gps notes

1. 1

2. A Seminar Report on
GPS Based
Location Based Services
Prepared by : Divvi Jothsna Narsimham Ramavataram
Roll No. : 10
Class : B.E.IV (Electronics & Communication Engineering.)
Semester : 8th Semester
Year : 2008-2009
Guided by : Prof Nehal N. Shah
Department of
Electronics & Communication Engineering
Sarvajanik College of Engineering & Technology
Dr R.K. Desai Road,
Athwalines, Surat - 395001,
India.
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3. Sarvajanik College of
Engineering & Technology
Dr R.K. Desai Road,
Athwalines, Surat - 395001,
India.
Department of
Electronics & Communication Engineering
CERTIFICATE
This is to certify that the Seminar report entitled GPS Based
Location Based Services is prepared & presented by Miss.
Divvi Jothsna Narsimham Ramavataram Class Roll No 10 of
B.E.IV Sem VIII Electronics & Communication Engineering
during year 2008-2009. Her work is satisfactory.
Signature of Guide Head of Department
Electronics Engineering
Signature of Jury Members
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4. Acknowledgment
I would like to express my sincere thanks to Prof Nehal Shah. She helped me in
selecting my seminar topic as per my capability. She encouraged me with worthy suggestions
and support. Without her guidance, I would have not been able to complete my seminar
report.
I rarely find words to express my gratitude towards my faculty members of the
Electronics Department who were constantly involved with me during my seminar
preparation.
And last but not the least, I am thankful to all my colleagues and friends, who have
directly or indirectly contributed in preparing this seminar.
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5. ABSRACT
GPS based Location Based Services
A location based service (LBS) is an information and entertainment service that is
accessible with mobile devices through the mobile network. These services utilize the ability
to make use of the geographical position of the mobile device.LBS services use a single base
station with a radius of inaccuracy to determine a phone’s location. Several categories of
methods can be used to find the location of the subscriber. They are:
1) GPS based LBS
2) GSM localization
3) Blutooth, WLAN, Infrared or RFID technologies
The simple and standard solution from these three methods is GPS based LBS. It is used
to maintain the knowledge of the exact location. It is the only fully functional Global
Navigational Satellite System (GNSS) in the world.
The GPS is made up of three parts:
1) Satellites orbiting the Eearth
2) Control and monitoring stations on the Earth
3) The GPS receivers owned by users
It uses a constellation of between 24 and 32 Medium Earth Orbit Satellites that broadcast
signals from space that are picked up and identified by GPS receivers. These satellites also
transmit piecewise microwave signals, which allows GPS receivers to determine their current
location, the time and their velocity.
There are numerous advantages of GPS systems. A GPS tracking device can be
incorporated in mobile phones, palmtops or personal digital assistants. Hence this feature is
used for direction finding purposes, distance calculations and lot more. These devices are
used to track or determine the location of something whether it is a stolen device, a lost pet or
monitoring the location of wild or endagered species.
GPS has become a mainstay of transportation systems worldwide providing navigation
for ground and maritime operations. Everyday activities such as banking, mobile phone
operations are facilitated by the accurate timing provided by GPS.
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6. INDEX
1 Introduction....................................................................................................................... 9
1.1 Location Based Services:.......................................................................................... 9
1.2 Types of Location Based Services:........................................................................... 9
1.3 Applications of Location Based Services: ................................................................ 9
1.4 Classification of Location Based Services:............................................................. 10
2 Global Positioning System.............................................................................................. 11
2.1 History: ................................................................................................................... 11
2.2 Reason of choosing Global Positioning System: .................................................... 12
3 The Parts of Global Positioning System ......................................................................... 13
3.1 Space Segment:....................................................................................................... 13
3.2 Control Segment: .................................................................................................... 14
3.3 User Segment:......................................................................................................... 15
4 Working of GPS.............................................................................................................. 16
4.1 Method of Triangulation:........................................................................................ 17
4.2 Need of four satellites: ............................................................................................ 19
4.3 Measuring distance from a satellite: ....................................................................... 19
5 GPS Satellite Signals ...................................................................................................... 20
5.1 The two L-band carriers:......................................................................................... 21
5.1.1 L1 component: ................................................................................................ 21
5.1.2 L2 component: ................................................................................................ 21
5.2 The Ranging Codes:................................................................................................ 21
5.2.1 Course Acquisition Code (C/A):..................................................................... 21
5.2.2 Precision Code (P): ......................................................................................... 21
5.3 The Navigation Message: ....................................................................................... 22
5.3.1 Structure of the Navigation Message:............................................................. 22
5.4 GPS Positioning Services: ...................................................................................... 27
5.4.1 Standard Positioning Service (SPS):............................................................... 27
5.4.2 Precise Positioning Service (PPS): ................................................................. 27
5.5 GPS signal transmission and reception:.................................................................. 27
5.5.1 Autocorrelation Technique: ............................................................................ 28
6 GPS Errors and Selective Availability............................................................................ 30
6.1 Selective Availability:............................................................................................. 30
6.2 Sources of errors: .................................................................................................... 31
6.2.1 Ionospheric Propagation Errors: ..................................................................... 31
6.2.2 Tropospheric Propagation Error: .................................................................... 31
6.2.3 Ephemeris Data Errors:................................................................................... 31
6.2.4 Signal Multi-path Error:.................................................................................. 32
6.2.5 Onboard clock errors: ..................................................................................... 32
6.2.6 Receiver clock errors: ..................................................................................... 33
7 Advantages and Disadvantages ...................................................................................... 34
7.1 Advantages:............................................................................................................. 34
7.2 Disadvantages [7]: .................................................................................................. 34
8 Assisted GPS................................................................................................................... 35
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7. 8.1 Need of Assisted GPS:............................................................................................ 35
8.2 Concept of Assisted GPS[9]: .................................................................................. 36
9 Applications of GPS ....................................................................................................... 38
9.1 Vehicle history tracking or “Bread-crumbing”:...................................................... 38
9.2 Real time tracking:.................................................................................................. 38
9.3 Turn by turn navigation or route guidance: ............................................................ 38
10 Road map devices using GPS ......................................................................................... 40
11 Conclusion ...................................................................................................................... 42
12 Bibliography ................................................................................................................... 43
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8. List of figures
Figure 3-1 Parts of GPS .......................................................................................................... 13
Figure 3-2 Space Segment of GPs .......................................................................................... 14
Figure 3-3Control Segment of GPS........................................................................................ 15
Figure 4-1 Working Of GPS ................................................................................................... 16
Figure 4-2 Step 1 of triangulation........................................................................................... 17
Figure 4-3 Step 2 of triangulation........................................................................................... 18
Figure 4-4 Step 3 of triangulation........................................................................................... 18
Figure 5-1 GPS Satellite Signal Components......................................................................... 20
Figure 5-2 Structure of entire Navigation message ................................................................ 23
Figure 5-3 TLM and HOW word formats............................................................................... 24
Figure 5-4 Time Line relationship of HOW word .................................................................. 25
Figure 5-5 A schematic diagram showing how the GPS pseudo range.................................. 28
Figure 6-1 Plot of position determination with....................................................................... 30
Figure 6-2 Multi-path effect.................................................................................................... 32
Figure 8-1 Distortions of signals due to unclear view of sky ................................................. 35
Figure 8-2 Concept of Assisted GPS ...................................................................................... 37
Figure 10-1 Road mapping using Nokia 5800........................................................................ 40
Figure 10-2 Structure of a road map ....................................................................................... 41
List of tables
Table 2-1 Disadvantages of various positioning techniques................................................... 12
Table 5-1 Sub-frame ID code of HOW word ......................................................................... 24
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9. 1 Introduction
1.1 Location Based Services:
Location Based Services[1] (LBSs) are IT services for providing information that has
been created, compiled, selected or filtered taking into consideration the current locations of
the users or those of other persons or mobile objects. They can also appear in conjunction
with conventional services like telephony and related added value features, to realize location
based routing of calls or location based charging. The main advantage of LBS is that the
participants do not have to enter location information manually, but they are automatically
pinpointed and tracked. Therefore, the key technology is positioning, for which various
methods exist differing from each other in a number of quality parameters and other
circumstances.
Once location information is derived, it needs to be processed in several ways, including
transformation into the format of another spatial reference system, its correlation with other
location information or geographic content, the generation of maps, or the calculation of
navigation instructions. Usually, these tasks are not carried out on a single mobile device or
PC but are adopted by many actors involved in the operation of the respective LBS. Thus, the
operation of LBSs is an inter-organizational matter for which various actors like network
operators, service and content providers have to co-operate on a distributed infrastructure.
Thus, location based services can be defined as “An information and entertainment
service that is accessible with mobile devices through the mobile network. These services
utilize the ability to make use of the geographical position of the mobile device”.
1.2 Types of Location Based Services:
Location Based Services can be classified into two broad categories:
• Reactive LBS:
The user always explicitly activates Reactive LBS. The user first invokes the
service and establishes a service session using any handheld device. Then the server
processes the request for certain functions or information, and the location dependent
information is returned to the user. Thus, reactive LBS are characterized by a
synchronous interaction pattern between user and service.
• Proactive LBS:
Proactive LBS are automatically initialized as soon as a predefined location
event occurs, for example, if the user enters, approaches or leaves a certain point of
interest. As an example, consider an electronic tourist guide that notifies tourists via
SMS as soon as they approach a landmark. Thus, the user does not explicitly request
proactive services, but the interaction between them happens asynchronously.
1.3 Applications of Location Based Services:
LBS applications can be classified into four main categories:
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10. 1) The first one deals with the safety applications including emergency services and
roadside assistance.
2) The second category deals with the information related applications like business
finder, traffic alerts and weather report.
3) The third category is tracking application, which includes friend finder, fleet tracking
management, asset tracking and child tracking.
4) The last is the location based billing, for example, billing with zone based or area
based pricing options
1.4 Classification of Location Based Services:
Location based services can be classified as Indoor, Outdoor and Hybrid services based
on positioning technologies used. Positioning techniques are classified as:
1) Satellite based positioning systems such as Global Positioning System (GPS) and
Galileo.
2) Cellular positioning based on GSM and CDMA networks.
3) Wireless positioning based on Wi-Fi, Bluetooth, RFID and Sensor networks.
4) Assisted-GPS uses assistance from cellular network
5) Hybrid positioning which uses combination of various techniques.
Depending on which positioning techniques to be used, the handheld device should be
enabled with GPS, GPRS, WAP, WLAN, Bluetooth and RFID.
Of these techniques we will be dealing mainly for satellite based positioning i.e. Global
Positioning System (GPS).
.
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11. 2 Global Positioning System
2.1 History:
The Global Positioning System [1] (GPS) is a satellite navigation system that provides
positioning and clock time to the terrestrial user. Its original name was NAVSTAR
(Navigation System for Timing and Ranging).
The GPS was developed in the 1970’s by the U.S Department of Defense (DOD) so that
the military units can always know their exact location as well as that of other units.
The GPS system is made of 24 NAVSTAR satellites and 5 ground stations. The ground
stations are responsible for keeping the satellites in precise orbit. The DOD places each of the
24 satellites in a precise orbit of an altitude of 10,900 miles. Each satellite weighs 2 tons, is
18.5 feet long and orbits the earth in a little less than 12 hours.
The GPS can be used in any type of weather and is used on land, in air and for marine
applications. There are no subscription fees or setup charges to use GPS.
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12. 2.2 Reason of choosing Global Positioning System:
GPS offers various advantages over other positioning systems. The disadvantages of
other positioning systems [3] are as shown in table below:
Sr No. Positioning System Disadvantages
1. Landmarks Works only in local area. Subjected to movement or
destruction by environmental factors.
2. Dead Reckoning Very complicated. Accuracy depends on
measurement tools which are usually crude. Errors
accumulate quickly.
3. Celestial Complicated. Only works at night in good weather.
Limited precision.
4. Omega Based on relatively few radio direction beacons.
Accuracy limited and subjected to radio
interference.
5. Loran Limited coverage (mostly coastal). Accuracy
variable, affected by geographical situation. Easy to
jam or disturb.
6. SatNav Based on low frequency Doppler measurements. So
it’s sensitive to small movements at receiver. Few
satellites are there so updates are infrequent.
Table 2-1 Disadvantages of various positioning techniques
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13. 3 The Parts of Global Positioning System
The GPS consists of three major segments [4].
1) Space segment
2) Control segment
3) User segment
Figure 3-1 Parts of GPS
The space and control segments are operated by Unites States Military and administered
by the U.S Air Force.
3.1 Space Segment:
The Space Segment of the system consists of the GPS satellites or the space vehicles
(SV). These space vehicles send radio signals from space. The GPS system constellation has
24 satellites revolving the Earth in six orbital planes. From these 24 satellites, 21 are working
satellites and the remaining three are reserved in case of the failure of any of the 21 working
satellites.
There are six orbital planes with four satellites in each plane. The planes are equally
spaced (60 degrees apart) and inclined at about 55 degrees with respect to the equatorial
plane. The orbit period of each satellite is approximately 12 hours at an altitude of 20,183
km. The average elevation of the satellites is approximately 20,000 km above the Earth.
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14. Figure 3-2 Space Segment of GPs
The satellite broadcast signal contains data which identifies the satellite and provides the
positioning, timing, ranging data, satellite status and corrected orbit parameters of the
satellites.
3.2 Control Segment:
The control segment of the Global Positioning System consists of:
1) One Master Control Station (MCS) located at Falcon Air Force Base in Colorado
Springs, Colorado.
2) Five unmanned monitor stations located strategically around the world.
3) Three primary ground antennas maintained by the Air Force and located more or less
equidistant around the equator.
4) Two back up master control stations, in the event of some catastrophic failure, one
located in Sunnyvale, California, and the other in Rockville, Maryland.
5)
The monitor stations passively track all GPS satellites visible to them at any given
moment, collecting signal data from each. This information is then passed on to the master
control station where the satellite position (“ephemeris”) and clock timing data are estimated
and predicted.
The master control station then periodically sends the corrected position and clock timing
data to the appropriate ground antennas which then uploads those data to each of the
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15. satellites. Finally, the satellites use the corrected information in their data transmissions down
to the end users.
This sequence of events occurs every few hours for each of the satellites in order to
ensure that any possibility of error creeping into the satellite position of their clocks is
minimized.
Figure 3-3Control Segment of GPS
3.3 User Segment:
The GPS user segment consists of GPS receiver. The receiver then collects and processes
signals from the GPS satellites that are in view and then use that information to determine
and display the location, speed, time and so on. The GPS receiver does not transmit any
information back to the satellites. However, the accuracy and reliability is enhanced as the
number of visible satellites increases.
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16. 4 Working of GPS
The principle behind GPS is the measurement of distance (range) between the satellites
and the receiver. The satellites tell us exactly where they are in their orbits by broadcasting
data and this data in turn is used by the receiver to compute their positions [5].
Figure 4-1 Working Of GPS
Each satellite transmits data that indicates its location and current time. All GPS satellites
synchronize operations so that these repeating signals are transmitted at the same instant.
These signals moving at the speed of light arrive at the GPS receiver at slightly different
times because some satellites are farther away than others. The distance to the GPS satellites
can be determined by estimating the amount of time it takes for their signals to reach the
receiver. When the receiver estimates the distance to at least four GPS satellites, it can
calculate its position in three dimensions.
The Global Positioning System uses various position techniques. They are:
1) Precise Point positioning (PPP):
It is a method to perform precise position determination using a single GPS receiver.
Combining precise satellite positions and clocks with a dual frequency GPS receiver,
PPP provides position solutions at centimeter to decimeter level which is useful in
applications like airborne mapping.
2) Differential positioning:
It uses a network of fixed, ground based reference stations to broadcast the difference
between the positions indicated by the satellite systems and known fixed positions.
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17. These stations broadcast the difference between the measured satellite pseudo-ranges
and actual pseudo-ranges, and receiver stations may correct their pseudo-ranges by
the same amount.
3) Real time kinematics positioning (RTK):
It is used in land survey and in hydrographic survey based on the use of carrier phase
measurements of the GPS. Here a signal reference station provides the real time
corrections of even to centimeter level of accuracy. RTK uses the satellite’s carrier as
its signal and not the messages contained within.
4) Positioning using triangulation method
Of these methods, the triangulation method is described as below.
4.1 Method of Triangulation:
In order to understand the method of triangulation, consider the following example:
1) Suppose that the distance of the receiver from the satellite is measured and it is
around 11,000 miles. Thus, knowing that the receiver is 11,000 miles from a
particular satellite, narrows down all the possible locations where the receiver could
be in the whole universe to the surface of the sphere that is centered on this satellite
and has a radius of 11,000 miles.
Figure 4-2 Step 1 of triangulation
2) Next the distance of the receiver from a second satellite is measured and it is around
10,000 miles away. Thus, now the receiver is not only on the first sphere, but also on
the sphere that is 10,000 miles from the second satellite.
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18. Figure 4-3 Step 2 of triangulation
3) If we then make a measurement from a third satellite and find that the receiver is
8,000 miles from that satellite, it narrows the position of the receiver even further to
the two points where the 8,000 mile sphere cuts through the circle that is the
intersection of the first two spheres.
Figure 4-4 Step 3 of triangulation
Thus, by ranging from three satellites we can narrow the position of the receiver to just
two points in space. From these two points, one is always out somewhere where it makes no
sense, like thousands of kilometers out in space. The receivers are smart enough to sense that
one of the two points is wrong and rejects that point.
Although three satellites give us the precise location in the universe, four satellites are
needed to ensure an accurate position.
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19. 4.2 Need of four satellites:
Here the time taken by the radio signal to travel from a satellite transmitter down to
the receiver. In order to acquire an accurate position, very precise time measurements must
be made.
Now the time taken by the satellite signal to travel from the orbit to the receiver on
the ground is about 1/15th of a second. Since the radio waves are traveling at about 300,000
km per second, only 1/1,000,000th (i.e. one millionth) of a second of error in measuring the
travel time translates approximately 300 meters of error in position.
In order to keep very accurate time, each satellite carries four atomic clocks on board
i.e. two rubidium and two cesium. These clocks are accurate to within billionths of a second
per month. This is very accurate but not practical for ground based receivers because of more
weight and more cost.
Now each satellite only carries “inexpensive” quartz clocks with much lower
accuracy. However, it is more critical that the satellite and the receiver both start “counting
time” at exactly the same moment and continue to count time at the same rate since it’s the
time taken for a signal to reach the receiver. Thus, this is ensured by adding a fourth satellite
that acts as a time “referee”.
4.3 Measuring distance from a satellite:
1) The distance to a satellite is determined by measuring how long a radio signal takes to
reach the receiver from that satellite.
2) In order to make this measurement, an assumption that both the satellite and the
receiver generate the same pseudo random codes at the same time is made.
3) By comparing how late the satellite’s pseudo random code appears with respect to
that of the receiver’s, then the time taken to reach the receiver is determined.
4) Thus, multiplying this travel time with the speed of light, the distance can be
calculated.
Velocity (mph) x Time (hour) = Distance (miles)
In this case of GPS, we are measuring a radio signal so the velocity will be equal to the speed
of light or approximately 186,000 miles per second.
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20. 5 GPS Satellite Signals
Each GPS satellite simultaneously transmits a unique navigational signal[6] centered on
two L-band frequencies of the electromagnetic spectrum, thus eliminating the ionospheric
effect on the signals. At these frequencies the signals are highly directional and hence are
easily blocked as well as reflected by solid objects and water surfaces. The satellite signal
consists of the following components:
1) The two L-band carriers
2) The ranging codes modulated on the carrier waves.
3) The navigation message.
Figure 5-1 GPS Satellite Signal Components
Modulated onto the carrier waves are the PRN ranging codes and navigation message for
the user. The primary function of the ranging code is to determine the signal transit time from
satellite to receiver. The transit time when multiplied by the velocity of light gives the
receiver-satellite range.
The navigation message contains the satellite orbit information and satellite clock
parameters. All signals are derived from the output of a highly stable atomic clock.
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21. 5.1 The two L-band carriers:
5.1.1 L1 component:
The frequency of this component is 1575.42 MHz. The carrier of L1 signal consists of an
in-phase and a quadrature-phase component. The in-phase component is bi-phase modulated
by a 50-bps data stream and a pseudorandom code. This code is known as Course
Acquisition (C/A) code. It consists of a 1023 chip sequence that has a period of 1ms and a
chipping rate of 1.023 MHz.
The quadrature-phase component is also bi-phase modulated by the same 50-bps data
stream but with a different pseudorandom code. This code is called the precise or the
protected (P) code. It has a 10.23MHz chipping rate and a one week period.
5.1.2 L2 component:
The frequency of this component is 1227.60 MHz. In contrast to the L1 signal, the L2 signal
is modulated with only the 50-bps data and the P-code, although there is the option of not
transmitting the 50-bps data stream.
5.2 The Ranging Codes:
5.2.1 Course Acquisition Code (C/A):
The C/A code is a 1,023 bit long pseudorandom number (PRN) which when transmitted
at 1.023 Mbps, repeats every millisecond. The Pseudorandom codes when properly aligned
correlate strongly. Each satellite transmits a unique PRN code, which does not correlate with
any other satellite’s PRN code. Thus, in other words the PRN codes are highly orthogonal to
each other.
5.2.2 Precision Code (P):
The P-code is also pseudorandom number (PRN) which when transmitted at 10.23 Mbps,
repeats once a week. Each satellite’s P-code PRN code is 6.1879 x 1012 bits long. Since this
code is very long and complex, the receiver cannot directly acquire and synchronize with this
signal alone. Thus, the receiver must first lock onto the relatively simple C/A code and then,
after obtaining the correct time and approximate position, synchronize with the P-code.
Whereas the C/A PRNs are unique for each satellite, the P-code PRN is actually a small
segment of a master P-code approximately 2.35 x 1014 bits in length and each satellite
repeatedly transmits its assigned segment of the master code.
In order to prevent unauthorized users from using or potentially interfering with the
military signal through a process called “Spoofing”, the P-code was encrypted. Thus, the P-
21

22. code was modulated with the W-code which is a special encryption sequence, to generate the
Y-code. Here the encrypted signal is referred to as the P(Y) code.
The W-code is applied to the P-code at approximately 500 KHz, which is a slower rate
than that of the P-code itself by a factor of 20.
5.3 The Navigation Message:
In addition to the PRN ranging codes, the satellite needs to know the detailed information
about each satellite’s position and network. The GPS design has this information modulated
on top of both the C/A and P(Y) ranging codes at 50 bps. This information is known as the
Navigation Message.
The navigation message is made up of three major components:
1) GPS date and time as well as the satellite’s status and an indication of its health.
2) Orbital information called “Ephemeris” data which allows the receiver to calculate
the position of the satellite.
3) Almanac data containing information and status concerning all the satellites; their
locations and PRN numbers.
Whereas ephemeris information is highly detailed and considered valid for no more than
four hours, almanac information is more general and is considered valid for up to 180 days.
The almanac assists the receiver in determining which satellites to search for, and once the
receiver picks up each satellite’s signal in turn, it then downloads the ephemeris data directly
from that satellite. A position fix using any satellite cannot be calculated until the receiver
has an accurate and complete copy of that satellite’s ephemeris data.
5.3.1 Structure of the Navigation Message:
Data Page Format:
A complete message consists of 25 frames, each containing 1500 bits. Each frame is
subdivided into five 300-bit sub-frames, and each sub-frame consists of 10 words of 30 bits
each with the most significant bit (MSB) of the word transmitted first.
Thus, at 50 bps rate it takes 6sec to transmit a sub-frame and 30sec to complete one
frame. Transmission of complete 25-frame navigation message requires 750sec or 12.5min.
Except for occasional updating, sub-frames 1, 2, and 3 are constant with each frame at the
30sec frame repetition rate. While on the other hand, sub-frames 4 and 5 are each sub-
commutated 25 times. The 25 versions of sub-frames 4 and 5 are referred to as pages 1-25.
Hence, except for occasional updating, each of these pages repeats every 750sec or 12.5min.
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23. Figure 5-2 Structure of entire Navigation message
Each sub-frame or page of a sub-frame starts with a Telemetry (TLM) word a Handover
word (HOW) pair. The TLM word is transmitted first, immediately followed by the HOW.
The later is then followed by 8 data words.
Telemetry Word (TLM):
Each TLM word is 30 bits long, occurs every 6sec in the data frame and is the first word
in each sub-frame or page. Each TLM word starts with a preamble of 8 bits that indicates the
beginning of a new sub-frame and is used by the receiver for synchronization purposes. In
addition, it carries information about the recent operations that have been performed on the
transmitting satellite by the control stations.
Hand over Word (HOW):
The HOW is 30 bits long and is the second word in each sub-frame or page, immediately
following the TLM word. A HOW occurs every 6sec in the data frame.
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24. Figure 5-3 TLM and HOW word formats
Within its structure it contains the start time for the next sub-frame, which is transmitted
as the time of the week (TOW). The TOW count begins with the value 0 at the beginning of
the GPS week (i.e. from Saturday 23:59:59 hours to Sunday 00:00:00 hours) and is increased
by a value of 1 every 6sec. The HOW is then transmitted in order to synchronize with the P-
code. Bit nos. 20 to 22 used in the HOW word identifies the sub-frame just transmitted. The
sub-frame ID code of the HOW word is as follows:
Sub-frame ID Code
1 001
2 010
3 011
4 100
5 101
Table 5-1 Sub-frame ID code of HOW word
GPS time and satellite Z-count:
24

25. GPS time is established by the Control Segment and is used as the primary time reference
for all GPS operations. The zero time point is defined as midnight on the night of January 5,
1980/ morning of January 6, 1980. The largest unit used in stating GPS time is one week
which is defined as 604,800 seconds. GPS time differs from UTC because GPS time is
continuous time scale, while UTC (Universal Coordinated Time) is corrected periodically
with an integer number of leap seconds.
In each satellite, an internally derived 1.5 second epoch provides a convenient unit for
precisely counting and communicating time. Time stated in this manner is referred to as a Z-
count. The Z count is provided to the user as a 29-bit binary number consisting of two parts
as follows:
1) Time of Week (TOW):
The binary number represented by the 19 least significant bits of the Z count is
known as the time of week (TOW) count and is defined as the number of 1.5 second
epochs that have occurred since the transition from the previous week. The range of
TOW count is from 0 to 403,199 1.5 second epochs and is reset to zero at the end of each
week. The TOW count’s zero state is defined as that 1.5 second epoch which is
coincident with the start of the present week. This epoch occurs at midnight Saturday
night-Sunday morning.
In order to aid in rapid ground lock on, the HOW of each sub-frame contains a
truncated time of week (TOW) count. The HOW message TOW count consists of the 17
MSB’s of the actual TOW count at the start of the next sub-frame. To convert from HOW
message TOW count to actual TOW count at the start of the next sub-frame, multiply it
by four.
Figure 5-4 Time Line relationship of HOW word
25

26. 2) GPS Week number:
The ten most significant bits of the Z count are a binary representation of the
sequential number assigned to the GPS week (Modulo 1024). The range of this count is
from 0 to 1023, with its zero state being defined as that week which starts with the 1.5 sec
epoch. At the end of GPS week number 1023, the GPS week number will roll-over to 0.
Information by Sub-frame:
1) Sub-frame 1:
The first sub-frame carries the current GPS week number, the health of the
transmitting satellite, and clock correction data. The health gives information about the
state of the satellite’s transmitted navigation data and signals. For example, it indicates
whether navigation data is corrupted, the sub-frames that are affected by corrupted data,
and if the satellite is or will be temporarily out. From the health information, the receiver
can thus decide whether to use a navigation and measurement data from this satellite for
position estimation. Clock correction data informs the receiver about the amount of the
drift of the satellite’s clock with regard to GPS time. The correction is specified by means
of polynomial coefficients, which are used by the receiver to compute the exact GPS
time.
2) Sub-frames 2 and 3:
These sub-frames contain the ephemeris data, which is used to determine the precise
satellite position and velocity required by the navigation solution. This data contains all
data needed by the receiver to compute the exact satellite position in space. The
ephemeris does not reflect the satellite position at the time of measurements but instead it
reflects the satellite position at the exact time. The receiver can then estimate the current
position taking into consideration the difference between current and reference time.
3) Sub-frame 4:
The 25 pages of this sub-frame contains the almanac data for satellites with
pseudorandom code (PRN) numbers 25 and higher, as well as special messages,
ionospheric correction terms, and coefficients to convert GPS time to UTC time. The
almanac is a subset of each satellite’s ephemeris and clock data. The almanac helps to
speed up the start-up time of the GPS receiver since it obtains a rough overview of the
current satellite constellation when the receiver is turned on, which replaces the time
consuming identification by means of C/A codes.
4) Sub-frame 5:
The 25 pages of this sub-frame contain the almanac for satellites with PRN numbers
from 1 to 24. All 25 pages are transmitted together with information on the health of
satellite from 1 to 24.
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27. 5.4 GPS Positioning Services:
There are two basic services offered by the GPS system: the Standard Positioning Service
(SPS) and the Precise Positioning Service (PPS). Though both can be requested from all over
the world at each time of day without being charged, they differ from each other in the
accuracy of delivered position data, the features associated with them and the groups of users
they address.
5.4.1 Standard Positioning Service (SPS):
The SPS is a positioning and timing service focusing on the civilian user. It is based on
the C/A code transmitted at the L1 carrier and the navigation message it transfers.
Published specifications for the Standard Positioning Service are:
1) 100 meter horizontal accuracy
2) 156 meter vertical accuracy
3) 167 nanoseconds time accuracy
5.4.2 Precise Positioning Service (PPS):
The PPS is a positioning, velocity, and timing service for military applications. It is based
on both the C/A and the P code transmitted on the L1 and L2 carriers.
Published specifications for the Precise Positioning Service are:
1) 17.8 meter horizontal accuracy
2) 22.7 meter vertical accuracy
3) 100 nanoseconds time accuracy
5.5 GPS signal transmission and reception:
Let us now summarize how the GPS signal is transmitted from space, and then received
on the ground. The GPS signal starts in the satellite as a voltage which oscillates at the
fundamental clock frequency of 10.23 MHz. The signal is then separately multiplied in
frequency by the integers 154 and 120, to create the L1 and L2 carrier signals. The signals
are then multiplied by +1 and -1 to generate the C/A code on L1 and the P-code on both L1
and L2. These codes are unique to each satellite.
Finally, the Navigation message is encoded onto the signal. The signals are boosted by an
amplifier, and then sent to transmitting antennas, which point towards the Earth. These
antennas are exposed electrical conductors which radiate the signal into space in the form of
electromagnetic waves.
These electromagnetic waves pass through space and the Earth’s atmosphere, at the speed
of light in a vacuum, until they reach the receiver’s antenna. The waves create a minute
signal in the antenna, in the form of an oscillating voltage. The signal is now pre-amplified at
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28. the antenna, to boost the signal strength, so that it is not overcome by noise by the time it gets
to the other end of the antenna cable. The signal then enters the receiver, which then
measures it using a process called “auto-correlation”.
5.5.1 Autocorrelation Technique:
Figure 5-5 A schematic diagram showing how the GPS pseudo range
Observation is related to satellite and receiver clocks
Also the receiver generates GPS like signals internally. The receiver knows precisely
what the transmitted GPS signal is supposed to look like at any given time, and it generates
an electronic replica, in synchronization with the receiver’s own clock. The receiver then
compares the replica signal with the actual signal. Since the GPS signal was created in the
satellite some time previously, the receiver’s replica signal must be delayed in to match up
the incoming signal with the replica signal. This time delay is measured by the receiver. This
represents the time taken for the signal to pass from the satellite to the receiver, but it
includes any error in the satellite and the receiver clock. This time delay is therefore related
to the range to the satellite. Now let us see how the receiver matches the two signals.
The time difference is computed by autocorrelation. The first bit from signal one is
multiplied by the first bit of signal two. For example, if the first bits from the two signals
both have values -1, then the result is (-1) x (-1) = +1. Similarly, if both bits have values +1,
then the result is +1. While on the other hand, if the two bits disagree, the result is (+1) x (-1)
= -1. This process is repeated for the second pair of bits, and so on. The result can be written
as a sequence of +1 (where the bits agree) and -1 (where the bits disagree). This sequence is
then summed, and divided by the total number of bits in each signal. For example, if the
signal A can be written (+1,-1,-1,+1,-1), and signal B can be written (+1,+1,-1,-1,+1), then
multiplication gives (+1,-1,+1,-1,-1); the sum of which gives -1; then dividing by the number
of bits (5) gives -0.2. If the two signals matched perfectly, the result would be +1. If the two
signals were completely random, the result will close to zero.
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29. Thus, larger the number of bits that are compared, the better is the resolution because the
random bits will average to zero, better as more bits are compared.
Now since that the peak autocorrelation is found, the inferred time displacement between
the two signals is multiplied by the speed of light. This observation is called pseudo range.
This pseudo range measurement is shown schematically in above figure.
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30. 6 GPS Errors and Selective Availability
6.1 Selective Availability:
GPS included a feature called Selective Availability (SA) that adds intentional, time
varying errors up to 100 meters to the publicly available navigation signals. This was
intended to deny an enemy the use of civilian GPS receivers for precision weapon guidance.
For example, terrorists should not be provided with the possibility of locating important
buildings with homemade remote control weapons.
Before it was turned off, typical SA errors were 10 meters horizontally and 30 meters
vertically. Because SA affects every GPS receiver in a given area almost equally, a fixed
station with an accurately known position can measure the SA error values and transmit them
to the local GPS receivers so they may correct their position fixes. This is called Differential
GPS (DGPS). DGPS also corrects for several other important sources of GPS errors,
particularly ionospheric delay, so it is widely used even though SA has been turned off.
The following two graphs show the improvement of position determination after
deactivation of SA. The edge length of the diagrams is 200 m. While with SA 95% of all
points are located within a radius of 45 m, without SA 95% of all points are within a radius
of 6.3 m.
Figure 6-1 Plot of position determination with
and without SA
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31. Selective Availability is an artificial falsification of the time in the L1 signal transmitted
by the satellite for civil GPS receivers that leads to a less accurate position determination.
Additionally the ephemeris data is transmitted with lower accuracy, meaning that the
transmitted satellite positions do not comply with the actual positions. In this way an
inaccuracy of the position of 50-150 m is achieved. Also from the deactivation of SA, the
determination of heights has improved.
One side effect of the Selective Availability is the capability to correct the frequency of
the GPS cesium and rubidium atomic clocks to an accuracy of approximately 2 x 10-13.
Hence, this represented a significant improvement over the raw accuracy of the clocks.
6.2 Sources of errors:
6.2.1 Ionospheric Propagation Errors:
The ionosphere, which extends from approximately 50 to 1000km above the surface of
the earth, consists of the gases that have been ionized by the solar radiation. The ionization
produces clouds of free electrons that acts as a dispersive medium for GPS signals in which
propagation velocity is a function of frequency.
The primary effect of the ionosphere on GPS signals is to change the signal propagation
speed as compared to that of free space. Satellite signal is slowed as it passes through the
ionosphere. This delay creates a miscalculation of the satellite’s distance resulting in receiver
position error.
6.2.2 Tropospheric Propagation Error:
The lower part of Earth’s atmosphere is composed of dry gases and water vapor, which in
turn lengthens the propagation path due to refraction. The magnitude of the resulting signal
delay depends on the refractive index of air along the propagation path. The troposphere is
non-dispersive at the GPS frequencies, so that the delay is independent of frequency.
In contrast to the ionosphere, the tropospheric path delay is consequently the same for
code and carrier signal components. Although a GPS receiver cannot measure pseudo range
error due to the troposphere, differential operation can usually reduce the error to small
values by taking advantage of the high spatial correlation of tropospheric at two points within
the 100-200 km on the earth surface.
6.2.3 Ephemeris Data Errors:
Small errors in the ephemeris data which are transmitted by each satellite, causes
corresponding errors in the computed position of the satellite. Satellite ephemerides are
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32. determined by the master control station of the GPS ground segment based on monitoring of
individual signals by four monitoring stations.
6.2.4 Signal Multi-path Error:
Multi-path propagation of the GPS signal is a dominant source of error in differential
positioning. Objects in the vicinity of the receiver antenna, such as tall buildings or large
rock surfaces, reflects GPS signals, resulting in one or more secondary propagation paths.
These secondary path signals, which are superimposed on the desired direct path signal,
always have a longer propagation time and can significantly distort the amplitude and phase
of the direct path signal.
Figure 6-2 Multi-path effect
6.2.5 Onboard clock errors:
Timing of the signal transmission from each satellite is directly controlled by its own
atomic clock without any corrections applied. This time frame is called Space Vehicle (SV)
time. Although the atomic clocks in the satellites are highly accurate, errors can be large
enough to require correction. Correction is needed partly because it is difficult to directly
synchronize the clocks closely in all the satellites.
Instead, the clocks are allowed some degree of relative drift that is estimated by ground
station observations and is used to generate clock correction data in the GPS navigation
message. When SV time is corrected using this data, the result is called GPS time. The time
of transmission used in calculating pseudo ranges must be in GPS time, which is common to
all satellites.
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33. 6.2.6 Receiver clock errors:
Similar to satellite clock errors, any error in the receiver clock causes inaccuracy in distance
measurements. However, it is not practical to equip receivers with very accurate atomic
clocks. Atomic clocks weigh more than 20 kg, cost about US$50,000, and require extensive
care in temperature control.
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34. 7 Advantages and Disadvantages
Of GPS
7.1 Advantages:
1) A GPS tracking system is more beneficial for travelers. For example, while taking
road trips to a distant location, a tracking system would be very advantageous. A
tracking system helps to find where one is located on the road.
2) Another benefit is having the ability to improve supervision over employees. The
GPS tracking system has been great assistance to several businesses in terms of
saving more money.
3) A GPS tracking system can provide safety for children. Finding a missing child
becomes much easier with a GPS tracking system.
7.2 Disadvantages [7]:
1) GPS signal reception:
Proper functioning of a GPS receiver requires the undisturbed reception of signals
from at least four GPS satellites. These signals propagate from the satellites to the
receiver antenna along the line of sight and cannot penetrate water, soil, walls or other
obstacles very well.
Therefore, GPS cannot be used for subsurface marine navigation and nor for
underground positioning and surveying
2) GPS signal integrity:
A GPS receiver computes position and time from range measurements to the GPS
satellites, using satellite positions derived from information encoded in the transmitted
signal i.e. the satellite message. With one measurement to each of the four measurements
there will be a unique receiver position solution.
However, wrong satellite positions or wrong range measurements will result in an
incorrect calculation of receiver position. If the faulty signals are not detected, the user
will not know that the displayed position is wrong.
3) GPS signal accuracy:
A GPS receiver essentially measures the time required for a signal to travel from the
satellite to the receiver. This travel time is converted to a range measurement by
multiplying it by speed of light. However this measurement is corrupted by a number of
different errors.
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35. 8 Assisted GPS
8.1 Need of Assisted GPS:
Upon activation, the GPS receiver scans for signals from the GPS satellites. The unit
must locate and receive signals from at least four satellites to be able to determine its
location. With unassisted GPS, this process of locating the satellites, receiving the data and
achieving a position fix can take several minutes. This delay can be problematic for many
GPS applications.
A second limitation of GPS is that the receiver needs a clear view of the sky to
successfully receive signals from the satellites. Under unfriendly RF conditions, such as in a
building or other RF shadowed environments, accuracy of the position fix can be
compromised. In some cases it is impossible to achieve a position fix.
Figure 8-1 Distortions of signals due to unclear view of sky
With AGPS [8], a wireless network sends information directly to the GPS receiver, which
in turn allows the receiver to quickly locate the four satellites and process the data contained
in their signals.
The AGPS information includes identification of the visible satellites. Since the receiver
is only searching for specific signals, the amount of time it takes for a GPS receiver to obtain
its first location or time to first fix (TTFF) is reduced from minutes to seconds.
Assistance is also provided to the GPS receiver by sending the ephemeris data for each
satellite so that this data does not have to be decoded from the GPS signals. The receiver
35

36. must still obtain signals from at least four satellites to determine the time it took each signal
to arrive at the receiver without the need of decoding the entire signal.
Assisted GPS effectively increases the sensitivity of the receiver so that it is able to
obtain and demodulate the satellite signals in areas where unassisted GPS could not. Further,
since ephemeris data is already provided to the receiver, it can determine more quickly than
if unassisted, even in clear view of the sky.
Assisted GPS is more advantageous when the device is in unfriendly RF environment.
For example, this situation prevails when the device is first powered. When first powered,
there is no valid ephemeris data on the GPS receiver, so the positions of the satellites in the
sky are unknown. Thus, in this case the Assistance information enables the receiver to obtain
a fix more quickly than an unassisted device and in some cases to obtain a position fix where
an unassisted device could not obtain.
If a GPS receiver has been functioning and has been demodulating the satellite signals
prior to entering an unfriendly RF environment, the assistance offers no advantage. However,
if the receiver remains in this unfriendly RF environment for a period of time, the satellites
viewable over its position will change. In addition, the ephemeris data of each satellite will
also change, as corrections are made to its orbit. For these reasons the ephemeris data
becomes stale and needs to be updated on the GPS receiver. Regular updates of ephemeris
data to the receiver enable the device to continue operation in conditions where an unassisted
device would cease to operate.
8.2 Concept of Assisted GPS[9]:
Compared to the GPS, assisted GPS technology offers superior accuracy, availability and
coverage at a reasonable cost. An AGPS consists of:
1) A wireless handset with a partial GPS receiver.
2) An AGPS server with a reference GPS receiver that can simultaneously view the
same satellites as the handset
3) A wireless network infrastructure consisting of base stations and a mobile switching
center.
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37. Figure 8-2 Concept of Assisted GPS
Since an A-GPS server can obtain from the MSC the handset’s position (up to the level of
cell and sector), and at the same time monitors signals from the GPS satellites seen by MS, it
can predict the signals received by the handset for any given time. Specifically, it can predict
the Doppler shift (due to satellite motion) of GPS signals experienced by the handset
receiver, as well as other signal parameters that are a function of the mobile’s location.
In a typical sector, the uncertainty in the predicted time of arrival of a satellite signal at
the mobile is about ±5 μs, which corresponds to ±5 chips of the C/A spreading code
sequence. Therefore, AGPS server can predict to within ±5 chips the phase of the PRN
sequence that the receiver should use to de-spread the C/A signal from a particular satellite
and communicate that prediction to the mobile. The search space for the actual Doppler shift
and PRN phase is therefore greatly reduced and the AGPS handset can accomplish the task in
a small fraction of the time required by conventional GPS receivers. In addition, the AGPS
server maintains a connection with the handset receiver over the wireless link, asking it to
make specific measurements, collect the results, and communicate them back.
After de-spreading, an AGPS receiver could pass the PRN phase information back to the
AGPS server, which would then calculate the mobile location coordinates. To reduce the
amount of information sent over the air-interface, a preferred solution is to perform
additional signal processing in the handset and return pseudo ranges instead.
An additional way to help the handset receiver in detecting GPS signals in “Sensitivity
Assistance (or modulation wipe-off)”. The sensitivity assistance message contains sets of
predicted data bits in the GPS navigation message, which are expressed to modulate GPS
signal of specific satellites at specified times. For optimal performance of sensitivity
assistance, AGPS server must communicate to the handset the PRN sequence timing with an
accuracy of several microseconds.
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38. 9 Applications of GPS
GPS applications[10] for mobile computing fall into the following categories:
9.1 Vehicle history tracking or “Bread-crumbing”:
“Bread-crumbing” captures and stores a detailed GPS history of vehicle travel
information and uploads it for later management review. However this does not enable the
managers to know where their mobile workers are at any given moment. Since, the data is
not transmitted in real time over a wide area wireless network, the added cost of wireless
airtime is unnecessary.
Bread-crumbing analyzes and reports on activity that occurred previously in the field,
providing information that can be extracted from vehicle travel history. This information can
be used to improve field performance and give managers an unprecedented view into what
actually happens in the field.
Bread-crumbing provides the following benefits:
1) Gain visibility into field operations for greater control
2) Capture a complete and detailed record of field activities.
3) Identify unproductive time in the field to increase overall productivity.
4) Identify and reduce out-of-route mileage for fuel cost savings.
5) No need for wireless airtime to transmit data.
9.2 Real time tracking:
Real time tracking is one of the original uses of GPS technology in the field. Also called
Automatic Vehicle Location (AVL), real time tracking systems enable vehicles in the field to
periodically report their location over a wide area wireless network in time intervals of
anywhere from one minute to fifteen minutes or more depending on the needs and airtime
budget of the user. AVL systems typically provide a map based interface where the
dispatcher can view can view and report on vehicle location and status. These systems also
provide features like “Geo-fencing” or the ability to alert the dispatcher when a vehicle enters
or leaves a predetermined area.
Real time tracking offers the following benefits:
1) Improved management of dispatch and fleet activities.
2) Increased daily efficiency, productivity and accountability.
3) Added security for vehicles.
9.3 Turn by turn navigation or route guidance:
One of the best known uses of GPS technology is to provide turn by turn driving
directions to the user in real-time. Vehicle navigation system use GPS to calculate the user’s
current position and navigation algorithms to calculate the best route to the user’s planned
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39. destination. The system then provides the driving directions, which are delivered as voice
instructions through text-to-speech or recorded audio output. If the driver does not follow an
instruction, say by missing a turn, the navigation system will automatically recalculate the
route without the need for any action by the driver.
Some of the benefits that the GPS navigation can provide are:
1) Reduce unnecessary mileage and fuel costs by providing accurate driving
directions.
2) Improve on time performance as drivers are less likely to get “lost” or follow
inefficient routes.
3) Reduce vehicle engine idling time as driver lookup customer locations on maps.
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40. 10 Road map devices using GPS
The Nokia 5800 has a built in GPS satellite navigation receiver, which tell our exact
location anywhere on the planet. The 5800 also has cell tower positioning technology which
finds the location using the position of the nearest phone network if a satellite is unavailable.
GPS is more accurate but can take some time depending on how obstructed the sky is to find
the position. Thus, Assisted GPS can be used, which downloads the expected GPS satellite
positions from the Internet and speeds up the finding of the position.
Figure 10-1 Road mapping using Nokia 5800
The 5800 also has a built in mapping application called Nokia Maps, which shows our
location on a street map, and tells us how to get to another location by driving or walking and
can give details of nearby amenities such as shops, fuel stations, museums etc. Nokia maps
use both GPS and cell tower positioning to tell us where we are. The latter gives an instant
rough position while GPS data is used after the phone has locked onto enough satellite
signals to calculate its position.
The various portions of road map is as shown below:
40

41. Figure 10-2 Structure of a road map
41

42. 11 Conclusion
Thus, from above discussion, it is evident that the Global Positioning System is the very
accurate method of positioning. It is very accurate in the position determination of the
receiver. The area covered by the GPS is the whole Earth and it uses the minimum number of
satellite required for it. Its time limitation was overcomed by using Assisted GPS which in
turn is less accurate than GPS but much faster. GPS has already been used in cell phones for
various applications like road mapping etc. In military application it is the only system relied
upon in providing data to very expensive guided weapons.
Thus, GPS can be considered as the most advanced accurate, commercially available and
multi-use satellite navigation system that has ever been existed.
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43. 12 Bibliography
1) Axel Kupper, “Location Based Services, Fundamentals and operation”, Wiley
Publication, 2005
2) http://www.kowama.de/en/gps
History and setup of GPS System
3) http://www.trimble.com/gps
GPS tutorials
4) Gregory T. French, “An Introduction to the Global Positioning System, What it is and
How it works, Geo-research, Inc Publication, 1st edition
5) http://www.aero.org/education/primers/gps/
Introduction and working of GPS
6) Mohinder S. Grewal, Lawrence R. Weill, Angus P. Andrews, “Global Positioning
Systems, Inertial Navigation, and Integration”, A John Wiley & Sons, Inc.
Publication,2001
7) A. Kleusberg and R.B. Langley, “Limitations of GPS”, University of New
Brunswick, March/April 1990, Vol. 1, No. 2, pp. 50-52.
8) www.skytel.com
Assisted GPS
9) Djuknic, Goran M. and Robert E. RichtonBell, “Geolocation and AGPS”,
Laboratories, Lucent Technologies, February 2001.
10) Michael Forbes, “Mobile GPS Applications”, Electric Compass, 2008, pp. 4-7
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