Earth station terminals are relevant to data gathering applications.
• Major subsystems comprising an Earth station include

(a) transmitter system whose complexity depends upon the number of different carrier frequencies and satellites simultaneously handled by the Earth station

Earth Station

(b) receiver system whose complexity again depends upon the

number of frequencies and satellites handled by the Earth station

(c) antenna system that is usually a single antenna used for both transmission and reception with a multiplex arrangement to allow simultaneous connection to multiple transmit and receive chains

(d) tracking system to ensure that the antenna points to the satellite; (e) terrestrial interface equipment

(f) primary power to run the Earth station and

(g) test equipment required for routine maintenance of the Earth station and terrestrial interface.

Earth Station

Types of Earth Station

• Earth stations are generally categorized on the basis of type of services or functions provided by them though they may sometimes be classified according to the size of the dish antenna.
• Based on the type of service provided by the Earth station, they are classified into the following three broad categories.

1. Fixed Satellite Service (FSS) Earth Stations

2. Broadcast Satellite Service (BSS) Earth Stations

3. Mobile Satellite Service (MSS) Earth Stations

Earth stations are also sometimes conveniently categorized into three major functional groups depending upon their usage.

These categories are the following.

1. Single function stations

2. Gateway stations

3. Teleports

Types of Earth Station

Fixed Satellite Service (FSS) Earth Station

• Fixed satellite service (FSS) is a term that is mainly used in North America.
• The service involves the use of geostationary communication satellites for telephony, data communications and radio and television broadcast feeds.
• FSS satellites operate in either the C band (3.7 GHz to 4.2 GHz) or the Ku band (11.45 GHz to 11.7 GHz and 12.5 GHz to 12.75 GHz in Europe, and 11.7 GHz to 12.2 GHz in the USA).
• FSS satellites operate at relatively lower power levels as compared to Broadcast Satellite Service (BSS) satellites and therefore consequently require a much larger dish.
• FSS satellite transponders use linear polarization as compared to circular polarization employed by BSS satellite transponders.

Large Earth station

Very Small terminal (Transmit/Receive)

Broadcast Satellite Service (BSS) Earth Stations

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• Under the group of BSS Earth stations, we have large Earth stations (G/T ∼=15 dB/K) used for community reception and small Earth stations (G/T ∼= 8 dB/K) used for individual reception.
• Technically broadcast satellite service or BSS as it is known by the International Telecommunications Union (ITU) refers only to the services offered by satellites in specific frequency bands. These frequency bands for different ITU regions include 10.7 GHz to 12.75 GHz in ITU region-1 (Europe, Russia, Africa), 12.2 GHz to 12.7 GHz in ITU region-2 (North and South America) and 11.7 GHz to 12.2 GHz in ITU region-3 (Asia, Australia).

• ITU adopted an international BSS plan in the year 1977.
• Under this plan, each country was allotted specific frequencies for use at specific orbital locations for domestic services.
• It is also known by the name of Direct Broadcast Service or DBS or more commonly as Direct-to-Home or DTH.
• The term DBS is often used interchangeably with DTH to cover both analog and digital video and audio services received by relatively small dishes.

Broadcast Satellite Service (BSS) Earth Stations

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Very small terminal (Receive only)

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Mobile Satellite Service (MSS) Earth Stations

• Under the group of MSS Earth stations, we have the large Earth stations (G/T

∼= −4 dB/K), medium Earth stations (G/T ∼= −12 dB/K) and small Earth stations (G/T ∼= −24 dB/K).

• While both large and medium Earth stations require tracking, small MSS Earth stations are without tracking equipment.
• Satellite phone is the most commonly used mobile satellite service.

• It is a type of mobile that connects to satellites instead of terrestrial cellular sites.
• Mobile satellite services are provided both by the geostationary as well as low Earth orbit satellites.
• In the case of the former, three or four satellites can maintain near continuous global coverage.
• These satellites are very heavy and therefore very expensive to build and launch.
• Geostationary satellite based mobile services also suffer from noticeable delay while making a telephone call or using data services.

Mobile Satellite Service (MSS) Earth Stations

Earth Station Architecture

• The major components of an Earth station include the RF section, the baseband equipment and the terrestrial interface.
• every Earth station has certain support facilities such as

power supply unit with adequate back-up, monitoring and control equipment and thermal and environment conditioning unit (heating, air-conditioning etc.).

• Though the actual architecture of an Earth station depends on the application

Block schematic arrangement of a generalized Earth station

• The RF section as shown in the block schematic arrangement mainly comprises of antenna subsystem, the up-converter and the high power amplifier (HPA) in the up-link channel and the antenna subsystem, low noise amplifier (LNA) and the down-converter in the down-link channel.
• In the case of an Earth station being a major hub of a network or if service reliability were a major concern; equipment redundancy is used in the RF section.

Block schematic arrangement of a generalized Earth station

Block schematic arrangement of a generalized Earth station

• RF section interfaces with the modem subsystem of the baseband section.
• The job of up-converter in the up-link channel is to up-convert the baseband signal to the desired frequency.
• The upconverted signal is then amplified to the desired level before it is fed to the feed system for subsequent transmission to the intended satellite.

• A low noise amplifier amplifies the weak signals received by the antenna. The amplified signal is then down converted to the intermediate frequency level before it is fed to the modem in the baseband section.
• The antenna feed system provides the necessary aperture illumination, introduces the desired polarization and also provides isolation between the transmitted and the received signals by connecting HPA output and LNA input to the cross-polarized ports of the feed.

Block schematic arrangement of a generalized Earth station

• The baseband section performs the modulation/demodulation function with the specific equipment required depending upon the modulation technique and the multiple access method employed.
• For example, in the case of a two-way digital communication link, the baseband section would comprise of a digital modem and a time division multiplexer.
• The baseband section input/output is connected to the terrestrial network through a suitable interface known as terrestrial interface.

Block schematic arrangement of a generalized Earth station

• It may be connected directly to the user in some applications.
• The terrestrial network could be a fibre optic cable link or a microwave link or even a combination of the two.
• Every Earth station has support facilities such as tracking, control and monitoring equipment, power supply with back-up and environmental conditioning unit.

Block schematic arrangement of a generalized Earth station

• The complexity of Earth station architecture depends upon the application.
• For example, a TVRO Earth station would be far less complex than a FSS Earth station interconnecting large traffic nodes.
• Figure shows the detailed block schematic of a typical large FSS Earth station.
• Redundancy of equipment as outlined earlier is evident in the RF and the baseband sections.
• The diagram shown is typical of the Earth station used in the INTELSAT network.

Block schematic arrangement of a generalized Earth station

Block schematic of a typical large FSS Earth station

• Figure shows the block schematic of a typical VSAT remote terminal showing both the outdoor and the indoor units along with the dish antenna.
• The outdoor unit is typically of the size of a shoe box or even smaller and contains different subsystems of the RF section.
• The dish antenna is typically 0.55 to 2.4 metre in diameter.
• The indoor unit, typically of the size of a domestic video recorder, contains different subsystems of the baseband section.
• These include modulator and demodulator, multiplexer and demultiplexer and user interfaces.

Block schematic of VSAT remote terminal

Block schematic of VSAT remote terminal

Earth Station Design Considerations

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• Design of an Earth station is generally a two-step process.
• The first step involves identification of Earth station requirement specifications, which in turn govern the choice of system parameters.
• The second step is about identifying the most cost effective architecture that achieves the desired specifications.

• Requirement specifications affecting the design of an Earth station include type of service offered (Fixed satellite service, Broadcast satellite service or Mobile satellite service), communication requirements (telephony, data, television etc.), required base band quality at the destination, system capacity and reliability.
• Major system parameters relevant to Earth station design include transmitter EIRP (Effective Isotropic Radiated Power), receiver figure-of-merit (G/T ), system noise and interference and allowable tracking error.

Earth Station Design Considerations

Key Performance Parametres

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• Key performance parameters governing Earth station design include the EIRP (Effective or Equivalent Isotropic Radiated Power) and the figure-of-merit (G/T ).
• While the former is a transmitter parameter, the latter is indicative of receiver performance in terms of sensitivity and the quality of the received signal.

Earth Station Design Considerations

Effective (or Equivalent) Isotropic Radiated Power (EIRP)

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• EIRP gives the combined performance of the high power amplifier (HPA) and the transmitting antenna.
• It is given by the product of the power output of HPA at the antenna and the gain of the transmitting antenna.
• Expressed in decibels, EIRP is the sum of the power output of HPA in dB and the gain of transmitting antenna in dB.

• If a particular HPA-transmitting antenna combine had an EIRP of 60 dBW, it would imply that the RF power radiated by the antenna is the same as that radiated by an isotropic radiator in that direction when fed with million times more power at its input.
• EIRP is defined for both Earth station transmitting antenna as well as satellite transmitting antenna.
• It is important to note that EIRP is always measured at the antenna.
• When we see a footprint map with EIRP numbers for a given transponder on a satellite, these numbers are indicative of the amount of power sent down to the Earth station and measured as it left the satellite’s down-link dish.

Effective (or Equivalent) Isotropic Radiated Power (EIRP)

Receiver Figure-of-merit (G/T )

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• Receiver figure-of-merit is indicative of how the receiving antenna performs together with the receiving electronics to produce a useful signal.
• While the EIRP gives the performance of the transmitting antenna and HPA combination, receiver figure-of-merit, tells us about the sensitivity of the receiving antenna and the Low Noise Amplifier (LNA) combine to weak received signals.
• As it is effectively a measurement of the sensitivity of the receiving antenna to weak signals, the larger the value of receiver figure-of-merit, the better it is.
• The response of the receiving system to weak signals is largely governed by the receiving system gain and the overall system noise.

• The figure-of-merit is therefore defined by a parameter called G/T ratio, which is the ratio of receiving antenna gain to system noise temperature. G/T is expressed in dB/K.
• G/T of the Earth station may be enhanced by increasing the receiving antenna gain or lowering the noise temperature or both.
• For any practical communication link, EIRP of the satellite transmitting antenna and the G/T of the Earth station receiving antenna and the EIRP of the Earth station transmitting antenna and G/T of the satellite receiving antenna have to work together to get the desired results.
• A poorer G/T necessitates a higher EIRP and vice versa.

Receiver Figure-of-merit (G/T )

Earth Station Design Optimization

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• The transmitter EIRP and receiver G/T together dictate the performance of the communication system and therefore one can be traded off against the other during the design optimization process.
• In the early days of development of satellite technology, available EIRP from satellites was pretty low, which made complex and expensive Earth stations a necessity.

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• Earth station antennas were several tens of metres in diameter and cost a few million US Dollars a piece.
• Current trend is to minimize Earth station complexity at the cost of a complex space segment.
• It is more so for applications that involve a large user

population such as direct broadcast, business use, mobile communication and so on.

Earth Station Design Optimization

Environmental and Site Considerations

• It is important to consider a number of environmental and locational factors while making a decision on the site of an Earth station.
• Environmental parameters of interest include external temperature and humidity, rainfall and snow, wind conditions, likelihood of Earth quakes, corrosive conditions of the atmosphere and so on.
• Careful site selection can take care of the ill effects of some but not all of these factors.
• Minimizing radio frequency interference (RFI) and electromagnetic interference (EMI) is another requirement.
• RFI and EMI produced by the Earth station can cause interference to other RF installations.

• RFI and EMI from external sources can adversely affect the Earth station performance.
• It is usually necessary to carry out a radio frequency survey at various possible sites before a final choice is made on the Earth station location.
• An essential requirement is to have a clear line-of-sight to the satellites of interest.
• Availability of sufficient space for the Earth station equipment, easy transportation to the Earth station and reliable electrical power are the other requirements.

Environmental and Site Considerations

Earth Station Testing

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• Having chosen the Earth station equipment, it is important to ensure that the equipment would not only meet the specified requirements of the intended Earth station
• It is also necessary to ensure that the Earth station would not cause any problems either to other users of the satellite or to any adjacent satellites.
• This is achieved by performing different levels of testing, which begins with testing at component or unit level followed up by subsystem level testing.

• These two levels of testing form part of Earth station hardware and software commissioning process and therefore precede any integrated testing of the overall Earth station.
• Overall Earth station testing also includes what is called line-up testing, which involves checking the performance of the Earth station in conjunction with the Earth stations, the newly commissioned Earth station is intended to work with.

Earth Station Testing

Unit and Subsystem Level Testing

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• Unit or component level testing is usually done at the manufacturer’s premises and the test data is made available to the subsystem designer making use of the components.
• The user may choose to witness the tests, if the component happens to be of a new design.

• In subsystem or equipment level testing, different subsystems are comprehensively tested for their electrical, mechanical and environmental specifications.
• The critical tests are witnessed by the user.
• Test data generated as a part of comprehensive testing is usually supplied to the user before some selected tests are repeated in the presence of the user.
• The selected tests carried out in the presence of the user are repeated once the equipment or subsystem is installed on the site.

Unit and Subsystem Level Testing

System Level Testing

• System level testing is carried out after subsystem testing and integration has been completed.
• In cases where the complete system has been ordered on a single supplier; as many subsystems as possible are integrated at the premises of the supplier and the performance of the integrated set verified.
• The rest of the integration job is carried out on site followed up by full system testing for a wide range of parameters. These tests are also called acceptance tests.

• Tests are carried out to verify that the system meets all the performance specifications and also all the mandatory requirements of the satellite system to be used.
• The system is also tested for its adherence to international regulatory standards and fulfillment of desired base band signal quality requirement.
• A wide range of transmit and receive tests are carried out to meet the abovementioned requirements.
• These tests fall into two broad categories namely the mandatory tests and the additional tests.

System Level Testing

Earth Station Hardware

• Most Earth station hardware can be categorized into one of the three groups namely RF equipment, IF and baseband equipment and terrestrial interface equipment.
• Basic functions performed by each one of these equipment classes were briefly outlined in Earth station architecture.

RF Equipment

• The RF equipment comprises of up-converters, high power amplifiers (HPA) and the transmit antenna in the transmit channel, and the receive antenna, low noise amplifiers (LNA) and down-converters in the receive channel.
• While the output of HPA feeds the transmit antenna, the receive antenna is connected to the input of the LNA.
• Transmit and receive antenna functions are almost invariably performed by the same antenna.

Earth Station Hardware

Block schematic of the RF portion of the Earth station

Satellite Tracking

• The Earth station antenna needs to track the satellite when the beam width of the antenna is only marginally wider than the satellite drift seen by it.
• Satellite drift is typically in the range of 0.5–3◦ per day, antennas with large beam widths such as DBS receivers do not require to track the satellite.
• The large Earth stations do need some form of tracking with tracking accuracy depending upon the intended application.
• The tasks performed by the Earth station’s satellite tracking system include some or all of the following.

1. Satellite acquisition

2. Manual tracking

3. Automatic tracking

4. Programme tracking

• The acquisition system acquires the desired satellite by either moving the antenna manually around the expected position of the satellite or by programming the antenna to perform a scan around the anticipated position of the satellite.
• Automatic tracking is initiated only after the received signal strength due to the beacon signal transmitted by the satellite is above a certain threshold value, which allows the tracking receiver to lock to the beacon.
• Manual track option is used in the event of total failure of auto track system.

Satellite Tracking

• Automatic tracking ensures continuous tracking of the satellite.
• In the case of programme tracking, the antenna is driven to the anticipated position of the satellite usually predicted by the satellite operator.
• Unlike automatic tracking, which is a closed loop system; programme track is an open loop system and therefore its accuracy is relatively much lower than that of auto track mode of operation.

Satellite Tracking

Satellite Tracking System – Block Diagram

• The Earth station antenna makes use of the beacon signal to track itself to the desired positions in both azimuth and elevation.
• The auto track receiver derives the tracking correction data or in some cases the estimated position of the satellite.
• The estimated position is compared with the measured position in the control subsystem whose output feeds the servomechanism.
• In the case of manual and programme track modes, the desired positions of the satellite in the two orthogonal axes are respectively set by the operator and the computer.
• The difference in actual and desired antenna positions constitutes the error signal that is used to drive the antenna.

Block schematic arrangement of satellite tracking system

Tracking Techniques

• Tracking techniques are classified on the basis of the methodology used to generate angular errors.
• Commonly used tracking techniques include the following.

1. Lobe switching

2. Sequential lobing

3. Conical scan

4. Monopulse track

5. Step track

6. Intelligent tracking

• Of all the abovementioned techniques, the last four are more common in the case of satellite tracking.
• Sequential lobing with the rapid switching of a single beam has also been tried insome cases.

Lobe Switching

• In the case of lobe switching tracking methodology, the antenna beam is rapidly switched two positions around the antenna axis in a single plane as shown in Figure.
• The amplitudes of the echo from the object to be tracked are compared for the two lobe positions.
• The difference between the two amplitudes is indicative of the location of the target with respect to the antenna axis.

Principle of lobe switching technique

• When the object to be tracked is on the axis, the echo amplitudes for the two positions of the beam are equal and the difference between the two is zero.
• When the object is on one side of the antenna axis, the amplitude and sense of the difference signal tells how much and what side of the antenna axis the object is located.
• The difference signal can then be used to generate correction signal, which with the help of servo control loop can be used to drive the antenna to bring the object on to the antenna axis.
• The lobe switching technique is prone to inaccuracies if the object cross-section as seen by the antenna changes between different returns in one scan.

Lobe Switching

Sequential Lobing

• In sequential lobing, the beam axis is slightly shifted off the antenna axis.
• This squinted beam is sequentially placed in discrete angular positions, usually four, around the antenna axis
• The angular information about the object to be tracked is determined by processing several echo signals.
• The track error information is contained in the echo signal amplitude variations.
• The squinting and beam switching is done with the help of electronically controlled feed and therefore can be done very rapidly practically simulating simultaneous lobing.

Principle of sequential lobing

Conical Scan

• This is similar to sequential lobing except that in the case of conical scan, the squinted beam is scanned rapidly and continuously in a circular path around the axis as shown in Figure.
• If the object to be tracked is off the antenna axis, the amplitude of the echo signal varieswith antenna’s scan position.

Principle of conical scan

• The tracking system senses the amplitude variations and the phase delay as function of scan position to determine the angular co-ordinates.
• The amplitude variation provides information on the amplitude of the angular error and the phase delay indicates direction.
• The angular error information is then used to steer the antenna axis to make it to coincide with the object location.
• The technique offers good tracking accuracy and an average response time.

Conical Scan

Monopulse Tracking

• One of the major disadvantages of sequential techniques including lobe switching, sequential lobing and conical scan is that the tracking accuracy is severely affected if the cross-section of the object to be tracked changes during the time the beam was being switched or scanned to get the desired number of samples.
• Monopulse tracking overcomes these shortcomings by generating the required information on the angular error by simultaneous lobing of the received beacon.
• There are two techniques of monopulse tracking namely amplitude comparison monopulse tracking and phase comparison monopulse tracking.

Step Track

• In the case of step track, antenna axes are moved in small incremental steps in an effort to maximize the received signal strength.
• Amplitude sensing is the basis of this tracking methodology.
• It is simple and low cost and RF phase stability is not important. It is best suited to small and medium Earth stations.
• The technique is susceptible to amplitude perturbations caused by scintillation, signal fading and so on.
• Tracking accuracy is primarily determined by the step size and signal to noise ratio.
• For a high signal-to-noise ratio, tracking error approaches the step size. Accuracy is sensitive to amplitude interference.

Intelligent Tracking

• In the case of intelligent tracking, the satellite position is obtained by optimally combining antenna position estimate data obtained from a gradient tracking algorithm with the prediction data on satellite position obtained from a satellite position model.
• In the case of signal amplitude fluctuations, the antenna position may be updated by using prediction data from satellite position model.
• Intelligent tracking offers all advantages of step track.
• It is however susceptible to amplitude fluctuations during initial acquisition.
• Full accuracy is achieved several hours after acquisition.
• Intelligent track may be used in small, medium and large Earth stations, particularly those susceptible to scintillation and signal fades.

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