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GPS SEGMENT

USERS SEGMENTDr.N.NAGAPPAN

SCIENTISTCERS

SPACE SEGMENTP.AMALADASM.ScGeoinformaticsCERS

Centre Of Excellence InRemote Sensing

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INTRODUCTION:

123

4567890

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History of the GPS

1969Defense Navigation Satellite

System (DNSS) formed

1973NAVSTAR Global PositioningSystem developed

1978first 4 satellites

launched

Delta rocket launch

199324th satellite launched; initial

operational capability

1995full operational capability

May 2000Military accuracy

available to all users

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GPS SEGMENT

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The first GPS segment is the space segment, which consists of 24 satellitevehicles that orbit the Earth in six orbital planes (see right)4. There are 4

equally spaced satellites per orbital plane, and the vehicles complete an orbitonce every 12 hours5. This means that there are always between 5 and 8

satellites visible anywhere on Earth at any given time6. The satellite vehiclesbroadcast radio signals that can be detected by GPS users 12,000 miles awayon Earth7. The GPS receiving units measure these digital transmissions that

arrive from the satellites to figure how far away each satellite is, and thenapply trigonometry and the concept of triangulation to pinpoint its location on

Earth8. In order to get an accurate measurement, the receiver must gathersignals from at least 4 different satellites

circularorbit s

witharadiu

sof2

6,560km

,

SPACE SEGMENT

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monitor and operate the GPS satellites. There are

monitoring stations spaced around the globe and oneMaster Control Station located in Colorado Springs,

Colorado . Each station sends information to the Control

Station which then updates and corrects the navigationalmessage of the satellites. There are actually five majormonitoring systems, the figure below does not include the

Hawaiian station.

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Kwajalein Atoll

US Space Command

Control SegmentControl Segment

Hawaii

Ascension Is.Diego Garcia

Ground AntennaMaster Control Station Monitor Station

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User Segment The user requires a GPS receiver in order to receive the transmissions from the

satellites. The GPS receiver calculates the location based on signals from thesatellites. The user does not transmit anything to the satellites and therefore the

satellites don't know the user is there. The only data the satellites receive is from theMaster Control Station in Colorado. The users consist of both the military and civilians.

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One of the effects of EinsteinsSpecial Relativity theory is time

dilation.Clocks moving with high

velocity run slower then clocks

with the smaller relative velocity.Therefore, clocks in the GPS

satellites will run slower compareto the clocks on the Earthbecause GPS satellites have a

large velocity.

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Conclusions

GPS is becoming a very popular and effective tool for the precise mapping and monitoringpurpose. Many and many organizations are inducting GPS for the survey and othervarious purposes. Though, many factors affect the accuracy of the GPS survey e.g.number of satellite observed, observation time, surroundings at the place of observation,distance of each observation points etc. Accuracy also depends upon the interactedsatellites geometry. The ultimate accuracy of a GPS determined by sum of severalsources of error. The contribution of each source may vary depending on atmospheric and

equipment conditions. Precision of observation such as landslide by the GPS depends onhow above said errors can be eliminate of minimize. GPS cannot be used for theunderground mapping as site of satellite is blocked. If such sites are falls in thecontinuous mapping, GPS survey can be switchover to electronic Total Station.

GPS industry is likely to continue to develop in the civilian community besides the specificuse in defense. Now, GPS trend is continuing to be towards smaller, less expensive, andmore easily operated devices for its wider use in the many aspects of community.

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T HAN

KU

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In and Out of the Classroom

T HAN

KU