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					                                     GPS & DGPS



ABSTRACT :                                            of GPS. This paper further discusses about
GPS has become a vital tool for outdoor
                                                      DGPS, concept of DGPS, DGPS using beacon
agricultural professionals (farmers, consultants,
                                                      receiver and applications of DGPS. GPS is
etc.) and many other professions. GPS, or the
                                                      proved to be all -weather, space based navigation
Global Positioning System, is basically an
                                                      system. The primary application of it is in
electronic tool that provides the user with
                                                      military nature. Next application is in civilian
accurate position information, usually in the
                                                      use. The multi purpose usage of GPS has
form   of     world   coordinates   (latitude   and
                                                      developed enormously within the last three
longitude). Even though GPS receivers are
                                                      decades. These GPS Satellites provide a platform
becoming very popular, few people have a good
                                                      for radio transmitter, atomic clock, computers
understanding of just how GPS works, wh at
                                                      and various equipment used for positioning and
GPS is capable of, etc. This paper will provide
                                                      for a series of other military projects (e.g.-atomic
you    with    a   simple   and     straightforward
                                                      flash detection).
explanation of how GPS works, GPS satellite
constellation, GPS errors, its accuracy and uses

                                                      INTRODUCTION              :-
CONTENTS :
                                                      GPS is a satellite based radio navigation system

1. Introduction                                       established by the US Department of defense for
                                                      Military positioning applications. It is a $12+
2. GPS Description
                                                      billion navigation system made up of satellites,
3. GPS Satellite Constellation                        ground-based control, and an end-user receiver

4. How does GPS work?                                 and antenna (the hardware we use). In the 1970s,
                                                      after much research, the U.S. Department of
5. GPS Errors
                                                      Defense (DOD) developed and began deploying
6. GPS Uses and advantages                            a satellite-based system to provide military

7. DGPS and its concept                               positioning, navigation and weapons aiming
                                                      capabilities — the Global Positioning System.
8. DGPS using beacon receiver
                                                      While the original use of GPS was solely
9. Conclusion                                         military, civilian use soon followed. This

10. Bibliography                                      technology is now available to everyone,
                                                      everywhere, at any time and, best of all, at no
                                                      cost. GPS operates 24 hours a day in all weather


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conditions, and can be used world -wide for
precise navigation on land, on water and even in     Space segment:
the air. Today, the civilian use of GPS l ikely      The GPS satellite constellation consists of 21
equals or surpasses that of the military.            main and 3 spare satellites in near circular orbits
Navigation, surveying and integration with           approximately 20,150km above the earth. The
Geographic Information Systems (GIS) are just a      nominal 24 -satellites are positioned in six earth
few of the fields which have seen the successful     centered orbital planes with four satellites in
application of GPS Technology. GPS is a              each plane. The orbits are equally spaced above
complex system which can be used to achieve          the equator at a 60 degree separ ation with an
positional accuracies ranging from 10 m to a         inclination relative to the equator nominally 55
parts per million (ppm) depending on the             degrees. The GPS satellites travel at a velocity of
equipment used and procedures followed.              3.9km/sec. The advantage of greater altitude is
                                                     that the orbits will be less affected by the
                                                     irregularities caused by unequal distribution of
GPS DESCRIPTION :-                                   mass in the earth.

The Global positioning system (GPS) consists of      Control Segment:

1) a constellation of radio -                        The control segment consists of the master

navigation satellites, 2) a ground control segment   control station located near Colorado Springs,

which manages satellite operation and 3) users       which is responsible for all the data processing,

with specialized receivers who use the satellite     world wide network of monitor stations and

data to satisfy a broad range of positioning         ground antennas. The position of the se monitor

requirements. The system was established by the      stations are precisely known. The radiometric

United States Department o f Defense (DOD) to        data   (pressure,    temperature,     humidity)   are

fulfill defense positioning needs and as a by -      tracked by the monitor stations and transmitted

product, to serve the civilian community.            to the master control station by communication
                                                     lines. For better system accuracy the predicted
                                                     navigation me ssage is up linked three times
                                                     daily. The other important aspect of the control
                                                     segment is to maintain health and welfare of the
                                                     satellites.
                                                     User Segment:
                                                     The user segment consists of five principle
                                                     components     -     antennas,      receiver,   signal
                                                     processing and data processing capabilities
                                                     input/output device such as control display unit
                                                     and a power supply. Satellite signals are received
                                                     via the antenna , which is right handed circularly


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polarized     (RHCP)       and     provides     near
hemispherical coverage. Since the satellites are
RHCP, a conical helix antenna or variation is
suitable.

GPS SATELLITE CONSTELLATION                   :-
The satellite constellation consists of 24 satellites
and three active spares
positioned 20,200 km (about three times the
earth's radius) above the earth. The satellites
were distributed in a manner that ensures at least
four satellites are visible almost anywhere in the
world at any time. Each satellite receives and
stores information from the control segment             HOW DOES GPS WORK?
maintains very accurate time through on board           The basic operating principle of GPS is quite
precise atomic clocks and transmits signals to the      simple. Each GPS satellite
earth.The control segments tracks all satellites,       broadcasts a coded radio signal (traveling at the
ensure they are operating properly and computes         speed of light) containing its orbit position and a
their position in space. If a satellite is not          very accurate time value (exact to a billionth of a
operating properly the ground control segment           second). The GPS receiver uses this data to
may set the satellite "unhealthy" and apply             calculate distance to the satellites by determining
measures to correct the problem. In such cases,         the time it took the signal to travel from the
the satellite should not be used for positioning        satellites to the receiver. A GPS receiver uses
until its status is returned to "healthy". The          this data from a minimum of four satellites to
computed positions of the satellites are used to        calculate the receiver’s distance from each
derive parameters, which in turn are used to            satellite and then its position o n the earth -
predict where the satellites will be later in time.     latitude, longitude and altitude (i.e. 3 -D
These parameters are uploaded from the control          position). This distance calculation (from each
segment to the satellites and are referred to as        satellite to the receiver) must be very precise to
broadcast E phemeredes.                                 determine position. Each satellite uses four
                                                        cesium –based atomic clocks to achieve their
                                                        high degree of timing accuracy, whereas
                                                        consumer -oriented GPS receivers use less
                                                        sophisticated and less expensive internal timing
                                                        circuitry. However, the receivers continuously
                                                        update their clocks from the satellite clocks thus
                                                        making them far more



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accurate than your wristwatch.Except for the             caused by slight satellite orbit variations that
ultra-precise “P-Code” signal (reserved for              must be     periodically adjusted     by ground
military use), every GPS satellite’s current             controllers. The earth’s atmosphere can also
navigational message is broadcast such that it           affect GPS accuracy . Electrically charged
can be detected, recognized, and processed by            particles in the ionosphere and water vapor in the
any GPS receiver operat ed by any user                   troposphere combine to slow satellite signals as
anywhere on earth. The 24 GPS satellites orbit           they pass through these regions. This slight
the earth twice in a 24-hour period at an altitude       slowing of the signals can introduce considerable
of about 12,000 miles.                                   position errors.




                                                          The atmosphere (A) and multipath (M) are
                                                                    two sources of GPS error.
GPS ERRORS :-
Don’t assume, however, that using a GPS
receiver to determine latitude, longitude
and altitude ensures absolute accuracy. Several
factors can cause positional errors that affect
GPS accuracy. The most common factors                    USES FOR GPS :-
affecting GPS accuracy include clock errors,
                                                         Just as there are numerous styles and models of
ephemeris        errors,      atmospheric      delays,
                                                         GPS receivers, there are many uses for GPS.
multipathing and satellite geometry. Clock errors
                                                         Hand-held GPS units (Class I) are useful for
occur because the previously mentioned internal
                                                         determining locations of features and creating
timing circuitry of GPS receivers is not as
                                                         boundaries and determining areas of boundaries.
accurate    as    satellite   clocks.   Very    small
                                                         Class II and Class III GPS receivers, in addition
differences between the receiver clock and
                                                         to uses described for Class I receivers, are useful
satellite clocks can cause noticeable errors in
                                                         for connecting to guidance systems like light
position. Ephemeris errors are position errors
                                                         bars and auto steer systems, for variable –rate

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applications, and for yield mapping. Class IV         whole GPS system by identifying certain errors
systems are designed for surveying, topographic       The DGPS system uses GPS receivers accurately
mapping, and for precise GPS guidance systems.        placed at known locations (base station). Base
                                                      station GPS receivers constantly calculate the
                                                      error between location determined from GPS
ADVANTAGES:-
                                                      signals and its known location. This error is then
Some of the predominant advantages of GPS are
                                                      relayed to a rover GPS receiver being used “in
Land based system problems like ground
                                                      the field” for normal GPS work. Today, users do
reflections, electromagnetic interference,
                                                      not need to establish their own base stations to
reflections from physical systems are mostly
                                                      obtain DGPS. Now there are several sources for
avoided in GPS since it is space
                                                      DGPS correction signals that enable DGPS-
constellation
                                                      equipped receivers to function over wide are as.
Intentional   interference      like   jamming,
                                                      The differences in these sources lie in the
unintentional interference will not affect GPS
                                                      delivery methods and reliability of the DGPS
easily as spread spectrum techniques are used in
                                                      signals.
it.
                                                      Principle:
System accuracy can be improved to the order
                                                      At the same time, the errors occurring at one
of centimeters using differential
                                                      location (Base station) are occurring
techniques.
                                                      everywhere (Rover) within the same vicinity.
Smaller size and reduced cost of the GPS
                                                      Concept:
receiver ena ble it to be used in #G
                                                      DGPS (Differential Global Positioning Systems)
Communications
                                                      is very useful instrument for
                                                      generating contours even at 10cms intervals. In
                                                      the normal GPS survey we will get the
DIFFERENTIAL GPS :-                                   positional accuracy with in the range of + 5 to
GPS receiver is about 100 feet. Obviously, this       10mts. In this study area it has been used DGPS
would “get you out of the woods” but                  technology for generating the conto urs at
would not be accurate enough for most                 1mt.interval. In the DGPS technology it will
applications in the agricultural or natural           have Base station and rover, base station will
resources areas.To offset the effects of GPS          continuously record the base station position and
errors and to increase the overall accuracy of        rover will collect the information in the field.
consumer grade GPS units, a system called             Base station position will be known and with
Differential GPS, or DGPS, is used. DGPS is a         reference to this known position it will calculate
method of improving the accuracy of your              the error at every instant of the time. Same time
receiver by adding a local reference station to       rover will collect the information from the field.
augment the information available from the            Same error that is calculated at the base station
satellites. It also improves the integrity of the


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will be applied to the rover files for the same        most of the error sources would be common
time.                                                  between the two locations. Beacon easily
DGPS        MODE       USING      A    BEACON          identifies satellite where the required corrections

RECEIVER                                               exceed a prescribed value and should not be

In DGPS operation a station, often called a            used.

beacon, transmits correction data in
real time that is received by a separate box,
called a beacon receiver, which sends the c
orrection information to the GPS receiver. In
principle this is quite simple. A GPS receiver
normally calculates it position by measuring the
time it takes for a signal from a satellite to reach
its position. By knowing where the satellite is,
how long it takes to send the signal, and knowing
the speed of the signal it can compute what is
called a pseudo range (distance) to the satellite.
This range must be corrected before it is used to
compute the final position. A DGPS beacon              For simple DGPS connections you can just wire
transmitter site has already calculated all of the     a beacon receiver output signal
pseudo range correction data based on the fact         along with its ground to the data input terminals
that it already knows exactly where it is and can      of the GPS. If you need to be able to control the
compute the errors in the satellite computed           beacon receiver from the GPS receiver then you
position from its known location. Once the             will also need to send the output for the GPS
pseudo range correction data is computed it is         receiver to the beacon receiver. A standard
sent to the GPS and used to compute a more             computer interface cable can usually be used for
accurate    fix.Each    beacon     transmitter    is   this connection. If you also need to talk to a pc at
autonomous and computes its own corrections            the same time things start to get a little more
based on its reception of GPS signals. It then         complicated. To talk to a pc in NMEA mode you
packages the correction data in groups of 3            can simply send the output of the GPS to both
satellites and sends the data to the GPS receiver.     units. Wire the output signal to the input on the
Note that the design of a beacon DGPS                  computer and the input on the beacon receiver.
transmitter will send corrections for up to 9          There is sufficient power in the signal from the
satellites and these are only those at least 7.5       Garmin to drive both units and even a third item.
degrees above the horizon. The assumption is           Note if the beacon receiver doesn't need to
that the GPS receiver will be close e nough to         receive commands from the GPS then there is no
have the same sort of errors that the beacon           reason to send the signal both places but the
station saw. This works well in practice since         ground wi re is still needed. Finally if the GPS


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needs to talk to the pc in Garmin mode or other       the system for civilian users was even more
proprietary mode and also to the beacon receiver      pronounced. These GPS Satellites provide a
you will need a switch to permit the beacon           platform for radio transmitter, atomic clock,
receiver to transmit difference signals or the pc     computers and various equipment used for
to interact with the GPS. You won't be able to do     positioning and for a series of other military
both at the same time. This should not present        projects (e.g.-atomic flash detection).DGPS has
any                                                   also been developed to reduce the offset of GPS
real   problems    since   the    bi   -directional   errors and improve its accuracy.
proprietary mode is used to upload and
download
waypoint, route, and track data which does not
need the beacon receiver to be operational.
                                                      BIBLIOGRAPHY:
                                                      1. El-Rabbany, A., “Introduction to GPS: The
                                                      Global Positioning System”, Artech House
                                                      Publishers, Boston, USA, 2002.

CONCLUSION:-                                          2. Kalpan, E.D., “Understanding GPS: Principles
                                                      and Applications”, Artech House, 1996.
We conclude that GPS is proved to be all -
weather, space based navigation                       3. Alkan. R.M.,H.kamman & M. Sahin, “Recent
                                                      Advances in Space Technologies” ,
system. The primary application of it is in           proceedings      of     2nd       International
military nature. Next application is in civilian      Conference,pp.390-394,dated:9-11 June,2005.

use. The multi purpose usage of GPS has               4. Misra,P and Enge, P., “Global Positioning
developed enormously within the last three            System”, Ganga -Jamuna press, New York,
                                                      2001.
decades. With the elimination of SA (Selective
availability) on May 2nd, 2000, the usefulness of




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