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Global Positioning System
Where am I? Where am I going? Where are you? What is the best way to get there? When will I get there? GPS technology can answer all these questions. GPS satellite can show you exact position on the earth any time, in any weather, no matter where you are! GPS technology has made an impact On navigation and positioning needs with the use of satellites and ground stations the ability to track aircrafts, cars, cell phones, boats and even individuals has become a reality. This paper describes the Global positioning system (GPS) satellite. It depicts what GPS satellite is, how it works and its tracking features. This paper also gives how the GPS satellite has been used to compute position and time, gives the details of various segments in which the GPS system is useful. The paper gives the benefits of GPS satellite such as ability to track an object, due to reduced cost it is more affordable for everyone and helps you to find out where you are and how to get to your destination, where ever you are going on land or sea., Applications such as military, car alarms, home security and home monitoring technologies have tried to simplify the task but everyone has had some disadvantages. Finally, the U.S. Department of Defense decided that the military had to have a super precise form of worldwide positioning. And fortunately they had the kind of money ($12 billion!) it took to build something really good. The result is the Global Positioning System, a system that's changed navigation forever. GPS initially created by the U.S Defense Department for the military has later been made available to the public. GPS technology is not just a handheld “help-mefind-my-way-home” operation anymore. GPS is finding its way into cars, boats, planes, construction equipment, moviemaking gear, farm machinery, even laptop computers. Move over Mr. Bell, it won’t be long until GPS will become as basic as the telephone.


Trying to figure out where you are and where you're going is probably one of man's oldest pastimes. Navigation and positioning are crucial to so many activities and yet the process has always been quite cumbersome. Over the years all kinds of A constellation of 24 satellites A system of satellites, computers, and receivers that is able to determine the latitude and longitude of a receiver on Earth by

calculating the time difference for signals from The Global Positioning different satellites to reach the receiver. System (GPS) is a worldwide radio-navigation system formed from a constellation of 24 satellites and their ground stations. GPS uses these "man-made stars" as reference points to calculate positions accurate to a matter of meters. In fact, with advanced forms of GPS you can make measurements to better than a centimeter! In a sense it's like giving every square meter on the planet a unique address. GPS receivers have been miniaturized to just a few integrated circuits and so are becoming very economical. And that makes the technology accessible to virtually everyone.

Technical description
The system consists of a "constellation" of at least 24 satellites in 6 orbital planes. The GPS satellites were initially manufactured by Rockwell; the first was launched in February 1978, and the most recent was launched November 6 2004. Each satellite circles the Earth twice every day at an altitude of 20,200 kilometers (12,600 miles). The satellites carry atomic clocks and constantly broadcast the precise time according to their own clock, along with administrative information including the Orbital elements of their own motion, as determined by a set of ground-based observatories. The receiver does not need a precise clock, but does need to have a clock with good short-term stability and receive signals from four satellites in order to find its own latitude, longitude, elevation, and the precise time. The receiver computes the distance to each of the four satellites from the difference between local time and the time the satellite signals were sent (this distance is called a pseudo range). It then decodes the satellites' locations from their radio signals and an internal database. The receiver should now be located at the intersection of four spheres, one around each satellite, with a radius equal

to the time delay between the satellite and the receiver multiplied by the speed of the radio signals. The receiver does not have a very precise clock and thus cannot know the time delays. However, it can measure with high precision the differences between the times when the various messages were received. This yields 3 hyperboloids of revolution of two sheets, whose intersection point gives the precise location of the receiver. This is why at least four satellites are needed: fewer than 4 satellites yield 2 hyperboloids, whose intersection is a curve; it is impossible to know where the receiver is located along the curve without supplemental information, such as elevation. If elevation information is already known, only signals from three satellites are needed (the point is then defined as the intersection of two hyperboloids and an ellipsoid representing the Earth at this altitude). The receiver contains a mathematical model to account for these influences, and the satellites also broadcast some related information, which helps the receiver in estimating the correct speed of propagation. High-end receiver /antenna systems make use of both L1 and L2 frequencies to aid in the determination of atmospheric delays.

GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this

information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. This delay is calculated, and the length of the delay tells the signal’s travel time. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map. A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset.

achieves with some tricks. 4. Along with distance, you need to know exactly where the satellites are in space. High orbits and careful monitoring are the secret. 5. Finally you must correct for any delays the signal experiences as it travels through the atmosphere. Improbable as it may seem, the whole idea behind GPS is to use satellites in space as reference points for location here on earth. That’s right by very, very accurately measuring our distance from the satellites we can “triangulate” our position anywhere on the earth. Forget for a moment how our receiver measures this distance. We'll get to that later. First consider how distance measurements from three satellites can pinpoint you in space.

The Big Idea Geometrically:
Suppose we measure our distance from a satellite and find it to be 11,000 miles. Knowing that we're 11,000 miles from a particular satellite narrows down all the possible locations we could be in the whole universe to the surface of a sphere that is centered on this satellite and has a radius of 11,000 miles. In Review: Triangulating

Here's how GPS works in five logical steps:
1. The basis of GPS is "triangulation" from satellites. 2. To "triangulate," a GPS receiver measures distance using the travel time of radio signals. 3. To measure travel time, GPS needs very accurate timing, which it

• • • •

Position is calculated from distance measurements (ranges) to satellites. Mathematically we need four satellite ranges to determine exact position. Three ranges are enough if we reject ridiculous answers or use other tricks. Another range is required for technical reasons to be discussed later

Several frequencies make up the GPS Electromagnetic spectrum:

• •

L1 (1575.42MHz): Caries a publicly usable coarse acquisition C/A) code as well as an encrypted position P (Y) code. L2 (1227.60MHz): Usually carries only the P(Y) code. The encryption keys required to directly use the P(Y) code are tightly controlled by the U.S. Government and are generally provided only for military use. The keys are changed on a daily basis. In spite of not having the P (Y) code encryption key, several high-end GPS receiver manufacturers have developed techniques for utilizing this signal (in a round-about manner) to increase accuracy and remove error caused by the ionosphere. L3 (1381.05MHz): Carries the signal for the GPS constellation’s alternative role of detecting missile/rocket launches (supplementing Defense Support Program satellites), nuclear detonations, and other high-energy infrared events. L4 (1841.40MHz): Being studied for additional ionospheric correction. L5 (1176.45MHz): Proposed for use as a civilian safety-of-life signal.

Techniques to improve GPS accuracy
The accuracy of GPS can be improved in a number of ways: • Using a network of fixed ground based reference stations. These stations broadcast the difference between the measured satellite pseudo ranges and actual (internally computed) pseudo ranges, and receiver stations may correct their pseudo ranges by the same amount. This method is called Differential GPS or DGPS.DGPS was especially

useful when GPS was still degraded (via the "Selective Availability” described below), since DGPS could nevertheless provide 5–10 meter accuracy. The DGPS network has been mainly developed by the Finnish and Swedish maritime administrations in order to improve safety in the archipelago between the two countries. • Exploitation of DGPS for Guidance Enhancement (EDGE) is an effort to integrate DGPS into precision-guided munitions such as the Joint Direct Attack Munitions (JDAM). • The Wide Area Augmentation System WAAS). This uses a series of ground reference stations to calculate GPS correction messages, which are uploaded to a series of additional satellites in geosynchronous orbit for transmission to GPS receivers, including information on ionospheric delays, individual satellite clock drift, and suchlike. Although only a few WAAS satellites are currently available as of 2004, it is hoped that eventually WAAS will provide sufficient reliability and accuracy that it can be used for critical applications such as GPS-based instrument approaches in aviation (landing an airplane in conditions of little or no visibility). The current WAAS system only works for North America (Where the reference stations are located), and due to the satellite location the system is only generally usable in the eastern and western coastal regions. However, variants of the WAAS system are being developed in Europe (EGNOS, the Euro Geostationary Navigation Overlay Service), and Japan (MSAS, the Multi-Functional Satellite Augmentation system), which are virtually identical to WAAS. Currently, WAAS satellite coverage is only available in North America. There are no ground reference stations in South

America, so even though GPS users there can receive WAAS, the signal has not been corrected and thus would not improve the accuracy of their unit. For some users in the U.S., the position of the satellites over the equator makes it difficult to receive the signals when trees or mountains obstruct the view of the horizon. WAAS signal reception is ideal for open land and marine applications. WAAS provides extended coverage both inland and offshore compared to the land based DGPS (differential GPS) system. Another benefit of WAAS is that it does not require additional receiving equipment, while DGPS does. • A Local-Area Augmentation System (LAAS). This is similar to WAAS, in that similar correction data are used. But in this case, the correction data are transmitted from a local source, typically at an airport or another location where accurate positioning is needed. These correction data are typically useful for only about a thirty to fifty kilometer radius around the transmitter. • Wide Area GPS Enhancement (WAGE) is an attempt to improve GPS accuracy by providing more accurate satellite clock and ephemeris (orbital) data to specially equipped receivers. • Relative Kinematic Positioning (RKP) is another approach for a precise GPS-based positioning system. In this approach, accurate determinination of range signal can be resolved to an accuracy of less than 10 centimeters. This is done by resolving the number of cycles in which the signal is transmitted and received by the receiver. This can be accomplished by using a combination of differential GPS (DGPS) correction data, transmitting GPS signal phase information and

ambiguity resolution techniques via statistical tests—possibly with processing in real-time (real-time kinematic positioning, RTK) Many automobile GPS systems combine the GPS unit with a gyroscope and speedometer pickup, allowing the computer to maintain a continuous navigation solution by dead reckoning when buildings, terrain, or tunnels block the satellite signals. This is similar principle to the combination of GPS and inertial navigation used in ships and aircraft, but less accurate and less expensive because it only fills in for short periods.

GPS tracking means to trace something or someone with the Global Positioning System. The below diagram illustrates the basic AVL system. It shows the GPS signal arriving from satellite to vehicle. The vehicle location is communicated to the PC (Control Center) via wireless network.

But for thousands of years Homo sapiens has had the opportunity to observe the movement and general habits of members of his own species as well as of wildlife, particularly by following their

tracks. It was a hard and particular unsafe affair. Hence the development of satellite tracking by the Argos consortium was a quantum leap in the human Tracking business. Since 1994 the Global Positioning System has been available for civilian use at no cost. Nowadays GPS makes it available to everyone to track nearly everything. Objects as well as persons can be tracked if they are fitted out with a GPS receiver estimating the respective location. The GPS location data is stored on board of the GPS receiver. Modern GPS tracking systems are able to send such GPS position data from the object directly to a receiving station. A receiving station can be a stationary receiver of a tracking service company (in case of car tracking f. ex.) or provider of a mobile phone company, or just a PC. Nowadays the GPS location data can be also received by small mobile gadgets like laptops, handsets etc. The AVL tracking system consists of a GPS receiver inside the vehicle and a communications link between the vehicle and the control Center as well as pc-based tracking software for dispatch. The communication system is usually a cellular network similar to the one used by your cellular phone. Currently all kind of communications networks permit Real-Time Tracking for mobile assets. Communications used by Laipac for Vehicle Tracking Systems.


GPS jamming
A large part of modern munitions, the so-called "smart bombs" or precision-guided munitions, use GPS. GPS jammers are available, from Russia, and are about the size of a cigarette box. The U.S. government believes that such jammers were used occasionally during the U.S. invasion Afghanistan. Some officials believe that jammers could be used to attract the precision-guided Munitions towards noncombatant infrastructure, other officials believe that the jammers are completely ineffective. In either case, the jammers are attractive targets for anti-radiation missiles.


GPS Segments
The GPS system is divided into three segments: space, control, and user. The space segment comprises the GPS satellite constellation. The control segment comprises ground stations around the world that are responsible for monitoring the flight paths of the GPS satellites, synchronizing the satellites' onboard atomic clocks, and uploading data for transmission by the satellites. The user segment consists of GPS receivers used for both military and civilian

Who uses GPS? GPS has a variety of applications on land, at sea and in the air. Basically, GPS is usable everywhere except where it's impossible to receive the signal such as inside most buildings, in caves and other subterranean locations, and underwater. The most common airborne applications are for navigation by general aviation and commercial aircraft. At sea, GPS is also typically used for navigation by recreational boaters, commercial fishermen, and professional mariners. Land-based applications are more diverse. The scientific

community uses GPS for its precision timing capability and position information. Surveyors use GPS for an increasing portion of their work. GPS offers cost savings by drastically reducing setup time at the survey site and providing incredible accuracy. Basic survey units, costing thousands of dollars, can offer accuracies down to one meter. More expensive systems are available that can provide accuracies to within a centimeter. Recreational uses of GPS are almost as varied as the number of recreational sports available. GPS is popular among hikers, hunters, snowmobilers, mountain bikers, and cross-country skiers, just to name a few. Anyone who needs to keep track of where he or she is, to find his or her way to a specified location, or know what direction and how fast he or she is going can utilize the benefits of the global positioning system. GPS is now commonplace in automobiles as well. Some basic systems are in place and provide emergency roadside assistance at the push of a button (by transmitting your current position to a dispatch center). More sophisticated systems that show your position on a street map are also available. Currently these systems allow a driver to keep track of where he or she is and suggest the best route to follow to reach a designated location. The primary military purposes are to allow improved command and control of forces through improved location awareness, and to facilitate accurate targeting of smart bombs, cruise missiles, or other munitions. The satellites also carry nuclear detonation detectors, which form a major portion of 5 the United States Nuclear Detonation Detection System. The system is used by countless civilians as well, who can use the GPS's Standard Positioning Service worldwide free of charge. Low cost GPS receivers (price $100 to $200) are widely available, combined in a bundle with a PDA or car computer. The system is used as a

navigation aid in airplanes, ships and cars. The system can be used by computercontrolled harvesters, mine trucks and other vehicles. Hand held devices are used by mountain climbers and hikers. Glider pilots use the logged signal to verify their arrival at turn points in competitions. Military (and selected civilian) users still enjoy some technical advantages, which can give quicker satellite lock and increased accuracy. The increased accuracy comes mostly from being able to use both the L1 and L2 frequencies and thus better compensate for the varying signal delay in the ionosphere (see above). Commercial GPS receivers are also required to have limits on the velocities and altitudes at which they will report fix coordinates; this is to prevent them from being used to create improvised cruise or ballistic missiles. Many synchronization systems use GPS as a source of accurate time, hence one of the most common applications of this use is that of GPS as a reference clock for time code generators or NTP clocks. For instance, when deploying sensors (for seismology or other monitoring application), GPS may be used to provide each recording apparatus with some precise time source, so that the time of events may be recorded accurately. Mapping is the art and science of using GPS to locate items, and then create maps and models of everything in the world. And we do mean everything. Mountains, rivers, forests and other landforms. Roads, routes, and city streets. Endangered animals, precious minerals and all sorts of resources. Damage and disasters, trash and archeological treasures. GPS is mapping the world. GPS for Private and commercial Use The GPS system is free for everyone to use, all that is needed is a GPS receiver, which costs about $90 and up (March 2005). This

has led to widespread private and commercial use. An example of private use is the popular activity Geocaching where a GPS unit is used to search for objects hidden in nature by traveling to the GPS coordinates. Commercial use can be land measurement, navigation and road construction.

there have been several research projects such as MoBIC, Drishti, and Brunel Navigation System for the Blind, NOPPA, Braille Note GPS and Trekker. MoBIC MoBIC means Mobility of Blind and Elderly people interacting with Computers, which was carried out from 1994 to 1996 supported by the Commission of the European Union. It was developing a route planning system, which is designed to allow a blind person access to information from many sources such as bus and train timetables as well as electronic maps of the locality. The planning system helps blind people to study and plan their routes in advance, indoors.

GPS on Air Planes
Most airline companies allow private use of ordinary GPS units on their flights, except during landing and take-off, like all other electronic devices. The unit does not transmit radio signals like mobile phones, it can only receive. Note, however, that some airline companies might disallow it for security reasons, such as unwillingness to let ordinary passengers track the flight route.

GPS in marine system

1. 2. 3. 4. Satellite Communications by “Dennis Roddy”.

Marine GPS receivers feature waterproof casings, marine chart plotter maps, and even fishing tables and celestial schedules. Most can also store highway map information, so you can use your marine GPS to get you to the marina and then out to the fish.

GPS For Visually Impaired
The projects of the navigation system using GPS for the visually impaired have been conducted quite a few times. GPS was introduced in the late 80’s and since then

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