Free Research Paper on GPS

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The Global Positioning System (GPS), comprised of three integral segments, is one of the newest and most efficient forms of locating a global position. It was developed by the United States of American department of Defense (DoD) for military purposes. Since its creation it has developed many different uses. The three distinct segments are the user, space, and control segments. This report will explain what each of these segments entail with an emphasis on how they work together to provide the user with a GPS position.

The Global Positioning System is a large system of satellites that orbit the earth. These satellites transmit signals that are received by the user. The user to find a precise position on the earth then processes the signals. All sorts of people for all kinds of reasons use GPS. They could be military reasons, for example using GPS to aim a missile. GPS is used in vehicle navigation whether that be a plane, automobile, or boat. Land surveyors also use it since it can now produce sub-centimetre accuracy.

The report will discuss the three segments of GPS, the user, space, and control segments. These three segments are all so intertwined that each would be useless on its own.

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2.0 THE THREE SEGMENTS OF GPS
GPS is a system of Satellites surrounding the earth. It is used to obtain accurate three-dimensional positions on the earth’s surface. GPS can be divided into three separate segments; the user, space, and control segment. The user segment is made up of those who use GPS and the receivers they use. They consist of the military users, land surveyors, recreational users, and many more. The satellites orbiting around the earth make up the space segment. Thirdly, the control segment is the five monitor stations located around the world.
This chapter will then take a look at the three segments of GPS. The user, space, and control segments can be found in section 2.1, 2.2, and 2.3 respectively.

2.1 User Segment
Anyone who uses GPS and the receivers they use make up the user segment of GPS. GPS has a very wide range of users. The list of users runs from the United States military to professional users, such as land surveyors, all the way to recreational users. Although originally intended for military use the number of civilian users far out numbers the military users. Each different user has different needs when it comes to uses and accuracy.

A land surveyor for instance may need sub-centimetre level measurements while a recreational boater would only need positions accurate two under 20 metres to find out the best possible route to a previous marked good fishing area. Although each user has various uses for GPS the receiver is the basic element of the user segment and they all work on similar principles. This chapter contains a basic run though about how a GPS works.

A GPS receiver is very complex and its operation and how it works is far from simple but is basically works as follows. A GPS receiver obtains signals sent from a system of satellites, which orbit around the earth (the space segment) to triangulate a position on earth. A GPS receiver determines the distance from satellites, known distances away, by calculating the time it takes a signal sent from the satellite to the receiver.

Although this sounds simple enough GPS uses some of today’s most complicated technology. Assuming that the satellites position in space is known then the distance from one satellite limits the receivers position to somewhere on a three dimensional sphere with a radius of the distance from that satellite. If you add another satellite than you further reduce the position of the receiver to some where on the circle formed by the intersection of two spheres. If a third satellite is added that limits the receiver’s position to one of two points. One of the points is extraneous, i.e. will be located off in space, hence you can obtain a three dimensional position using three satellites (see Figure 2.1). More satellites are preferable to add redundancy to the data and to eliminate any errors as well as the extraneous measurements. To determine the distance from the satellite in space to the receiver located on the earth a radio signal is sent from the satellite. The signals are coded by time so that when the receiver receives the signal it compares it with the signal being produced by the receiver. The offset in these two signals will determine how long it took this signal to reach the receiver. The radio signals travel at the speed of light, approximately 300,000 kilometres per second, multiplied by the time it took the signal to reach the receiver equals the distance from the satellite to the receiver [Logsdon, 1992]. The Satellites have atomic clocks to help keep very accurate time but they are too expensive to be found in simple receiver. This can prove to be a problem because measuring the time it takes the radio signal to reach the receiver requires incredibly accurate time. For example, the radio signal travels at approximately 300,000 kilometres per hour and a timing error of one hundredth of a second would cause a positioning error of 300 kilometres [Hurn, 1989]. This problem can be solved by an extra distance measurement taken from and additional satellite. In theory three measurements can locate a point in three dimensional space but a fourth measurement is needed to correct for the imperfections in the timing, as long as the time offset is consistent.

2.2 The Space Segment
The Space segment is made up of a system of at least 24 satellites orbiting the earth (see Figure 2.2). The number of satellites fluctuates as old satellites are taken out of orbit and new satellites are placed into orbit. Each satellite orbits around the earth approximately two times in a 24-hour period. The average satellite orbit is 20,183 kilometres above the earth [Logsdon, 1992]. The satellites’ orbits are strategically designed so at any given time and at any given place on the earth there will be sufficient coverage of at least four satellites. The space segment can alter the trajectories of the satellites in order to obtain increased coverage over a certain area. For example, the United States may wish to have better coverage over the Middle East during their current war with Iraq. They would do this so they can aim their missiles with increased accuracy.

The satellites have wing like solar panels protruding from either side of the satellite (see Figure 2.3). The solar panels are movable and are positioned so that they are constantly perpendicular to the suns rays [Logsdon, 1992]. There are antenna fixed to the lower edge and they are always pointed towards earth.

As previously mentioned in the section 2.1 the user segment uses the time it takes the satellites signal to obtain a distance measurement. In order to ensure near perfect timing, the satellites come equipped with four atomic clocks. An atomic clock uses the vibration frequencies of atoms to accurately keep time. Although even these high tech clock experience some drift due to the effects of hurling through the atmosphere. The clocks are allowed to drift up to one millisecond until the control segment corrects them. These clocks are very expensive and cost around one hundred thousand dollars apiece [Van Sickle, 1996].

2.3 The Control Segment
The stations located at various positions around the globe make up what is called the control segment. Their purpose is to monitor and maintain the space segment constantly. Since the United States created GPS for their use the DoD is what makes up the control segment.

There is one master station and four other stations located around the world. The master station is located in the United States twelve miles east of Colorado Springs, Colorado, at the Falcon Air Force Base [Shank, 1999]. The other four stations are spread out around the world near the equator. They are located in Hawaii, Ascension, Diego Garcia, and Kwajalein (see Figure 2.4). The responsibility of these stations is to monitor the satellitesЎЇ positions and atomic clocks. The control segment is also responsible for the replacement old satellites and launching new ones.

In order for the user segment to obtain an accurate GPS position the user segment must be able to determine the exact position of the satellite in space. Determining the location of a satellite that orbits around the earth at an average of 20,183 kilometres above the earth is another matter altogether [Logsdon, 1992]. The fact that the satellites are actually that high above the earth is a benefit. They are high enough that they are well clear of the earth’s atmosphere so their orbit is easily predicted with a large accuracy. To ensure near perfection the control segment constantly monitors the satellites to make sure that they are following their orbit. As the satellites orbit over one of the control stations, the control segment precisely measures their altitude, position, and speed for any variations called ‘ephemeris’  errors. The errors are usually minor and are caused by such things as pressure of solar radiation and gravitational pull from the sun and moon. As stated in section 2.3 the atomic clocks time tends to drift slightly. Instead of constantly correcting the atomic clocks of each satellite their time is constantly monitored by the control segment and their error is transmitted to the receiver on earth, this is known as the broadcast clock correction. Both the ephemeris and timing errors are then sent up to the satellite, which in turn broadcasts these minor corrections to the user segment.

CHAPTER 3
SUMMARY
The Global Positioning system can be broken down into three separate categories. They comprise of the user, space, and control segment. The user segment is made up of all those who use GPS and the receivers they use. The kinds of receivers vary as much as the different kinds of users but all receivers work on basically the same principles.

Receiving signals from the space segment and using these signals to obtain a global position is the basic function of the user segment. It finds a position by determining how long it takes the signals, from at least three different satellites, to reach the receiver. It then multiplies the time it took the signal to reach the receiver by the speed of the signal.

A large system, around 24 satellites orbiting the earth, comprises the space segment. These satellites send the signals to the user segment. The satellites orbit around the earth approximately twice a day. They are positioned so that you can receive signals from at least four satellites at almost any given time and place. Accurate time is a key element in GPS positioning so each satellite is equipped with four atomic clocks. The satellite signals sent contain timing information as well as the clock and ephmeris corrections.

The control segment constantly monitors and maintains the satellites. This segment is owned and operated by the United States Department of Defense (DoD). They have a master control station and four other stations located thought out the world.

The control segment monitors the position of the satellites and the satellites atomic clock and broadcast corrections to the satellites. These corrections are then sent out in the satellites signals. These three segments all work together simultaneously to form what is now called GPS.

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