01-03-2013, 12:44 PM
SATELLITE COMMUNICATIONS
[attachment=52298]
ABSTRACT
A satellite is any object that orbits another object. In popular usage, the term 'satellite' normally refers to an artificial satellite. A planet, or any other body, which finds itself at any distance from the sun with no "sideways" velocity will quickly fall without missing the sun. Only our sideways motion saves us. The planet, which is at a larger distance, requires longer falling to where it would strike the sun. As a result, it takes a longer time to complete the ¼ trip around the sun, which is necessary to make a circular orbit. A satellite orbiting at an altitude of 22,300 miles would require exactly 24 hours orbiting the earth. Hence such an orbit is called "geosynchronous" or "geostationary." Both radio and television frequency signals can propagate directly from transmitter to receiver. Telstar's orbit was such that it could "see" Europe" and the US simultaneously during one part of its orbit.
INTRODUCTION
What Is a Satellite?
A satellite is any object that orbits another object. All masses that are part of the solar system, including the Earth, are satellites either of the Sun, or satellites of those objects, such as the Moon. It is not always a simple matter to decide which is the 'satellite' in a pair of bodies. Because all objects exert gravity, the satellite also affects the motion of the primary object. If two objects are sufficiently similar in mass, they are generally referred to as a binary system rather than a primary object and satellite. The general criterion for an object to be a satellite is that the center of mass of the two objects is inside the primary object. In popular usage, the term 'satellite' normally refers to an artificial satellite. However, scientists may also use the term to refer to natural satellites, or moons.
climate of the Earth.
What Keeps Objects in Orbit?
For 10,000 years man has wondered about questions such as "What holds the sun up in the sky?" "Why doesn't the moon fall on us?" and "How do they return from the far west back to the far east to rise again each day?” It is only last 300 years that we have developed a scientific description of how those bodies travel. Our description of course is based on fundamental laws put forth by the English genius Sir Isaac Newton in the late 17th century. Newton's law of gravity means that the sun pulls on the earth and the earth pulls on the sun. Furthermore, since both are quite large the force must also be quite large. "If the sun and the planets are pulling on each other with such a large force, why don't the planets fall into the sun?" THEY ARE! The Earth, Mars, Venus, Jupiter and Saturn are continuously falling into the Sun. The Moon is continuously falling into the Earth. Our salvation is that they are also moving "sideways" with a sufficiently large velocity that by the time the earth has fallen the 93,000,000 miles to the sun it has also moved "sideways" about 93,000,000 miles - far enough to miss the sun. By the time the moon has fallen the 240,000 miles to the earth, it has moved sideways about 240,000 miles - far enough to miss the earth. This process is repeated continuously. A planet, or any other body, which finds itself at any distance from the sun with no "sideways" velocity will quickly fall without missing the sun. Only our sideways motion saves us. “Does the time required to complete an orbit depend on the distance at which the object is orbiting?” The planet, which is at a larger distance, requires longer falling to where it would strike the sun. As a result, it takes a longer time to complete the ¼ trip around the sun, which is necessary to make a circular orbit
Can We Imitate Nature? (Artificial Satellites)
We can launch an artificial satellite, which would orbit the earth just as the moon does. A simple calculation, however, using the equations, which we developed above, will show that an artificial satellite, orbiting near the surface of the earth will have a period of approximately 90 minutes. This corresponds to a sideways velocity of approximately 17,000 miles/hour. To visualize the "missing the earth" feature, let's imagine cannon firing a cannonball.
Why Satellites for Communications?
We had, of course, been able to do transatlantic telephone calls and telegraphs via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required transmitting television pictures required that they operate at much higher frequencies than radio stations. A typical television station would operate at a frequency of 175 MHz As a result; television signals would not propagate the way radio signals did. Both radio and television frequency signals can propagate directly from transmitter to receiver.
Basic Communications Satellite Components:
Every communications satellite in its simplest form involves the transmission of information from an originating ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. Hence the satellite must have a receiver and a receive antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components
Transmitters:
The amount of power, which a satellite transmitter needs to send out, depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same.
Antennas:
One of the biggest differences between a low earth satellite and a geosynchronous satellite is in their antennas. Virtually all antennas in use today radiate energy preferentially in some direction. The commercial station will use an antenna that radiates very little power straight up or straight down. They have very few listeners in those directions power sent out in those directions would be totally wasted.
The communications satellite carries this principle even further. All of its listeners are located in an even smaller area, and a properly designed antenna will concentrate most of the transmitter power within that area, wasting none in directions where there are no listeners. The easiest way to do this is simply to make the antenna larger. Doubling the diameter of a reflector antenna (a big "dish") will reduce the area of the beam spot to one fourth of what it would be with a smaller reflector. We describe this in terms of the gain of the antenna. The larger antenna described above would have four times the gain of the smaller one. This is one of the primary ways that the geosynchronous satellite makes up for the apparently larger transmitter power, which it requires.
Power Generation:
You might wonder why we don't actually use transmitters with thousands of watts of power. There simply isn't that much power available on the spacecraft. There is no line from the power company to the satellite. The satellite must generate all of its own power. For a communications satellite, that power usually is generated by large solar panels covered with solars cells - just like the ones in your solar-powered calculator. These convert sunlight into electricity. Since there is a practical limit to the how big a solar panel can be, there is also a practical limit to the amount of power which can generated. In addition, unfortunately, transmitters are not very good at converting input power to radiated power so that 1000 watts of power into the transmitter will probably result in only 100 or 150 watts of power being radiated. We say that transmitters are only 10 or 15% efficient. In practice the solar cells on the most "powerful" satellites generate only a few thousand watts of electrical power.
Conclusion:
It will be necessary to mass-produce communications satellites, so that they can turn out quickly and cheaply, the way VCRs are manufactured now. This seems a truly ambitious goal since until now the average communications satellite might require 6 months to 2 years to manufacture. Nevertheless, at the present time, several companies indicate their intent to undertake such a system.
[attachment=52298]
ABSTRACT
A satellite is any object that orbits another object. In popular usage, the term 'satellite' normally refers to an artificial satellite. A planet, or any other body, which finds itself at any distance from the sun with no "sideways" velocity will quickly fall without missing the sun. Only our sideways motion saves us. The planet, which is at a larger distance, requires longer falling to where it would strike the sun. As a result, it takes a longer time to complete the ¼ trip around the sun, which is necessary to make a circular orbit. A satellite orbiting at an altitude of 22,300 miles would require exactly 24 hours orbiting the earth. Hence such an orbit is called "geosynchronous" or "geostationary." Both radio and television frequency signals can propagate directly from transmitter to receiver. Telstar's orbit was such that it could "see" Europe" and the US simultaneously during one part of its orbit.
INTRODUCTION
What Is a Satellite?
A satellite is any object that orbits another object. All masses that are part of the solar system, including the Earth, are satellites either of the Sun, or satellites of those objects, such as the Moon. It is not always a simple matter to decide which is the 'satellite' in a pair of bodies. Because all objects exert gravity, the satellite also affects the motion of the primary object. If two objects are sufficiently similar in mass, they are generally referred to as a binary system rather than a primary object and satellite. The general criterion for an object to be a satellite is that the center of mass of the two objects is inside the primary object. In popular usage, the term 'satellite' normally refers to an artificial satellite. However, scientists may also use the term to refer to natural satellites, or moons.
climate of the Earth.
What Keeps Objects in Orbit?
For 10,000 years man has wondered about questions such as "What holds the sun up in the sky?" "Why doesn't the moon fall on us?" and "How do they return from the far west back to the far east to rise again each day?” It is only last 300 years that we have developed a scientific description of how those bodies travel. Our description of course is based on fundamental laws put forth by the English genius Sir Isaac Newton in the late 17th century. Newton's law of gravity means that the sun pulls on the earth and the earth pulls on the sun. Furthermore, since both are quite large the force must also be quite large. "If the sun and the planets are pulling on each other with such a large force, why don't the planets fall into the sun?" THEY ARE! The Earth, Mars, Venus, Jupiter and Saturn are continuously falling into the Sun. The Moon is continuously falling into the Earth. Our salvation is that they are also moving "sideways" with a sufficiently large velocity that by the time the earth has fallen the 93,000,000 miles to the sun it has also moved "sideways" about 93,000,000 miles - far enough to miss the sun. By the time the moon has fallen the 240,000 miles to the earth, it has moved sideways about 240,000 miles - far enough to miss the earth. This process is repeated continuously. A planet, or any other body, which finds itself at any distance from the sun with no "sideways" velocity will quickly fall without missing the sun. Only our sideways motion saves us. “Does the time required to complete an orbit depend on the distance at which the object is orbiting?” The planet, which is at a larger distance, requires longer falling to where it would strike the sun. As a result, it takes a longer time to complete the ¼ trip around the sun, which is necessary to make a circular orbit
Can We Imitate Nature? (Artificial Satellites)
We can launch an artificial satellite, which would orbit the earth just as the moon does. A simple calculation, however, using the equations, which we developed above, will show that an artificial satellite, orbiting near the surface of the earth will have a period of approximately 90 minutes. This corresponds to a sideways velocity of approximately 17,000 miles/hour. To visualize the "missing the earth" feature, let's imagine cannon firing a cannonball.
Why Satellites for Communications?
We had, of course, been able to do transatlantic telephone calls and telegraphs via underwater cables for almost 50 years. At exactly this time, however, a new phenomenon was born. The first television programs were being broadcast, but the greater amount of information required transmitting television pictures required that they operate at much higher frequencies than radio stations. A typical television station would operate at a frequency of 175 MHz As a result; television signals would not propagate the way radio signals did. Both radio and television frequency signals can propagate directly from transmitter to receiver.
Basic Communications Satellite Components:
Every communications satellite in its simplest form involves the transmission of information from an originating ground station to the satellite (the uplink), followed by a retransmission of the information from the satellite back to the ground (the downlink). The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. Hence the satellite must have a receiver and a receive antenna, a transmitter and a transmit antenna, some method for connecting the uplink to the downlink for retransmission, and prime electrical power to run all of the electronics. The exact nature of these components will differ, depending on the orbit and the system architecture, but every communications satellite must have these basic components
Transmitters:
The amount of power, which a satellite transmitter needs to send out, depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. This is a result of the fact that the geosynchronous satellite is at an altitude of 22,300 miles, while the low earth satellite is only a few hundred miles. The geosynchronous satellite is nearly 100 times as far away as the low earth satellite. We can show fairly easily that this means the higher satellite would need almost 10,000 times as much power as the low-orbiting one, if everything else were the same.
Antennas:
One of the biggest differences between a low earth satellite and a geosynchronous satellite is in their antennas. Virtually all antennas in use today radiate energy preferentially in some direction. The commercial station will use an antenna that radiates very little power straight up or straight down. They have very few listeners in those directions power sent out in those directions would be totally wasted.
The communications satellite carries this principle even further. All of its listeners are located in an even smaller area, and a properly designed antenna will concentrate most of the transmitter power within that area, wasting none in directions where there are no listeners. The easiest way to do this is simply to make the antenna larger. Doubling the diameter of a reflector antenna (a big "dish") will reduce the area of the beam spot to one fourth of what it would be with a smaller reflector. We describe this in terms of the gain of the antenna. The larger antenna described above would have four times the gain of the smaller one. This is one of the primary ways that the geosynchronous satellite makes up for the apparently larger transmitter power, which it requires.
Power Generation:
You might wonder why we don't actually use transmitters with thousands of watts of power. There simply isn't that much power available on the spacecraft. There is no line from the power company to the satellite. The satellite must generate all of its own power. For a communications satellite, that power usually is generated by large solar panels covered with solars cells - just like the ones in your solar-powered calculator. These convert sunlight into electricity. Since there is a practical limit to the how big a solar panel can be, there is also a practical limit to the amount of power which can generated. In addition, unfortunately, transmitters are not very good at converting input power to radiated power so that 1000 watts of power into the transmitter will probably result in only 100 or 150 watts of power being radiated. We say that transmitters are only 10 or 15% efficient. In practice the solar cells on the most "powerful" satellites generate only a few thousand watts of electrical power.
Conclusion:
It will be necessary to mass-produce communications satellites, so that they can turn out quickly and cheaply, the way VCRs are manufactured now. This seems a truly ambitious goal since until now the average communications satellite might require 6 months to 2 years to manufacture. Nevertheless, at the present time, several companies indicate their intent to undertake such a system.