tracking and positioning of mobiles in telecommunication
DESCRIPTION
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.TRANSCRIPT
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
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.
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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 location-
based services.
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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.
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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.
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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).
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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.
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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 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.
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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’
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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.
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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.
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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.
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.
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Basic concept of GPS operation
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
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be computed. Knowing the distance from GPS receiver to a satellite and the position of a
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
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sign indicates that you are 500 km from city A. But this is not of much help, as you could
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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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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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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Space segment
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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
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
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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.
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
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.
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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.
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
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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. 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.
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:
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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 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 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.
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 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.
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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.
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
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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 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.
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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
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
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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
Relativistic Effects
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Tracking and Positioning of Mobiles in Telecommunication
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
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.
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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.
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.
Ionospheric effects ± 5 meter
Shifts in the satellite orbits ± 2.5
meter
Clock errors of the satellites' clocks ± 2 meter
Multipath effect ± 1 meter
Tropospheric effects ± 0.5
meter
rounding errors ± 1 meter
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Artificial sources
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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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
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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.
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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.
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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.
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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.
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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.
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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.
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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
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