24-01-2010, 03:49 PM
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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 to orbit 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.
The downlink may either be to a select number of ground stations or it may be broadcast to everyone in a large area. 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. One of the biggest differences between a low earth satellite and a geosynchronous satellite is in their antennas.
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. We say that transmitters are only 10 or 15% efficient. The ACTS concept involves a single, rather complicated, and expensive geosynchronous satellite. An alternative approach is to deploy a "constellation" of low earth orbiting satellites. It will be necessary to mass-produce communications satellites, so that they can turn out quickly and cheaply
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.
Types of satellites Astronomical satellites are satellites used for observation of distant planets, galaxies, and other outer space objects. Communications satellites are artificial satellites stationed in space for the purposes of telecommunications using radio at microwave frequencies. Most communications satellites use geosynchronous orbits .
Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc.
Navigation satellites are satellites, which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few meters in real time.
Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years. Weather satellites are satellites that primarily are used to monitor the weather and/or 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.
The Earth Orbits the Sun With Angular Velocity
" 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 a cannon firing a cannonball.
Launching an Artificial Satellite In the first frame of the cartoon, we see it firing fairly weakly. The cannonball describes a parabolic arc as we expect and lands perhaps a few hundred yards away.
In the second frame, we bring up a little larger cannon, load a little more powder and shoot a little farther. The ball lands perhaps a few hundred miles away. We can see just a little of the earth's curvature, but it doesn't really affect anything. In the third frame, we use our super-shooter and the cannonball is shot hard enough that it travels several thousand miles. Clearly the curvature of the earth has had an effect. The ball travels much farther than it would have had the earth been flat. Finally, our mega-super-big cannon fires the cannonball at the unbelievable velocity of 5 miles/second or nearly 17,000 miles/hour. The result of this prodigious shot is that the ball misses the earth as it falls. Nevertheless, the earth's gravitational pull causes it to continuously change direction and continuously fall. The result is a "cannonball" which is orbiting the earth. In the absence of gravity, however, the original throw would have continued in a straight line, leaving the earth far behind.
Finally, in 1957, the first artificial satellite, called Sputnik, was launched by the Soviets. Consisting of little more than a spherical case with a radio transmitter, it caused quite a stir. A satellite orbiting at an altitude of 22,300 miles would require exactly 24 hours to orbit the earth. Hence such an orbit is called "geosynchronous" or "geostationary." Why Satellites for Communications? We had, of course, been able to do transatlantic telephone calls and telegraph 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. This is a very dependable signal, but it is more or less limited to line of sight communication. The mode of propagation employed for long distance radio communication was a signal, which traveled by bouncing off the charged layers of the atmosphere (ionosphere) and returning to earth. The higher frequency television signals did not bounce off the ionosphere and as a result disappeared into space in a relatively short distance. Radio Signals Reflect Off the Ionosphere; TV Signals Do Not. Low Earth-Orbiting Communications Satellites: In 1960, the simplest communications satellite ever conceived was launched. It was called Echo, because it consisted only of a large (100 feet in diameter) aluminized plastic balloon. Radio and TV signals transmitted to the satellite would be reflected back to earth and could be received by any station within view of the satellite.
Echo Satellite
Unfortunately, in its low earth orbit, the Echo satellite circled the earth every ninety minutes. This meant that although virtually everybody on earth would eventually see it, no one person, ever saw it for more than 10 minutes or so out of every 90 minute orbit. In 1958, the Score satellite had been put into orbit. It carried a tape recorder, which would record messages as it passed over an originating station and then rebroadcast them as it passed over the destination. Once more, however, it appeared only briefly every 90 minutes - a serious impediment to real communications. In 1962, NASA launched the Telstar satellite for AT&T. Telstar Communications Satellite
Telstar's orbit was such that it could "see" Europe" and the US simultaneously during one part of its orbit. During another part of its orbit it could see both Japan and the U.S. As a result, it provided real- time communications between the United States and those two areas - for a few minutes out of every hour. Geo-synchronous Communications Satellites:
The solution to the problem of availability, of course, lay in the use of the geo-synchronous orbit. In 1963, the necessary rocket booster power was available for the first time and NASA launched the geo-synchronous satellite, Syncom 2. For those who could "see" it, the satellite was available 100% of the time, 24 hours a day. The satellite could view approximately 42% of the earth. For those outside of that viewing area, of course, the satellite was NEVER available. Syncom II Communications Satellite
Today, there are approximately 150 communications satellites in orbit, with over 100 in geo-synchronous orbit. it possible to transmit 1000s of phone calls between almost any two points on the earth. It was also possible for the first time, due to the large capacity of the satellites, to transmit live television pictures between virtually any two points on earth. 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.
Basic Components of a Communications Satellite Link
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.
Future communication satellite:
The nature of future satellite communications systems will depend on the demands of the market place the costs of manufacturing, launching, and operating various satellite configurations; and the costs and capabilities of competing systems - especially fiber optic cables, which can carry a huge number of telephone conversations or television channels. In any case, however, several approaches are now being tested or discussed by satellite system designers.
One approach, which is being tested experimentally, is the "switchboard in the sky" concept. NASA's Advanced Communications Technology Satellite (ACTS) consists of a relatively large geosynchronous satellite with many uplink beams and many downlink beams, each of which covers a rather small spot on the earth. However, many of the beams are "steer able". The ACTS satellite is also unique in that it operates at frequencies of 30 GHz on the uplink and 20 GHz on the downlink. It is one of the first systems to demonstrate and test such high frequencies for satellite communications The ACTS concept involves a single, rather complicated, and expensive geosynchronous satellite. An alternative approach is to deploy a "constellation" of low earth orbiting satellites. By planning the orbits carefully, some number of satellites could provide continuous contact with the entire earth, including the poles. By providing relay links between satellites, it would be possible to provide communications between any two points on earth, even though the user might only be able to see any one satellite for a few minutes every hour. Obviously, the success of such a system depends critically on the cost of manufacturing and launching the satellites.
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.
