Tracking and Positioning of Mobiles in Telecommunication

Tracking and Positioning of Mobiles in Telecommunication

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   positi positioni oning ng techno technolog logies ies.. The major major challe challenge nge to accurat accuratee locati location on estima estimatio tion n is in creating techniques that yield acceptable performance when the direct path from the transmitter to the receiver is intermittently blocked.

1

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 the radi radio o sign signal al that that dete determ rmin ines es the the vect vector or to the the mobi mobile le tar target, get, a twotwo-wa way y communi communicati cations ons system system and a compute computerr. The mobile mobile transm transmit it and receiv receivee statio stations ns exch exchan ange ge thei theirr posi positi tion on and and dire direct ctio ion n to tar target get info inform rmat atio ion n via via the the twotwo-wa way y 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 b y multilateration based on the signal strength to nearby antenna masts.

Mobile positioning, i.e. location based service that discloses the actual coordin coordinate atess of a mobile mobile phone phone bearer bearer,, is a technol technology ogy used used by telecom telecommun munica icatio tion n 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 location based services.

2

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 To locate the phone, it must emit at least the roaming signal to contact the next next nearby nearby ante antenna nna towe towerr, but the the proc proces esss does does not not requ requir iree an activ activee call. call. GSM 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 one of the the clos closes estt base base stat statio ions ns,, so if you you know know whic which h base base stat statio ion n the the phon phonee 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. precisely.

3

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 o n GSM localisation.

4

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 signaling and speech channels are digital, and thus is considered considered a second generation 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 nu mber..

Newer versions of the standard standard were backward-compa backward-compatible tible with the origin original al GSM GSM phones. phones. For example example,, Releas Releasee '97 of the standa standard rd added added packet packet data 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.

6

Tracking and Positioning of Mobiles in Telecommunication

GSM is designed to provide recommendations reco mmendations only. 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 link of the network with the Mobile Station. Station. The Network Network Subsystem Subsystem controls the main part i.e. the Mobile Mobile services 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.

7

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 commonly known known as a SIM card. The SIM is a detacha detachable ble smart card contain containing ing 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   priva private te servic services es to remove remove the lock, or make make use of ample ample softwa software re and websit websites es 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 secu securi rity ty mode modell ther theref efor oree offe offers rs confi confide dent ntia iali lity ty and auth authent entic icat atio ion, n, but limi limite ted d authorization authorization capabilities, capabilities, and no non-repudiat non-repudiation. ion. GSM uses several several cryptographi cryptographicc algorithms for security.

9

Tracking and Positioning of Mobiles in Telecommunication

Specifications of GSM

The foll follow owin ing g list list give given n below below is a brie brieff descr descrip ipti tion on of the the spec specif ific icat atio ions ns and and characteristics of GSM.

1. Frequency Frequency Band-The Band-The frequenc frequency y range range specified specified for for GSM is 1,850 1,850 to 1,990 1,990 MHz. 2. Duplex Duplex distan distancece-The The duplex duplex distance distance is 80 MHz. This This is the distan distance ce between between the uplink and downlink frequencies. 3. Channel separat separationion- In GSM GSM there there is 200 kHz kHz separation separation between between the adjacent adjacent carrie carrier  r  frequencies. 4. ModulationModulation- It It is the process process of sending sending a signal signal by changing changing the characte characterist ristics ics of a carrier frequency. frequency. Gaussian minimum shift keying (GMSK) is used for this pu rpose in GSM. 5. Transmis Transmission sion rate-G rate-GSM SM has has an over-theover-the-air air bit rate of 270 kbps. kbps. 6. Acce Access ss method method-T -TDM DMA A is used used in GSM. GSM. TDMA TDMA is a tech techni nique que in whic which h seve severa rall different calls may share the same carrier. A particular slot is made available to each call. 7. Speech codercoder-GSM GSM uses uses linear linear predictive predictive coding, coding, LPC. LPC. The main main purpose purpose of LPC is is to reduce the bit rate. Speech is encoded at 13 kbps.

10

Tracking and Positioning of Mobiles in Telecommunication

Additional Services in GSM

There are many supplementary supplementary services services provided by GSM to generate generate more revenue. This comprehensive set of supplementary services include--

1. Call forwardin forwarding-This g-This service service allows allows to forward forward the the incoming incoming calls to another another number  number  if the called mobile unit is not reachable, reachable, busy or there is no reply. reply. This will happen only if the call ca ll forwarding is allowed unconditionally unc onditionally.. 2. Barring Barring of outgoing outgoing calls-Thi calls-Thiss service service allows allows to prevent prevent all outgoing outgoing calls. calls. 3. Barring Barring of incoming incoming calls-Thi calls-Thiss allows allows the subscriber subscriber to to prevent incomi incoming ng calls. calls. 4. Advice of chargecharge-This This service service provides provides the mobile mobile subscri subscriber ber with an estimat estimatee of the call charges. 5. Call Call hold-This hold-This servic servicee enables enables the subscri subscriber ber to interrupt interrupt an ongoing ongoing call and then subsequently switch to another call. 6. Call waitingwaiting-This This service service allows allows the mobile mobile subscriber subscriber to to be notified notified of an incoming 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 Multiparty serviceservice-The The multiparty multiparty service service or conference conference enables enables a mobile subscribe subscriber  r  to establish a conversation of callers, that is, a simultaneous conversation between three and six subscribers.

11

Tracking and Positioning of Mobiles in Telecommunication 8. Callin Calling g line line identif identifica icatio tionn- This allows allows the called called subscrib subscriber er with the integr integrate ated d 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 position is, and the return return contact will tell it within 16 feet. No localization 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 pho ne at high speeds of travel.

12

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 however,, more more precis precisee accurac accuracies ies are unatta unattaina inable ble with with standa standard rd GPS. GPS. Atmosp Atmospher heric ic 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 receive receiverr is able able to determ determine ine the satell satellite ite position positionss and time sent. 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. below. Two 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 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 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.

16

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 accoun t 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.

17

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).

18

Tracking and Positioning of Mobiles in Telecommunication

 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 separated by 60° right ascension ascension of the ascending node (angle along the equator  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 

19

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 provid providing ing redunda redundant nt measur measuremen ements. ts. With the increa increased sed number number of satell satellite ites, s, the constel constellat lation ion was changed changed to a nonuni nonunifor form m arrange arrangemen ment. t. Such Such an arrange arrangemen mentt was shown to improve reliability and availability of the system, relative to a uniform system, when multiple satellites fail.

 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 Earth and Moon. Moon. The The contr control ol segm segmen entt updat updates es the the atom atomic ic cloc clock k and and adju adjust stss the 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 wa y sphere surfaces intersect in the GPS problem.

It is likely the surfaces surfaces of the three spheres intersect intersect since the circle 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.

21

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.



User segment This component consists of the GPS receivers and the user community. community. GPS receivers receivers convert the signal signal into position, position, velocity velocity and time estimates. estimates. This process process requires four satellites to compute the four dimension of X, Y, Z (position) and time. With With this ability, ability, GPS has three main functions; functions; navigation (for aircraft, aircraft, ships, etc),   preci precise se posi positi tion onin ing g (for (for surv survey eyin ing, g, plat platee tect tectoni onics cs,, etc, etc,)) and and time time and and freq frequen uency cy 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.

22

Tracking and Positioning of Mobiles in Telecommunication

Error sources in GPS Apart from the inaccuracy inaccuracy of the clock in the GPS receiver receiver,,

there

can

be

other 

factors that affect the quality of the GPS signal and cause c alculation errors. These are:



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. 

Signal reflection: Here the signal hits and is reflected off objects like tall buildings, rocks etc. This causes the signal to be delayed d elayed before it reaches the receiver.



Ephemeris errors: Ephemeris errors are also known as orbital errors. These are errors in the satellite’s reported position against its actual position.



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.



Visibility of Satellites: The more the number of satellites a GPS receiver can lock with, the better better its accuracy accuracy.. Buildi Buildings ngs,, rocks rocks and mounta mountains ins,, dense dense foliag foliage, e, electr electroni onicc 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.

23

Tracking and Positioning of Mobiles in Telecommunication 

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.



Intentional Intentional degradation: degradation: This was used till May 2000 by the US Department Department of Defense 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 delay, the receiv receiver er compar compares es the bit sequen sequence ce receiv received ed from from the satell satellite ite with 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.

24

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 appl y 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, satellites, which was first launched in 2005. It allows a direct comparison 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 comparing the GPS-measured GPS-measured position to a known surveyed location. location. This correction 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 corre correct ctio ions ns.. The The iono ionosp sphe heri ricc data data are are tran transm smit itte ted d via via sate satell llit itee in Sate Satell llit itee Base Based d 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. 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  buildings or other elevations. elevations. The reflected reflected signal takes more time to reach the receiver  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   posi positi tion on deter determi mina nati tions ons by GPS. GPS. It is calle called d Sagna Sagnacc-Ef Effe fect ct and is caus caused ed by the the movement 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

± 5 meter 

Shif Shifts ts in the the sate satell llit itee orbi orbits ts

±

2.5 2.5

meter  Clock errors of the satellites' clocks

± 2 meter 

Multipath effect

± 1 meter 

Tropospheric effects

±

0 .5

meter  rounding errors

29 ± 1 meter 

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 electromagn electromagnetic etic radiation to desensitize desensitize the receiver receiver,, making acquiring and tracking the satellite signals difficult or impossible.

Solar Solar flares flares are one such such natura naturally lly occurring occurring emissi emission on with with the potent potential ial to degrade GPS reception, and their impact can affect reception over the half of the Earth faci facing ng the the sun. sun. GPS GPS sign signal alss can also also be inte interf rfer ered ed with with by natur natural ally ly occur occurri ring ng 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 accu rate 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 even the the mobi mobile le virt virtual ual network  automat automatica ically lly tracks tracks and alert alert people people about about their  their  location. Another service like Geocaching let mobile phone users to par ticipate in a 33

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, longi longitu tude de with with maxi maximu mum m accur accuracy acy up to 15 mete meters rs in radi radius us.. GPS GPS techn technol ology ogy 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 ne arer.. Safety, Security & Privacy - With the growing demand of mobile services the

safety and user’s privacy issues holds a huge importance for both o perators 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.

34

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.

35

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, military, industry, transportation, recreation and science.

Location

This category is for position determination and is the most obvious use of the Global Positionin Positioning g System. 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. Measur Measuring ing the movem movement ent of volcano volcanoes es and glaci glaciers ers.. 2. Measur Measuring ing the growth growth of moun mountai tains. ns. 3. Measur Measuring ing the location location of iceberg icebergss - this this is very valuabl valuablee to ship ship captains captains helping helping them to avoid possible disasters. 4. Storing Storing the location location of where you you were - most most GPS receiver receiverss on the market market will will allow 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.

36

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  precise atomic clock. This is why we can all synchronize synchronize our watches 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 ob tained.

37

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.

38

Tracking and Positioning of Mobiles in Telecommunication

CONCLUSION

Mobile positioning technology has become an important area of research, for emerge emergency ncy as well well as for commer commercia ciall servic services. es. Mobile Mobile positi positioni oning ng in cellul cellular  ar  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.

39

Tracking and Positioning of Mobiles in Telecommunication

REFERENCES •

Steven R. Strom. "Charting a Course toward Global Navigation”. The Aerospace Corporation. Retrieved on 2008-06-27.

•

en.wikipedia.org

•

www.landairsea.com

•

www.roseindia.net

40