Seminar Global Positioning System Main
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seminar report 2008-2009
global positioning system
INTRODUCTION Today , the development of the science and technology is rapid.To compete with this , an individual must utilize innovative intelligence. Now the whole world are running to discover latest technics for each and every field. The Global Positioning System (GPS)is anapplication of computer and microcontroller technology.Today’s fast growing life standards demands such a navigation system.The main purpose of GPS is the military navigation and tracking. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but now available for civilian use also. GPS is changing the lifestyle of newgeneration and having its influence in almost all areas such as military ,agriculture, aerospace , reserch, industrial etc. It is now widely used for navigation using cell phones and in vehicles. Also to track the path , GPS is used.
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HISTORY OF GPS
1940 - LORAN & DECCA – Radio Navigation( Ground based )
1960 - Transit - U S Navy- five satellites
1967 - Timation -U.S.Navy- for time info
1970 - Omega Navigation System-1st satellite based radio navigation
1983 - First version of GPS tested
1989 - GPS satelites setting to 1993
1993 - GPS is ready to use only for military
1997 - GPS is ready to use for public also , but not clear signals
2000 - GPS is available to civilians in clear form.
Main inspiration for the GPS came when the Soviet Union launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.
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How it workS 24 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. The time difference tells the GPS receiver how far away the satellite is. 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.
24 satelites revolving earth
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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 time and more.
Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible. KGPTC Dept. of Tool & Die Engineering
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principles A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information. It might seem three satellites are enough to solve for position, since space has three dimensions. However a very small clock error multiplied by the very large speed of light the speed at which satellite signals propagate results in a large positional error.
The receiver uses a fourth satellite to solve for x, y, z, and t While most GPS applications use the computed location only and effectively hide the very accurately computed time, it is used in a few specialized GPS applications such as time transfer and traffic signal timing.
Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as xi,yi,zi,ti ; Knowing the indicated time the message was received tr, the GPS receiver can compute the indicated transit time, tr-ti. of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as . (tr-ti)c . GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the KGPTC Dept. of Tool & Die Engineering
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indicated position of the GPS receiver is at the intersection of the surfaces of four spheres ( If no error )
A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivable that they could intersect at one point—or not at all. Another figure, Surface of Sphere Intersecting a Circle at Two Points, shows this intersection. The two intersections are marked with dots. Again trilateration clearly shows this mathematically.
The correct position of the GPS receiver is the intersection that is closest to the surface of the earth for automobiles and other near-Earth vehicles. The correct position of the GPS receiver is also the intersection which is closest to the surface of the sphere corresponding to the fourth satellite. (The two intersections are symmetrical with respect to the plane containing the three satellites. If the three satellites are not in the same orbital plane, the plane containing the three satellites will not be a vertical plane passing through the center of the Earth. In this case one of the intersections will be closer to the earth than the other. The near-Earth intersection will be the correct position for the case of a near-Earth vehicle. The intersection which is farthest from Earth may be the correct position for space vehicles.)
The method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this KGPTC Dept. of Tool & Die Engineering
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dilemma is based on the way sphere surfaces intersect in the GPS problem.
System Details There are 3 main segments in a GPS system. They are
1. Space segment(SS).
2. Controll Segment(CS).
3. User Segment(US).
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Space Segment(SS) The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes, but this was modified to six planes with four satellites each. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. KGPTC Dept. of Tool & Die Engineering
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A GPS Satelite- Space segmant
Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within sight (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day. The ground track of each satellite therefore repeats each day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.
As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS KGPTC Dept. of Tool & Die Engineering
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receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.
GPS Satelite sending signal to control station
Control segment The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA). The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado KGPTC Dept. of Tool & Die Engineering
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Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF).
There are 3 stations for the control segment. They are
• A)Master Controll Stations (MCS) • B)Monitor Stations(MS) • C)Ground Antennas(GA)
Master Controll Stations Its located near Colorado Springs in US.They receives the signals from Monitor Stations and pass it to antennas.They are the intermediator between the Monitor Stations and Antennas. The Demodulation and transmission of signals are done here. KGPTC Dept. of Tool & Die Engineering
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Monitor Stations There are 6 Monitor Stations all over world.
They
receives signals from satelites & pass to Master Controll Stations. They can receive the modulated signal. They are the one for the maintenance of the Satelite and for the control.
Ground Antennas They receives demodulated signals from MCS and transmits to air and receivers receives those signals. Thus the desired result will be shown in the display .
USER segment The user's GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highlystable clock (often a crystal oscillator). KGPTC Dept. of Tool & Die Engineering
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They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels
Gps receivers
GPS Signals
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-week, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part KGPTC Dept. of Tool & Die Engineering
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of the message). The messages are sent in frames, each taking 30 seconds to transmit 1500 bits.
Transmission of each 30 second frame begins precisely on the minute and half minute as indicated by the satellite's atomic clock according to Satellite message format. Each frame contains 5 subframes of length 6 seconds and with 300 bits. Each subframe contains 10 words of 30 bits with length 0.6 seconds each.
Words 1 and 2 of every subframe have the same type of data. The first word is the telemetry word which indicates the beginning of a subframe and is used by the receiver to synch with the navigation message. The second word is the HOW or handover word and it contains timing information which enables the receiver to identify the subframe and provides the time the next subframe was sent.
Words 3 through 10 of subframe 1 contain data describing the satellite clock and its relationship to GPS time. Words 3 through 10 of subframes 2 and 3, contain the ephemeris data, giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate.
The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to KGPTC Dept. of Tool & Die Engineering
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relate GPS derived time to Coordinated Universal Time (UTC). Words 3 through 10 of subframes 4 and 5 contain a new part of the almanac. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the entire almanac from a single satellite.[23] The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite.
All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses a code division multiple access (CDMA) spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual message data. The message data is transmitted at 50 bits per second. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code (a so-called Gold code) at 1.023 million chips per second, and the precise (P) code at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[24] The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.
SIGNAL STRUCTURE
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L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.
L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
L4 (1379.913 MHz): Being studied for additional ionospheric correction.
L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009
Sources of errors KGPTC Dept. of Tool & Die Engineering
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User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, , of about 1 meter. The standard deviations, , for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e. RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error.
The main sources are 1. Signal arrival time measurement 2. Atmospheric effects 3. Multipath effects 4. Ephemeris and clock errors 5. Geometric dilution of precision computation (DOP)
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Sources of Interference There are mainly two sorces.
1.
Natural sources
2.
Artificial sources
Natural sources Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.
Artificial sources In automotive GPS receivers, metallic features in windshields,such as defrosters, or car window tinting filmscan act as a Faraday cage, degrading reception just inside the car.
Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well documented case, the entire harbor of Moss Landing, Calif. was unable to receive GPS signals due to unintentional jamming caused by malfunctioning TV antenna preamplifiers. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range KGPTC Dept. of Tool & Die Engineering
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DIFFERENTIAL GPS The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal. Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution, DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV. These corrections are passed to the remote, or rover, receiver which must be capable of applying these individual pseudo-range corrections to each SV used in the navigation solution. Applying a simple position correction from the reference receiver to the remote receiver has limited effect at useful ranges because both receivers would have to be using the same set of SVs in their navigation solutions and have identical GDOP terms (not possible at different locations) to be identically affected by bias errors.
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APPLICATIONS OF GPS
Agriculture (proper soil selection , Fertilizer application)
Aviation (to view roots and air ports , pads etc)
Environment (mapping , weather forecast )
Transportation (vehicle tracking,personal )
Marine (Root navigation,prediction of risks)
Military (target navigatn,mapping,routing)
Rail (passage navign,building up of new tracks)
Space (Space research,forecasting)
Surveying ( for survey purposes)
Timing (for accurate time calculation)
Industial ( for selecting plant locations , transportation , etc. )
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CONCLUSION The GPS is now changing the way we live , standards and very useful. There should be more and more applications for GPS in the future years. Also its available in any part of the world. GPS functionality has now started to move into mobile phones en masse. The first handsets with integrated GPS were launched already in the late 1990’s, and were available for broader consumer availability on networks such as those run by Nextel, Sprint and Verizon in 2002 in response to US FCC mandates for handset positioning in emergency calls. Capabilities for access by third party software developers to these features were slower in coming, with Nextel opening up those APIs upon launch to any developer, Sprint following in 2006, and Verizon soon thereafter.
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REFERENCE 1.
1999 Federal Radionavigation Plan, February 2000 Washington, US Dept.of Defence.
2. GPS Std Positioning Service Specification, 2nd Edn June1995. onlinefrom U.S Coast Guard Navign Center 3.
GPS Joint Program Office. 1997. ICD-GPS-200: GPS Interface Control Document. ARINC Research
4.
Hoffmann-Wellenhof, B. H. Lichtenegger, & Collins. 1994. GPS: Theory and Practice. 3rd ed.New York.
5.
Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New
6.
http://www.nasa.com/globalpositioningsystem
7.
http://www.wikipedia.org/search#gps#articles.php
8. http://www.aero.org/publications/crosslink/summer2002/01.html. 9. http://www.icao.int/cgi/goto_m.pl?icao/en/trivia/kal_flight_007.htm. 10. http://www.colorado.edu/geography/gcraft/notes/gps/gif/oplanes 11 http://www.google.com/imagesearch/gps KGPTC Dept. of Tool & Die Engineering
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12. http://www.navstar.com/cgi/goto_m.pl?icao/en
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global positioning system
INTRODUCTION Today , the development of the science and technology is rapid.To compete with this , an individual must utilize innovative intelligence. Now the whole world are running to discover latest technics for each and every field. The Global Positioning System (GPS)is anapplication of computer and microcontroller technology.Today’s fast growing life standards demands such a navigation system.The main purpose of GPS is the military navigation and tracking. The Global Positioning System (GPS) is a satellite-based navigation system made up of a network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally intended for military applications, but now available for civilian use also. GPS is changing the lifestyle of newgeneration and having its influence in almost all areas such as military ,agriculture, aerospace , reserch, industrial etc. It is now widely used for navigation using cell phones and in vehicles. Also to track the path , GPS is used.
KGPTC Dept. of Tool & Die Engineering
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HISTORY OF GPS
1940 - LORAN & DECCA – Radio Navigation( Ground based )
1960 - Transit - U S Navy- five satellites
1967 - Timation -U.S.Navy- for time info
1970 - Omega Navigation System-1st satellite based radio navigation
1983 - First version of GPS tested
1989 - GPS satelites setting to 1993
1993 - GPS is ready to use only for military
1997 - GPS is ready to use for public also , but not clear signals
2000 - GPS is available to civilians in clear form.
Main inspiration for the GPS came when the Soviet Union launched the first Sputnik in 1957. A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio transmissions. They discovered that, because of the Doppler effect, the frequency of the signal being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away from them. They realized that since they knew their exact location on the globe, they could pinpoint where the satellite was along its orbit by measuring the Doppler distortion.
KGPTC Dept. of Tool & Die Engineering
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How it workS 24 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. The time difference tells the GPS receiver how far away the satellite is. 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.
24 satelites revolving earth
KGPTC Dept. of Tool & Die Engineering
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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 time and more.
Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known (for example, a ship or plane may have known elevation), a receiver can determine its position using only three satellites. Some GPS receivers may use additional clues or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or including information from the vehicle computer) to give a degraded position when fewer than four satellites are visible. KGPTC Dept. of Tool & Die Engineering
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principles A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, precise orbital information. It might seem three satellites are enough to solve for position, since space has three dimensions. However a very small clock error multiplied by the very large speed of light the speed at which satellite signals propagate results in a large positional error.
The receiver uses a fourth satellite to solve for x, y, z, and t While most GPS applications use the computed location only and effectively hide the very accurately computed time, it is used in a few specialized GPS applications such as time transfer and traffic signal timing.
Using messages received from a minimum of four visible satellites, a GPS receiver is able to determine the satellite positions and time sent. The x, y, and z components of position and the time sent are designated as xi,yi,zi,ti ; Knowing the indicated time the message was received tr, the GPS receiver can compute the indicated transit time, tr-ti. of the message. Assuming the message traveled at the speed of light, c, the distance traveled, can be computed as . (tr-ti)c . GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the KGPTC Dept. of Tool & Die Engineering
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indicated position of the GPS receiver is at the intersection of the surfaces of four spheres ( If no error )
A circle and sphere surface in most cases of practical interest intersect at two points, although it is conceivable that they could intersect at one point—or not at all. Another figure, Surface of Sphere Intersecting a Circle at Two Points, shows this intersection. The two intersections are marked with dots. Again trilateration clearly shows this mathematically.
The correct position of the GPS receiver is the intersection that is closest to the surface of the earth for automobiles and other near-Earth vehicles. The correct position of the GPS receiver is also the intersection which is closest to the surface of the sphere corresponding to the fourth satellite. (The two intersections are symmetrical with respect to the plane containing the three satellites. If the three satellites are not in the same orbital plane, the plane containing the three satellites will not be a vertical plane passing through the center of the Earth. In this case one of the intersections will be closer to the earth than the other. The near-Earth intersection will be the correct position for the case of a near-Earth vehicle. The intersection which is farthest from Earth may be the correct position for space vehicles.)
The method of calculating position for the case of no errors has been explained. One of the most significant error sources is the GPS receiver's clock. Because of the very large value of the speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudoranges, are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer to build inexpensive GPS receivers for mass markets. The solution for this KGPTC Dept. of Tool & Die Engineering
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dilemma is based on the way sphere surfaces intersect in the GPS problem.
System Details There are 3 main segments in a GPS system. They are
1. Space segment(SS).
2. Controll Segment(CS).
3. User Segment(US).
KGPTC Dept. of Tool & Die Engineering
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Space Segment(SS) The space segment (SS) comprises the orbiting GPS satellites, or Space Vehicles (SV) in GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital planes, but this was modified to six planes with four satellites each. The orbital planes are centered on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55° inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending node (angle along the equator from a reference point to the orbit's intersection). The orbits are arranged so that at least six satellites are always within line of sight from almost everywhere on Earth's surface. KGPTC Dept. of Tool & Die Engineering
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A GPS Satelite- Space segmant
Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible within sight (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or 14,400 NM)), each SV makes two complete orbits each sidereal day. The ground track of each satellite therefore repeats each day. This was very helpful during development, since even with just four satellites, correct alignment means all four are visible from one spot for a few hours each day. For military operations, the ground track repeat can be used to ensure good coverage in combat zones.
As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation. The additional satellites improve the precision of GPS KGPTC Dept. of Tool & Die Engineering
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receiver calculations by providing redundant measurements. With the increased number of satellites, the constellation was changed to a nonuniform arrangement. Such an arrangement was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.
GPS Satelite sending signal to control station
Control segment The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii, Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor stations operated by the National Geospatial-Intelligence Agency (NGA). The tracking information is sent to the Air Force Space Command's master control station at Schriever Air Force Base in Colorado KGPTC Dept. of Tool & Die Engineering
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Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United States Air Force (USAF).
There are 3 stations for the control segment. They are
• A)Master Controll Stations (MCS) • B)Monitor Stations(MS) • C)Ground Antennas(GA)
Master Controll Stations Its located near Colorado Springs in US.They receives the signals from Monitor Stations and pass it to antennas.They are the intermediator between the Monitor Stations and Antennas. The Demodulation and transmission of signals are done here. KGPTC Dept. of Tool & Die Engineering
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Monitor Stations There are 6 Monitor Stations all over world.
They
receives signals from satelites & pass to Master Controll Stations. They can receive the modulated signal. They are the one for the maintenance of the Satelite and for the control.
Ground Antennas They receives demodulated signals from MCS and transmits to air and receivers receives those signals. Thus the desired result will be shown in the display .
USER segment The user's GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors, and a highlystable clock (often a crystal oscillator). KGPTC Dept. of Tool & Die Engineering
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They may also include a display for providing location and speed information to the user. A receiver is often described by its number of channels: this signifies how many satellites it can monitor simultaneously. Originally limited to four or five, this has progressively increased over the years so that, as of 2007, receivers typically have between 12 and 20 channels
Gps receivers
GPS Signals
Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-of-week, GPS week number and satellite health information (all transmitted in the first part of the message), an ephemeris (transmitted in the second part of the message) and an almanac (later part KGPTC Dept. of Tool & Die Engineering
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of the message). The messages are sent in frames, each taking 30 seconds to transmit 1500 bits.
Transmission of each 30 second frame begins precisely on the minute and half minute as indicated by the satellite's atomic clock according to Satellite message format. Each frame contains 5 subframes of length 6 seconds and with 300 bits. Each subframe contains 10 words of 30 bits with length 0.6 seconds each.
Words 1 and 2 of every subframe have the same type of data. The first word is the telemetry word which indicates the beginning of a subframe and is used by the receiver to synch with the navigation message. The second word is the HOW or handover word and it contains timing information which enables the receiver to identify the subframe and provides the time the next subframe was sent.
Words 3 through 10 of subframe 1 contain data describing the satellite clock and its relationship to GPS time. Words 3 through 10 of subframes 2 and 3, contain the ephemeris data, giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions. The time needed to acquire the ephemeris is becoming a significant element of the delay to first position fix, because, as the hardware becomes more capable, the time to lock onto the satellite signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to the low data transmission rate.
The almanac consists of coarse orbit and status information for each satellite in the constellation, an ionospheric model, and information to KGPTC Dept. of Tool & Die Engineering
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relate GPS derived time to Coordinated Universal Time (UTC). Words 3 through 10 of subframes 4 and 5 contain a new part of the almanac. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the entire almanac from a single satellite.[23] The almanac serves several purposes. The first is to assist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible satellites based on stored position and time, while an ephemeris from each satellite is needed to compute position fixes using that satellite.
All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276 GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses a code division multiple access (CDMA) spread-spectrum technique where the low-bitrate message data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite. The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual message data. The message data is transmitted at 50 bits per second. Two distinct CDMA encodings are used: the coarse/acquisition (C/A) code (a so-called Gold code) at 1.023 million chips per second, and the precise (P) code at 10.23 million chips per second. The L1 carrier is modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[24] The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a so-called P(Y) code which is only available to military equipment with a proper decryption key. Both the C/A and P(Y) codes impart the precise time-of-day to the user.
SIGNAL STRUCTURE
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L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and encrypted precision P(Y) code, plus the new L1C on future Block III satellites.
L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer satellites.
L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload (NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to enforce nuclear test ban treaties.
L4 (1379.913 MHz): Being studied for additional ionospheric correction.
L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009
Sources of errors KGPTC Dept. of Tool & Die Engineering
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User equivalent range errors (UERE) are shown in the table. There is also a numerical error with an estimated value, , of about 1 meter. The standard deviations, , for the coarse/acquisition and precise codes are also shown in the table. These standard deviations are computed by taking the square root of the sum of the squares of the individual components (i.e. RSS for root sum squares). To get the standard deviation of receiver position estimate, these range errors must be multiplied by the appropriate dilution of precision terms and then RSS'ed with the numerical error.
The main sources are 1. Signal arrival time measurement 2. Atmospheric effects 3. Multipath effects 4. Ephemeris and clock errors 5. Geometric dilution of precision computation (DOP)
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Sources of Interference There are mainly two sorces.
1.
Natural sources
2.
Artificial sources
Natural sources Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible.
Artificial sources In automotive GPS receivers, metallic features in windshields,such as defrosters, or car window tinting filmscan act as a Faraday cage, degrading reception just inside the car.
Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well documented case, the entire harbor of Moss Landing, Calif. was unable to receive GPS signals due to unintentional jamming caused by malfunctioning TV antenna preamplifiers. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range KGPTC Dept. of Tool & Die Engineering
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DIFFERENTIAL GPS The idea behind all differential positioning is to correct bias errors at one location with measured bias errors at a known position. A reference receiver, or base station, computes corrections for each satellite signal. Because individual pseudo-ranges must be corrected prior to the formation of a navigation solution, DGPS implementations require software in the reference receiver that can track all SVs in view and form individual pseudo-range corrections for each SV. These corrections are passed to the remote, or rover, receiver which must be capable of applying these individual pseudo-range corrections to each SV used in the navigation solution. Applying a simple position correction from the reference receiver to the remote receiver has limited effect at useful ranges because both receivers would have to be using the same set of SVs in their navigation solutions and have identical GDOP terms (not possible at different locations) to be identically affected by bias errors.
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APPLICATIONS OF GPS
Agriculture (proper soil selection , Fertilizer application)
Aviation (to view roots and air ports , pads etc)
Environment (mapping , weather forecast )
Transportation (vehicle tracking,personal )
Marine (Root navigation,prediction of risks)
Military (target navigatn,mapping,routing)
Rail (passage navign,building up of new tracks)
Space (Space research,forecasting)
Surveying ( for survey purposes)
Timing (for accurate time calculation)
Industial ( for selecting plant locations , transportation , etc. )
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CONCLUSION The GPS is now changing the way we live , standards and very useful. There should be more and more applications for GPS in the future years. Also its available in any part of the world. GPS functionality has now started to move into mobile phones en masse. The first handsets with integrated GPS were launched already in the late 1990’s, and were available for broader consumer availability on networks such as those run by Nextel, Sprint and Verizon in 2002 in response to US FCC mandates for handset positioning in emergency calls. Capabilities for access by third party software developers to these features were slower in coming, with Nextel opening up those APIs upon launch to any developer, Sprint following in 2006, and Verizon soon thereafter.
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REFERENCE 1.
1999 Federal Radionavigation Plan, February 2000 Washington, US Dept.of Defence.
2. GPS Std Positioning Service Specification, 2nd Edn June1995. onlinefrom U.S Coast Guard Navign Center 3.
GPS Joint Program Office. 1997. ICD-GPS-200: GPS Interface Control Document. ARINC Research
4.
Hoffmann-Wellenhof, B. H. Lichtenegger, & Collins. 1994. GPS: Theory and Practice. 3rd ed.New York.
5.
Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New
6.
http://www.nasa.com/globalpositioningsystem
7.
http://www.wikipedia.org/search#gps#articles.php
8. http://www.aero.org/publications/crosslink/summer2002/01.html. 9. http://www.icao.int/cgi/goto_m.pl?icao/en/trivia/kal_flight_007.htm. 10. http://www.colorado.edu/geography/gcraft/notes/gps/gif/oplanes 11 http://www.google.com/imagesearch/gps KGPTC Dept. of Tool & Die Engineering
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12. http://www.navstar.com/cgi/goto_m.pl?icao/en
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