Book Source: Digital Library of India Item medical-site.info: Dr. D. C. medical-site.infoioned. Satellite Communications. Front Cover. D. C. Agarwal. Khanna, - Artificial satellites in telecommunication - pages. 1 Review. Satellite Communications [Agarwal Dc] on medical-site.info *FREE* shipping on This book deals with the subject of satellite communications in detail. Read more .
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Dr. D.C. Agarwal. medical-site.info Tech. This book or part thereof cannot be translated or reproduced in any Satellite communication is of much importance for both the. Satellite Communications by Agarwal Dc, , available at Book Depository with free delivery worldwide. Title: Satellite Communications. Author: D. C. Agarwal. Publisher: Eqn Equation (Particular equation of the above book). AP Appendix to.
Find the earth station GT ratio. Calculate the new GT ratio. Calculate the power output of an uplink transmitter. Calculate the uplink transmitter power required. Calculate the symbol rate. Estimate the BW occupied by the RF signal and the frequency range of transmitted signal. Calculate the required carrier to noise ratio.
Calculate the earth station transmitter power needed for transmission of a baseband signal. Calculate the uplink power increase required for TDMA operation. Calculate the EIRP of a satellite downlink. Calculate the gain of 3 m paraboidal antenna. Calculate the overall noise temperature of the system. Calculate the carrier to noise spectral density ratio. Calculate the earth station EIRP required for saturation assuming clear sky conditions.
Calculate the eective isotropic radiated power in watts as seen by the antenna. Calculate the noise power for a BW of 36 MHz.
Calculate the total link loss. Satellite's inherent strength as a broadcast medium makes it perfect. Since satellite with new services. To implement this frequency planning, the world is divided into three regions: Europe, Africa and Mongolia Region 2: North and South America and Greenland Region 3: Asia excluding region 1 areas , Australia and south-west Pacific.
Within these regions, he frequency bands are allocated to various satellite services. Some of them are listed below. Provides Direct Broadcast to homes. This includes services for: Certain satellites are specifically designed to monitor the climatic conditions of earth.
They continuously monitor the assigned areas of earth and predict the weather conditions of that region. This is done by taking images of earth from the satellite. These images are transferred using assigned radio frequency to the earth station.
Earth Station: These satellites are exceptionally useful in predicting disasters like hurricanes, and monitor the changes in the Earth's vegetation, sea state, ocean color, and ice fields.
These dedicated satellites are responsible for making s of channels across the globe available for everyone. They are also responsible for broadcasting live matches, news, world-wide radio services. These satellites require a cm sized dish to make these channels available globally. These satellites are often used for gathering intelligence, as a communications satellite used for military purposes, or as a military weapon.
A satellite by itself is neither military nor civil. It is the kind of payload it carries that enables one to arrive at a decision regarding its military or civilian character. The system allows for precise localization world-wide, and with some additional techniques, the precision is in the range of some meters. Ships and aircraft rely on GPS as an addition to traditional navigation systems. Many vehicles come with installed GPS receivers. This system is also used, e.
One of the first applications of satellites for communication was the establishment of international telephone backbones. Instead of using cables it was sometimes faster to launch a new satellite. But, fiber optic cables are still replacing satellite communication across long distance as in fiber optic cable, light is used instead of radio frequency, hence making the communication much faster and of course, reducing the delay caused due to the amount of distance a signal needs to travel before reaching the destination.
Using satellites, to typically reach a distance approximately 10, kms away, the signal needs to travel almost 72, kms, that is, sending data from ground to satellite and mostly from satellite to another location on earth. Due to their geographical location many places all over the world do not have direct wired connection to the telephone network or the internet e.
Here the satellite provides a complete coverage and generally there is one satellite always present across a horizon. The basic purpose of satellites for mobile communication is to extend the area of coverage. Areas that are not covered usually have low population where it is too expensive to install a base station.
With the integration of satellite communication, however, the mobile phone can switch to satellites offering world-wide connectivity to a customer. Satellites cover a certain area on the earth. Within the footprint, communication with that satellite is possible for mobile users.
Sometimes it becomes necessary for satellite to create a communication link between users belonging to two different footprints. By going through the above slides we came to know that satellite is mostly responsible for: Forces involved in orbital mechanics There are two relevant forces involved in this problem 1.
The laws of motion are described through three fundamental principles. If no other forces are acting on the satellite, either intentionally by orbit control or unintentionally as in gravity forces from other bodies, the satellite will eventually settle in an elliptical orbit, with the earth as one of the foci of the ellipse. The shaded area A1 shows the area swept out in the orbital plane by the orbiting satellite in a one hour time period at a location near the earth.
This result also shows that the satellite orbital velocity is not constant; the satellite is moving much faster at locations near the earth, and slows down as it approaches apogee. This factor will be discussed in more detail later when specific satellite orbit types are introduced. This allows the satellite designer to select orbit periods that best meet particular application requirements by locating the satellite at the proper orbit altitude.
The altitudes required to obtain a specific number of repeatable ground traces with a circular orbit are listed in Table 2. Orbital Elements: A point for a satellite farthest from the Earth. It is denoted as ha. A point for a satellite closest from the Earth.
It is denoted as hp. Line of Apsides: Line joining perigee and apogee through centre of the Earth. It is the major axis of the orbit. Ascending Node: The point where the orbit crosses the equatorial plane going from north to south. Descending Node: The point where the orbit crosses the equatorial plane going from south to north.
Its measured at the ascending node from the equator to the orbit, going from East to North. Also, this angle is commonly denoted as i. Line of Nodes: Prograde Orbit: Its inclination is always between 0 to Retrograde Orbit: Argument of Perigee: Right ascension of ascending node: The definition of an orbit in space, the position of ascending node is specified.
But as the Earth spins, the longitude of ascending node changes and cannot be used for reference. Thus for practical determination of an orbit, the longitude and time of crossing the ascending node is used. For absolute measurement, a fixed reference point in space is required. Mean anamoly: It gives the average value to the angular position of the satellite with reference to the perigee.
True anamoly: Mean anomaly M0 It denotes the position of a satellite in its orbit at a given reference time. As the equatorial bulge causes a slow variation in argument of perigee and right ascension of ascending node, and because other perturbing forces may alter the orbital elements slightly, the values are specified for the reference time or epoch.
They are required at the antenna so that it points directly at the satellite. Look angles are calculated by considering the elliptical orbit. These angles change in order to track the satellite. For geostationary orbit, these angels values does not change as the satellites are stationary with respect to earth.
Thus large earth stations are used for commercial communications, these antennas beamwidth is very narrow and the tracking mechanism is required to compensate for the movement of the satellite about the nominal geostationary position. For home antennas, antenna beamwidth is quite broad and hence no tracking is essential. This leads to a fixed position for these antennas. Sub satellite point: The following information is needed to determine the look angles of geostationary orbit.
Position of Earth Station SS: Sub-Satellite Point S: This leads to rotation of the line of apsides. The non-spherical shape leads to the small value of eccentricity at the equatorial plane. The impact of this drag is maximum at the point of perigee. Satellites maneuvered by the earth station back to their original orbital position.
The control earth stations used to measure the angular position of the satellites also carryout range measurements using unique time stamps in the telemetry stream or communication carrier. There is little point in orbiting the correct height and not having the appropriate velocity component in the correct direction to achieve the desired orbit.
A geostationary satellite for example must be in an orbit at height 35, The further out from the earth the orbit is greater the energy required from the launch vehicle to reach that orbit. In any earth satellite launch, the largest fraction of the energy expanded by the rocket is used to accelerate the vehicle from rest until it is about 20miles 32 km above the earth. To make the most efficient use of the fuel, it is common to shed excess mass from the launcher as it moves upward on launch; this is called staging.
Most launch vehicles have multiple stage and as each stage is completed that portion of the launcher is expended until the final stage places the satellite into the desired trajectory. Hence the term: The solid rocket boosters are recovered and refurbished for future mission and the shuttle vehicle itself is flown back to earth for refurbishment and reuse.
Launch vehicles are hence used to set these satellites in their orbits. These vehicles are reusable.
When the orbital altitude is greater than 1, km it becomes expensive to directly inject the satellite in its orbit. For this purpose, a satellite must be placed in to a transfer orbit between the initial lower orbit and destination orbit. This manoeuvre is named for the German civil engineer who first proposed it, Walter Hohmann, who was born in He didn't work in rocketry professionally and wasn't associated with military rocketry , but was a key member of Germany's pioneering Society for Space Travel that included people such as Willy Ley, Hermann, and Werner von Braun.
He published his concept of how to transfer between orbits in his book, The Attainability of Celestial Bodies. The transfer orbit is selected to minimize the energy required for the transfer. This orbit forms a tangent to the low attitude orbit at the point of its perigee and tangent to high altitude orbit at the point of its apogee.
Similarly, an apogee kick motor AKM is used to inject the satellite in its destination orbit. Generally it takes months for the satellite to become fully functional. It is a reaction force described quantitatively by Newton's second and third laws.
When a system expels or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite direction on that system. As the vast majority of geostationary satellite launches are carried out from spaceports at a significant distance away from Earth's equator, the carrier rocket would only be able to launch the satellite into an elliptical orbit of maximum apogee 35,kilometres and with a non-zero inclination approximately equal to the latitude of the launch site.
It is better to launch rockets closer to the equator because the Earth rotates at a greater speed here than that at either pole. This extra speed at the equator means a rocket needs less thrust and therefore less fuel to launch into orbit. This speed bonus means the vehicle needs less fuel, and that freed space can be used to carry more pay load.
The most effecting ones are gravitational fields of sun and moon, non-spherical shape of the Earth, reaction of the satellite itself to motor movements within the satellites. Thus the earth station keeps manoeuvring the satellite to maintain its position. Within a set of nominal geostationary coordinates. Thus the exact GEO is not attainable in practice and the orbital parameters vary with time. If the true transmitter frequency i. Range variations Even with the best station keeping systems available for geostationary satellites, the position of a satellite with respect to earth exhibits a cyclic daily variation.
The variation in position will lead to a variation in range between the satellite and user terminals. If time division multiple access TDMA is being used, careful attention must be paid to the timing of the frames within the TDMA bursts so that the individual user frames arrive at the satellite in the correct sequence and at the correct time. This happens for some duration of time every day. These eclipses begin 23 days before the equinox and end 23 days after the equinox.
They last for almost 10 minutes at the beginning and end of equinox and increase for a maximum period of 72 minutes at a full eclipse. The solar cells of the satellite become non-functional during the eclipse period and the satellite is made to operate with the help of power supplied from the batteries. The eclipse will happen at night but for satellites in the east it will happen late evening local time.
An earth caused eclipse will normally not happen during peak viewing hours if the satellite is located near the longitude of the coverage area. Modern satellites are well equipped with batteries for operation during eclipse. A satellite east of the earth station enters eclipse during daylight busy hours at the earth station. A Satellite west of earth station enters eclipse during night and early morning hours non busy time.
Sun Transit Outage Sun transit outage is an interruption in or distortion of geostationary satellite signals caused by interference from solar radiation. Sun appears to be an extremely noisy source which completely blanks out the signal from satellite. This effect lasts for 6 days around the equinoxes. They occur for a maximum period of 10 minutes. Generally, sun outages occur in February, March, September and October, that is, around the time of the equinoxes.
At these times, the apparent path of the sun across the sky takes it directly behind the line of sight between an earth station and a satellite. As the sun radiates strongly at the microwave frequencies used to communicate with satellites C-band, Ka band and Ku band the sun swamps the signal from the satellite. The effects of a sun outage can include partial degradation, that is, an increase in the error rate, or total destruction of the signal. Richharia, Mobile Satellite Communication: Richharia, BS Publications, 2 Edition, Satellite communication- D.
C Agarwal, Khanna Publications, 5th Ed. The particular application of the satellite system, for example fixed satellite service, mobile service, or broadcast service, will determine the specific elements of the system.
A generic satellite system, applicable to most satellite applications, can be described by the elements shown in Figure 3. The basic system consists of a satellite or satellites in space, relaying information between two or more users through ground terminals and the satellite. The information relayed may be voice, data, video, or a combination of the three. The user information may require transmission via terrestrial means to connect with the ground terminal.
The satellite is controlled from the ground through a satellite control facility, often called the master control center MCC , which provides tracking, telemetry, command, and monitoring functions for the system.
The space segment of the satellite system consists of the orbiting satellite or satellites and the ground satellite control facilities necessary to keep the satellites operational. The ground segment, or earth segment, of the satellite system consists of the transmit and receive earth stations and the associated equipment to interface with the user network.
Ground segment elements are unique to the type of communications satellite application, such as fixed service, mobile service, broadcast service, or satellite broadband, and will be covered in later chapters where the specific applications are discussed.
The space segment equipment carried aboard the satellite can be classified under two functional areas: The bus subsystems are: Payload The payload on a satellite is the equipment that provides the service or services intended for the satellite.
A communications satellite payload consists of the communications equipment that provides the relay link between the up- and downlinks from the ground. The communications payload can be further divided into the transponder and the antenna subsystems. A satellite may have more than one payload. The basic shape of the structure depends of the method of stabilization employed to keep the satellite stable and pointing in the desired direction, usually to keep the antennas properly oriented toward earth.
Two methods are commonly employed: Figure 3. Spin Stabilization A spin stabilized satellite is usually cylindrical in shape, because the satellite is required to be mechanically balanced about an axis, so that it can be maintained in orbit by spinning on its axis. For GSO satellites, the spin axis is maintained parallel to the spin axis of the earth, with spin rates in the range of 30 to revolutions per minute.
The spinning satellite will maintain its correct attitude without additional effort, unless disturbance torques are introduced. External forces such as solar radiation, gravitational gradients, and meteorite impacts can generate undesired torques. Internal effects such as motor bearing friction and antenna subsystem movement can also produce unwanted torque in the system. Impulse type thrusters, or jets, are used to maintain spin rate and correct any wobbling or nutation to the satellite spin axis.
The entire spacecraft rotates for spin-stabilized satellites that employ omnidirectional antennas. When directional antennas are used, which is the prevalent case, the antenna subsystem must be despun, so that the antenna is kept properly pointed towards earth.
The antenna subsystem is mounted on a platform or shelf, which may also contain some of the transponder equipment. The satellite is spun-up by small radial gas jets on the surface of the drum.
The rotation, ranging from 30 to rpm, provides gyroscopic force stability for the satellite. The propellants used include heated hydrazine or a bipropellant mix of hydrazine and nitrogen tetroxide. The despun platform is driven by an electric motor in the opposite direction of the satellite spin, on the same spin axis and at the same spin rate as the satellite body, to maintain a fixed orientation for the antennas, relative to earth. Three-axis Stabilization A three-axis stabilized satellite is maintained in space with stabilizing elements for each of the three axes, referred to as roll, pitch, and yaw, in conformance with the definitions first used in the aircraft industry.
The entire body of the spacecraft remains fixed in space, relative to the earth, which is why the three-axis stabilized satellite is also referred to as a body- stabilized satellite. Active attitude control is required with three-axis stabilization. Control jets or reaction wheels are used, either separately or in combination, to provide correction and control for ach of the three axes.
A reaction wheel is basically a flywheel that absorbs the undesired torques that would shift spacecraft orientation. The three-axis stabilized satellite does not need to be symmetric or cylindrical, and most tend be box-like, with numerous appendages attached.
Typical appendages include antenna systems and solar cell panels, which are often unfurled after placement at the on-orbit location. Attitude Control The attitude of a satellite refers to its orientation in space with respect to earth.
Orientation is monitored on the spacecraft by infrared horizon detectors, which detect the rim of earth against the background of space.
Four detectors are used to establish a reference point, usually the center of the earth, and any shift in orientation is detected by one or more of the sensors.
Acontrol signal is generated that activates attitude control devices to restore proper orientation. Gas jets, ion thrusters, or momentum wheels are used to provide active attitude control on communications satellites. If no orbit control station keeping is provided, the satellite will drift to and eventually settle at one of the stable points. This could take several years and several passes through the stable point before the satellite finally comes to rest at a stable point.
Orbital Control Orbital control, often called station keeping, is the process required to maintain a satellite in its proper orbit location. It is similar to, although not functionally the same as, attitude control, discussed in the previous section. Orbital control is usually maintained with the same thruster system as is attitude control.
The non-spherical oblate properties of the earth, primarily exhibited as an equatorial bulge, cause the satellite to drift slowly in longitude along the equatorial plane. Control jets are pulsed to impart an opposite velocity component to the satellite, which causes the satellite to drift back to its nominal position. These corrections are referred to as east-west station keeping maneuvers, which are accomplished periodically every two to three weeks.
For a nominal geostationary radius of 42 km, the total longitude variation would be about km for C-band and about 75 km for Ku-band. Latitude drift will be induced primarily by gravitational forces from the sun and the moon. These forces cause the satellite inclination to change about 0. Periodic pulsing to compensate for these forces, called north-south station keeping maneuvers, must also be accomplished periodically to maintain the nominal satellite orbit location.
North south station-keeping tolerance requirements are similar to those for east-west station keeping, 0. TheKu-band satellite requires a box with approximately equal sides of 75 km. North-south station keeping requires much more fuel than east-west station keeping, and often satellites are maintained with little or no north-south station keeping to extend on-orbit life.
The expendable fuel that must be carried on-board the satellite to provide orbital and attitude control is usually the determining factor in the on-orbit lifetime of a communications satellite. As much as one-half of the satellite launch weight is station-keeping fuel. The lifetimes of most of the critical electronic and mechanical components usually exceed the allowable time for active orbit control, which is limited by the weight of fuel that can be carried to orbit with current conventional launch vehicles.
Thermal Control Orbiting satellites will experience large temperature variations, which must be controlled in the harsh environment of outer space. Thermal radiation from the sun heats one side of the spacecraft, while the side facing outer space is exposed to the extremely low temperatures of space. Much of the equipment in the satellite itself generates heat, which must be controlled. Low orbiting satellites can also be affected by thermal radiation reflected from the earth itself.
The satellite thermal control system is designed to control the large thermal gradients generated in the satellite by removing or relocating the heat to provide an as stable as possible temperature environment for the satellite.
Several techniques are employed to provide thermal control in a satellite. Thermal blankets and thermal shields are placed at critical locations to provide insulation.
Radiation mirrors are placed around electronic subsystems, particularly for spin-stabilized satellites, to protect critical equipment. Heat pumps are used to relocate heat from power devices such as traveling wave power amplifiers to outer walls or heat sinks to provide a more effective thermal path for heat to escape. Thermal heaters may also be used to maintain adequate temperature conditions for some components, such as propulsion lines or thrusters, where low temperatures would cause severe problems.
The satellite antenna structure is one of the critical components that can be affected by thermal radiation from the sun.
Large aperture antennas can be twisted or contorted as the sun moves around the satellite, heating and cooling various portions of the structure. Telemetry data are received from the other subsystems of the spacecraft, such as the payload, power, attitude control, and thermal control.
Command data are relayed from the command receiver to other subsystems to control such parameters as antenna pointing, transponder modes of operation, battery and solar cell changes, etc. Satellite control and monitoring is accomplished through monitors and keyboard interface. Tracking refers to the determination of the current orbit, position, and movement of the spacecraft.
The Doppler shift of the beacon or the telemetry carrier is monitored to determine the rate at which the range is changing the range rate.
Angular measurements from one or more earth terminals can be used to determine spacecraft location. The range can be determined by observing the time delay of a pulse or sequence of pulses transmitted from the satellite. Acceleration and velocity sensors on the satellite can be used to monitor orbital location and changes in orbital location. The telemetry function involves the collection of data from sensors on-board the spacecraft and the relay of this information to the ground.
A typical communications satellite telemetry link could involve over channels of sensor information, usually in digital form, but occasionally in analog form for diagnostic evaluations. The telemetry carrier modulation is typically frequency or phase shift keying FSK or PSK , with the telemetry channels transmitted in a time division multiplex TDM format.
Telemetry channel data rates are low, usually only a few kbps. Command is the complementary function to telemetry. The command system relays specific control and operations information from the ground to the spacecraft, often in response to telemetry information received from the spacecraft. Security is an important factor in the command system for a communications satellite.
The structure of the command system must contain safeguards against intentional or unintentional signals corrupting the command link, or unauthorized commands from being transmitted and accepted by the spacecraft.
Command links are nearly always encrypted with a secure code format to maintain the health and safety of the satellite. The command procedure also involves multiple transmissions to the spacecraft, to assure the validity and correct reception of the command, before the execute instruction is transmitted. The backup system usually operates with an omnidirectional antenna, at UHF or S-band, with sufficient margin to allow operation in the most adverse conditions. The radiation on a satellite from the sun has an intensity averaging about 1.
Because of this, large numbers of cells, connected in serial-parallel arrays, are required to support the communications satellite electronic systems, which often require more than one to two kilowatts of prime power to function. The spin-stabilized satellite usually has cylindrical panels, which may be extended after deployment to provide additional exposure area. The three-axis stabilized satellite configuration allows for better utilization of solar cell area, because the cells can be arranged in flat panels, or sails, which can be rotated to maintain normal exposure to the sun — levels up to 10kW are attainable with rotating panels.
All spacecraft must also carry storage batteries to provide power during launch and during eclipse periods when sun blockage occurs. Daily eclipses start about 23 days before the equinox, and end the same number of days after. The daily eclipse duration increases a few minutes each day to about a minute peak on equinox day, then decreases a similar amount each day following the peak. Sealed nickel cadmium Ni-Cd batteries are most often used for satellite battery systems.
They have good reliability and long life, and do not outgas when in a charging cycle. Nickel-hydrogen NiH2 batteries, which provide a significant improvement in power-to-weight ratio, are also used. A power conditioning unit is also included in the power subsystem, for the control of battery charging and for power regulation and monitoring. Transponder The transponder in a communications satellite is the series of components that provides the communications channel, or link, between the uplink signal received at the uplink antenna, and the downlink signal transmitted by the downlink antenna.
A typical communications satellite will contain several transponders, and some of the equipment may be common to more than one transponder. Each transponder generally operates in a different frequency band, with the allocated frequency spectrum band divided into slots, with a specified center frequency and operating bandwidth. A typical design would accommodate 12 transponders, each with a bandwidth of 36 MHz, with guard bands of 4MHz between each.
Atypical commercial communications satellite today can have 24 to 48 transponders, operating in the C-band, Ku-band, or Ka-bands. The number of transponders can be doubled by the use of polarization frequency reuse, where two carriers at the same frequency, but with orthogonal polarization, are used. Both linear polarization horizontal and vertical sense and circular polarization right-hand and left-hand sense have been used. Additional frequency reuse may be achieved through spatial separation of the signals, in the form of narrow spot beams, which allow the reuse of the same frequency carrier for physically separate locations on the earth.
The communications satellite transponder is implemented in one of two general types of configurations: Frequency Translation Transponder The first type, which has been the dominant configuration since the inception of satellite communications, is the frequency translation transponder.
The frequency translation transponder, also referred to as a non-regenerative repeater, or bent pipe, receives the uplink signal and, after amplification, retransmits it with only a translation in carrier frequency. The uplinks and downlinks are codependent, meaning that any degradation introduced on the uplink will be transferred to the downlink, affecting the total communications link. This has significant impact on the performance of the end-to-end link On-board Processing Transponder Figure 3.
The uplink signal at fup is demodulated to baseband, fbaseband. The baseband signal is available for processing on-board, including reformatting and error-correction.
Thus the uplinks and downlinks are independent with respect to evaluation of overall link performance, unlike the frequency translation transponder where uplink degradations are codependent, as discussed earlier. On-board processing satellites tend to be more complex and expensive than frequency translation satellites; however, they offer significant performance advantages, particularly for small terminal users or for large diverse networks.
The TWTA is a slow wave structure device, which operates in a vacuum envelope, and requires permanent magnet focusing and high voltage DC power supply support systems. The major advantage of the TWTA is its wide bandwidth capability at microwave frequencies. SSPAs are used when power requirements in the 2—20 watt region are required.
SSPAs operate with slightly better power efficiency than the TWTA, however both are nonlinear devices, which directly impacts system performance, as we shall see when RF link performance is discussed in later chapters. Other devices may be included in the basic transponder configurations of Figures 3. Each device must be considered when evaluating the signal losses and system performance of the space segment of the satellite network.
The antenna system is a critical part of the satellite communications system, because it is the essential element in increasing the strength of the transmitted or received signal to allow amplification, processing, and eventual retransmission.
The most important parameters that define the performance of an antenna are antenna gain, antenna beamwidth, and antenna sidelobes. The antenna gain is usually expressed in dBi, decibels above an isotropic antenna, which is an antenna that radiates uniformly in all directions. The beamwidth is usually expressed as the half-power beamwidth or the 3-dB beamwidth, which is a measure of the angle over which maximum gain occurs. The sidelobes define the amount of gain in the off-axis directions.
Most satellite communications applications require an antenna to be highly directional high gain, narrow beamwidth with negligibly small sidelobes. The common types of antennas used in satellite systems are the linear dipole, the horn antenna, the parabolic reflector, and the array antenna. The linear dipole antenna is an isotropic radiator that radiates uniformly in all directions. Four or more dipole antennas are placed on the spacecraft to obtain a nearly omni-directional pattern.
Dipole antennas are also important during launch operations, where the spacecraft attitude has not yet been established, and for satellites that operate without attitude control or body stabilization particularly for LEO systems. Horn antennas are used at frequencies from about 4 GHz and up, when relatively wide beams are required, such as global coverage from a GSO satellite. If higher gains or narrower bandwidths are required, a reflector or array antenna must be used.
The most often used antenna for satellite systems, particularly for those operating above 10 GHz, is the parabolic reflector antenna. Parabolic reflector antennas are usually illuminated by one or more horn antenna feeds at the focus of the paroboloid. Parabolic reflectors offer a much higher gain than that achievable by the horn antenna alone. Narrow beam antennas usually require physical pointing mechanisms gimbals on the spacecraft to point the beam in the desired direction.
There is increasing interest in the use of array antennas for satellite communications applications. A steerable, focused beam can be formed by combining the radiation from several small elements made up of dipoles, helices, or horns.
Beam forming can be achieved by electronically phase shifting the signal at each element. Proper selection of the phase characteristics between the elements allows the direction and beamwidth to be controlled, without physical movement of the antenna system. The array antenna gain increases with the square of the number of elements. Gains and beamwidths comparable to those available from parabolic reflector antennas can be achieved with array antennas. Once a satellite is in geo stationary orbit, there is little possibility of repairing components that fail or adding more fuel for station keeping.
The components that make up the satellite must therefore have very high reliability in the hostile environment of outer space, and a strategy must be devised that allows some components to fail without causing the entire communication capacity of the satellite to be lost. Two separate approaches are used: Space Qualification: Outer space, at geostationary orbit distances is a harsh environment.
There is a total vacuum and the sun irradiates the satellite with 1. Electronic equipment cannot operate at such extremes of temperature and must be housed within the satellite and heated or cooled so that its temperature stays within 0 0 the range 0 to 75 C. This requires a thermal control system that manages heat flow throughout a GEO satellite as the sun moves around once every 24hr.
When a satellite is designed, three prototype models are often built and tested. The thermal model contains all the electronics packages and other components that must be maintained at correct temperature. The electrical model contains all electronic parts of the satellite and is tested for correct electrical performance under total vacuum and a wide range of temperatures. Many of the electronic and mechanical components that are used in satellite are known to have limited life times, or a finite probability of failure.
If failure of one of these components will jeopardize the mission or reduce the communication capacity of the satellite, a backup, or redundant, unit will provided. The design of the system must be such that when one unit fails, the backup can automatically take over or be switched into operation by a command from the ground.
Reliability Reliability is counted by considering the proper working of satellites critical components. Reliability could be improved by making the critical components redundant. Components with a limited lifetime such as travelling wave tube amplifier etc should be made redundant.
The main attraction of these devices is their very high gain dB , linear characteristics and octave bandwidth. They are quite widely used professionally, but are still rather scarce in amateur circles. This article describes a little of the theory of twts, and explains how to use them, in the hope that more amateurs may be able to acquire and use these fascinating components. When used as receiver RF amplifiers they are characterized by high gain, low noise figure and wide bandwidth, and are known as low noise amplifiers LNAS.
These usually come with tube, mount and power supply in one integral unit, with no external adjustments to make-just input socket, output socket and mains supply connections. A typical LNA has an octave bandwidth eg GHz , 30 dB gain, 8 dB noise figure, and a saturated power output of 10 mW, within a volume of 2 in by 2 in by 10 in. Transmitter TWTAs are naturally somewhat bulkier, and often have the poweror supplies as a separate unit. Medium-power tubes have outputs of up to about 10 W, while high-power tubes deliver several hundred watts.
Such tubes have gains of the order of 30 or 40 dB, and bandwidths of up to an octave. The failure rate for all components is calculated and they are categorized into the following three categories: Certainly early failures criteria is eliminated as most of the components are tested before used in the satellite.
Random failures are more seen. They could be reduced by using reliable engineering techniques. The life-spam of component could be increased by improving manufacturing techniques and the type of material used to reduce the number of worn out parts and hence reducing the high failure rate criteria.
It is sent that the failure rate is constant over time and is looking at this reliability can be determined. The system is made of several components, connected in a series, then the overall reliability is determined.
By duplicating the less reliable and critical components, the overall reliability of the system could be improved. If any failure occurs in operational unit, then the standby unit takes over to develop a system with redundant components, its redundant elements are considered in parallel.
Parallel redundancy is useful when the reliability of an individual sub-system is high. If Qi is the unreliability of the ith parallel element, then the probability that all units will fail is the product of the individual un-reliabilities: By doing a complete failure analysis, one could find out which failure occurs more than the rest and such analysis help in finding out the manufacturing defects in the product of a given batch of components or probably a design defect.
This analysis is done to reduce the overall reliability to a value less than that predicted by the above analysis. Co-related failures could also be reduced by using units from different manufacturers. The design defects are generic to all satellite produced in a series.
Generally these defects are detected and corrected to minimize their impact. This is done when a complete design change cannot be implemented.
Even through the reliability can be improved by adding redundant devices and components, the weight of the satellite increases which again becomes a problem. Redundant component also increase the cost of the satellite.
The two major cost components are: Optimization techniques are performed for cost minimization purpose. The parallel connection of two TWTs as shown above raises the reliability of the amplifier stage to 0. To further improve the reliability of the transponder, a second redundant transponder may be provided with switching between the two systems.
Note that a combination of parallel and switched redundancy is used to combat failures that are catastrophic to one transponder channel and to the complete communication system. References 1. Principles and Trends, Pearson Education 4.
Rappaort, Wireless Communications Principals and Practices 5. Nicopolitidis ,Wireless Networks, John Wiley 7. The basic communications link, shown in Figure 4. The parameters of the link are defined as: The radiowave is characterized by variations of its electric and magnetic fields.
The oscillating motion of the field intensities vibrating at a particular point in space at a frequency f excites similar vibrations at neighbouring points, and the radiowave is said to travel or to propagate. The frequency and wavelength in free space are related by Where c is the phase velocity of light in a vacuum.
The wave is isotropic in space, i. This relationship demonstrates the well-known inverse square law of radiation: Effective Isotropic Radiated Power An important parameter in the evaluation of the RF link is the effective isotropic radiated power, eirp. The eirp, using the parameters introduced in Figure 4. The pfd is an important parameter in the evaluation of power requirements and interference levels for satellite communications networks.
Antenna Gain Isotropic power radiation is usually not effective for satellite communications links, because the power density levels will be low for most applications there are some exceptions, such as for mobile satellite networks, some directivity gain is desirable for both the transmit and receive antennas. Consider first a lossless ideal antenna with a physical aperture area of A m2. Physical antennas are not ideal — some energy is reflected away by the structure, some energy is absorbed by lossy components feeds, struts, subreflectors.
Therefore, Note also that the effective aperture can be expressed as The aperture efficiency for a circular parabolic antenna typically runs about 0. Circular Parabolic Reflector Antenna The circular parabolic reflector is the most common type of antenna used for satellite earth station and spacecraft antennas.
It is easy to construct, and has good gain and beamwidth characteristics for a large range of applications. The physical area of the aperture of a circular parabolic aperture is given by where d is the physical diameter of the antenna. The boresight direction refers to the direction of maximum gain, for which the value g is determined from the above equations. The antenna pattern shows the gain as a function of the distance from the boresight direction.
Most antennas have sidelobes, or regions where the gain may increase due to physical structure elements or the characteristics of the antenna design.
It is also possible that some energy may be present behind the physical antenna reflector. Sidelobes are a concern as a possible source for noise and interference, particularly for satellite ground antennas located near to other antennas or sources of power in the same frequency band as the satellite link.
Free-Space Path Loss Consider now a receiver with an antenna of gain gr located a distance r from a transmitter of pt watts and antenna gain gt, as shown in Figure 4. Replacing Ae with the representation A rearranging of terms describes the interrelationship of several parameters used in link analysis: Basic Link Equation for Received Power We now have all the elements necessary to define the basic link equation for determining the received power at the receiver antenna terminals for a satellite communications link.
We refer again to the basic communications link Figure 4. Noise temperature is useful concept in communication receivers, since it provides a way of determining how much thermal noise is generated by active and passive devices in the receiving system. At microwave frequencies, a black body with a physical temperature, Tp degrees kelvin, generates electrical noise over a wide bandwidth.
The term kTp is a noise power spectral density, in watts per hertz. We need a way to describe the noise produced by the components of a low noise receiver.
This can conveniently be done by equating the components to a black body radiator with an equivalent noise temperature, Tn kelvins.
To determine the performance of a receiving system we need to be able to find the total thermal noise power against which the signal must be demodulated. We do this by determining the system noise temperature, Ts.
Ts is the noise temperature of a noise source, located at the input of a noiseless receiver, which gives the same noise power as the original receiver, measured at the output of the receiver and usually includes noise from the antenna. The signal power at the demodulator input is PrGrx watts, representing the power contained in the carrier and sidebands after amplification and frequency conversion within the receiver. This is convenient, because a link budget will find Pr at this point.
Using a single parameter to encompass all of the sources of noise in receiving terminals is very useful because it replaces several sources of noise in the receiver by a single system noise temperature, Ts. Calculation of system Noise Temperature The above figure shows a simplified communication receiver with an RF amplifier and single frequency conversion, from its RF input to the IF output.
This is the form used for all radio receivers with few exceptions, known as the superhet.
The superhet receiver has three main subsystems: The RF amplifier in a satellite communications receiver must generate as little noise as possible, so it is called a low noise amplifier or LNA. The mixer and local oscillator from a frequency conversion stage that downconverts the RF signal to a fixed intermediate frequency IF , where the signal can be amplified and filtered accurately.
Tin is the noise temperature of the antenna, measured at its output port. Noise figure and noise source Noise figure is frequently used to specify the noise generated within a device.
Because noise temperature is more useful in satellite communication system, it is best to convert noise figure to noise temperature, Td. Design of downlink: Linear transponder: Ku band 80 W Transponder bandwidth: Ku band 54 MHz Signal: Compressed digital video signals with transmitted symbol rate of We will first find the noise power in thc transponder for At Hence Pt - Hence the receiving antenna must have a gain Gr,where Gr - Voice, Data, Video: Communications satellites are used to carry telephone, video, and data signals, and can use both analog and digital modulation techniques.
The modulation and multiplexing techniques that were used at this time were analog, adapted from the technology developed for The change to digital voice signals made it easier for long-distance. Primarily for video provided that a satellite link's overall carrier-to-noise but in to older receiving equipment at System and Satellite Specification Ku-band satellite parameters.
Modulation And Multiplexing: In analog television TV transmission by satellite, the baseband video signal and one or two audio subcarriers constitute a composite video signal. Digital modulation is obviously the modulation of choice for transmitting digital data are digitized analog signals may conveniently share a channel with digital data, allowing a link to carry a varying mix of voice and data traffic.
Hybrid multiple access schemes can use time division multiplexing of baseband channels which are then modulate. Analog — digital transmission system: Analog vs. Digital Transmission: Compare at two levels: Data—continuous audio vs. Signaling—continuously varying electromagnetic wave vs. Also Transmission—transmit without regard to signal content vs.
Difference in how attenuation is handled, but not focus on this. Seeing a shift towards digital transmission despite large analog base. Repeaters take out cumulative problems in transmission. Can thus transmit longer distances. Can integrate voice, video and digital data. Must convert digital data to analog signal such device is a modem to translate between bit-serial and modulated carrier signals?
To send digital data using analog technology, the sender generates a carrier signal at some continuous tone e. The following techniques are used to encode digital data into analog signals. Used to transmit digital data over optical fiber. Used in a full-duplex modem signals in both directions. For instance, the wave could be shifted by 45, , , degree at each timing mark. In this case, each timing interval carries 2 bits of information.
Why not shift by 0, 90, , ? Shifting zero degrees means no shift, and an extended set of no shifts leads to clock synchronization difficulties. Frequency division multiplexing FDM: Divide the frequency spectrum into smaller subchannels, giving each user exclusive use of a subchannel e.
One problem with FDM is that a user is given all of the frequency to use, and if the user has no data to send, bandwidth is wasted — it cannot be used by another user.
Time division multiplexing TDM: Use time slicing to give each user the full bandwidth, but for only a fraction of a second at a time analogous to time sharing in operating systems. Statistical multiplexing: Allocate bandwidth to arriving packets on demand. This leads to the most efficient use of channel bandwidth because it only carries useful data. Digital Video Broadcasting DVB has become the synonym for digital television and for data broadcasting world-wide.
This article aims at describing what DVB is all about and at introducing some of the technical background of a technology that makes possible the broadcasting. Multiple access is a technique in which the satellite resource bandwidth or time is divided into a number of nonoverlapping segments and each segment is allocated exclusively to each of the large number of earth stations who seek to communicate with each other. There are three known multiple access techniques. They are: In FDMA, the available satellite bandwidth is divided into portions of non-overlapping frequency slots which are assigned exclusively to individual earth stations.
Because of the nonlinear mode of the transponder, FDMA signals interact with each other causing intermodulation products intermodulation noise which are signals at all combinations of sum and difference frequencies as shown in the example given in Fig.
The power of these intermodulation products represents a loss in the desired signal power. In addition, if these intermodulation products appear within the bandwidth of the other signals, they act as interference for these signals and as a result the BER performances will be degraded.
The other major disadvantage of the FDMA system is the need for accurate uplink power control among network stations in order to mitigate the weak signal suppression effect caused by disproportionate power sharing of the transponder power. Intermodulation Intermodulation products are generated whenever more than one signal is carried by nonlinear device.
Sometimes filtering can be used to remove the IM products, but if they are within the bandwidth of the transponder they cannot be filtered out. The saturation characteristic of a transponder can be modeled by a cubic curve to illustrate the generation of third —order intermodulation. Third -order IM is important because third —order products often have frequencies close to the signals that generate the intermodulation, and are therefore likely to be within the transponder bandwidth.
Intermodulation Example Consider the case of a 36 -MHz bandwidth C -band transponder which has an output spectrum for downlink signals in the frequency range MHz. The transponder carries two unmodulated carriers at and MHz with equal magnitudes at the input to the HPA. Using Eq.