satelite communication - ii

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Presentation Transcript

MODULE 2 : 

MODULE 2

Communication Satellites : 

Communication Satellites

Communication satellites (comsat) : 

Communication satellites (comsat) Satellite is a RF repeater in orbit. The design of a satellite is governed by the communication capacity, physical environment in which it is operated and state of technology. Main considerations of a comm. satellite are:- i) Type of service to be provided (eg: mobile communication, DTH) ii) communication capacity (transponder BW and satellite EIRP) iii) coverage area iv) Technological limitations Basic specifications are laid out for satellite depending on the communication requirement. A domestic fixed satellite service it is the EIRP per carrier, number of carriers and coverage area. A direct broadcast satellite it is the number of television channels and coverage area.

First TV image of weather (1960) : 

First TV image of weather (1960)

First complete view of world’s weather, photographed by TIROS 9 (13/2/1965).Image assembled from 450 individual photographs : 

First complete view of world’s weather, photographed by TIROS 9 (13/2/1965).Image assembled from 450 individual photographs

SATELLITE SERVICES : 

SATELLITE SERVICES Satellite links: Trunking / Point to Point (Telecom , Telephony …) Broad cast Services (TV, Radio…) Multi cast Services (IP, Multimedia …)

Unicast / Point to Point : 

Unicast / Point to Point

Broadcast : 

Broadcast

Multicast : 

Multicast

Slide 10: 

Analog/digital TV Transponder Lease SNG (Sat News Gathering) PPV (Pay Per View) Services Public Services Private Services Telecommunications (Voice & Data) Fixed Communications Television (Video) Internet Direct PC ’s Remote area link Virtual backbone

Satellite Telecom Services : 

Satellite Telecom Services Classical Communications Via Ground Station existing Networks Fax Telephone V-SAT Indoor Unit Indoor Unit Fax Telephone HUB at Ground Station Sound Broadcasting Organization Host computer Video conferencing Monitor Multimedia Applications Internet INTERACTIVE TV

Advantages of Satellite Services : 

Advantages of Satellite Services Advantages of Satellite Services : Fast deployment Large footprint / coverage Adaptable and scalable solutions Low Capital expenditures Complementary to terrestrial network, independent from terrestrial network bottle neck, congestions, damages or default Optimum of Satellite Services : Optimum for broadcast and multicast services Optimum for remote access unicast services

Environmental Conditions: : 

Environmental Conditions: A spacecraft must be reliable in all types of environments beginning from launch to the in-orbit deployment and throughout its operation phase. Most important stresses are- a) Zero gravity :- At GEO, gravitational force is negligible giving rise to zero gravity effects. Major effect is on liquid fuel flow and hence external means are to be provided for liquid flow. The absence of gravity facilitates operation of the deployment mechanisms used for stowing antennas and solar panels during launch. b) Atmospheric pressure and temperature :- At high altitudes, atmospheric pressure is extremely low (10-7 torr). This makes thermal conduction negligible and increase friction between surfaces. Hence special materials are used for lubrication of moving parts. However, pressure inside the spacecraft is higher The temperature of a spacecraft is mainly affected by heat from sun and various spacecraft subsystems. The excursion in the external temperature varies from 330-350K during sunlight and 95-120K during eclipses.

Slide 14: 

c) Space Particles :- Various types of particles like cosmic rays, protons, electrons, meteoroids, manmade debris etc exist in space. Main effect of bombardment of particles on a satellite is the degradation of solar cells and certain solid state components within the satellite. Effect of meteoroids is negligible in GEO satellites. d) Magnetic fields :- Magnitude of earth’s magnetic field is very weak at GEO (1/300 of earths surface). The effect of magnetic field can be compensated by the use of large coil. While Satellites passing through Van Allen belt ,deflected charged particles that are trapped in this region affect electronic components Hence special manufacturing mechanisms are used to harden the components against radiations. e) Other Considerations :- Due to the variation of distance of earth from sun, a variation in DC generation capability must be taken into account in design of satellite power system. Also satellites must be prepared for loss of power during eclipses and may result in gradual degradation of solar cell efficiency. There are several perturbations affecting the satellites due to movement of mechanical parts and fuel within it. There may be a small drift in position of antennas.

Life time and reliability : 

Life time and reliability Lifetime of a geostationary satellite is determined by the maximum acceptable deviation in inclination and orbital location. Satellite is maintained in its orbital location by firing thrusters regularly, using stored fuel Hence the operational lifetime of a satellite is determined by- a) Increasing fuel capacity b) Saving fuel by accepting orbital deviation to the maximum extent possible. However there is a practical limit to a satellites fuel storage capacity. Hence satellite lifetime is between 12-15 years.

Reliability : 

Reliability The overall reliability of a satellite is governed by its critical components. Reliability is improved by employing redundancy in the critical sub systems and in components such as TWT amplifiers. Reliability is defined as the probability that a given component/system performs its function within a specified time t. R= where λ= failure rate of a component Unit of λ is specified as FIT, the number of failures in 109 Hr.

Slide 17: 

Three regions can be identified An early high failure rate region attributed to manufacturing faults, defects in materials etc A region of low failure attributed to random component failures A region of high failure rate attributed to component wear-out. In a satellite system, early failures are eliminated to a large extent during testing and burn-in. The main aim is to minimize the random failures which occur during the operational phase of the satellite by using reliability engineering techniques. The beginning of wear-out failure can best be delayed by improving the manufacturing technique and the type of material used.

Slide 18: 

The reliability can be expressed as R = e-λt = e-t/m ; where m =1/ λ (mean time between failures) When several components or sub-systems are connected in series, the overall reliability is Rs=R1 R2…….Rn where Ri is the reliability of the ith component. In terms of the failure rate : Rs= e-(λ1+ λ2+… λn)t Parallel redundancy is useful when the reliability of an individual subsystem is high. If Qi is the unreliability of the ith parallel element, the probability that all units will fail is the product of the individual unreliabilities Qs=Q1 Q2…Qi When the unreliabilities of all elements are equal, this expression reduces to Qs = Qi ;Where Q is the unreliability of each element. Therefore the reliability is R = 1-Qs = 1- Qi =1- (1- R)i =1- (1- e-λt)i

A Typical reliability model of a Geostationary Satellite:All the major sub-systems are shown in series : 

A Typical reliability model of a Geostationary Satellite:All the major sub-systems are shown in series Communication (R1) TT&C (R2) Power (R3) Altitude & orbit control (R4) Thermal (R5) Structure (R7) Propulsion (R6) Spacecraft reliability (R1R2R3R4R5R6R7)

Slide 20: 

. Simplified reliability model Applying the equation for series and parallel combination, the reliability of the communication system is obtained as Rs =RRXRTX [1-(1-RT)2] When RT=0.9,reliability of transponder increases to 0.99 Figure of merit, Fγ = r/M ;where r = R’/R R’= reliability with redundancy employed R= reliability without employing redundancy M= increase in mass due to added redundancy The addition of redundant equipment increases the cost of the transponder

Spacecraft : 

Spacecraft A spacecraft is a vehicle or device designed for spaceflight. On a sub-orbital spaceflight, a spacecraft enters outer space but then returns to the planetary surface (such as Earth) without making a complete orbit. For an orbital spaceflight, a spacecraft enters a closed orbit around the planetary body. Spacecraft used for human spaceflights carry people on board as crew or passengers. Spacecraft used for robotic space missions operate either autonomously or telerobotically. Robotic spacecraft that leave the vicinity of the planetary body are space probes. Robotic spacecraft that remain in orbit around the planetary body are artificial satellites. Spacecraft are used for a variety of purposes, including communications, earth observation, meteorology, navigation, planetary exploration and space tourism. Spacecraft and space travel are common themes in works of science fiction.

Slide 22: 

Tsiolkovsky "the father of human space flight"

Examples of spacecraft : 

Examples of spacecraft Apollo 15 A Russian Soyuz Jules Verne Automated Transfer Vehicle (ATV) Luna 9 soft landing capsule (NASA)

Slide 24: 

Fastest spacecraft Helios I & II Solar Probes (252,792 km/h/157,078 mph) Furthest spacecraft from Earth Voyager 1 at 9.5824234 billion miles. Pioneer 10 at 8.3445237 billion miles. Voyager 2 at 7.4351695 billion miles. Heaviest spacecraft NASA STS Space Shuttle/Orbiter (109,000 kilograms/107 long tons/120 short tons)

Spacecraft sub-systems : 

Spacecraft sub-systems A communication satellite essentially consists of two main functional units - payload and bus. The primary function of the payload is to provide communication. The bus provides all the necessary electrical and mechanical support to the payload. Structure Thermal Propulsion Attitude & Orbit control Electrical power Repeater Telemetry, Tracking & command Payload Bus Spacecraft management Antenna

Slide 26: 

The payload is made up of a repeater and an antenna sub-system. The repeater performs the required processing to the received signals, and the antenna system is used to receive signals from and transmit signals to the ground stations in the coverage area. The bus (sometimes called a platform) consists of several sub-systems - the attitude and orbit control system (AOCS) stabilizes the spacecraft and controls its orbit. The propulsion system provides the necessary velocity increments and torques to the AOCS. The thermal control system maintains the temperature of various sub-systems within tolerable limits. The structure provides the necessary mechanical support during all the phases of the mission. The telemetry, tracking and command (TT&C) subsystem transmits the status of various sub-systems of the satellite to the satellite control centre and accepts commands from the control centre for performing essential functions on the spacecraft. This system also provides tracking support to ground stations. The electrical power supply system provides the necessary DC power.

Payload : 

Payload The payload comprises of repeater and antenna sub-systems and performs the primary function of communication. Repeater The function of a repeater is to receive the uplink RF signals, and convert these signals to the appropriate downlink frequency and power for transmission towards the service area. Two types of repeater architectures are possible: (i) transparent and (ii) regenerative. A transparent repeater ( ‘bent-pipe’) - only translates the uplink frequency to a suitable downlink frequency and power without in anyway processing the baseband signal. A regenerative repeater - translate and amplifies and also provides capabilities of demodulation, baseband processing and remodulation. A regenerative repeater can demodulate individual carriers and time multiplex the signals on to one (or a few) carriers, and remodulate the signal using a better optimized modulation and/or multiple access scheme for the downlink before retransmission. Thus the uplink and the downlink can be optimized separately resulting in a more efficient system. A regenerative repeater is best suited for digital systems and offers several advantages over a transparent responder However, regenerative transponders are heavier and more complex than transparent transponders.

Transparent repeater : 

Transparent repeater With high end-to-end gain requirements, a repeater tends to become unstable unless the isolation between the receive and the transmit section is made high (typically 40 dB more than the loop again). Isolation is achieved by using filters with high isolation between the transmit and the receive bands and careful RF shielding between the paths to avoid radiative coupling. However, even with such measures the desired high isolation is only possible when the frequency separation between the transmit and the receive bands is large. Therefore the uplink and downlink frequencies are usually assigned in separate bands - such as 6 GHz uplink and 4GHz downlink. Generally the uplink frequency is larger than the downlink frequency : One reason for such a choice of frequency was that power was at a premium on earlier satellites. Assuming that global coverage is required, With beamwidth fixed, the antenna gain is constant with frequency, but note that the path loss reduces with frequency. Thus for a specified receive flux density on the ground, the satellite transmitter power requirement reduces as the frequency is reduced.

The main elements of a typical transparent repeater : 

The main elements of a typical transparent repeater

Bent-pipe relay system : 

Bent-pipe relay system Most commercial communication satellite systems have this kind of repeater All signals received at the satellite are amplified and sent back to base station Supports Point-to-Point link

Slide 31: 

Signals from the antenna and the feed systems are fed into a low-noise amplifier through a band pass filter (BPF) . The BPF attenuates all out-of-band signals such as transmission from the ground stations of adjacent satellite systems. The low-noise amplifier provides amplification to the weak received signals. The spacecraft antenna is pointed towards a relatively warm Earth having a noise temperature of about 300K. Tunnel diodes were used in earlier satellites. At present bipolar transistors are used below 2GHz and FETs above 2GHz. Down-conversion is achieved by mixing the amplified uplink signal with a local oscillator. The local oscillator consists of a stable crystal oscillator operating at a relatively low frequency (for example, 100MHz) followed by a multiplier chain, band pass filters and amplifiers

Slide 32: 

The conversion to the downlink frequency can be performed either in a single stage using one intermediate frequency (IF) or in multiple stages using more than one intermediate frequency. A single-state conversion is simple to implement. Multiple-stage down-conversions are preferred when there are special requirements. 1. it may be necessary to interconnect earth stations operating in different frequency bands 2. the required transponder gain or filtering cannot be achieved if direct conversion is used because of technological limitations. A multiple-stage down-conversion scheme is useful, because the necessary amplification and filtering can be conveniently performed at an intermediate frequency.

Slide 33: 

A band pass filter is used at the output of the mixer to remove the unwanted out-of-band frequencies generated in the mixing process. It is essential that the amplification in the LNA and the down-conversion be performed linearly to avoid intermodulation noise being generated by the signals arriving at the satellite from various earth stations of the network. When a single conversion down-conversion is used, the down-converted signal is at the desired downlink frequency. The signal is fed into the input multiplexer ,which divides the channels into sub-bands. Advantages of Channelization : 1. reduces the number of carriers passing through the high-power amplifier simultaneously, thereby minimizing the intermodulation noise. 2. since satellite high-power amplifiers have a limited power handling capacity, it is desirable to split the signals into sub-bands. 3. In a satellite using spot beams, channelization is also used for interconnecting spot beams.

Slide 34: 

The input multiplexer essentially consists of a filter bank, each filter permitting a specific band of signal to pass through. Signals in each transponder is amplified to boost up the relatively low power level of signals .BJT/FET amplifiers are commonly used. Electronically switchable attenuators are introduced in the signal path to control the gain of the transponder. they are controlled through the telecommands. The IF is amplified and up converted to the downlink RF frequency after suitable amplification. RF signals are then fed to high power amplifiers, consisting of one or more stages of driver amplifiers and a power stage. The outputs from the transponders are combined into a composite signal in the output multiplexer. Multiplexer consists of a bank of bandpass filters. Finally the output of the multiplexer is delivered to the antenna for transmission.

Slide 35: 

In bent type transponder, the uplink noise and interference are amplified at thetransponder and transmitted back to receiving earth station. As a result, the uplink and downlink noise and interference are added at the earth station receiver. e.g. Globalstar, Skybridge, Ellipso

Slide 36: 

e.g. Iridium, Teledesic LEOs - Astrolink GEOs The regenerative repeaters includes the additional subsystems.

Regenerative Relay + Onboard Switching : 

Regenerative Relay + Onboard Switching All signals received at the satellite are demodulated, switched, re-modulated and sent back to the base station Multi points-to-Multi points. 3-dB power gain Boost the total system bandwidth by the statistical multiplexing effect by using the onboard baseband switch Flexible link design Already been tested and demonstrated with experimental satellites Still few commercial satellites use this type of transponder

Antenna : 

Antenna The function of the antenna is to transmit signals to and receive signals from ground stations located within the coverage area of the satellite. The choice of the antenna system is therefore governed by the size and the shape of the coverage area. At present, the practical maximal size of antenna which can be launched is about 4m.However, it is possible to deploy larger antenna in space by folding them to fit the launcher diameters during the launch, using complex mechanical arrangements, and unfurling them in space. ATS-6 have demonstrated this concept by unfurling antennas upto 10m

ATS-6 Satellite : 

ATS-6 Satellite ATS-6 (Applications Technology Satellite-6) was the world's first educational satellite as well as world's first experimental Direct Broadcast Satellite as part of the Satellite Instructional Television Experiment between NASA and ISRO. It was launched May 30, 1974 and decommissioned July 1979. ATS-6 transmitted educational programming to various countries, including India, the United States and other regions

Slide 40: 

The coverage area of a satellite (footprint) is best represented by contours of constant received power on the ground, usually shown relative to the power at the beam centre. When the complete disk of the earth, as seen from the geostationary orbit, is illuminated, the coverage is known as global. Global coverage is mainly used for international communication,where communication needs to be established between widely separated earth stations. If the service area is confined to a country or a region, it is wasteful to transmit power outside this area in terms of satellite EIRP and spectrum utilization. The coverage in such systems is best provided by spot beams. Therefore for the same size of HPA, a spot beam gives a higher EIRP than a global beam. Thus spot beams are well suited for applications when satellite EIRP requirements are high, as in direct broadcast or mobile systems, and high spectrum utilization is vital.

Satellite Contours : 

Satellite Contours

Slide 43: 

Further refinements in coverage are achieved through the use of shaped beams - the antenna pattern is shaped to follow the contour of the coverage region as closely as possible. Multiple spot beams are used when service is provided to and between zones of high traffic concentration separated by a large distance, such as between Europe and America and when available spectrum in the service area is severely limited. Dual-polarized systems use antennas which permit orthogonally polarized transmissions. Optimizing coverage areas involves the advantages gained in terms of increase in EIRP, frequency reuse, minimizing interference and the resulting complexity in the antenna system.

Slide 44: 

INTELSAT VII (operates in the C -6/4 GHz and Ku -14/11 or 12 GHz bands) provides a fourfold reuse of C-band frequency spectrum through the use of orthogonal polarization and spatial beam isolation, and a two-fold increase in Ku band spectrum through dual polarization.

Slide 45: 

elliptical beam shapes have been recommended by the ITU for its worldwide direct broadcast plan Several techniques are used to synthesize shaped beams: 1. The desired beam shape can be synthesized by a combination of small elemental circular beams. 2. Phased array techniques are well suited for synthesizing complex coverage shapes. 3. The desired shape can be obtained by controlling the amplitude and phase of each element of the feed.

Slide 46: 

Implementation issues: Fundamental-mode conical horn antennas are light and low cost, and therefore commonly used for providing global coverage. These types of horn, do not exhibit good cross-polar isolation and are therefore unsuitable for use in dual polarized systems. Hybrid-mode corrugated horns which provide high cross-polar discrimination are useful for such applications.

Conical Horn Antenna (CHA) are designed for superior side lobe rejection , operate in high terrestrial interference areas where parabolic antennas fail to work. : 

Conical Horn Antenna (CHA) are designed for superior side lobe rejection , operate in high terrestrial interference areas where parabolic antennas fail to work.

Broad-Band Horn Antenna 0.8 – 18 GHz : 

Broad-Band Horn Antenna 0.8 – 18 GHz Bell Labs' Horn Antenna

Slide 49: 

Reflector antennas are well suited for obtaining spot beams. The offset reflector configurations eliminates the inefficiency introduced by feed blockage. Its geometry permits the feed horns to be located close to the spacecraft body which has the advantage of reducing power pass in feeder lines.

Slide 50: 

Gridded reflectors :Such reflectors are made up of two sets of wire grids at right angles to each other, each set of grids permitting transmission of a single polarization.

Slide 51: 

Multiple beams are formed by using an array of feeds illuminating a single reflector. Each beam is formed by a cluster of feeds, each of which is fed by RF signals of appropriate amplitude and phase. Another consideration is the routing of the received signal from any spot beam. Multi Beam Antennas: This antenna is able to accommodate up to 20 different satellites simultaneously in C or Ku band.

Helical antenna working frequency app. 2.5 GHz : 

Helical antenna working frequency app. 2.5 GHz Linden lad antennas

Slide 53: 

Flat Panel Helical antenna

Slide 54: 

BUS The function of a satellite platform (bus) is to support the payload operation reliably throughout the mission. The main demands on a satellite platform are as follows: The function performed by the attitude and orbit control system: a) Maintain the position and orientation of a satellite at any specified orbital location b) keep the antennas correctly pointed towards the service area The functions performed by the telemetry, tracking and command: a) Provide data to the ground control centre for monitoring the performance of the various spacecraft sub-systems. b) Accept commands from the ground control centre for altering spacecraft configurations and performing vital manoeuvres. c) Support ground station tracking requirements. The function performed by the power sub-system: Provide DC power to all active components of a spacecraft The function performed by the thermal sub-systems: Maintain the temperature of the various spacecraft sub-systems within specified limits The function performed by the structure sub-systems: Provide the required mechanical and structural support.

Slide 57: 

A satellite maintains the desired orientation and orbital position through its attitude control sub-system. The ACS performs the following functions: 1. Controls the rotation of solar arrays. 2. Provides thruster commands for repositioning and station keeping. 3. Controls orientation of the Single Access antennas to slew between user satellites and track user satellites in response to ground commands. 4. Controls the orientation of the Space to Ground Link antenna to point at the ground terminal in response to ground command. 5. Provides sun and earth sensor telemetry data for ground based satellite attitude determination. 6. Provides attitude sensor data and solar array angles for actuation of thrusters, solar array drives and momentum wheels by direct ground control. 7. Provides the capability to recover the satellite from failure conditions which cause loss of attitude control. To improve reliability, provision is made for the system to switch automatically to redundant equipment in case of failure of the operational equipment The Attitude Control System (ACS)

Slide 58: 

The demands on the attitude and orbit control system (AOCS) differ during the two main phases of the mission- the orbit-raising phase and the operational phase. Two types of attitude control systems are in common use- 1. spin stabilization and 2. Three-axis stabilization. The specifications of the attitude-control system depend on the desired spacecraft pointing accuracy which is a function of the satellite antenna beam width. The attitude control may be either active or passive. A passive attitude-control system maintains the attitude by obtaining an equilibrium at the desired orientation without the use of active attitude devices. An active control system maintains the attitude by the use of active devices in the control loop.

Slide 59: 

The local orbital reference system is defined at each point of the orbit by three unit vectors. These vectors are derived from the satellite position and velocity vectors: Vector L is collinear with position vector P (on the axis between the Earth's centre and the satellite). It defines the yaw axis. Vector T is perpendicular to the orbital plane (vector L, vector V). It defines the pitch axis. Vector R completes the set of orthogonal axes. it lies in the plane defined by Vectors L and V and defines the roll axis. It does not coincide exactly with the velocity vector due to the eccentricity of the orbit.

Axes Xs, Ys, Zs represent an orthogonal reference frame related to the satellite (satellite axes).nominal attitude pointing consists of the best possible alignment of this set of axes with the local orbital reference system (while ensuring stability and limiting angular rates about this position).With perfect geocentric pointing, this gives: Xs = -TYs = -RZs =  L : 

Axes Xs, Ys, Zs represent an orthogonal reference frame related to the satellite (satellite axes).nominal attitude pointing consists of the best possible alignment of this set of axes with the local orbital reference system (while ensuring stability and limiting angular rates about this position).With perfect geocentric pointing, this gives: Xs = -TYs = -RZs =  L

Slide 61: 

The attitude is continuously controlled by a programmed control loop: sensors measure the satellite's attitude, the onboard computer then processes these measurements and generates commands which are carried out by the actuator, to ensure correct pointing. Actuators may also be activated by command from the ground. The satellite is also perturbed by several external forces, causing it to drift away from its allocated location. This drift is nullified by the orbit-control system. Disturbances on a geo stationary satellite includes – The main external perturbation caused by solar radiation pressure (satellites with large surface area suffer large torques). Weak disturbing torques are, generated by the Earth’s magnetic fields and gravity. disturbing torques are generated by internal sources - misalignment of thrusters Switching operation of mechanical relays or fuel sloshing can also give rise to disturbing torques

Actuator : 

Actuator

Slide 63: 

Sensors for Attitude Control Sensors are used to estimate the position of the axis with respect to the specified reference directions. The choice of sensor depends on the desired attitude accuracy and the axis to be controlled. Earth sensors are commonly used for maintaining the roll and pitch axes. Orientation of the Earth with respect to the yaw axis is unsuitable for providing yaw errors, and therefore a Sun or star sensor is used in conjunction for controlling this axis. Infra-red emissions from the Earth, are much higher than the background noise from cold space behind the Earth. Therefore the edges of Earth can be detected by sensors sensitive to infra-red emissions. In a scanning sensor scheme, a mirror is made to scan the Earth continuously and the edges of the Earth are detected on an infra-red detector. A feedback control system is used to maintain the Earth centered correctly. The infra-red Earth sensors have limited accuracy-about ±0.10, because the edges of the Earth are not defined clearly. The control accuracy can be improved by using the RF sensing method. In this method the satellite attitude is determined by tracking a RF signal transmitted from the Earth, much in the same way as earth stations track a satellite.

Digital Earth sensors : 

Digital Earth sensors Two digital Earth sensors, one nominal and one redundant, measure the direction of the Earth's geocentre, i.e., the local vertical.

Slide 65: 

A fixed focusing lens projects the image of earth on to the infrared detector Detectors provide equal outputs along each axis when earth is centered, otherwise a voltage proportional to the offset is generated and feedback to the control system for generating necessary corrections

Slide 66: 

A sun sensor has the advantage of availability of high flux density which reduces the sensitivity requirements of the sensor. The Sun, however, cannot be used in situation where it is in the same straight line with the Earth and the satellite. A star sensor overcomes this limitation but has the disadvantage that the received signal is of low intensity. The gyroscope can also be used as an attitude sensor. The attitude information available from a gyroscope is relative to its spin axis. Under equilibrium conditions, the gyroscope gives zero error voltage. If the spacecraft orientation changes relative to this spin axis, a relative motion is developed in the gyroscope. This change can be detected and used for the attitude control. The achievable accuracy is high but a gyroscope requires the use of extra power for its operation and is less reliable.

Autonomous Wireless Sun Sensor : 

Autonomous Wireless Sun Sensor

A gyroscope : 

A gyroscope

Gyroscope invented by Léon Foucault, and built by Dumoulin-Froment, 1852 : 

Gyroscope invented by Léon Foucault, and built by Dumoulin-Froment, 1852

Thrusters : 

Thrusters Small thruster manoeuvres are performed roughly once a month to correct the semi-major axis of the orbit. About once every six months, a larger correction manoeuvre rotates the satellite and fires its thrusters to correct orbit inclination.The hydrazine thrusters used for these manoeuvres can generate 15 newtons of thrust.

Slide 71: 

To obtain the correction torque the raw data obtained from the error sensors must be processed using a suitable control law. The control algorithm used for the purpose is of the form N = Iθ Where N = correcting torque I = moment of inertia of the spacecraft for the axis under consideration θ = angular acceleration. The algorithm takes into account factors such as natural resources occurring on the satellite structure and may include the use of past attitude information. The complexity of the control law is a trade-off between the control accuracy and the on-board processing complexity.

Techniques of attitude control : 

Techniques of attitude control Two main techniques: 1.Spin stabilization: Provide necessary gyroscopic stiffness by spinning either a part or whole of the spacecraft To retain continuous earth pointing the antenna must posses a toroidal beam shape or employ electronic steering In Dual spin satellites the antenna platform is spun in the opposite direction with respect to the main spinning body Solar arrays are mounted around the spinning drum Despun platform uses a closed control loop to ensure that antennas are continuously pointing the earth Solar radiation pressure, spin rate decay with time etc. causes satellite to deviate from normal values Thrusters are utilized to control the attitude in two axes Station keeping in north-south direction is maintained by firing the thrusters parallel to the spin axis and east-west station keeping is obtained by firing thrusters perpendicular to the spin

Slide 73: 

Disadvantage: 1. the satellite cannot use large solar arrays to obtain power from the Sun. Thus, it requires large amounts of battery power. 2. the instruments or antennas also must perform “despin” maneuvers so that antennas or optical instruments point at their desired targets. Spin stabilization was used for NASA's Pioneer 10 and 11 spacecraft, the Lunar Prospector, and the Galileo Jupiter orbiter

Slide 74: 

Credits - NASA

Slide 75: 

2.Body stabilized / three axis: The body of the satellite remains fixed in space Required gyroscopic stiffness is obtained by using momentum wheels which rotate within the body They have to use different sensors for transfer orbit phase and on station phase Static earth sensors are used for pitch and roll axes and separate yaw axis sensors are used for yaw attitude control or roll and yaw can share same sensor Station keeping is achieved by firing thrusters in east-west or north-south direction in continuous mode to provide solar array sun tracking a sun sensor and a control loop is used

Slide 76: 

Voyagers 1 and 2 stay in position using 3-axis stabilization. An advantage of 3-axis stabilization is that optical instruments and antennas can point at desired targets without having to perform “despin” maneuvers.

Credits - NASA : 

Credits - NASA

Propulsion system : 

Propulsion system They generate the thrust required for the attitude and orbit corrections Thrust requirement for orbit control is generally large Mono or bi-propellant fuels are commonly used For attitude control ,thrusters are positioned away from the centre of mass to achieve maximal thrust For orbit control ,the thrusters are mounted so that the thrust vector passes through the centre of mass Force applied by a thruster F = ωIsp : where ω = weight flow rate Isp = specific impulse Therefore total impulse applied by a thruster in time t is I= ∫0t Isp ω dt

Slide 79: 

Nitrogen hydrazine filter valve Combustion chamber Propellant tank Control signals nozzle Mono- propellant propulsion system

Hydrazine is commonly used fuel because it is stable chemical with a large storage time and provides large specific impulse(230 sec) In the absence of gravity, it is necessary to apply external pressure to force out hydrazine. This is done by storing nitrogen under pressure When valve is released ,the pressure of nitrogen forces the fuel to pass through the line Fuel is filtered to remove impurities and passed into the combustion chamber via a valve which permits only a one way fuel transfer Hydrazine is finally passed through a catalyst which decomposes it in the combustion chamber and release the energy through the nozzle to provide required thrust A bi-propellant system consists of a fuel and an oxidizer each stored in separate champers They can provide large thrusts hence are used in apogee kick motors and for north-south station keeping : 

Hydrazine is commonly used fuel because it is stable chemical with a large storage time and provides large specific impulse(230 sec) In the absence of gravity, it is necessary to apply external pressure to force out hydrazine. This is done by storing nitrogen under pressure When valve is released ,the pressure of nitrogen forces the fuel to pass through the line Fuel is filtered to remove impurities and passed into the combustion chamber via a valve which permits only a one way fuel transfer Hydrazine is finally passed through a catalyst which decomposes it in the combustion chamber and release the energy through the nozzle to provide required thrust A bi-propellant system consists of a fuel and an oxidizer each stored in separate champers They can provide large thrusts hence are used in apogee kick motors and for north-south station keeping

Telemetry, tracking and command (TT&C) : 

Telemetry, tracking and command (TT&C) They support the function of spacecraft management Main functions are: 1. monitor the performance of all subsystems and transmit the data to control centre 2. support the determination of orbital parameters 3. provide a source to earth stations for tracking 4. receive commands from control centre for performing various functions of satellite

Slide 82: 

Telemetry subsystem: Monitors various spacecraft parameters such as voltage, current, temp, equipment status and transmit them to control stations Telemetered datas are analyzed and are used for routine operational and failure diagnostics purposes Commonly monitored datas are- Voltage, current, temperature Switch status of transponder Pressure of propulsion tanks Outputs from attitude sensors Reaction wheel speed

Slide 83: 

Telemetry subsystem: Sensor outputs A/D converter formatter Telemetry transmitter modulator Digital outputs Ranging signals

Slide 84: 

Monitored signals are multiplexed and transmitted as a continuous digital stream Analog signals are digitally encoded and multiplexed with other digital outputs Typical telemetry data rates are in the range of 150-100bps In Low bit rate telemetry, a sub carrier modulated with PSK / FSK is used before RF modulation In Distributed telemetry systems , digital encoders are located in each subsystem and data from each encoder are sent to a central encoder via a common, time shared bus This reduces the number of wire connections and also permits easy expansion of initial design and facilitates testing during assembling

Command subsystem : 

Command subsystem Receives commands transmitted from control station, verifies reception and executes these commands Typically over 300 commands are used in a communication Satellites Common commands are: Transponder switching Switch matrix reconfiguration Antenna pointing control Controlling direction and speed of solar array drive Battery reconditioning Thruster firing It is vital that commands are decoded and executed correctly Normally for safety, all commands are verified before execution to reduce the impact of high bit error rate, coding and repetition of datas are employed

Slide 86: 

Command receiver Command decoder Verification process Ranging Base band extraction Command execution Verification data To telemetry transmitter Receiver converts RF signals into base band Command decoder decodes the commands verification process involves transmission of decoded data back to control centre via telemetry carrier Command is stored in a memory and is executed only after verification Tele command receiver also provides the base band output of the ranging tone

Tracking satellite position : 

Tracking satellite position Regular estimation of orbital parameters are necessary to maintain a satellite in its assigned orbit and to provide look angle information to the earth station Orbital parameters are obtained by measuring the angular position and range of the satellite from the ground station During orbital raising period a network of ground stations are distributed throughout the globe to obtain these parameters Mono pulse technique is most commonly used Angular positions measured through a single station is adequate for determining the orbital parameters Measuring the round trip time delay (modulation of a tone-demodulation-remodulation) of a signal gives range of a satellite

Slide 88: 

Multi tone ranging system Stable Reference source Tone generator transmitter Phase Comparison And data processing range From receiver Multiple tones are transmitted to resolve the errors in multiples of tone time period (phase difference of transmitted and received signals are more than 3600) Lower tones resolve the ambiguity and higher tones provide desired accuracy

Slide 89: 

Total phase shift in degrees: Φ=360n+ΔΦ ; where n = unknown integer , ΔΦ = measured phase shift Range R=λn + ( ΔΦ/360) λ ; λ = wavelength The error is estimated as ΔRc = (ΔΦc / 360) λ ; where ΔΦc =error in estimating the phase shift Transmission of Pseudo random digital data is an alternate method Here received signal sequence is correlated with the replica of transmitted sequence Time difference between correlation peaks provides the estimate of the range

Power sub system : 

Power sub system To provide DC power to all subsystems throughout the life of a spacecraft Thus system should generate power, regulate power and also provide alternate energy source for periods when power cannot be generated Solar cells are used because solar power is available for over 99% of on station lifetime Nuclear powered electricity is used for military purpose Amount of power generated by a cell depends on the conversion efficiency and the intensity of incident solar radiation Average power above earth’s atmosphere is 137mW/cm2 (approx.)

Slide 91: 

Solar radiation intensity reduction is caused by either a small eccentricity (0.0167 approx.) in earths orbit around sun and variation in tracking sun position by solar arrays Silicon solar cells are widely used Efficiency varies between 10%-15% and are inversely proportional to temp. An increase in 100c to 700 can reduce power output by 25% approx. Also efficiency reduces by bombardment of cells with electrons and proton particles and damage caused to semiconductor structure

Electrical Power Subsystem (EPS) : 

Electrical Power Subsystem (EPS) responsible for generating electrical power and supplying it to the various loads in an appropriate form

Thin film solar cell : 

Thin film solar cell

Slide 94: 

Thousands of Cells are connected in series to provide the desired bus voltage In series connection a failure of a cell would cause discontinuity and total failure Hence a stack is formed by connecting few cells in parrell and many such stacks are connected in series A geostationary satellite undergoes 88 eclipses an year and rechargeable batteries provide power during this period Mass of the battery constitutes a significant portion of satellite mass Ni-Cd batteries are commonly used because of their high reliability and long lifetime Ni-H Cells have higher capacity per unit mass and tolerance to higher depths of discharge and are preferred nowadays

voltage regulation : 

voltage regulation Voltages generated from solar cells undergo short term and long term variations Short term are caused by rapid change in temp. on the spacecraft While satellite emerges from an eclipse the temp. changes from -180 to 600 in a few minutes and array voltage changes by a factor of 2.5 Long term is due to periodic yearly variation in the solar radiation intensity and gradual deterioration of efficiency of solar cells over time Two regulation approaches are commonly used a) centralized b) decentralized

Slide 96: 

Solar array Charge control battery Re conditioner regulator load Monitor and control Control signals Control signals bus switch Array drive Monitoring signals Control signals Telemetry and command centralized regulation

Slide 97: 

Centralized regulation : Solar array voltage is regulated centrally and the regulated voltage is available to all loads on the bus A fraction of voltage is used for charging the battery and is controlled by ground commands battery output can be switched to reconditioner or to main bus regulator may be series or shunt Monitor and control sub system provides various control signals, sends monitored data to ground via telemetry subsystem and receives commands from command subsystem Adv: availability of well defined voltage simplified power conditioning for loads Disadv: possibility of a single point failure large demands to regulator circuit due to high variation in load requirements

Slide 98: 

De-centralized regulation Each load regulates its own voltage Voltage limiters are used with each load to avoid high voltage while emerging from eclipse Adv: simplicity in overall power regulation lower overall mass Disadv: complex power conditioning requirements on individual loads need to provide over voltage protection to each load In multiple bus concept both regulated and unregulated schemes are used This avoids complete satellite failure when one section of subsystem fails

Slide 99: 

A geostationary satellite undergo a wide range of variations of temp. The main sources of heat affecting a geostationary satellite during its operational phase are: i) solar radiation (ii) heat generated within a satellite due to dissipation. since satellite operates in vacuum conduction and radiation are the only mechanisms for dissipation of heat. The heat lost by a body through radiation is governed by the Stefan-Boltzmann law of radiation as follows: H = ЄAσT4 Where Є = emissivity A = surface area σ = Stefan-Boltzmann constant T = absolute temperature. In satellite average temperature is estimated by integrating the average temperatures of the individual surfaces of different materials. Thermal Control

Slide 100: 

Thermal control can be achieved either by passive or active techniques. Passive techniques are simple and reliable and therefore preferred wherever feasible. Such techniques involve the use of materials with the desired emissitivity / absorptivity to achieve thermal equilibrium according to requirements. Insulation blankets are used around the apogee motor to provide protection against damaging temperatures caused during apogee motor firing. Fillers are used to provide high conductivity paths from the hot spots Thermal Control Techniques

Slide 101: 

Active thermal control techniques are used i) to supplement passive techniques ii) during an eclipse when the low ambient temperature could cause temperatures to fall below the tolerance Commonly used techniques include: 1. Electric heaters - used during eclipses. controlled either by heat sensors / activated by ground commands. 2. hinged pipes- used to expose or cover specific areas to/from cold space for heat dissipation. 3. heat pipes- used to transfer heat from a heat source to a radiator. Fluid is vaporized at the hot end. The vapour travels towards the cold end and condenses, thus giving off heat.

Slide 102: 

A heat sink (aluminium) with heat pipe (copper)

Structure : 

Structure A spacecraft undergoes severe shock and vibrations during the launch phase, which must be absorbed by the structure of the spacecraft. On-station, the structure must provide accurate alignment of antennas and sensors. Diameter of existing expandable launch vehicle is 2.5 - 3.5 m it should include a mechanical adapter to interface with the launch vehicle. The structure design is also affected by the needs of the attitude-control system – eg: a spin-stabilized satellite requires the centre of gravity to be in certain preferred directions.

Slide 104: 

The structure design usually employs a mathematical technique known as the finite element method. Satellite is modeled as a large number of small elements each with required mechanical properties Computer simulation is made to study the performance under various stressed conditions The materials chosen for structure must be such that no deformity can occur under the worst loading conditions Aluminium and magnesium alloys, carbon-fibre reinforced plastics and beryllium products are the most commonly used materials.

Slide 105: 

Structures subsystem: the physical structure of the spacecraft, to which all electronics boxes, thrusters, sensors, propellant tanks, and other components are mount

Earth Stations : 

Earth Stations Earth station receives/transmits information to the satellite network in the most cost-effective and reliable manner while retaining the desired signal quality. A fundamental parameter of an earth station is the G/ T (antenna gain to system noise temperature ratio). This figure of merit represents the sensitivity of an earth station. A higher value implies a more sensitive station. Design Considerations Type of service: Fixed Satellite Service, Mobile Satellite Service or Broadcast Satellite Service. Type of communication requirements: telephony, data, television, etc. Required base band signal quality at the destination Traffic requirements: number of channels, type of traffic-continuous or bursty Cost, reliability.

Intelsat Standard A Gateway : 

Intelsat Standard A Gateway

An Earth Station with several antennas : 

An Earth Station with several antennas A satellite earth station is a communications facility with a microwave radio transmitting and receiving antenna and required receiving and transmitting equipment for communicating with satellites

Slide 109: 

interface Baseband processing Upconvertor modulator Down convertor demodulator HPA Feed system LNA Tracking system Drive motor From/to terrestrial system antenna General configuration of an Earth Station

Slide 110: 

General configuration signals from terrestrial n/w or user are fed to an earth station via interface Baseband signals are processed, modulated and upconverted to the desired frequency and amplified to the required level, combined with other carriers and transmitted via antennas feed system provide necessary aperture illumination, introduces required polarization and provides isolation between transmitted and received signals signals received are amplified in LNA,downconverted to IF, demodulated and transferred to terrestrial n/w via an interface other subsystems tracking, control, monitoring and power supply provide necessary support Major sub systems of an earth station: a) antenna b) feed c) tracking d) low noise amplifier e) high power amplifier

Antenna : 

Antenna Most earth stations use reflector antennas - high gain - the desirable side lobe characteristics. A reflector antenna consists of a parabolic reflector which is illuminated by a primary radiator- usually a horn. The reflector diameter varies from 30m ( INTELSAT Standard-A earth station) to 60cm (a direct broadcast satellite receiver/ Ka band personal stations. The antennas should possess high efficiency - cost is sensitive to the size of its diameter radiation pattern with low side lobes - to minimize interference from and to other radio systems. Based on the geometry Earth stations may have either axi-symmetric or asymmetric antenna configurations.

Slide 112: 

Axi-symmetric configuration the antenna axes are symmetrical with respect to the reflector relatively simple mechanical structure and antenna mount. Depending on the feed arrangement two of the most commonly used arrangements are: 1. Prime-focus feed 2. The Cassegrain and Gregorian systems. Prime-focus feed The parabolic reflector antenna is fed from a primary feed source located at the focus of the parabolic reflector. the signal reflected from the parabolic reflector possesses a planar wavefront in the aperture plane producing the desired radiation pattern. Although simple, this results in a larger antenna noise temperature because the feed horn is pointed towards a relatively hot Earth Additional thermal noise is added by the dissipative loss in the cable/ waveguide located between the feed and the low-noise amplifier (LNA)

Primary feed configuration : 

Primary feed configuration

Prime-focus feed : 

Prime-focus feed Some power is lost in the cable/waveguide used to connect the high-power amplifier (HPA) to the antenna. These reasons the prime-focus feed is used in small earth stations (< 3 - 5m) where the inherent simplicity of the scheme provides an economic solution.

Slide 115: 

ii) Cassegrain and Gregorian systems consists of a parabolic reflector and a hyperbolic sub-reflector sharing the same focal point F1. The primary feed is located at the second focal point F2 of the sub-reflector. The electromagnetic waves from the primary radiator are reflected off the sub-reflector to the main reflector. The geometry of the arrangement ensures the desired planar wavefront in the aperture plane. Adv: 1. A low-noise system - low magnitude of its main noise components 2. Easy accessibility of the feed - LNAs can be mounted close to the feed, reducing noise contributions from the feeder line loss. 3. A reduction in power requirements of the earth station HPA 4. Easy access to the electronic unit which can be mounted at the base of the reflector. Most large earth stations use the Cassegrain system.

Cassegrain configuration : 

Cassegrain configuration

Cassegrain : 

Cassegrain

Slide 118: 

The hyperbolic sub-reflector of the Cassegrain system can be replaced by an ellipsoid. This configuration is known as the Gregorian configuration. Here the focal point of the main parabolic reflector and the ellipsoid are at common point F1. This configuration is less commonly used than the Cassegrain system. Asymmetric configuration In Axi-symmetric configuration the blockage of the aperture by the feed and the sub-reflector assembly reduces the antenna efficiency and an increase in the side lobe levels. Asymmetric configuration removes this limitation by offsetting the mounting arrangement of the feed so that it does not obstruct the main beam. As a result, the efficiency and side lobe level performance are improved.

Asymmetric configuration : 

Asymmetric configuration

Slide 120: 

Antenna mounts commonly used mounts in medium and large earth stations are: 1. the azimuth-elevation mount; 2. the X-Y mount. In azimuth-elevation mount rotation around primary vertical axis controls the azimuth angle and the horizontal axis mounted over the primary axis, provides the elevation angle control. The X-Y mount consists of a horizontal axis (X-axis) and a secondary axis (Y-axis) mounted on the X-axis and at right-angles to it. Movement around these axes provides the necessary steering.

Slide 121: 

In recent years polar mounts are beginning to be used increasingly in small earth stations. A polar mount permits scanning of the complete geostationary arc by rotation around a single axis which is made parallel to the Earth’s polar axis.

Slide 122: 

Feed system Functions: i) illuminate the main reflector ii) separate the transmit and receive bands iii) separate and combine polarizations in a dual polarized system iv) provide error signals for some types of satellite tracking system. A horn antenna is common as the primary feed at microwave frequencies. Here an open waveguide is flared at the transmitting end to match the impedance of the free space with waveguide. This ensures an efficient transfer of power. Circular aperture horns, known as conical horns, are widely used as primary feeds in earth stations.

Slide 123: 

Orthogonal mode transducer Orthogonal Mode transducer polarizer polarizer orthogonal Mode Junction assembly Mode Extraction LHCP. RHCP RHCP LHCP receive transmit Tracking error LHCP= Left Hand Circular Polarization RHCP= Right Hand Circular Polarization Orthogonal polarized feed assembly

Slide 124: 

A higher- mode coupler (mode extractor) provides the error signals to the tracking system The orthogonal mode junction (OMJ) assembly is used to separate the dually polarized transmit and receive signals. The orthogonal mode transducer (OMT) separates the two linear orthogonally polarized signals into separate ports on the receive side and combines two linearly polarized signals into a composite linear orthogonally polarized signal on the transmit side. since OMT operates on linearly polarized signals, polarizer's are used to convert a circular polarization to a linear.

Slide 125: 

Tracking system Tracking is essential when the satellite drift δθs > ψhp / N Where δθs = Maximum satellite drift over a day as seen from earth station ψhp = antenna half-power beamwidth N = application-dependent constant (typically 5-10). Existing satellites typically move in the range 0.5-30 / day. Antennas with large beamwidths do not require satellite tracking. Functions performed by tracking system: Satellite acquisition Automatic tracking Manual track Program track

Slide 126: 

Satellite acquisition Before communication can be established it is necessary to ‘acquire’ a satellite. program the antenna to perform a scan around the predicted position of the satellite. In simplest form, a satellite can be acquired by moving the antenna manually around the expected satellite position. Automatic tracking After acquisition a satellite needs to be tracked continuously. Auto-track systems are closed-loop control systems and are highly accurate. This tracking mode is the preferred configuration when accuracy is the dominant criterion.

Slide 127: 

Manual track earth stations have manual mode to avoid a total loss of communication due to a failure in the tracking system. In this mode an antenna is moved through manual commands. Program track here the antenna is driven to the predicted satellite position by a computer. since a program track system is an open-loop control system, its accuracy is mainly governed by the accuracy of the prediction data.

Slide 128: 

Antenna drive Servo amplifier Antenna Control system Antenna And Feed system Auto - track receiver Measured position Estimated position error corrections Manual track data Program track data beacon satellite Main elements of a tracking system

Slide 129: 

Main functional elements 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-track techniques the feed system provides the required components of error signals. The outputs of the auto-track receivers are processed and used to drive each axis of the antenna to the estimated satellite position. In the 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 from a computer. The difference in the actual and the desired antenna positions constitutes the error and is used to drive the antenna.

Slide 130: 

Auto-track system There are three main types of auto-track system conical scan mono pulse step-track Conical scan The conical scan technique has evolved from the lobing technique used in Radars an antenna beam is switched between two positions. When target is at the centre of these beams the echoes from each beam are equal in magnitude, but at other positions unequal. The antenna position is adjusted such that the amplitudes of echoes are equalized.

Slide 131: 

Adv: One RF channel Good Tracking accuracy Medium response time Phase stability not important Disadv: Accuracy sensitive to amplitude interference Mechanical moving parts with difficult maintenance Applications: Medium/ large earth stations (not common nowadays)

Slide 132: 

Monopulse technique the errors for driving the antenna system are derived by simultaneous lobing of the received beacon -hence the name static split or monopulse. The inherent susceptibility of the conical scan technique to amplitude fluctuations is eliminated since errors are derived from simultaneous measurements. Several monopulse schemes such as amplitude comparison, phase comparison or amplitude and phase comparison are possible. The amplitude comparison technique is the simplest and is commonly used for satellite tracking.

Slide 133: 

Adv: No mechanical moving parts, hence little maintenance Very high tracking accuracy Fast response Disadv: At least two channel coherent receivers required Good RF phase stability required Expensive Feed system large and complex Application: Large earth stations (INTELSAT-Std A)

Slide 134: 

Step-track system error signals are derived from amplitude sensing. The operation is based on maximization of the received signal by moving the axes in small steps until a maximization is effected. Adv: simple design Low cost One RF channel RF Phase stability not required Disadv: Tracking accuracy low Slow response time Accuracy sensitive to amplitude interference Applications: Low cost and simple earth stations (INTEL Std B) Ship earth stations

Slide 135: 

Intelligent tracking the satellite position is computed by optimal control techniques. The satellite position is obtained by optimally combining the antenna position estimate obtained from an accurate gradient tracking algorithm with predictions obtained from a simple, self-learning satellite position model. The algorithm can switch between either of the two schemes or optimally combine satellite position estimates depending on the system state. Adv: Highly accurate simplicity marginal extra cost. Disadv: Slow response time Susceptible to amplitude interference Applications: Large, medium or small earth stations

Slide 136: 

Recent tracking techniques Electronic beam squinting : the sequential lobing technique has been implemented by using rapid electronic switching of a single beam which effectively approximates simultaneous lobing. The high rate of switching is achieved by the use of an electronically controlled feed. requires a simple single channel receiver tracking accuracy approaching that of the mono pulse technique.

Slide 137: 

Low-noise amplifier (LNA) a special type of electronic amplifier used in communication systems to amplify very weak signals captured by an antenna. it is an electronic device used to filter out the noise of input signals received at the front ends of communication systems It is often located very close to the antenna, so that losses in the feedline become less critical. JFETs and HEMT (hetrojunction FET)are used and distributed amplifiers could be used In the earliest earth stations, MASERs were used as the front-end amplifier. These devices are relatively narrow band (40-120 MHz), require liquid helium temperatures expensive and difficult maintenance requirements. these were replaced by parametric amplifiers provide wide bandwidths with the required low-noise temp. lower cost and complexity. The advent of gallium arsenide FET has greatly simplified the front-end amplifier design of earth stations.

Slide 138: 

High-power amplifier Earliest stations required very high transmitted power and hence used microwave tubes such as a klystron. At present, the smallest stations use solid-state power amplifiers (0.25 watt) and the largest use traveling wave tube (TWT) amplifiers or klystrons (10-15 kW). TWT amplifiers offer bandwidths of the order of 500 MHz capable of providing powers of up to 10 kW. a linearizer is used to improve the linearity of the amplifier, especially if a multi-carrier operation is desired. Klystrons narrow-band devices typically offering bandwidths of the order of 40 MHz tunable over the entire 500 MHz bandwidth Maximal powers are of the order of 3 kW. multiple amplifier configuration is essential when a larger BW is required. higher efficiency, longer tube life, lower cost simpler to maintain and operate than TWTs.

Slide 139: 

High-power klystron used at the Canberra Deep Space Communications Complex

Slide 140: 

The L-3403 Klystron is a four-cavity, modulating anode, pulsed klystron amplifier which can be mechanically tuned to amply any frequency within the range from 400 to 450 MHz

Slide 141: 

Cutaway view of a TWT. (1) Electron gun; (2) RF input; (3) Magnets; (4) Attenuator; (5) Helix coil; (6) RF output; (7) Vacuum tube; (8) Collector.

Slide 142: 

For a Multi-carrier operation, two types of configurations are used In a single amplifier configuration all the carriers are combined before the amplifier to minimize the intemodulation noise HPA is operated in a linear portion of the HPA characteristic. In a multiple amplifier configuration each HPA amplifies one or a few of the total carriers. the amplified signals are then combined at the output of the HPAs. improves the overall efficiency of the earth station.

Slide 143: 

C O M B I N E R HPA HPA To feed system switch (standby) carriers 1 n Matched termination Single amplifier configuration

Slide 144: 

HPA HPA HPA HPA combiner C O M B I N E R Multi amplifiers configuration Wide band carriers switch Narrow band carriers To feed system

Classification of earth stations : 

Classification of earth stations Depending upon the service: 1. Fixed satellite service earth station a) Large earth stations b) Very small aperture terminal (VSAT) 2. Mobile satellite service earth station a) Large mobile earth station ( Inmarsat-B) b) Small mobile earth station ( Inmarsat-C)

Slide 146: 

Very small aperture terminal (VSAT) VSATs are used for information exchange from a single point to multi-point in broadcast applications multi-point to a single point in data collection systems. Because of its large coverage, it is cost-effective The VSAT n/w has a large fixed earth station ( hub) for communicating with one / more VSAT n/w a n/w control centre which is often co-located with the hub. The total user population in a single network may be of the order of several hundreds. The earliest VSATs used C band and employed the spread spectrum technique to counter interference in this heavily congested band. The current trend is to utilize the less congested Ku band which also permits the use of small antennas.

Slide 147: 

The main cost-sensitive sub-systems are antenna (typically 1.2-1.8m in the Ku band) solid-state power amplifiers (1-2W) the low-noise amplifier. A cost-effective solution is to use a low-noise converter, in which the low-noise amplification and down-conversion are performed in the same unit. Coding is usually employed because of the powerlimited downlink. Block codes are simple to implement but offer lower coding gain, whereas convolution coding, although complex, can provide higher gain. At present, convolution coding with Viterbi soft-decision decoding is cost-effective. Several modulation schemes such as some form of BPSK (differential BPSK) is favoured owing to cost, reliability and implementation considerations.

Slide 148: 

HVNET the High Speed Satellite based VSAT network of Department of Telecom Services, provides high speed data transfers (up to 64 Kbps) and voice communication service covering the entire country. VSATs are most commonly used to transmit narrowband data (credit card, polling or RFID data; or SCADA), or broadband data (Satellite Internet access to remote locations, VoIP or video).

Slide 149: 

Mobile satellite service earth stations The main features in design optimization of an MSS earth station: limited mounting space hence the antenna size on mobiles is restricted minimization of cost is important for service uptake(personal s/m) traffic flow is low (a single channel is adequate) Transmitted power should conform to radiation safety standards. Examples of mobile earth stations are Inmarsat-B and Inmarsat-C Inmarsat-B terminals are relatively large in size with tracking antennas used as shipborne terminals terminals can be interfaced with voice, telex or data networks, Inmarsat-C terminals are small and inexpensive with non-tracking antennas terminals can be interfaced with telex or data networks.

Slide 150: 

Large mobile earth station (Inmarsat-B) Inmarsat B provides communications services between a ship and the outside world. It is an efficient, all-digital system that supports telephone and telefax services telex and Internet and LAN access emergency services for deep-sea and passenger ships. The hardware consists of a sensor-stabilized antenna above decks an electronics unit its peripherals below decks A land version of the Inmarsat B system is also available.

Slide 152: 

Small mobile earth station (Inmarsat-C) These are used on small vessels and in land mobile applications. main objective of this standard is to minimize the cost. Low G/T requirement permits the use of simple non-tracking antennas. disadvantage for this reduced antenna complexity is the very low-bit rate they can provide store- and-forward messages end-to-end services. In the store-and-forward service complete messages are formatted and transmitted on a simplex basis when a channel is available. In the end-to-end service permanent or semi-permanent connections are established for the duration of a call.

Slide 154: 

Terminals for personal communication systems These systems may be either the fixed satellite service or mobile satellite services. Their size and communication capability will depend on the extent of mobility. portable sizes will offer broadband services hand-held offers low bit rate data and voice. For eg: five terminal types are envisaged in UMTS – hand-held, vehicular, transportable, fixed and paging. main requirements: availability of light weight low-cost hand-held terminals.

Slide 155: 

main features: small omni-directional or hemispheric antenna, low power consumption low radiated power for radiation safety dual mode which can use either terrestrial / satellite systems. functions: service provision and access location registration and updating paging domain updating database and handover.

Slide 156: 

Satellite television receivers These terminals are commonly called Television Receive Only or TVRO terminals they are relatively expensive because of the low-power flux density of satellite transmissions available on the ground. features: small antennas are used at homes The terminals are receive-only low cost, reliability The antenna sizes for a DBS receiver are generally between 60 and 90cm. Polar or simple fixed mounts are commonly used. DBS transmissions are circularly polarized

Slide 157: 

Parabolic antennas with offset feed provide the most reliable and cost effective solution at present. Flat antennas, which merge well with surroundings, are also used. A low-noise block down-converter (LNB) is attached close to the antenna to minimize the degradation in system noise. Different transmission techniques are used in various parts of the world. ( PAL, NTSC and SEACOM systems) A technique called Multiplexed Analog Components (MAC) is now being implemented in Europe. In MAC ,the audio, chrominance and luminance components are digitized, multiplexed and transmitted. many operating companies scramble and/or encrypt the transmitted picture to avoid unauthorized viewing.