Satellite Communication Seminar and PPT with PDF Report: The outer space has every time enchanted people on the earth and the exchange. ronaldweinland.info A. Seminar report. On. Satellite Communications. Submitted in partial fulfillment of the requirement for the award of degree of Electronics. PDF | Al-Ahliyya Amman University Electronics and Communication Engineering.
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Page |1 CONTENTS Serial no. Topics Page No. Chapter 1: History of satellites 1 Introduction Satellite Communication 2 Advantages of Satellite. Othet Reports on Recent Advances in Networking · Back to Raj Satellite communications are comprised of 2 main components: The Satellite. Project Report on Satellite Communication - Free download as Word Doc .doc), PDF File .pdf), Text File .txt) or read online for free.
The outer space has every time enchanted people on the earth and the exchange of information through space developed as an offshoot of suggestions for the space travel. The ancient suggestion of availing artificial satellites for the purpose of exchanging information was found in a science fiction called Brick Moon which was published by the Evert Hale in the year of In the field of satellite communications, the applied science has been suitable to the implausible dreams and it is also expected that the new inventions of technology will lead the development of satellite communications towards the vision of the present day. Also See: The Merriam-Webster dictionary explains satellite as a heavenly body orbiting other heavenly or celestial bodies of larger size or it is a vehicle intended to earth or it is a vehicle intended to orbit the moon.
Beginning with the Mars Exploration Rovers , landers on the surface of Mars have used orbiting spacecraft as communications satellites for relaying their data to Earth. The landers use UHF transmitters to send their data to the orbiters, which then relay the data to Earth using either X band or Ka band frequencies. These higher frequencies, along with more powerful transmitters and larger antennas, permit the orbiters to send the data much faster than the landers could manage transmitting directly to Earth, which conserves valuable time on the NASA Deep Space Network.
This orbit has the special characteristic that the apparent position of the satellite in the sky when viewed by a ground observer does not change, the satellite appears to "stand still" in the sky. This is because the satellite's orbital period is the same as the rotation rate of the Earth. The advantage of this orbit is that ground antennas do not have to track the satellite across the sky, they can be fixed to point at the location in the sky the satellite appears.
As satellites in MEO and LEO orbit the Earth faster, they do not remain visible in the sky to a fixed point on Earth continually like a geostationary satellite, but appear to a ground observer to cross the sky and "set" when they go behind the Earth. Therefore, to provide continuous communications capability with these lower orbits requires a larger number of satellites, so one will always be in the sky for transmission of communication signals.
However, due to their relatively small distance to the Earth their signals are stronger. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity. Low-Earth-orbiting satellites are less expensive to launch into orbit than geostationary satellites and, due to proximity to the ground, do not require as high signal strength Recall that signal strength falls off as the square of the distance from the source, so the effect is dramatic.
Thus there is a trade off between the number of satellites and their cost.
In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions. Main article: Satellite constellation A group of satellites working in concert is known as a satellite constellation. Two such constellations, intended to provide satellite phone services, primarily to remote areas, are the Iridium and Globalstar systems.
The Iridium system has 66 satellites. It is also possible to offer discontinuous coverage using a low-Earth-orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. Another system using this store and forward method is Orbcomm. MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 and 8 hours.
One disadvantage is that a MEO satellite's distance gives it a longer time delay and weaker signal than a LEO satellite, although these limitations are not as severe as those of a GEO satellite.
Like LEOs, these satellites do not maintain a stationary distance from the earth. In various patterns, these satellites make the trip around earth in anywhere from 2 to 8 hours.
Example[ edit ] In , the first communications satellite, Telstar, was launched. It was a medium earth orbit satellite designed to help facilitate high-speed telephone signals. Although it was the first practical way to transmit signals over the horizon, its major drawback was soon realized. Because its orbital period of about 2. It was apparent that multiple MEOs needed to be used in order to provide continuous coverage.
Geostationary orbit GEO [ edit ] Geostationary orbit To an observer on Earth, a satellite in a geostationary orbit appears motionless, in a fixed position in the sky.
This is because it revolves around the Earth at Earth's own angular velocity one revolution per sidereal day , in an equatorial orbit. A geostationary orbit is useful for communications because ground antennas can be aimed at the satellite without their having to track the satellite's motion.
This is relatively inexpensive. In applications that require a large number of ground antennas, such as DirecTV distribution, the savings in ground equipment can more than outweigh the cost and complexity of placing a satellite into orbit.
Examples[ edit ] The first geostationary satellite was Syncom 3 , launched on August 19, , and used for communication across the Pacific starting with television coverage of the Summer Olympics. Fig 2.
Satellite Subsystem Within the satellite are two major sections: The bus is a metal or composite frame on which the other elements are mounted. Because it bears the stresses of launch, the bus is generally resilient. It may be painted with reflective paint to limit the solar heat it absorbs, which could also provide some protection from laser attacks.
Within the bus we find: Telemetry The telemetry system collects data from many sensors within the spacecraft and sends these data to the controlling earth station. Typically as many as sensors may be located on the spacecraft to monitor pressure in the fuel tanks, voltage and current in the power conditioning unit, current drawn by each subsystem, and critical voltages and current sin the communications electronics.
The sighting devices used to maintain spacecraft attitude are also monitored via the telemetry link: A low data rate is normally used to allow the receiver at the earth station to have a narrow bandwidth and thus maintain a high carrier-to-noise ratio. The entire TDM frame may contain thousands of bits of data and take several seconds to transmit. At the earth station a computer is used to monitor, store and decode the telemetry data so that the status of any system or sensor on the satellite can be determined immediately by the controller on earth.
Tracking A number of techniques can be used to determine the current orbit of a spacecraft. Velocity and acceleration sensors on the spacecraft can be used to establish the change in orbit from the last known position, by integration of the data. Together with accurate angular measurements from the earth station antenna, range is used to determine the orbital elements.
Active determination of range can be achieved by transmitting a pulse or sequence of pulses to the satellite and observing the time delay before the pulse is received again.
If a sufficient number of earth stations with an adequate separation are observing the satellite, its position can be established by triangulation from the earth station look angles or by simultaneous range measurements. The command system is used to make changes in attitude and corrections to the orbit and to control the communication system.
During launch, it is used to control the firing of the apogee boost motor and to spin up a spinner or extend the solar sails of a three axis stabilized spacecraft.
The control word is converted into a command word, which is sent in a TDM frame to the satellite. After checking for validity in the spacecraft, the word is sent back to the control station via the telemetry link where it is checked again in the computer. If it is found to have been received correctly, an execute instruction will be sent to the satellite so that the command is executed.
Technological improvements in battery technology have led to new battery types with high specific energy energy stored per unit mass and high reliability. Power source Solar cells are mounted on the body of a satellite or on flat panels. The solar panels often have a large surface area compared with the rest of the satellite, so they sustain a relatively large number of collisions with debris particles.
Solar panels are fragile and can be damaged easily, but partial damage to a solar panel may not disable the satellite. Satellites often can continue to function with partially working solar panels, albeit with diminished capacity. However, if the solar panels fail to deploy or are torn off, a satellite without another power source would cease functioning fairly quickly.
A malfunction of the power distribution system could also totally impair the satellite. Page 9 2. If the propulsion system does not function, because of damage or lack of propellant, the satellite may still be functional. However, in orbits dense with other satellites, such as geostationary orbit, satellites must be able to maintain their position very accurately or they will be a danger to their neighbors and to themselves. Spacecraft attitude determination is the process of estimating the orientation of a spacecraft by making remote observations of other celestial bodies or reference points.
Combinations of these sensor observations are used to generate a more accurate estimate of spacecraft rotational attitude. Attitude estimates must be calculated quickly and continuously during the entire operational life of the mission.
During normal operations, the problem is recursive—the attitude filter basing new predictions on present and prior sensor information. The attitude filter must also estimate from activation when the spacecraft is first initiated and no prior data is available.
Attitude determination The systems designed to carry out 3-axis attitude determination are inevitably complex, but must still be designed with the utmost care to perform the task as reliably as possible. Any, even temporary malfunction is potentially serious, damaging fragile instruments, breaking communications links, upsetting measurements and disrupting power generation.
In addition to the communication equipment needed to operate the satellite, a satellite may carry similar equipment for other tasks. It may carry a radio antenna to collect radio signals, such as telephone or television signals, and to relay or rebroadcast them. The antenna serves to receive and transmit signals. It may be a parabolic dish similar to satellite TV dishes , a feed horn a conical or cowbell shaped structure , or a minimal metal construction similar to a rooftop TV antennae.
Radio receiver, transmitter and transponder When a system is designed to automatically receive a transmission, amplify it, and send it back to Earth, possibly at a different frequency, it is called a transponder. A satellite-based radar system is also composed in part of transmitters and receivers used to send and then receive the radio waves.
Receivers are also used by the military for signals intelligence i. Similarly, a satellite may carry transmitters to send out radio signals, such as the navigation signals from the Global Positioning System. A satellite may be designed to transmit a signal to a specific receiver on the Earth, or to broadcast it over a large area. The whole point of the thermal system is to regulate the temperature of the satellite's components.
Too hot or too cold, or too great a swing in temperature will prematurely end the useful life of a satellite.
This system dissipates the heat away from earth, out into space, so as not to interfere with the satellite's operation. The payload is the business-end of the satellite, consisting of: The antennas create "footprint" coverage but require the repeater to receive and transmit the actual signals from and to the ground. The on-board computer monitors the state of the satellite subsystems, controls its actions, and processes data.
High-value satellites may incorporate sophisticated anti-jamming hardware that is operated by the computer. Computer systems are also sensitive to their electromagnetic environment and may shut down or reboot during solar storms or if barraged by high levels of electromagnetic radiation.
Satellites are monitored and controlled from their ground stations. One type of ground station is the control station, which monitors the health and status of the satellite, sends it commands of various kinds, and receives data sent by the satellite. The antenna that the control station uses to communicate with the satellite may be located with the station, but it need not be: Satellites may also have other types of ground stations.
Military communications satellites have ground stations that range from large, permanent command headquarters to small, mobile field terminals.
Ground stations are generally not highly protected from physical attack. Disabling a control station may have an immediate disruptive effect, but the disruption can be reduced by having redundant capabilities, such as alternate control centers.
Computers at control centers may be vulnerable to attack and interference, especially if they are connected to the Internet. However, high value command computers will have high security, and many of the military command center computers are isolated from the Internet. The coordinate system used is called geocentric coordinate system. The origin of the system is at the centre of the earth.
This is the direction of a line from the center of the earth through the center of sun at vertical equinox, the instant when the sub solar point crosses the equator south to north.
The coordinate system moves through space along with the revolution of earth.
Fig 3. A satellite's position at a specific time can be determined using seven distinct orbit characteristics named the "Keplerian orbit elements". These orbit elements define the satellite orbit's orientation with respect to the Earth, the satellite's last known position within its orbit, the shape of the satellite's orbit and the satellite's orbital speed. Orbital elements are used to specify the absolute or inertial coordinates of the satellite at time t.
The set commonly used in satellite communications is — 1. Eccentricity e 2. Semi major axis a 3. Inclination i 4. Mean anomaly at epoch M 7. Look angles The main two elements that define the shape and size of the ellipse: The Eccentricity e defines how oval the satellite's orbit is. It is mathematically defined as the ratio of the orbit's focus distance c to the orbit's semi-major axis a. The eccentricity of a satellite orbit is a unit less value that lies between 0 circular orbit and 1 parabolic orbit.
Eccentricity 3. Semi major axis a is the sum of the perigee and apogee distances divided by two. For circular orbits, the semi major axis is the distance between the centers of the bodies, not the distance of the bodies from the center of mass. Two elements define the orientation of the orbital plane in which the ellipse is embedded: Inclination i: The angle that the orbital plane makes with the equatorial plane is the inclination angle i. It can also be thought as the vertical tilt of the ellipse with respect to equatorial plane reference plane.
The two points at which the orbit penetrates the equatorial plane are called nodes. Vertical tilt of the ellipse with respect to the reference plane, measured at the ascending node where the orbit passes upward through the reference plane. It defines the orientation of the ellipse in the orbital plane, as an angle measured from the ascending node to the perigee the closest point the second body comes to the first during an orbit. Orbital parameters In this diagram, the orbital plane yellow intersects a reference plane gray.
For earth-orbiting satellites, the reference plane is usually the Earth's equatorial plane, and for satellites in solar orbits it is the ecliptic plane. The intersection is called the line of nodes, as it connects the center of mass with the ascending and descending nodes. Mean Anomaly M: The Mean Anomaly indicates where the satellite was located within its orbit at a particular Epoch. The Mean Anomaly at any time t, M t , can be determined by adding the last known Mean Anomaly, Mo, to the orbit's Mean Motion multiplied by the time that has elapsed t - to: P a g e 17 For a perfectly circular orbit Eccentricity of 0 , the Mean Anomaly is exactly equal to the True Anomaly throughout the orbit.
The Mean Anomaly can range anywhere from 0 to degrees. The coordinate to which the earth station antennas must be pointed to communicate with a satellite are called look angles. These are — a Azimuth angle Az: The angle measured eastward from geographic north to the projection of the satellite path on a local horizontal plane on earth station.
A so called footprint can be defined as the area on earth where the signals of the satellite can be received.
An inclination angle of 0 degrees means that the satellite is exactly above the equator. Azimuthal Angle Fig 3. If the earth and the satellite are considered as point masses influenced only by mutual gravitational attraction, then Keplerian Orbit results.
These interfering forces cause the true orbit to be different from a Keplerian ellipse. The effect of this term is to cause an unconstrained geosynchronous satellite to drift toward and circulate around the nearer of two stable points. Gravitational attraction by the sun and moon causes the orbital inclination of a geosynchronous satellite to change with time. If not countered by north-south station keeping, these forces would increase the orbital inclination from an initial 0o at launch to Satellite Orbits 4.
This unit discusses the basics of satellite and elaborating the parameters which are needed to calculate the distance of an orbit to which a satellite is to be launched and the other factors which are necessary to define an orbit. Further this unit discusses the applications of satellites and elaborates on the global communication which has now become possible due to the presence of satellites.
Going further, this unit also elaborates on the types of orbits a satellite can follow to provide communication. Satellites orbit around the earth. Depending on the application, these orbits can be circular or elliptical. The orbit of a satellite is an ellipse with the centre of the earth at one of the foci.
The line joining the centre of the earth and the satellite sweeps over equal areas in equal intervals of time. The square of the orbital period of 2 satellites have the same ratio as the cubes of the mean distance from the centre of the earth, i. The attractive force Fg of the earth due to gravity equals: The centrifugal force Fc trying to pull the satellite away equals.
Looking at this equation the first thing to notice is that the mass m of a satellite is irrelevant it appears on both sides of the equation. Solving the equation for the distance r of the satellite to the centre of the earth results in the following equation: Fig 4.
All satellites today get into orbit by riding on a rocket or in the cargo bay of the space shuttle. Satellites have no independent means of breaking through the atmosphere and reaching space, so they require an external vehicle, such as a rocket or space shuttle, to carry them to their orbit. Several countries and businesses have rocket launch capabilities, and satellites as large as several tons make it into orbit regularly and safely.
For most satellite launches, the scheduled launch rocket is aimed straight up at first.
This gets the rocket through the thickest part of the atmosphere most quickly and best minimizes fuel consumption. After a rocket launches straight up, the rocket control mechanism uses the inertial guidance system to calculate necessary adjustments to the rocket's nozzles to tilt the rocket to the course described in the flight plan.
The IGS determines a rocket's exact location and orientation by precisely measuring all of the accelerations the rocket experiences, using gyroscopes and accelerometers. Mounted in gimbals, the gyroscopes' axes stay pointing in the same direction.
This gyroscopically stable platform contains accelerometers that measure changes in acceleration on three different axes. If it knows exactly where the rocket was at launch and the accelerations the rocket experiences during flight, the IGS can calculate the rocket's position and orientation in space.
In most cases, the flight plan calls for the rocket to head east because Earth rotates to the east, giving the launch vehicle a free boost. The strength of this boost depends on the rotational velocity of Earth at the launch location. The boost is greatest at the equator, where the distance around Earth is greatest and so rotation is fastest. P a g e 21 Once the rocket reaches extremely thin air, at about miles kilometres up, the rocket's navigational system fires small rockets, just enough to turn the launch vehicle into a horizontal position.
The satellite is then released. At that point, rockets are fired again to ensure some separation between the launch vehicle and the satellite itself. Most ELV Expendable Launch Vehicles launchers put the satellite in an inclined elliptical orbit called a transfer orbit with an apogee at geosynchronous altitude and a perigee. At the transfer orbit apogee, a rocket engine called the Apogee Kick Motor AKM puts the satellite into a circular geosynchronous orbit with ideally zero inclination.
The AKM must be capable of increasing the satellite velocity from 1. Placing of a satellite in orbit 4. A Low Earth orbit LEO is an orbit around Earth with an altitude between kilometres 99 mi , with an orbital period of about 88 minutes, and 2, kilometres 1, mi , with an orbital period of about minutes.
Objects below approximately kilometres 99 mi will experience very rapid orbital decay and altitude loss. With the exception of the manned lunar flights of the Apollo program, all human spaceflights have taken place in LEO. These satellites in this orbit are placed kilometres above the surface of the earth. As LEOs circulate on a lower orbit, hence they exhibit a much shorter period that is 95 to minutes. LEO systems try to ensure a high elevation for every spot on earth to provide a high quality communication link.
Each LEO satellite will only be visible from the earth for around ten minutes. The orbital velocity needed to maintain a stable low earth orbit is about 7. The delta-v needed to achieve low earth orbit starts around 9. It is a scalar that has the units of speed. Delta-v is produced by the use of propellant by reaction engines to produce a thrust that accelerates the vehicle.
Atmospheric and gravity drag associated with launch typically adds 1.
Present day mobile communication systems use LEO satellites. The advantages of LEO satellites are: The delay for packets delivered via a LEO is relatively low approximately 10 milliseconds.
The delay is comparable to long-distance wired connections about 5—10 milliseconds. Link diversity is better and satellites are relatively small.
LEOs provide this bandwidth for mobile terminals with Omni-directional antennas using low transmit power in the range of 1W. Smaller footprints of LEOs allow for better frequency reuse, similar to the concepts used for cellular networks. LEOs can provide a much higher elevation in Polar Regions and so better global coverage.
Much less power is required for satellite communication because of reduced height. These satellites are mainly used in remote sensing an providing mobile communication services 7. The LEO satellites less area coverage saves bandwidth. The disadvantages of LEO satellites are: The biggest problem of the LEO concept is the need for many satellites if global coverage is to be reached. Several concepts involve 50— or even more satellites in orbit.
The short time of visibility with a high elevation requires additional mechanisms for connection handover between different satellites. The high number of satellites combined with the fast movements resulting in a high complexity of the whole satellite system. Objects in LEO encounter atmospheric drag in the form of gases in the thermosphere approximately km up or exosphere approximately km and up , depending on orbit height.
There is a need for routing of data packets from satellite to another satellite if a user wants to communicate around the world, due to small footprint. Due to the large footprint, a GEO typically does not need this type of routing, as senders and receivers are most likely in the same footprint.
Examples of LEO are: A Sun-synchronous orbit sometimes called a heliosynchronous orbit is a geocentric orbit which combines altitude and inclination in such a way that an object on that orbit ascends or descends over any given Earth latitude at the same local mean solar time.
These satellites rise and set with the sun. Their orbit is defined in such a way that they are always facing the sun and hence they never go through an eclipse.
For these satellites, the surface illumination angle will be nearly the same every time. Surface illumination angle is the angle between the inward surface normal and the direction of light. This means that the illumination angle of a certain point of the Earth's surface is zero if the Sun is precisely overhead and that it is 90 degrees at sunset and at sunrise.
This consistent lighting is a useful characteristic for satellites that image the Earth's surface in visible or infrared wavelengths e. Examples of Sun-Synchronous Satellites: Medium Earth orbit MEO , sometimes called intermediate circular orbit ICO , is the region of space around the Earth above Low Earth orbit altitude of 2, kilometres 1, mi and below geostationary orbit altitude of 35, kilometres 22, mi. The satellites in these orbits are can be positioned somewhere between LEOs and GEOs, both in terms of their orbit and due to their advantages and disadvantages.
Period of revolution of a satellite is 12 hours. Using orbits around 10, km, the system only requires a dozen satellites which is more than a GEO system, but much less than a LEO system. Depending on the inclination, a MEO can cover larger populations, so requiring few handovers. Minimum 4 satellites are always visible from the Earth. Two frequencies are used for transmission: The latter is reserved for secure operations. Advantages of MEO satellites: It gives uniform global coverage and link diversity can be employed.
Main purpose is position fixing, but velocity and acceleration can also be determined which is not possible with GEO. Disadvantages of MEO satellites: Propagation delay is more than LEO. Doppler offset is larger. Ground segment and network control is more complex than GEO. The signal is weaker than LEO. Example of MEO satellites: GPS, Oddyssey, etc. A geosynchronous orbit sometimes abbreviated GSO is an orbit around the Earth with an orbital period of one sidereal day approximately 23 hours 56 minutes and 4 seconds , matching the Earth's sidereal rotation period.
The synchronization of rotation and orbital period means that, for an observer on the surface of the Earth, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky traces out a path, typically in the form of an analemma, whose precise characteristics depend on the orbit's inclination and eccentricity.
A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit at zero inclination that is, directly above the equator. A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to ground observers.
Popularly or loosely, the term "geosynchronous" may be used to mean geostationary. Specifically, Geosynchronous Earth orbit GEO may be a synonym for geosynchronous equatorial orbit, or geostationary earth orbit. A geostationary orbit is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately 42, km 26, mi measured from the centre of the Earth.
P a g e 26 There is a difference between the geostationary and geosynchronous orbits. Sometimes we send a satellite in the space which though has a period of revolution is equal to period of rotation of earth, but its orbit is neither equatorial nor Circular. So, this satellite will finish one revolution around the earth in exactly one day i.
It looks oscillating but NOT stationary and that is why it is called Geosynchronous. Features of a geosynchronous satellite: The orbit is NOT circular 2. The orbit is NOT in equatorial plane i. The angular velocity of the satellite is equal to angular velocity of earth 4. Period of revolution is equal to period of rotation of earth.
Finish one revolution around the earth in exactly one day i. There are many geosynchronous orbits.