18-11-2009, 03:08 PM
i need full report on ' mobile phone trcaking and positioning'...please send me as soon as possibile..
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mobile phone tracking and positioning
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i need full report on ' mobile phone trcaking and positioning'...please send me as soon as possibile..
18-11-2009, 08:19 PM
please make a good seminar report based on the information given below
see
http://tech-faqcell-phone-tracking.shtml
http://en.wikipediawiki/Mobile_phone_tracking
ABSTRACT
Mobile positioning technology has become an important area of research, for emergency as well as for commercial services. Mobile positioning in cellular networks will provide several services such as, locating stolen mobiles, emergency calls, different billing tariffs depending on where the call is originated, and methods to predict the user movement inside a region.
The evolution to location-dependent services and applications in wireless systems continues to require the development of more accurate and reliable mobile positioning technologies. The major challenge to accurate location estimation is in creating techniques that yield acceptable performance when the direct path from the transmitter to the receiver is intermittently blocked.
Tracking and Positioning of Mobiles in Telecommunication
INTRODUCTION
A mobile tracking and positioning system includes a plurality of mobile transmit and receive stations that track a mobile target which emits a radio signal in response to the occurrence of a tracking effort initiation event. The tracking stations have a GPS receiver or like means for determining their position, a radio direction finder responsive to the radio signal that determines the vector to the mobile target, a two-way communications system and a computer. The mobile transmit and receive stations exchange their position and direction to target information via the two-way communications systems, enabling the stations to triangulate the location of the target with their computers. Mobile phone tracking tracks the current position of a mobile phone even on the move. To locate the phone, it must emit at least the roaming signal to contact the next nearby antenna tower, but the process does not require an active call. GSM localisation is then done by multilateration based on the signal strength to nearby antenna masts. Mobile positioning, i.e. location based service that discloses the actual coordinates of a mobile phone bearer, is a technology used by telecommunication companies to approximate where a mobile phone, and thereby also its user (bearer), temporarily resides. The more properly applied term locating refers to the purpose rather than a positioning process. Such service is offered as an option of the class of locationbased services.
Tracking and Positioning of Mobiles in Telecommunication
MOBILE TRACKING
Mobile phone tracking tracks the current position of a mobile phone even on the move. To locate the phone, it must emit at least the roaming signal to contact the next nearby antenna tower, but the process does not require an active call. GSM localisation is then done by multilateration based on the signal strength to nearby antenna masts. In order to route calls to your phone the cell towers listen for a signal sent from the phone and negotiate which tower is best able to communicate with the phone. As the phone changes location, the towers monitor the signal and the phone is switched to a different tower as appropriate. By comparing the relative signal strength from multiple towers a general location of a phone can be determined. The technology of tracking is based on measuring power levels and antenna patterns and uses the concept that a mobile phone always communicates wirelessly with one of the closest base stations, so if you know which base station the phone communicates with, you know that the phone is close to the respective base station. Advanced systems determine the sector in which the mobile phone resides and roughly estimate also the distance to the base station. Further approximation can be done by interpolating signals between adjacent antenna towers. Qualified services may achieve a precision of down to 50 meters in urban areas where mobile traffic and density of antenna towers (base stations) is sufficiently high. Rural and desolate areas may see miles between base stations and therefore determine locations less precisely.
Tracking and Positioning of Mobiles in Telecommunication
Operational purpose
In order to route calls to a phone the cell towers listen for a signal sent from the phone and negotiate which tower is best able to communicate with the phone. As the phone changes location, the antenna towers monitor the signal and the phone is roamed to an adjacent tower as appropriate. By comparing the relative signal strength from multiple antenna towers a general location of a phone can be roughly determined. Other means is the antenna pattern that supports angular determination and phase discrimination. Newer phones may also allow the tracking of the phone even when turned on and not active in a telephone call-. This results from the roaming procedures that perform hand over of the phone from one base station to another. The principle of tracking is based on GSM localisation.
Tracking and Positioning of Mobiles in Telecommunication
GLOBAL SYSTEM FOR MOBILE COMMUNICATION (GSM)
GSM (Global System for Mobile communications) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 82% of the global mobile market uses the standard. GSM is used by over 3 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital, and thus is considered a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system. The ubiquity of the GSM standard has been an advantage to both consumers (who benefit from the ability to roam and switch carriers without switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM).GSM also pioneered a low-cost (to the network carrier) alternative to voice calls, the Short message service (SMS, also called "text messaging"), which is now supported on other mobile standards as well. Another advantage is that the standard includes one worldwide Emergency telephone number; 112This makes it easier for international travelers to connect to emergency services without knowing the local emergency number. Newer versions of the standard were backward-compatible with the original GSM phones. For example, Release '97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). Release '99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution (EDGE). 5
Tracking and Positioning of Mobiles in Telecommunication
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
The Base Station Subsystem (the base stations and their controllers). The Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
The GPRS Core Network (the optional part which allows packet based Internet connections).
All of the elements in the system combine to produce many GSM services such as voice calls and SMS.
Tracking and Positioning of Mobiles in Telecommunication
GSM is designed to provide recommendations only. It does not cater to the requirements. The specification does not include any hardware details but only define the functions and interface requirements. It has been done intentionally so that there is limit to the designers but still they are able to make it possible for the operators to buy the instrument or handset from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system(OSS). The Mobile Station is contained in the handset only which is carried by the subscriber. The Base Station Subsystem sets and controls the radio page link of the network with the Mobile Station. The Network Subsystem controls the main part i.e. the Mobile services Switching Center (MSC). The MSC performs the switching of calls between the mobile users and between mobile and fixed network users. The MSC also takes care of the the mobility regarding the various management operations.
MobileStation
The mobile station (MS) consists of the mobile equipment and Subscriber Identity Module (SIM) card. The SIM identifies the network and provides personal authentication. One can insert the SIM card to any other handset and still be able to receive call, make calls from that terminal, and receive other subscribed services or services offered by the network The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI).The SIM card contains the International Mobile Subscriber Identity (IMSI) and uses this to identify the subscriber by secret key.
Tracking and Positioning of Mobiles in Telecommunication
BaseStationSubsystem The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The Base Transceiver Station contains radio transceivers defining a cell and handles radio-link protocols. NetworkSubsystem Central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN. Besides this it also provides the required functionality to handle a mobile subscriber. This may include registration, authentication, location updating, handovers, etc. The MSC provides the connection to the fixed networks such as the PSTN or ISDN.
Subscriber Identity Module
One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries. In Australia, North America and Europe many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically subsidized with revenue from subscriptions, and operators want to try to avoid subsidizing competitors€„¢ 8
Tracking and Positioning of Mobiles in Telecommunication
mobiles. A subscriber can usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card).
GSM security
GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security.
Tracking and Positioning of Mobiles in Telecommunication
Specifications of GSM
The following list given below is a brief description of the specifications and characteristics of GSM. 1. Frequency Band-The frequency range specified for GSM is 1,850 to 1,990 MHz. 2. Duplex distance-The duplex distance is 80 MHz. This is the distance between the uplink and downlink frequencies. 3. Channel separation- In GSM there is 200 kHz separation between the adjacent carrier frequencies. 4. Modulation- It is the process of sending a signal by changing the characteristics of a carrier frequency. Gaussian minimum shift keying (GMSK) is used for this purpose in GSM. 5. Transmission rate-GSM has an over-the-air bit rate of 270 kbps. 6. Access method-TDMA is used in GSM. TDMA is a technique in which several different calls may share the same carrier. A particular slot is made available to each call. 7. Speech coder-GSM uses linear predictive coding, LPC. The main purpose of LPC is to reduce the bit rate. Speech is encoded at 13 kbps.
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Tracking and Positioning of Mobiles in Telecommunication
Additional Services in GSM
There are many supplementary services provided by GSM to generate more revenue. This comprehensive set of supplementary services include-1. Call forwarding-This service allows to forward the incoming calls to another number if the called mobile unit is not reachable, busy or there is no reply. This will happen only if the call forwarding is allowed unconditionally. 2. Barring of outgoing calls-This service allows to prevent all outgoing calls. 3. Barring of incoming calls-This allows the subscriber to prevent incoming calls. 4. Advice of charge-This service provides the mobile subscriber with an estimate of the call charges. 5. Call hold-This service enables the subscriber to interrupt an ongoing call and then subsequently switch to another call. 6. Call waiting-This service allows the mobile subscriber to be notified of an incoming call during a conversation which is already in progress. The subscriber can answer, reject, or ignore the incoming call. This service is applicable in all GSM connections. 7. Multiparty service-The multiparty service or conference enables a mobile subscriber to establish a conversation of callers, that is, a simultaneous conversation between three and six subscribers.
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Tracking and Positioning of Mobiles in Telecommunication 8. Calling line identification- This allows the called subscriber with the integrated services digital network (ISDN) which presents the number of the calling party. It is an optional feature and you can deactivate your calling line process.
MOBILE POSITIONING
It is now not necessary to use localization or tracking any longer. Since all phones have been converted to GPS able, the receiving tower can just ask the phone where it's position is, and the return contact will tell it within 16 feet. No localization is required any longer, since all phones now are required to have built in GPS abilities. This helps the computer to track which direction one is traveling so it can be determined when to best switch contact to the next tower. This improves service and reception and even makes it possible to use a phone at high speeds of travel.
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Tracking and Positioning of Mobiles in Telecommunication
GLOBAL POSITIONING SYSTEM
The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). The GPS uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, which enable GPS receivers to determine their current location, the time, and their velocity (including direction). GPS was developed by the United States Department of Defense. Global Positioning System (GPS) is comprised of 24 U.S. government owned satellites that circle 12,000 miles above the earth, twice a day in precise orbits, so that several are always in view from any position. The system is designed to provide worldwide positioning services with an accuracy ranging from 10 to 15 meters. Instant location information enables users to ascertain exactly where their vehicles or assets are at anytime, anywhere in the world. Due to minor timing errors and satellite orbit errors, however, more precise accuracies are unattainable with standard GPS. Atmospheric conditions can also affect GPS signals and their arrival time on Earth.
Basic concept of GPS operation
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Tracking and Positioning of Mobiles in Telecommunication A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, a precise orbit for the satellite sending the message (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac). These signals travel at the speed of light (which varies between vacuum and the atmosphere). The receiver uses the arrival time of each
message to measure the distance to each satellite, from which it determines the position of the receiver. The resulting coordinates are converted to more user-friendly forms such as latitude and longitude, or location on a map, and then displayed to the user. It might seem that three satellites would be enough to solve for a position, since space has three dimensions. However, a three satellite solution requires the time be known to a nanosecond or so, far better than any non-laboratory clock can provide. Using four or more satellites allows the receiver to solve for time as well as geographical position, eliminating the need for a very accurate clock.
Position calculation
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 where the subscript i denotes the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received, the GPS receiver can compute the indicated transit time of the message. Assuming the message traveled at the speed of light, the distance traveled, can be computed. Knowing the distance from GPS receiver to a satellite and the position of a 14
Tracking and Positioning of Mobiles in Telecommunication satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres if they intersect in more than one point intersect in a circle. A figure, two sphere surfaces intersecting in a circle, is shown below. Two points at which the surfaces
of the spheres intersect are clearly shown in the figure. The distance between these two points is the diameter of the circle of intersection. Now consider how a side view of the intersecting spheres would look. This view would look exactly the same as the figure because of the symmetry of the spheres. And in fact a view from any horizontal direction would look exactly the same. This should make it clear to the reader that the surfaces of the two spheres actually do intersect in a circle.
Two sphere surfaces intersecting in a circle
2-D Trilateration
The concept of trilateration is easy to understand through an example. Imagine that you are driving through an unfamiliar country and that you are lost. A road sign indicates that you are 500 km from city A. But this is not of much help, as you could 15
Tracking and Positioning of Mobiles in Telecommunication be anywhere in a circle of 500 km radius from the city A. A person you stop by to ask for directions then volunteers that you are 450 km from city B. Now you are in a better position to locate yourself- you are at one of the two intersecting points of the two circles surrounding city A and city B. Now if you could also get your distance from another place say city C, you can locate yourself very precisely, as these three circles can intersect each other at just one point. This is the principle behind 2D trilateration.
3-D Trilateration
The fundamental principles are the same for 2D and 3D trilateration, but in 3D trilateration we are dealing with spheres instead of circles. It is a little tricky to visualize. Here, we have to imagine the radii from the previous example going in all directions that is in three dimensional space thus forming spheres around the predefined points. Therefore the location of an object has to be defined with reference to the intersecting point of three spheres. Thus if you learn that the object is at a distance of 100 km from satellite A, it simply says that the object could be on surface of a huge imaginary sphere of 100 km radius around satellite A. Now you are also informed that the object is 150 km from satellite B. The imaginary spheres of 100km and 150 km around satellites A and B respectively intersect in a perfect circle. The position of the object defined from a third satellite C intersects this circle at just two points. The Earth acts as the fourth sphere, making us able to eliminate one of the two intersection points of the first three spheres. This makes it possible to identify the exact location of the object.
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Tracking and Positioning of Mobiles in Telecommunication However GPS receivers take into account four or more satellites to improve accuracy and provide extra information like altitude of the object.
Thus the GPS receiver needs the following information for its calculations.
€¢
The location of a minimum of three satellites that lock in with the object to be located or tracked.
€¢
The distance between the object and each of these satellites.
The GPS receiver works this out by analyzing high-frequency radio signals from GPS satellites. The more sophisticated the GPS, the more its number of receivers, so that signals from a larger number of satellites are taken into account for the calculations.
GPS signal
There are two frequencies of low power radio signals that GPS satellites transmit. These are called L1 and L2. The L1 frequency at 1575.42 MHz in the UHF band is what comes into play for civilian applications. These signals can pass through clouds, glass, plastic and such light objects, but cannot go through more solid objects like buildings and mountains.
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Tracking and Positioning of Mobiles in Telecommunication Every GPS signal packs three bits of information- these are the pseudorandom code, ephemeris data and almanac data. The pseudorandom code is the identification code of the individual satellite. The ephemeris data identifies the location of each GPS satellite at any particular time of the day. Each satellite transmits this data for the GPS receivers as well as for the other satellites in the network. The almanac data has information about the status of the satellite as well as current date and time. The almanac part of the signal is essential for determining the position.
System segmentation
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).
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Tracking and Positioning of Mobiles in Telecommunication
Ã¯Æ’Ë Space segment
A visual example of the GPS constellation in motion with the Earth rotating is shown. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time. 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. Orbiting at an altitude of approximately 20,200 kilometers (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 (sidereal) day. This was very helpful during development, since even with just four
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Tracking and Positioning of Mobiles in Telecommunication
o 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 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.
Ã¯Æ’Ë Control segment
The control segment of the Global Positioning System is a network of ground stations that monitors the shape and velocity of the satellites' orbits. The accuracy of GPS data depends on knowing the positions of the satellites at all times. The orbits of the satellites are sometimes disturbed by the interplay of the gravitational forces of the Earth and Moon. The control segment updates the atomic clock and adjusts the ephemeris. Correcting GPS clock The method of calculating position for the case of no errors has been explained. One of the most important errors is the error in the GPS receiver clock. Because of the very large value of the speed of light, the estimated distances from the 20
Tracking and Positioning of Mobiles in Telecommunication GPS receiver to the satellites, the pseudo ranges, are very sensitive to errors in the GPS receiver clock. This seems to suggest that an extremely accurate and expensive clock is
required for the GPS receiver to work. On the other hand, manufacturers would like to make an inexpensive GPS receiver which can be mass marketed. The manufacturers were thus faced with a difficult design problem. The technique that solves this problem is based on the way sphere surfaces intersect in the GPS problem. It is likely the surfaces of the three spheres intersect since the circle of intersection of the first two spheres is normally quite large and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three since any clock error could cause it to miss intersecting a point. However the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Note the distance from the valid estimate of GPS receiver position to the fourth satellite and denote the pseudo range of the fourth satellite. This is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient provides an estimate of correct time (time indicated by the receiver's on-board clock), and the GPS receiver clock can be advanced if is positive or delayed if is negative.
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Tracking and Positioning of Mobiles in Telecommunication
Adjusting the ephemeris
Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked 'unhealthy', so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.
Ã¯Æ’Ë User segment
This component consists of the GPS receivers and the user community. GPS receivers convert the signal into position, velocity and time estimates. This process requires four satellites to compute the four dimension of X, Y, Z (position) and time. With this ability, GPS has three main functions; navigation (for aircraft, ships, etc), precise positioning (for surveying, plate tectonics, etc,) and time and frequency dissemination (for astronomical observatories, telecommunications facilities, etc.) The user requires a GPS receiver in order to receive the transmissions from the satellites. The GPS receiver calculates the location based on signals from the satellites. The user does not transmit anything to the satellites and therefore the satellites don't know the user is there. The only data the satellites receive is from the Master Control Station in Colorado. The users consist of both the military and civilians.
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Tracking and Positioning of Mobiles in Telecommunication
Error sources in GPS
Apart from the inaccuracy of the clock in the GPS receiver, there can be other
factors that affect the quality of the GPS signal and cause calculation errors. These are:
ï ¶ Ionosphere and troposphere disturbances: These cause the satellite signal to slow
down
as it passes through the atmosphere. However the GPS system has a built in model that accounts for an average amount of these disturbances.
ï ¶ Signal reflection: Here the signal hits and is reflected off objects like tall buildings, rocks
etc. This causes the signal to be delayed before it reaches the receiver.
ï ¶ Ephemeris errors: Ephemeris errors are also known as orbital errors. These are errors in
the satellite€„¢s reported position against its actual position.
ï ¶ Clock errors: The built in clock of the GPS receiver is not as accurate as the atomic
clocks of the satellites and the slight timing errors leads to corresponding errors in calculations.
ï ¶ Visibility of Satellites: The more the number of satellites a GPS receiver can lock with,
the better its accuracy. Buildings, rocks and mountains, dense foliage, electronic interference, in short everything that comes in the line of sight cause position errors and sometimes make it unable to take any reading at all. GPS receivers do not work indoors, underwater and underground.
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Tracking and Positioning of Mobiles in Telecommunication ï ¶ Satellite Shading: For the signals to work properly the satellites have to be placed at wide angles from each other. Poor geometry resulting from tight grouping can result in signal interference.
ï ¶ Intentional degradation: This was used till May 2000 by the US Department of Defense
so that military adversaries could not use the GPS signals. This has been turned off since May 2000, which has improved the accuracy of readings in civilian equipment. The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay. To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version.
Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
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Tracking and Positioning of Mobiles in Telecommunication Due to the ionospheric delay affects the speed of microwave signals differently depending on their frequency € a characteristic known as dispersion - delays measured on two or more frequency bands can be used to measure dispersion, and this measurement can then be used to estimate the delay at each frequency. Some military and expensive survey-grade civilian receivers measure the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This
can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave. The effects of the ionosphere generally change slowly, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required. Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect both is more localized and 25
Tracking and Positioning of Mobiles in Telecommunication changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects. Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct, either by
applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter. Propagation of radio waves through atmosphere
Ephemeris and clock errors
While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. Data up to four hours old is considered valid for calculating positions, but may not indicate the satellites actual position. If a fast TTFF is needed, it is possible to upload valid ephemeris to a receiver, and in addition to setting
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Tracking and Positioning of Mobiles in Telecommunication the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices. The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state. These problems tend to be very small, but may add up to a few meters (10s of feet) of inaccuracy.
Multipath Effects
The multipath effect is caused by reflection of satellite signals (radio waves) on objects. It was the same effect that caused ghost images on television when antennae on the roof were still more common instead of today€„¢s satellite dishes. For GPS signals this effect mainly appears in the neighbourhood of large buildings or other elevations. The reflected signal takes more time to reach the receiver than the direct signal. The resulting error typically lies in the range of a few meters.
Interference caused by reflection of signals
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Relativistic Effects
The theory of relativity also says that time moves the slower the stronger the field of gravitation is. For an observer on the earth surface the clock on board of a satellite is running faster (as the satellite in 20000 km height is exposed to a much weaker field of gravitation than the observer). And this second effect is six times stronger than the time dilation explained above. Altogether, the clocks of the satellites seem to run a little faster. The shift of time to the observer on earth would be about 38 milliseconds per day and would make up for a total error of approximately 10 km per day. In order that those errors do not have
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Tracking and Positioning of Mobiles in Telecommunication to be corrected constantly, the clocks of the satellites were set to 10.229999995453 MHz instead of 10.23 MHz but they are operated as if they had 10.23 MHz. By this trick the relativistic effects are compensated once and for all.
There is another relativistic effect, which is not considered for normal position determinations by GPS. It is called Sagnac-Effect and is caused by the movement of the observer on the earth surface, who also moves with a velocity of up to 500 m/s (at the equator) due to the rotation of the globe. The influence of this effect is very small and complicate to calculate as it depends on the directions of the movement. Therefore it is only considered in special cases.
The errors of the GPS system are summarized in the following table. The individual values are no constant values, but are subject to variances. All numbers are approximate values. Ionospheric effects Shifts in the satellite orbits meter Clock errors of the satellites' clocks Multipath effect Tropospheric effects meter rounding errors ± 1 meter ± 2 meter ± 1 meter ± 0.5 29 ± 5 meter ± 2.5
Tracking and Positioning of Mobiles in Telecommunication
Interference €¢ Natural sources
Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible. Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field.
€¢
Artificial sources
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Tracking and Positioning of Mobiles in Telecommunication In automotive GPS receivers, metallic features in windshields, such as defrosters, or car window tinting films can 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, an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning TV antenna preamplifier. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight.
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Accuracy enhancement
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Clouds and other atmospheric phenomena, and objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.
Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location.
Augmentation
Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation.
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Tracking and Positioning of Mobiles in Telecommunication
Precise monitoring
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways. The largest error in GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency process.
GPS services in mobile phones
For nearly a decade ago technology visionaries were talking about a technology that people can use in their cell phones to get direction, track their friends, keep an eye on any special object, kids tracking or simply find the nearest hotel or hospital. Now finally those kinds of services are finally starting with the help of Global Positioning System (GPS) that makes our life simpler and smother.
Besides the so-called social networking service that involves talking in phone or mobiles to simply typing a message people can now easily use their mobile phone in location-based service. So GPS really plays an important role in directing the use of mobile phone a new dimension. GPS enables users to type a location and broadcast it to their friends and even the mobile virtual network automatically tracks and alert people about their location. Another service like Geocaching let mobile phone users to participate in a
Tracking and Positioning of Mobiles in Telecommunication
treasure haunt game that guides them to a particular place in search of a cache, whose coordinates are saved on the mobile GPS unit. Mobile GPS unit identifies user€„¢s position information with details including latitude, longitude with maximum accuracy up to 15 meters in radius. GPS technology is measuring the exact position of the mobile user more accurately by calculating user's coordinates with the help of satellite signals. Subscriber location service € This requires the user to type their address and ZIP code to broadcast their location or to find local business points. If someone is driving in a foreign area and want to find out the closest restaurant or Movie Theater you might not know the ZIP code of the place and hence several manufacturers are now working closely with leading mobile operators in providing technologies that can track location in case of emergency. The mobile tracking service in cell phone allows the user to share their real-time location status, messages, photos and other information with their friends from a mobile phone. The GPS enable mobile phone can automatically updates and displays the location of users directly on a map on the phone. Also the user can get an alert when any of his/her friends comes nearer. Safety, Security & Privacy - With the growing demand of mobile services the safety and user€„¢s privacy issues holds a huge importance for both operators and users. But with new kinds of technological innovation the issues does not stand firm. Privacy and security are two of most concerned factor as far as establishing mobility through mobile is concerned. The technology also safeguards user€„¢s privacy by allowing merely the known people to track and only when they want to be found.
Tracking and Positioning of Mobiles in Telecommunication For example, if a subscriber wants to track a phone then the phone number must be used or a text message must be sent to the owner of that phone, who must reply in order to enable tracking. Beside, individual privacy setting allows users to hide him to any particular person.
Tracking and Positioning of Mobiles in Telecommunication
Applications
The applications of the Global Positioning System fall into five categories: location, navigation, timing, mapping, and tracking. Each category contains uses for the military, industry, transportation, recreation and science. Location This category is for position determination and is the most obvious use of the Global Positioning System. GPS is the first system that can give accurate and precise measurements anytime, anywhere and under any weather conditions. Some examples of applications within this category are: 1. Measuring the movement of volcanoes and glaciers. 2. Measuring the growth of mountains. 3. Measuring the location of icebergs - this is very valuable to ship captains helping them to avoid possible disasters. 4. Storing the location of where you were - most GPS receivers on the market will allow you to record a certain location. This allows you to find it again with minimal effort and would prove useful in a hard to navigate place such as a dense forest.
Tracking and Positioning of Mobiles in Telecommunication
Navigation Navigation is the process of getting from one location to another. This was what the Global Positioning System was designed for. The GPS system allows us to navigate on water, air, or land. It allows planes to land in the middle of mountains and helps medical evacuation helicopters save precious time by taking the best route. Timing GPS brings precise timing. Each satellite is equipped with an extremely precise atomic clock. This is why we can all synchronize our watches so well and make sure international events are actually happening at the same time. Mapping This is used for creating maps by recording a series of locations. The best example is surveying where the DGPS technique is applied but with a twist. Instead of making error corrections in real time, both the stationary and moving receivers calculate their positions using the satellite signals. When the roving receiver is through making measurements, it then takes them back to the ground station which has already calculated the errors for each moment in time. At this time, the accurate measurements are obtained.
Tracking and Positioning of Mobiles in Telecommunication
Tracking The applications in this category are ways of monitoring people and things such as packages. This has been used along with wireless communications to keep track of some criminals. The suspect agrees to keep a GPS receiver and transmitting device with him at all times. If he goes where he's not allowed to, the authorities will be notified. This can also be used to track animals.
Tracking and Positioning of Mobiles in Telecommunication
CONCLUSION
Mobile positioning technology has become an important area of research, for emergency as well as for commercial services. Mobile positioning in cellular networks will provide several services such as, locating stolen mobiles, emergency calls, different billing tariffs depending on where the call is originated, and methods to predict the user movement inside a region.
see
http://tech-faqcell-phone-tracking.shtml
http://en.wikipediawiki/Mobile_phone_tracking
ABSTRACT
Mobile positioning technology has become an important area of research, for emergency as well as for commercial services. Mobile positioning in cellular networks will provide several services such as, locating stolen mobiles, emergency calls, different billing tariffs depending on where the call is originated, and methods to predict the user movement inside a region.
The evolution to location-dependent services and applications in wireless systems continues to require the development of more accurate and reliable mobile positioning technologies. The major challenge to accurate location estimation is in creating techniques that yield acceptable performance when the direct path from the transmitter to the receiver is intermittently blocked.
Tracking and Positioning of Mobiles in Telecommunication
INTRODUCTION
A mobile tracking and positioning system includes a plurality of mobile transmit and receive stations that track a mobile target which emits a radio signal in response to the occurrence of a tracking effort initiation event. The tracking stations have a GPS receiver or like means for determining their position, a radio direction finder responsive to the radio signal that determines the vector to the mobile target, a two-way communications system and a computer. The mobile transmit and receive stations exchange their position and direction to target information via the two-way communications systems, enabling the stations to triangulate the location of the target with their computers. Mobile phone tracking tracks the current position of a mobile phone even on the move. To locate the phone, it must emit at least the roaming signal to contact the next nearby antenna tower, but the process does not require an active call. GSM localisation is then done by multilateration based on the signal strength to nearby antenna masts. Mobile positioning, i.e. location based service that discloses the actual coordinates of a mobile phone bearer, is a technology used by telecommunication companies to approximate where a mobile phone, and thereby also its user (bearer), temporarily resides. The more properly applied term locating refers to the purpose rather than a positioning process. Such service is offered as an option of the class of locationbased services.
Tracking and Positioning of Mobiles in Telecommunication
MOBILE TRACKING
Mobile phone tracking tracks the current position of a mobile phone even on the move. To locate the phone, it must emit at least the roaming signal to contact the next nearby antenna tower, but the process does not require an active call. GSM localisation is then done by multilateration based on the signal strength to nearby antenna masts. In order to route calls to your phone the cell towers listen for a signal sent from the phone and negotiate which tower is best able to communicate with the phone. As the phone changes location, the towers monitor the signal and the phone is switched to a different tower as appropriate. By comparing the relative signal strength from multiple towers a general location of a phone can be determined. The technology of tracking is based on measuring power levels and antenna patterns and uses the concept that a mobile phone always communicates wirelessly with one of the closest base stations, so if you know which base station the phone communicates with, you know that the phone is close to the respective base station. Advanced systems determine the sector in which the mobile phone resides and roughly estimate also the distance to the base station. Further approximation can be done by interpolating signals between adjacent antenna towers. Qualified services may achieve a precision of down to 50 meters in urban areas where mobile traffic and density of antenna towers (base stations) is sufficiently high. Rural and desolate areas may see miles between base stations and therefore determine locations less precisely.
Tracking and Positioning of Mobiles in Telecommunication
Operational purpose
In order to route calls to a phone the cell towers listen for a signal sent from the phone and negotiate which tower is best able to communicate with the phone. As the phone changes location, the antenna towers monitor the signal and the phone is roamed to an adjacent tower as appropriate. By comparing the relative signal strength from multiple antenna towers a general location of a phone can be roughly determined. Other means is the antenna pattern that supports angular determination and phase discrimination. Newer phones may also allow the tracking of the phone even when turned on and not active in a telephone call-. This results from the roaming procedures that perform hand over of the phone from one base station to another. The principle of tracking is based on GSM localisation.
Tracking and Positioning of Mobiles in Telecommunication
GLOBAL SYSTEM FOR MOBILE COMMUNICATION (GSM)
GSM (Global System for Mobile communications) is the most popular standard for mobile phones in the world. Its promoter, the GSM Association, estimates that 82% of the global mobile market uses the standard. GSM is used by over 3 billion people across more than 212 countries and territories. Its ubiquity makes international roaming very common between mobile phone operators, enabling subscribers to use their phones in many parts of the world. GSM differs from its predecessors in that both signaling and speech channels are digital, and thus is considered a second generation (2G) mobile phone system. This has also meant that data communication was easy to build into the system. The ubiquity of the GSM standard has been an advantage to both consumers (who benefit from the ability to roam and switch carriers without switching phones) and also to network operators (who can choose equipment from any of the many vendors implementing GSM).GSM also pioneered a low-cost (to the network carrier) alternative to voice calls, the Short message service (SMS, also called "text messaging"), which is now supported on other mobile standards as well. Another advantage is that the standard includes one worldwide Emergency telephone number; 112This makes it easier for international travelers to connect to emergency services without knowing the local emergency number. Newer versions of the standard were backward-compatible with the original GSM phones. For example, Release '97 of the standard added packet data capabilities, by means of General Packet Radio Service (GPRS). Release '99 introduced higher speed data transmission using Enhanced Data Rates for GSM Evolution (EDGE). 5
Tracking and Positioning of Mobiles in Telecommunication
The network behind the GSM system seen by the customer is large and complicated in order to provide all of the services which are required. It is divided into a number of sections and these are each covered in separate articles.
The Base Station Subsystem (the base stations and their controllers). The Network and Switching Subsystem (the part of the network most similar to a fixed network). This is sometimes also just called the core network.
The GPRS Core Network (the optional part which allows packet based Internet connections).
All of the elements in the system combine to produce many GSM services such as voice calls and SMS.
Tracking and Positioning of Mobiles in Telecommunication
GSM is designed to provide recommendations only. It does not cater to the requirements. The specification does not include any hardware details but only define the functions and interface requirements. It has been done intentionally so that there is limit to the designers but still they are able to make it possible for the operators to buy the instrument or handset from different suppliers. The GSM network is divided into three major systems: the switching system (SS), the base station system (BSS), and the operation and support system(OSS). The Mobile Station is contained in the handset only which is carried by the subscriber. The Base Station Subsystem sets and controls the radio page link of the network with the Mobile Station. The Network Subsystem controls the main part i.e. the Mobile services Switching Center (MSC). The MSC performs the switching of calls between the mobile users and between mobile and fixed network users. The MSC also takes care of the the mobility regarding the various management operations.
MobileStation
The mobile station (MS) consists of the mobile equipment and Subscriber Identity Module (SIM) card. The SIM identifies the network and provides personal authentication. One can insert the SIM card to any other handset and still be able to receive call, make calls from that terminal, and receive other subscribed services or services offered by the network The mobile equipment is uniquely identified by the International Mobile Equipment Identity (IMEI).The SIM card contains the International Mobile Subscriber Identity (IMSI) and uses this to identify the subscriber by secret key.
Tracking and Positioning of Mobiles in Telecommunication
BaseStationSubsystem The Base Station Subsystem is composed of two parts, the Base Transceiver Station (BTS) and the Base Station Controller (BSC). The Base Transceiver Station contains radio transceivers defining a cell and handles radio-link protocols. NetworkSubsystem Central component of the Network Subsystem is the Mobile services Switching Center (MSC). It acts like a normal switching node of the PSTN or ISDN. Besides this it also provides the required functionality to handle a mobile subscriber. This may include registration, authentication, location updating, handovers, etc. The MSC provides the connection to the fixed networks such as the PSTN or ISDN.
Subscriber Identity Module
One of the key features of GSM is the Subscriber Identity Module (SIM), commonly known as a SIM card. The SIM is a detachable smart card containing the user's subscription information and phone book. This allows the user to retain his or her information after switching handsets. Alternatively, the user can also change operators while retaining the handset simply by changing the SIM. Some operators will block this by allowing the phone to use only a single SIM, or only a SIM issued by them; this practice is known as SIM locking, and is illegal in some countries. In Australia, North America and Europe many operators lock the mobiles they sell. This is done because the price of the mobile phone is typically subsidized with revenue from subscriptions, and operators want to try to avoid subsidizing competitors€„¢ 8
Tracking and Positioning of Mobiles in Telecommunication
mobiles. A subscriber can usually contact the provider to remove the lock for a fee, utilize private services to remove the lock, or make use of ample software and websites available on the Internet to unlock the handset themselves. While most web sites offer the unlocking for a fee, some do it for free. The locking applies to the handset, identified by its International Mobile Equipment Identity (IMEI) number, not to the account (which is identified by the SIM card).
GSM security
GSM was designed with a moderate level of security. The system was designed to authenticate the subscriber using a pre-shared key and challenge-response. Communications between the subscriber and the base station can be encrypted. The development of UMTS introduces an optional USIM, that uses a longer authentication key to give greater security, as well as mutually authenticating the network and the user whereas GSM only authenticates the user to the network (and not vice versa). The security model therefore offers confidentiality and authentication, but limited authorization capabilities, and no non-repudiation. GSM uses several cryptographic algorithms for security.
Tracking and Positioning of Mobiles in Telecommunication
Specifications of GSM
The following list given below is a brief description of the specifications and characteristics of GSM. 1. Frequency Band-The frequency range specified for GSM is 1,850 to 1,990 MHz. 2. Duplex distance-The duplex distance is 80 MHz. This is the distance between the uplink and downlink frequencies. 3. Channel separation- In GSM there is 200 kHz separation between the adjacent carrier frequencies. 4. Modulation- It is the process of sending a signal by changing the characteristics of a carrier frequency. Gaussian minimum shift keying (GMSK) is used for this purpose in GSM. 5. Transmission rate-GSM has an over-the-air bit rate of 270 kbps. 6. Access method-TDMA is used in GSM. TDMA is a technique in which several different calls may share the same carrier. A particular slot is made available to each call. 7. Speech coder-GSM uses linear predictive coding, LPC. The main purpose of LPC is to reduce the bit rate. Speech is encoded at 13 kbps.
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Additional Services in GSM
There are many supplementary services provided by GSM to generate more revenue. This comprehensive set of supplementary services include-1. Call forwarding-This service allows to forward the incoming calls to another number if the called mobile unit is not reachable, busy or there is no reply. This will happen only if the call forwarding is allowed unconditionally. 2. Barring of outgoing calls-This service allows to prevent all outgoing calls. 3. Barring of incoming calls-This allows the subscriber to prevent incoming calls. 4. Advice of charge-This service provides the mobile subscriber with an estimate of the call charges. 5. Call hold-This service enables the subscriber to interrupt an ongoing call and then subsequently switch to another call. 6. Call waiting-This service allows the mobile subscriber to be notified of an incoming call during a conversation which is already in progress. The subscriber can answer, reject, or ignore the incoming call. This service is applicable in all GSM connections. 7. Multiparty service-The multiparty service or conference enables a mobile subscriber to establish a conversation of callers, that is, a simultaneous conversation between three and six subscribers.
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Tracking and Positioning of Mobiles in Telecommunication 8. Calling line identification- This allows the called subscriber with the integrated services digital network (ISDN) which presents the number of the calling party. It is an optional feature and you can deactivate your calling line process.
MOBILE POSITIONING
It is now not necessary to use localization or tracking any longer. Since all phones have been converted to GPS able, the receiving tower can just ask the phone where it's position is, and the return contact will tell it within 16 feet. No localization is required any longer, since all phones now are required to have built in GPS abilities. This helps the computer to track which direction one is traveling so it can be determined when to best switch contact to the next tower. This improves service and reception and even makes it possible to use a phone at high speeds of travel.
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GLOBAL POSITIONING SYSTEM
The Global Positioning System (GPS) is the only fully functional Global Navigation Satellite System (GNSS). The GPS uses a constellation of between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave signals, which enable GPS receivers to determine their current location, the time, and their velocity (including direction). GPS was developed by the United States Department of Defense. Global Positioning System (GPS) is comprised of 24 U.S. government owned satellites that circle 12,000 miles above the earth, twice a day in precise orbits, so that several are always in view from any position. The system is designed to provide worldwide positioning services with an accuracy ranging from 10 to 15 meters. Instant location information enables users to ascertain exactly where their vehicles or assets are at anytime, anywhere in the world. Due to minor timing errors and satellite orbit errors, however, more precise accuracies are unattainable with standard GPS. Atmospheric conditions can also affect GPS signals and their arrival time on Earth.
Basic concept of GPS operation
13
Tracking and Positioning of Mobiles in Telecommunication A GPS receiver calculates its position by carefully timing the signals sent by the constellation of GPS satellites high above the Earth. Each satellite continually transmits messages containing the time the message was sent, a precise orbit for the satellite sending the message (the ephemeris), and the general system health and rough orbits of all GPS satellites (the almanac). These signals travel at the speed of light (which varies between vacuum and the atmosphere). The receiver uses the arrival time of each
message to measure the distance to each satellite, from which it determines the position of the receiver. The resulting coordinates are converted to more user-friendly forms such as latitude and longitude, or location on a map, and then displayed to the user. It might seem that three satellites would be enough to solve for a position, since space has three dimensions. However, a three satellite solution requires the time be known to a nanosecond or so, far better than any non-laboratory clock can provide. Using four or more satellites allows the receiver to solve for time as well as geographical position, eliminating the need for a very accurate clock.
Position calculation
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 where the subscript i denotes the satellite number and has the value 1, 2, 3, or 4. Knowing the indicated time the message was received, the GPS receiver can compute the indicated transit time of the message. Assuming the message traveled at the speed of light, the distance traveled, can be computed. Knowing the distance from GPS receiver to a satellite and the position of a 14
Tracking and Positioning of Mobiles in Telecommunication satellite implies that the GPS receiver is on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated position of the GPS receiver is at or near the intersection of the surfaces of four spheres. In the ideal case of no errors, the GPS receiver will be at an intersection of the surfaces of four spheres. The surfaces of two spheres if they intersect in more than one point intersect in a circle. A figure, two sphere surfaces intersecting in a circle, is shown below. Two points at which the surfaces
of the spheres intersect are clearly shown in the figure. The distance between these two points is the diameter of the circle of intersection. Now consider how a side view of the intersecting spheres would look. This view would look exactly the same as the figure because of the symmetry of the spheres. And in fact a view from any horizontal direction would look exactly the same. This should make it clear to the reader that the surfaces of the two spheres actually do intersect in a circle.
Two sphere surfaces intersecting in a circle
2-D Trilateration
The concept of trilateration is easy to understand through an example. Imagine that you are driving through an unfamiliar country and that you are lost. A road sign indicates that you are 500 km from city A. But this is not of much help, as you could 15
Tracking and Positioning of Mobiles in Telecommunication be anywhere in a circle of 500 km radius from the city A. A person you stop by to ask for directions then volunteers that you are 450 km from city B. Now you are in a better position to locate yourself- you are at one of the two intersecting points of the two circles surrounding city A and city B. Now if you could also get your distance from another place say city C, you can locate yourself very precisely, as these three circles can intersect each other at just one point. This is the principle behind 2D trilateration.
3-D Trilateration
The fundamental principles are the same for 2D and 3D trilateration, but in 3D trilateration we are dealing with spheres instead of circles. It is a little tricky to visualize. Here, we have to imagine the radii from the previous example going in all directions that is in three dimensional space thus forming spheres around the predefined points. Therefore the location of an object has to be defined with reference to the intersecting point of three spheres. Thus if you learn that the object is at a distance of 100 km from satellite A, it simply says that the object could be on surface of a huge imaginary sphere of 100 km radius around satellite A. Now you are also informed that the object is 150 km from satellite B. The imaginary spheres of 100km and 150 km around satellites A and B respectively intersect in a perfect circle. The position of the object defined from a third satellite C intersects this circle at just two points. The Earth acts as the fourth sphere, making us able to eliminate one of the two intersection points of the first three spheres. This makes it possible to identify the exact location of the object.
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Tracking and Positioning of Mobiles in Telecommunication However GPS receivers take into account four or more satellites to improve accuracy and provide extra information like altitude of the object.
Thus the GPS receiver needs the following information for its calculations.
€¢
The location of a minimum of three satellites that lock in with the object to be located or tracked.
€¢
The distance between the object and each of these satellites.
The GPS receiver works this out by analyzing high-frequency radio signals from GPS satellites. The more sophisticated the GPS, the more its number of receivers, so that signals from a larger number of satellites are taken into account for the calculations.
GPS signal
There are two frequencies of low power radio signals that GPS satellites transmit. These are called L1 and L2. The L1 frequency at 1575.42 MHz in the UHF band is what comes into play for civilian applications. These signals can pass through clouds, glass, plastic and such light objects, but cannot go through more solid objects like buildings and mountains.
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Tracking and Positioning of Mobiles in Telecommunication Every GPS signal packs three bits of information- these are the pseudorandom code, ephemeris data and almanac data. The pseudorandom code is the identification code of the individual satellite. The ephemeris data identifies the location of each GPS satellite at any particular time of the day. Each satellite transmits this data for the GPS receivers as well as for the other satellites in the network. The almanac data has information about the status of the satellite as well as current date and time. The almanac part of the signal is essential for determining the position.
System segmentation
The current GPS consists of three major segments. These are the space segment (SS), a control segment (CS), and a user segment (US).
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Ã¯Æ’Ë Space segment
A visual example of the GPS constellation in motion with the Earth rotating is shown. Notice how the number of satellites in view from a given point on the Earth's surface, in this example at 45°N, changes with time. 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. Orbiting at an altitude of approximately 20,200 kilometers (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 (sidereal) day. This was very helpful during development, since even with just four
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o 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 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.
Ã¯Æ’Ë Control segment
The control segment of the Global Positioning System is a network of ground stations that monitors the shape and velocity of the satellites' orbits. The accuracy of GPS data depends on knowing the positions of the satellites at all times. The orbits of the satellites are sometimes disturbed by the interplay of the gravitational forces of the Earth and Moon. The control segment updates the atomic clock and adjusts the ephemeris. Correcting GPS clock The method of calculating position for the case of no errors has been explained. One of the most important errors is the error in the GPS receiver clock. Because of the very large value of the speed of light, the estimated distances from the 20
Tracking and Positioning of Mobiles in Telecommunication GPS receiver to the satellites, the pseudo ranges, are very sensitive to errors in the GPS receiver clock. This seems to suggest that an extremely accurate and expensive clock is
required for the GPS receiver to work. On the other hand, manufacturers would like to make an inexpensive GPS receiver which can be mass marketed. The manufacturers were thus faced with a difficult design problem. The technique that solves this problem is based on the way sphere surfaces intersect in the GPS problem. It is likely the surfaces of the three spheres intersect since the circle of intersection of the first two spheres is normally quite large and thus the third sphere surface is likely to intersect this large circle. It is very unlikely that the surface of the sphere corresponding to the fourth satellite will intersect either of the two points of intersection of the first three since any clock error could cause it to miss intersecting a point. However the distance from the valid estimate of GPS receiver position to the surface of the sphere corresponding to the fourth satellite can be used to compute a clock correction. Note the distance from the valid estimate of GPS receiver position to the fourth satellite and denote the pseudo range of the fourth satellite. This is the distance from the computed GPS receiver position to the surface of the sphere corresponding to the fourth satellite. Thus the quotient provides an estimate of correct time (time indicated by the receiver's on-board clock), and the GPS receiver clock can be advanced if is positive or delayed if is negative.
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Adjusting the ephemeris
Satellite maneuvers are not precise by GPS standards. So to change the orbit of a satellite, the satellite must be marked 'unhealthy', so receivers will not use it in their calculation. Then the maneuver can be carried out, and the resulting orbit tracked from the ground. Then the new ephemeris is uploaded and the satellite marked healthy again.
Ã¯Æ’Ë User segment
This component consists of the GPS receivers and the user community. GPS receivers convert the signal into position, velocity and time estimates. This process requires four satellites to compute the four dimension of X, Y, Z (position) and time. With this ability, GPS has three main functions; navigation (for aircraft, ships, etc), precise positioning (for surveying, plate tectonics, etc,) and time and frequency dissemination (for astronomical observatories, telecommunications facilities, etc.) The user requires a GPS receiver in order to receive the transmissions from the satellites. The GPS receiver calculates the location based on signals from the satellites. The user does not transmit anything to the satellites and therefore the satellites don't know the user is there. The only data the satellites receive is from the Master Control Station in Colorado. The users consist of both the military and civilians.
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Error sources in GPS
Apart from the inaccuracy of the clock in the GPS receiver, there can be other
factors that affect the quality of the GPS signal and cause calculation errors. These are:
ï ¶ Ionosphere and troposphere disturbances: These cause the satellite signal to slow
down
as it passes through the atmosphere. However the GPS system has a built in model that accounts for an average amount of these disturbances.
ï ¶ Signal reflection: Here the signal hits and is reflected off objects like tall buildings, rocks
etc. This causes the signal to be delayed before it reaches the receiver.
ï ¶ Ephemeris errors: Ephemeris errors are also known as orbital errors. These are errors in
the satellite€„¢s reported position against its actual position.
ï ¶ Clock errors: The built in clock of the GPS receiver is not as accurate as the atomic
clocks of the satellites and the slight timing errors leads to corresponding errors in calculations.
ï ¶ Visibility of Satellites: The more the number of satellites a GPS receiver can lock with,
the better its accuracy. Buildings, rocks and mountains, dense foliage, electronic interference, in short everything that comes in the line of sight cause position errors and sometimes make it unable to take any reading at all. GPS receivers do not work indoors, underwater and underground.
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Tracking and Positioning of Mobiles in Telecommunication ï ¶ Satellite Shading: For the signals to work properly the satellites have to be placed at wide angles from each other. Poor geometry resulting from tight grouping can result in signal interference.
ï ¶ Intentional degradation: This was used till May 2000 by the US Department of Defense
so that military adversaries could not use the GPS signals. This has been turned off since May 2000, which has improved the accuracy of readings in civilian equipment. The position calculated by a GPS receiver requires the current time, the position of the satellite and the measured delay of the received signal. The position accuracy is primarily dependent on the satellite position and signal delay. To measure the delay, the receiver compares the bit sequence received from the satellite with an internally generated version.
Atmospheric effects
Inconsistencies of atmospheric conditions affect the speed of the GPS signals as they pass through the Earth's atmosphere, especially the ionosphere. Correcting these errors is a significant challenge to improving GPS position accuracy. These effects are smallest when the satellite is directly overhead and become greater for satellites nearer the horizon since the path through the atmosphere is longer. Once the receiver's approximate location is known, a mathematical model can be used to estimate and compensate for these errors.
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Tracking and Positioning of Mobiles in Telecommunication Due to the ionospheric delay affects the speed of microwave signals differently depending on their frequency € a characteristic known as dispersion - delays measured on two or more frequency bands can be used to measure dispersion, and this measurement can then be used to estimate the delay at each frequency. Some military and expensive survey-grade civilian receivers measure the different delays in the L1 and L2 frequencies to measure atmospheric dispersion, and apply a more precise correction. This
can be done in civilian receivers without decrypting the P(Y) signal carried on L2, by tracking the carrier wave instead of the modulated code. To facilitate this on lower cost receivers, a new civilian code signal on L2, called L2C, was added to the Block IIR-M satellites, which was first launched in 2005. It allows a direct comparison of the L1 and L2 signals using the coded signal instead of the carrier wave. The effects of the ionosphere generally change slowly, and can be averaged over time. The effects for any particular geographical area can be easily calculated by comparing the GPS-measured position to a known surveyed location. This correction is also valid for other receivers in the same general location. Several systems send this information over radio or other links to allow L1-only receivers to make ionospheric corrections. The ionospheric data are transmitted via satellite in Satellite Based Augmentation Systems such as WAAS, which transmits it on the GPS frequency using a special pseudo-random noise sequence (PRN), so only one receiver and antenna are required. Humidity also causes a variable delay, resulting in errors similar to ionospheric delay, but occurring in the troposphere. This effect both is more localized and 25
Tracking and Positioning of Mobiles in Telecommunication changes more quickly than ionospheric effects, and is not frequency dependent. These traits make precise measurement and compensation of humidity errors more difficult than ionospheric effects. Changes in receiver altitude also change the amount of delay, due to the signal passing through less of the atmosphere at higher elevations. Since the GPS receiver computes its approximate altitude, this error is relatively simple to correct, either by
applying a function regression or correlating margin of atmospheric error to ambient pressure using a barometric altimeter. Propagation of radio waves through atmosphere
Ephemeris and clock errors
While the ephemeris data is transmitted every 30 seconds, the information itself may be up to two hours old. Data up to four hours old is considered valid for calculating positions, but may not indicate the satellites actual position. If a fast TTFF is needed, it is possible to upload valid ephemeris to a receiver, and in addition to setting
26
Tracking and Positioning of Mobiles in Telecommunication the time, a position fix can be obtained in under ten seconds. It is feasible to put such ephemeris data on the web so it can be loaded into mobile GPS devices. The satellite's atomic clocks experience noise and clock drift errors. The navigation message contains corrections for these errors and estimates of the accuracy of the atomic clock. However, they are based on observations and may not indicate the clock's current state. These problems tend to be very small, but may add up to a few meters (10s of feet) of inaccuracy.
Multipath Effects
The multipath effect is caused by reflection of satellite signals (radio waves) on objects. It was the same effect that caused ghost images on television when antennae on the roof were still more common instead of today€„¢s satellite dishes. For GPS signals this effect mainly appears in the neighbourhood of large buildings or other elevations. The reflected signal takes more time to reach the receiver than the direct signal. The resulting error typically lies in the range of a few meters.
Interference caused by reflection of signals
27
Tracking and Positioning of Mobiles in Telecommunication
Relativistic Effects
The theory of relativity also says that time moves the slower the stronger the field of gravitation is. For an observer on the earth surface the clock on board of a satellite is running faster (as the satellite in 20000 km height is exposed to a much weaker field of gravitation than the observer). And this second effect is six times stronger than the time dilation explained above. Altogether, the clocks of the satellites seem to run a little faster. The shift of time to the observer on earth would be about 38 milliseconds per day and would make up for a total error of approximately 10 km per day. In order that those errors do not have
28
Tracking and Positioning of Mobiles in Telecommunication to be corrected constantly, the clocks of the satellites were set to 10.229999995453 MHz instead of 10.23 MHz but they are operated as if they had 10.23 MHz. By this trick the relativistic effects are compensated once and for all.
There is another relativistic effect, which is not considered for normal position determinations by GPS. It is called Sagnac-Effect and is caused by the movement of the observer on the earth surface, who also moves with a velocity of up to 500 m/s (at the equator) due to the rotation of the globe. The influence of this effect is very small and complicate to calculate as it depends on the directions of the movement. Therefore it is only considered in special cases.
The errors of the GPS system are summarized in the following table. The individual values are no constant values, but are subject to variances. All numbers are approximate values. Ionospheric effects Shifts in the satellite orbits meter Clock errors of the satellites' clocks Multipath effect Tropospheric effects meter rounding errors ± 1 meter ± 2 meter ± 1 meter ± 0.5 29 ± 5 meter ± 2.5
Tracking and Positioning of Mobiles in Telecommunication
Interference €¢ Natural sources
Since GPS signals at terrestrial receivers tend to be relatively weak, it is easy for other sources of electromagnetic radiation to desensitize the receiver, making acquiring and tracking the satellite signals difficult or impossible. Solar flares are one such naturally occurring emission with the potential to degrade GPS reception, and their impact can affect reception over the half of the Earth facing the sun. GPS signals can also be interfered with by naturally occurring geomagnetic storms, predominantly found near the poles of the Earth's magnetic field.
€¢
Artificial sources
30
Tracking and Positioning of Mobiles in Telecommunication In automotive GPS receivers, metallic features in windshields, such as defrosters, or car window tinting films can 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, an entire harbor was unable to receive GPS signals due to unintentional jamming caused by a malfunctioning TV antenna preamplifier. Intentional jamming is also possible. Generally, stronger signals can interfere with GPS receivers when they are within radio range, or line of sight.
31
Tracking and Positioning of Mobiles in Telecommunication
Accuracy enhancement
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Clouds and other atmospheric phenomena, and objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.
Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location.
Augmentation
Augmentation methods of improving accuracy rely on external information being integrated into the calculation process. There are many such systems in place and they are generally named or described based on how the GPS sensor receives the information. Some systems transmit additional information about sources of error (such as clock drift, ephemeris, or ionospheric delay), others provide direct measurements of how much the signal was off in the past, while a third group provide additional navigational or vehicle information to be integrated in the calculation.
32
Tracking and Positioning of Mobiles in Telecommunication
Precise monitoring
The accuracy of a calculation can also be improved through precise monitoring and measuring of the existing GPS signals in additional or alternate ways. The largest error in GPS is usually the unpredictable delay through the ionosphere. The spacecraft broadcast ionospheric model parameters, but errors remain. This is one reason the GPS spacecraft transmit on at least two frequencies, L1 and L2. Ionospheric delay is a well-defined function of frequency and the total electron content (TEC) along the path, so measuring the arrival time difference between the frequencies determines TEC and thus the precise ionospheric delay at each frequency process.
GPS services in mobile phones
For nearly a decade ago technology visionaries were talking about a technology that people can use in their cell phones to get direction, track their friends, keep an eye on any special object, kids tracking or simply find the nearest hotel or hospital. Now finally those kinds of services are finally starting with the help of Global Positioning System (GPS) that makes our life simpler and smother.
Besides the so-called social networking service that involves talking in phone or mobiles to simply typing a message people can now easily use their mobile phone in location-based service. So GPS really plays an important role in directing the use of mobile phone a new dimension. GPS enables users to type a location and broadcast it to their friends and even the mobile virtual network automatically tracks and alert people about their location. Another service like Geocaching let mobile phone users to participate in a
Tracking and Positioning of Mobiles in Telecommunication
treasure haunt game that guides them to a particular place in search of a cache, whose coordinates are saved on the mobile GPS unit. Mobile GPS unit identifies user€„¢s position information with details including latitude, longitude with maximum accuracy up to 15 meters in radius. GPS technology is measuring the exact position of the mobile user more accurately by calculating user's coordinates with the help of satellite signals. Subscriber location service € This requires the user to type their address and ZIP code to broadcast their location or to find local business points. If someone is driving in a foreign area and want to find out the closest restaurant or Movie Theater you might not know the ZIP code of the place and hence several manufacturers are now working closely with leading mobile operators in providing technologies that can track location in case of emergency. The mobile tracking service in cell phone allows the user to share their real-time location status, messages, photos and other information with their friends from a mobile phone. The GPS enable mobile phone can automatically updates and displays the location of users directly on a map on the phone. Also the user can get an alert when any of his/her friends comes nearer. Safety, Security & Privacy - With the growing demand of mobile services the safety and user€„¢s privacy issues holds a huge importance for both operators and users. But with new kinds of technological innovation the issues does not stand firm. Privacy and security are two of most concerned factor as far as establishing mobility through mobile is concerned. The technology also safeguards user€„¢s privacy by allowing merely the known people to track and only when they want to be found.
Tracking and Positioning of Mobiles in Telecommunication For example, if a subscriber wants to track a phone then the phone number must be used or a text message must be sent to the owner of that phone, who must reply in order to enable tracking. Beside, individual privacy setting allows users to hide him to any particular person.
Tracking and Positioning of Mobiles in Telecommunication
Applications
The applications of the Global Positioning System fall into five categories: location, navigation, timing, mapping, and tracking. Each category contains uses for the military, industry, transportation, recreation and science. Location This category is for position determination and is the most obvious use of the Global Positioning System. GPS is the first system that can give accurate and precise measurements anytime, anywhere and under any weather conditions. Some examples of applications within this category are: 1. Measuring the movement of volcanoes and glaciers. 2. Measuring the growth of mountains. 3. Measuring the location of icebergs - this is very valuable to ship captains helping them to avoid possible disasters. 4. Storing the location of where you were - most GPS receivers on the market will allow you to record a certain location. This allows you to find it again with minimal effort and would prove useful in a hard to navigate place such as a dense forest.
Tracking and Positioning of Mobiles in Telecommunication
Navigation Navigation is the process of getting from one location to another. This was what the Global Positioning System was designed for. The GPS system allows us to navigate on water, air, or land. It allows planes to land in the middle of mountains and helps medical evacuation helicopters save precious time by taking the best route. Timing GPS brings precise timing. Each satellite is equipped with an extremely precise atomic clock. This is why we can all synchronize our watches so well and make sure international events are actually happening at the same time. Mapping This is used for creating maps by recording a series of locations. The best example is surveying where the DGPS technique is applied but with a twist. Instead of making error corrections in real time, both the stationary and moving receivers calculate their positions using the satellite signals. When the roving receiver is through making measurements, it then takes them back to the ground station which has already calculated the errors for each moment in time. At this time, the accurate measurements are obtained.
Tracking and Positioning of Mobiles in Telecommunication
Tracking The applications in this category are ways of monitoring people and things such as packages. This has been used along with wireless communications to keep track of some criminals. The suspect agrees to keep a GPS receiver and transmitting device with him at all times. If he goes where he's not allowed to, the authorities will be notified. This can also be used to track animals.
Tracking and Positioning of Mobiles in Telecommunication
CONCLUSION
Mobile positioning technology has become an important area of research, for emergency as well as for commercial services. Mobile positioning in cellular networks will provide several services such as, locating stolen mobiles, emergency calls, different billing tariffs depending on where the call is originated, and methods to predict the user movement inside a region.
18-02-2010, 04:55 PM
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23-04-2010, 09:58 AM
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10-10-2011, 07:29 PM
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01-03-2013, 04:56 PM
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