The Ways Satellites Are Controlled
Most satellites operate under the direction of a control center that is located on the earth. Computers and human operators at the control center monitor the satellite's position, send instructions to its computers and retrieve information that the satellite has gathered. A satellite can not always receive constant direction from the control center. So it has to be able to act like an orbiting robot. It also has to be able to control its solar panels to them keep them pointed toward the sun and keep its antennas ready to receive commands. "Satellites in a high altitude, Geosynchronous orbit are always in contact with the earth. Ground stations can contact satellites in low orbits as often as 12 times a day, so they do not need constant direction (Oberright 3).
Satellites play a very big part in our lives. Without the use of satellites we wouldn't be able to watch television, be able to communicate to the other side of the earth. Satellites aren't just in space they have to be put there. This part is very important step on using a satellite. What goes up must come down, so there comes the problem of bringing the satellite back to earth.
Launching
The launching of a satellite varies in two ways. The first way of launching a satellite is to carry it into space on a space shuttle. "Space shuttles carry some satellites into space."(Oberright,3) Then when the satellite is raised to the required height it is given a thrust into orbit. There is another way of launching a satellite into space, this method of launching is done with a rocket. A satellite is put into a rocket and launched into space. "The method of launching is to raise the projectile to the required height and then iv it a side ways thrust which will throw it into the right orbit."(Oberright,3) When rocket is launched upward into space the rockets fuel is spent, the satellite separates from the rocket and the rocket falls from space and into the ocean. The satellite is left in space. The satellite requires minor adjustment, this is done by built in rockets called thrusters. When a satellite is up in orbit it stays in orbit for a long time.
Retrieving of Satellites
Satellites don't last for ever. When a satellite has out lived its usefulness or needs to be repaired or replaced it needs to be retrieved. There are also two different ways of retrieving a satellite. One way of retrieving a satellite is by it falling from its orbit. "A satellite remains in orbit until its velocity decreases and the gravitational forces pulls it down into a relatively dense part of the atmosphere. A satellite slows down due to friction of air particles in the upper atmosphere and the gentle pressure of the sun's energy. When the gravitational force pulls the satellite down far enough into the atmosphere, the satellite rapidly compresses the air in front of it. This air becomes so hot that most or all of the satellite burns up."(Oberright,4)
The other way a satellite is retrieved is with a space shuttles This isn't something that happens often. If this does occur it is because the satellite needs to be repaired or reprogrammed. When it does happen a space shuttles crew go up into space and try to repair the satellite. The satellite is repaired in space or it is taken back to earth for its repairs or reprogramming. If the satellite isn't repairable reprogrammable the operator at the control center shuts the satellite off and it falls out of orbit and either drifts into space or falls to earth.
Conclusion
Without satellites everything would be different, for example television. Without a satellite to transmit the programs we wouldn't have any television. There for satellites due play a big role in our lives and without being able to launch a satellite into space or be able to retrieve it from space we wouldn't have such things as television.
Satellites Use Solar Energy
A satellite can receive its energy and power through the use of solar energy also. Solar energy comes from thermonuclear fusion reactions which occur in the sun. There are many different types of collectors which gather the solar energy. Photoelectric panels have been used to give power to satellites. By using photoelectric cells, or solar cells, solar energy can be changed into electricity. For as long as the light shines on the cells they will produce electrical voltage (Compton's Interactive Encyclopedia).
Satellites Use Nuclear Power
Another method used to give satellites energy and power is by the use of nuclear power. Nuclear power produces enough energy to operate a Space Tug as well as the laser power. Solar power, unfortunately, does not produce enough energy to operate the Space Tug. But nuclear power is often very hard to get licensed (Spellman).
Satellite Energy Benefits Humans
Satellite energy can be used to benefit humans in many ways. Satellites can radio information down to Earth. This ability has been utilized to use Global Positioning Systems (GPS). Global Positioning Systems can be used for many purposes. GPS are used to find locations on Earth. A series of three satellites are needed to find a precise location on Earth. GPS satellites send data to Earth every thirty seconds on its location, velocity, and when it was transmitted. This information can be used to figure out how much time the signal took to get to your receiver from the satellite. When this is figured out, the distance between the satellite and receiver can be found. This process is very accurate. The signal that is used for civilian use is accurate to 76 meters. Although this is very accurate for most purposes it is not accurate enough to be used for the automated landing of a 747 air plane or to find other precise locations that might be needed for other purposes. Other methods have been developed combining information from Global Positioning Systems with other signals from land based stations. These have been developed to narrow down the position to about 1 meter. This system is called Differential GPS. A system known as Survey GPS is even more precise. This system can be used to find a position down to millimeters. These measurements are so accurate they can be used to measure such thing as continental drift or changes in ocean levels. These systems can be used for commercial uses like automobile navigation systems. Automobile navigation systems like these are popular in Japan, and will become an option in US cars as well (Rozmiarek).
Commercial Use of Satellite Energy
One such car company that uses satellite energy in the form of Global Positioning systems is Cadillac. Their system is known as Onstar. It utilizes satellite technology to aid people. It will use Global Positioning Systems to help people and possibly save their life. If the airbag in the vehicle deploys, a signal is sent. If staff are not able to communicate with the people in the car, emergency help is then sent. This is only of the many things Onstar utilizes GPS to do. This service, which uses satellite energy, can be used for navigation assistance. They can also be used to unlock the doors if the keys are locked in the car, and the car can be tracked if it is stolen. All this is made possible by a Global Positioning System antenna and the Onstar transmitter/receiver. The use of satellite energy through things like Global Positioning Systems is used to help us in a variety of ways. It has made our lives a lot easier and can save and protect us (World of Wheels Online).
A satellite is a device which resides in space, intended for observation, research, or communication that is in orbit around the earth. In order for it do a specific job it requires a certain orbit.
Different Types of Satellite Orbits
Most satellites are placed into orbit by multistage booster rockets that are released after the fuel is consumed. Once the satellite is about two hundred miles from the earth's surface it is free from the earth's drag on it. Some orbits are circular while others are elliptical, or oval shaped. Satellites that have an elliptical orbit, have both an apogee and a perigee. An apogee is the satellites farthest point from the center of the earth. The perigee is when it is closest to the earth's center. The altitude of the orbit decides on the period of time it takes to complete one orbit, this is called the orbital period. In order for the satellite to remain in orbit, the satellite needs to maintain a constant velocity while the earth's pull of gravity keeps it in orbit. The velocity helps the satellite to try to escape the atmosphere but the earth's gravity holds it in orbit.
Satellites that have low altitude orbits are a couple hundred miles above the earth's surface. The satellites are in the earth's atmosphere, but in the highest level and can orbit the earth in ninety minutes. The satellite can be very large with this type of orbit and less propulsion is needed. There is little air resistance against them but they only stay in orbit for a short time, about a couple of weeks to a month.
An orbit that takes about twenty-four hours to revolve around the earth is called a geosynchronous orbit or a high altitude orbit. It is about 22,282 miles from the earth's surface and stays in the same spot over the earth at all times, because it is moving at the same speed as the earth. A satellite in this orbit moves in the same direction as the earth along the equator. While moving along the equator line, the satellite moves slightly up and down along the line forming a sort of figure eight. For a satellite to be put into this type of orbit a very powerful launch vehicle is needed.
Another type of orbit that is similar to the geosynchronous orbit is the geostationary orbit. A satellite in the geostationary orbit circles the earth at the same altitude as the geosynchronous orbit, at 22,282 miles but orbits directly over the equator with out moving up or down. To get a satellite to maintain this orbit is very difficult and often impossible. Both the geosynchronous and the geostationary orbits are good for communication, broadcasting, and weather satellites.
A third type of orbit is the sun-synchronous orbit or the polar orbit. It passes over the North and South poles. Its "orbit requires one entire earth year to make a full revolution about the sun. As observed from the position of the sun, the satellite orbit plane remains in the same apparent orientation throughout the year" (Parker 42). In a day a satellite in this orbit makes about fifteen revolutions and a good use for it is reconnaissance satellites.
Placing a Satellite in Orbit
"In order for a satellite to be orbited, it must be propelled at a velocity that imparts enough energy to keep it in that orbit with out the application of additional force. If the orbit is low, the resistance of the outer most atmosphere will cause the satellite to lose orbital speed and reenter the atmosphere" (Encyclopedia America 288). Once a satellites velocity decreases it will start to fall from orbit and the gravitational force of the earth pulls it down into a dense part of the atmosphere. Eventually the satellite is pulled down far enough into the atmosphere. When this happens the satellite rapidly compresses the air in front of it. This causes the air to become so hot, that all or most of the satellite will burn up.
Conclusion
For a satellite the type of orbit and the amount of velocity are two key roles in order for a satellite to do its job. The type of orbit the satellite takes depends upon the job it has to do. The velocity plays a key role in a satellites orbit, in that the speed that the satellite is going will decide if it will remain in orbit.
Everyday, satellites are either being launched , repaired, or retrieved in space. Satellites help transmit communications at the speed of light, serve for navigation for the military and help in exploring the universe. They are very important to the world of communication, but it takes a great deal of work to get them in and out of orbit. Launching is essential to satellites , but many problems might be encountered during the launching or retrieving of a satellite, which might cause for repairs to be made in space.
Where Are They Launched From?
The government uses a number of launching areas around the country depending upon the type and orbit of satellite that is to be launched. Most launches occur from Cape Canaveral, Florida for regular satellites. NASA's (National Aeronautics and Space Administration) Wallops Flight Center, located in Virginia is used to launch regular satellites. To launch polar orbits, or satellites which orbit over the North and South Poles, these take place from the Vandenberg Air Force Base in California.
Putting Satellites Into Orbit
Satellites are carried into orbit by shuttles, where they go through a process of being launched. Shuttles flying into an orbit are inclined at 28.5 degrees to the Equator (Broad, A22). Even though a good number of satellites are carried into orbit by space shuttles, most satellites are carried by rockets. The rockets fall back into the ocean after they have launched the satellite and their fuel has run out. Sometimes they are retrieved and are used for other launches. Rockets have the capability to put satellites into orbit which are 120 miles above the Earth. Satellites which need to be put into higher orbits are first put into one orbit around the Earth and then with their motors, are pushed into higher orbits. Once the satellite gets away from the rockets own protective shroud, they are then controlled by signals from ground controllers in the launching center. Space shuttles can carry a maximum of 3 or 4 satellites. These satellites are either pushed into orbit by a space arm or can be gently pushed out into orbit while spinning.
Types of Launching Vehicles
Different types of launching vehicles are used to put satellites into orbit. Two types of launching vehicles used are either solid or liquid fuel propellants, or motors. The solid propellants can be compared to a rocket used in fireworks displays and the liquid propellants, which are forms of kerosene and liquid oxygen are used to provide movement for launching. Another type of launch vehicle includes the use of volatile hydrogen and oxygen combination as fuel. At liftoff, the three large liquid motors are assisted by the solid rocket boosters. The two solid boosters burn out after some time and fall back into the ocean. The other liquid motors are used during the rest of the flight to put the satellite into the proper orbit, to control the satellites orbit and to return the rockets to Earth.
Problems Involved With Launching
Problems such as vibrations and the emission of gasses cause problems during a launch. During launching, vibrations which are transmitted from the launch vehicles and shock loading take over the launching process as they enter the upper stages of launching. To add to the problems, some of the materials, in the launch vehicles, can emit gas or give off particles of their substance such as kerosene or liquid oxygen under a vacuum (How It Works, 2038). This transmission of particles has to be avoided because the particles could be confused as stars or they might contaminate the solar system atmosphere. Satellites which require minor adjustments due to launching difficulties must be adjusted otherwise they will not function. Thrusters or built in rockets have been placed in satellites to assist in making these minor adjustments.
Another problem at launch time is the weather at the launching pad and tracking stations around the world. Unless weather is ideal, where there is very little wind and no rain to affect the rocket, a launch will not take place. Further the tracking stations must be able to follow the rocket into orbit.
During the countdown prior to actual launch, a major problem can occur while the computers are checking all the systems. If the computer encounters an error, the countdown is stopped to fix the problem so the launch can either go on, be scrubbed, or canceled to take place at a later date. To launch with the problem could mean the loss of a satellite and millions of dollars invested in the launch, and the lives of the astronauts who are flying the mission.
Life Span
Once in orbit, satellites may last for a few years to a few hundred years depending on its' size and its' orbital distance from Earth. Since outer space has become crowded with satellites, meteors, and space junk from failed launches and non-working satellites, many of the working satellites might have to be raised to higher orbits by the ground controllers with the use of the thrusters to escape the crowding in various orbits to extend their lives.
The Death of a Satellite
Many factors can contribute to the slowing down or even the death of a satellite. Even before some satellites reach space, 1 in 20 are rendered useless by either being jolted on lift-off, perish in fires of defective rocket blasts or are put into improper orbits (Canby, 282). The atmosphere in space is less dense than on Earth and approximately 124 miles above the Earth. There is no part of the space's atmosphere which is an absolute vacuum. In time, a satellite will lose velocity or speed and won't be able to go high enough and will eventually fall out of orbit and disintegrate falling towards Earth because of the tremendous speed and friction. Some satellites will begin to lose the balance in the pull of the Earth's gravity and will fall toward the Earth. Friction will be a major cause for the death of most satellites. The friction of air particles in the high atmosphere and the gentle pressure of the sun's energy will cause a satellite to fall back in the Earth's atmosphere. Falling into the atmosphere means that the satellite will be exposed to intense heat as it collides with large numbers of molecules and ions and this causing the satellite to burn up in flames even before it reaches Earth. Satellites that escape the Earth's gravitational field have a good chance to crash with the moon or planets and they might even begin to orbit the sun.
Conclusion
Satellites are a very important part of our communication system and without them, the lives of many people would be totally different. The life and death of satellites are a very crucial factor to the world today. Everyday, new discoveries are being made on how to make the launching and retrieving process a little more easier, while at the same time, technicians are learning new ways on how to extend the life span of a satellite.
Physics play a major role in the launching and retrieving of satellites. The launching and retrieving of a satellite are the most important part of a functioning satellite. Satellites are launched in many different ways including the use of space shuttles and rockets. There are many different ways of using the earth around you making it easier to launch a satellite, for instance the use of equatorial and polar orbits. The forces found in these orbits are also used for the re-entry of a satellite. Launching and retrieving of satellites depends on many factors which make it useful in the atmosphere.
Launching a Satellite
There are a variety of ways a satellite can be launched into orbit, mainly two. Space shuttles carry some satellites into space, but most satellites are launched by rockets that fall into the ocean after their fuel is spent. Many satellites require minor adjustments of their orbit before they begin to form their function. Built-in rockets called thrusters - some as small as a mechanical pencil - make these adjustments. Once a satellite is placed into orbit, it can remain there for a long period of time without further adjustments. Many satellites, when launched, are projected into the earth's atmosphere much like a space shuttle launched by NASA, except satellites are launched mainly for the use of scientists.
To take advantage of the speed of the earth's rotation, satellites are normally launched in a easterly direction. When they are launched into polar orbits, which means to move in a direction from the North Pole to the South Pole, their booster thrust must provide the whole orbital velocity. Polar orbits have certain advantages. In time, because the earth is rotating beneath it, the satellite will pass over its entire surface. In addition, as in an equatorial orbit, the movement of direction from east to west or vice versa. The satellite can be observed once in each revolution by a single tracking station, in this case at one of the poles. Polar and equatorial orbits help a satellite into orbit, using the earth's motion to give the satellite a extra "boost" into the atmosphere.
Falling From Orbit
Without any plan of re-entry, a satellite has forces acting on it , causing it to return to earth on its own. A satellite remains in orbit until its velocity decreases and the gravitational force pulls it down into the relatively dense part of the atmosphere. A satellite slows down due to the friction of air particles in the upper atmosphere and the gentle pressure of the sun's energy. When the gravitational force pulls the satellite down far enough, the air in front of it becomes so hot that most or all of the satellite burns up. There is not always a definite chance a satellite will make it back to the earth's surface in its entirety. It cannot with-stand the pressure of the earth's atmospheric pressure during movement through the orbit.
Re-entry of a Satellite
There are many elements which need to work for a satellite to return successfully. When re-entering, "the satellite tends to lose velocity faster and faster, because, as its apogee shortens, its ellipse loses more and more of its eccentricity, approaching a circular orbit and passing through perigee" (Jackson 202). Apogee is the point in the orbit of the moon or of a artificial satellite where it is farthest from the earth. Perigee is the exact opposite, it is the point nearest the earth. When the ellipse finally degenerates into a circle, the atmosphere is dragging continuously on the satellite. The perigee height becomes the apogee height, and the two switch back and forth as the satellite spirals in through the atmosphere.
" The satellite that broke free from the shuttle Columbia is to plunge back to the earth's atmosphere as in expected to burn up completely and not hit the ground. The fiery re-entry will be bright because of the 12 mile long tether dangling from the half- ton satellite" (NY Times C5).
The launching and retrieving of satellites involves many aspects of physics. These include the use of the earth itself and its motion as well as the atmosphere surrounding it. As stated, a satellite does not have to be launched or retrieved manually but can use the natural elements to provide us with the information we are looking for.
After the launching of a satellite there has to be a way to get the satellite into the correct position so that it will attain the correct orbit pattern. There are different stages and position placement tactics used to set the satellite in it's correct orbit. This is accomplished from the signals sent from the command center to the satellite and the on-board rockets move the satellite into position. Most of the time the people behind the scenes that are controlling the satellites like to put the satellite into a geosynchronous orbit. This is one kind of orbit that is in time with the Earth's rotation. There are certain times in which this type of orbit will be used to position the satellite.
Initial Positioning of Satellites
To begin the process of putting the satellite in orbit there are three stages. The first stage raises the satellite to an elevation of about 50 miles. The second stage raises the satellite 100 miles and the third stage places it into the transfer orbit. The satellite is placed in its final geosynchronous orbital slot by the AKM, a type of rocket used to move the satellite, which is fired on-command to allow the satellite to attain the apogee, the farthest point from Earth, of its elliptical orbit. A final thing to check is to make sure that a satellite that is to orbit the Earth is positioned at least 100 miles above the Earth's surface so that the atmospheric drag will not slow the satellite down (Cook 1).
Adjusting While in Orbit
Also important in the orbit of a satellite is the adjusting of it while in orbit. This is done through the use of rockets. Throughout the life of the satellite the orbit and attitude, the direction it points, must be adjusted. Generally they have on-board rockets for this purpose. Sometimes the rocket may be fired to accelerate the satellite and move it to a higher orbit or to decelerate the satellite and move it to a lower orbit. It can also be changed by firing the rocket to the side which changes the direction the satellite points. With the use of these special rockets the satellite can be moved at any time to any necessary position for any reason or another. The rockets help keep the satellite on its course without the possibility of it losing altitude or falling right out of the orbit in which it was in. A satellite's position has much to do with the time. For example, a satellite located 22,300 miles above the Earth's surface which is said to be synchronous, takes exactly 24 hours to orbit around the Earth once. This is said to be synchronous because it is synchronized with the Earth's rotation. Not all orbits are said to be synchronous and the time required for a satellite to complete one revolution around the Earth depends on its altitude. The laws that govern a satellite in orbit are the same laws that govern the motion of natural satellites and it travels around the planet in a nearly circular orbit when it reaches the 100 mile mark above the Earth's surface (Compton's Encyclopedia).
Command and Control
The command and control centers for the satellites play a big role in the keeping of a satellite in a particular orbit. As explained by Burke when he says, "Satellite command and control is exercised from the operational control nodes located at Orizuka Air Station, California, and Falcon Air Force Base, Colorado." From there, many of the techniques of positioning are controlled. Burke also says, "The AFSCN, Air Force Satellite Control Network, has a command and control segment that enables execution of phases used to position satellites." In conclusion Burke says in his article that the command center has tracking and command support from launch preparation to on-orbit operations (1).
Conclusion
It is interesting to know that the people who work with satellites have a large amount of control from a small room located on an Air Force base. Through their work the satellite can be launched, positioned, and adjusted with the touch of a few buttons. Although it is not as easy as it may seem. These people have to know exactly what there are doing or the satellite may not attain the proper orbit. The job is hard, but many satellites have been put in orbit without any problems.
In order for satellites to work properly, controlling them is very important and necessary. People on the ground have complete control over their satellite and need to be able to know where the satellite is at all times. They also should be in contact with the satellite at all times. Controlling a satellite can be accomplished by rocket engines, specific types of tracking, guidance systems and Telemetry links.
Rocket Engines:
Rocket engines are one of many ways to control satellites. They help to keep satellites from drifting off course. In order to do this, some systems use small bursts from the rocket engines which are placed around the satellite. One specific rocket engine is the Agena rocket. This rocket is on a satellite called the Seasat. It keeps pointing toward the Earth and gives the satellite an elongated shape which helps provide stabilization.
Tracking:
Satellites are tracked from tracking stations. "Tracking stations are usually ground based but have been placed aboard ships, aircrafts and satellites" (Dooling). There are also specific types of tracking. One type is optical tracking. "Optical tracking is used mainly for orbit determination of inactive spacecraft or of satellites equipped with optical ranging" (Dooling). Another type of tracking is radio tracking. "Radio tracking is used extensively to acquire data from spacecrafts and to transmit commands" (Dooling).
Guidance Systems:
Guidance systems are also a very important part of controlling satellites. They help the satellite to function and to stay on target. There are different types of guidance systems as well. Radio-command is one example. "In the radio-command guidance system, changes in velocity and direction are broadcasted by the spacecraft to a station on the ground. The data is fed into computers that make the calculations required to keep the craft on course. The necessary adjustments are then broadcasted back to the craft and control motors on the craft to carry out the instructions. This procedure may be directed by the ground station or it may be automatic" (Waters 305). One other type of guidance system is the inertial-guidance system. "In an inertial-guidance system, the velocity and direction are measured by an to board sensor system. This information if then fed onboard computers and flight controls" (Waters 305).
Telemetry links:
Telemetry links aid in the way information is transmitted to and from the satellites. "Commands from the ground and details of how the satellite is functioning are transmitted to and from the satellite by Telemetry links. Many scientific satellites collect data continuously but are only in view of the ground stations for a short time. In this case, the data will be recorded on a tape recorder and played back at a higher speed when in contact with the ground station" ("Satellite" 2040).
Conclusion:
As you can see, there are many factors involved in controlling a satellite. Some ways that satellites are controlled are by the use of rocket engines, tracking stations, guidance systems and Telemetry links. These are just some examples of how to control a satellite. There are many other ways because the world of satellites is a big field of physics and is being expanded everyday.
Gravitation and Satellites
Another major category of forces that produce uniform circular motion is gravitation. At a radius, r, from the center of the Earth, the gravitational field strength, g, can be calculated as:
Alternatively, we can also state that for a satellite in circular orbit that gravity supplies the unbalanced central force required to keep it in orbit.
Thus, the field strength of the Earth's gravitational field, g, is equivalent to the centripetal acceleration experienced by a satellite in circular orbit at radius r from the center of the Earth.
The formula derived above is the speed of a satellite orbiting a central mass, M, at a center-to-center distance of r. This speed can also be expressed as
where T is the period, or time for the satellite to make one complete revolution.
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