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					G P S

GLOBAL POSITIONING SYSTEM
HONEY SINGH & ASHISH SRIVASTAVA
B.tech 3rd year (EC)

CONTENTS
INTRODUCTION WHAT IS GPS? GPS:SATELLITE N.S. GPS ARCHITECTURE
GPS SATELLITE SIGNAL POSITION &TIME FROM GPS RECIVER POSITION VELOCITY &TIME CARRIER PHASE TRACKING GPS ERROR SOURCE APPLICATION OF GPS FUTURE OF GPS (((((((((((((((((((((((((((((((((((((((

What is GPS ?
GPS stand for global positioning system “ A network of satellites that continuously transmit coded information,which makes it possible to precisely identify locations on earth by measuring distance from the satellites “
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GPS refers to a group of U.S. Department of defense satellites constantly circling the earth. The satellites transmit very low power radio signals allowing any one with a GPS receiver to determine their location on earth.

GPS is a Satellite Navigation System
•GPS provides specially coded satellite signals that can be processed in a GPS receiver, enabling the receiver to compute position, velocity and time. •Four GPS satellite signals are used to compute positions in three dimensions and the time offset in the receiver clock.

Position and time from four GPS satellite signals

GPS ARCHITECTURE:


GPS is based on time difference of arrival concept using precise satellite position & onboard atomic clocks to generate navigation messages.
It comprise of three major segments: 1:SPACE SEGMENT 2:CONTROL SEGMENT 3:USER SEGMENT

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SPACE SEGMENT:
1. The Space Segment of the system consists of the GPS satellites. These space vehicles (SVs) send radio signals from space.
2. The nominal GPS Operational Constellation consists of 24 satellites that orbit the earth in 12 hours. The satellite orbits repeat almost the same ground track (as the earth turns beneath them) once each day. The orbit altitude is such that the satellites repeat the same track and configuration over any point approximately each 24 hours (4 minutes earlier each day). There are six orbital planes (with nominally four SVs in each), equally spaced (60 degrees apart), and inclined at about fiftyfive degrees with respect to the equatorial plane. This constellation provides the user with between five and eight SVs visible from any point on the earth.

GPS CONSTELLATION

GPS SATELLITES AND GROUND TRACKS

CONTROL SEGMENT:
1.The Control Segment consists of a system of tracking stations located around the world. 2. For the highest accuracy of GPS, great care must be taken to model and correct any error in the received time delay such as clock drift and propagation delay.The ground control segment monitors the broadcast satellite signals and uplinks corrections to ensure predefined accuracies.

USER SEGMENT:
1.The GPS User Segment consists of the GPS receivers and the user community. GPS receivers convert SV signals into position, velocity, and time estimates. Four satellites are required to compute the four dimensions of X, Y, Z (position) and Time. GPS receivers are used for navigation, positioning, time dissemination, and other research. 2.Navigation in three dimensions is the primary function of GPS. Navigation receivers are made for aircraft, ships, ground vehicles, and for hand carrying by individuals. 3.Time and frequency dissemination, based on the precise clocks on board the SVs and controlled by the monitor stations, is another use for GPS. Astronomical observatories, telecommunications facilities, and laboratory standards can be set to precise time signals or controlled to accurate frequencies by special purpose GPS receivers. 4.Research projects have used GPS signals to measure atmospheric parameters.

GPS SATELLITE SIGNALS:
• The SVs transmit two microwave carrier signals. The L1 frequency (1575.42 MHz) carries the navigation message and the SPS code signals. The L2 frequency (1227.60 MHz) is used to measure the ionospheric delay by PPS equipped receivers.
• Three binary codes shift the L1 and/or L2 carrier phase.

GPS Error Sources
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GPS errors are a combination of noise, bias, blunders.  Noise errors are the combined effect of PRN code noise (around 1 meter) and noise within the receiver noise (around 1 meter). Bias errors result from Selective Availability and other factors  Other Bias Error sources;  SV clock errors uncorrected by Control Segment can result in one meter errors.  Ephemeris data errors: 1 meter  unmodeled ionosphere delays:1 meter  Multipath: 0.5 meters.  Blunders can result in errors of hundred of kilometers.  Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution.

Poor GDOP, a large value representing a small unit vector volume, results when angles from receiver to the set of SVs used are similar

GOOD GDOP, a small value representing a large unit vector volume ,results when angles from receiver to SVs are different

APPLICATIONS OF GPS
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ELECTRICAL POWER DISTRIBUTION: Electric
power companies primarily use GPS signals for monitoring stability of line frequencies, synchronising frequency with the adjacent power company network and isolating faults in their own transmitting network.

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TELE COMMUNICATION: Over the past decade, global
timing and communication infrastructure has adopted GPS as the primary distribution mechanism for time &frequency synchronisation

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ELECTRONIC COMMERCE & FINANCE: Many
banking & financial firms employ GPS timing for synchronising their encrypted computer network.

FUTURE OF GPS
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CIVIL LAND NAVIGATION:
Car manufacturers are working on integrating GPS receivers with moving map displays . Soon cars will be available with display units that can be used to plan routes and display present location using GPS.

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INTERNET TRANSACTIONS:
With recent interest of the govt. in taxing internet transactions, it is necessary to legally authenticate GPS locations as a vehicle for assigning sales taxes to internet purchases that are currently location less. Processing the combination of constantly varying GPS signals and precise time creates a’ location signature.

THANK YOU


				
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posted:11/12/2007
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