global positioning satellite system (gps)
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Global PositioningSatellite System (GPS)
Presented byPresented by
Chellasundari & PournaChellasundari & Pourna
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GPS SYSTEMS
NAVSTAR GLONASS
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GPS Segments
Space Segment: the constellation ofsatellites
Control Segment: control the satellites
User Segment: users with receivers
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GPS SEGMENTS
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Space Segment
System consists of 27 satellites in the
operational mode: 24 in use and 3spares
3 other satellites are used for testing
Altitude: 20,200 Km
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GPS Orbits
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Control Segment
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GPS Transmitted SignalTwo signals are transmitted on carriers:L1 = 1575.42 MHz
L2 = 1227.60 MHz
These are derived from the system clock of
10.23 MHz (phase quadrature)
Modulation used is Direct Sequence Spread
Spectrum(code division multiple access - CDMA)
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Transmission of data
Amplitude modulation
Frequency modulation
Phasemodulation
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Amplitude modulation of a data signal onto acarrier signal
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quency modulation of a data signal onto a carriernal
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Phase modulation of a data signal onto a carrier
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GPS Clock SignalsTwo types of clock signals are
transmitted
C/A Code - Coarse/Acquisition Codeavailable for civilian use on L1provides 300 m resolution
P Code - Precise Code on L1 and L2used by the military provides 3mresolution
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GPS SignalsThe satellites transmit as part of their uniqueSpread Spectrum signal a clock or timing signal
The range or distance to the satellite is obtained
by measuring how long it takes for thetransmitted signal to reach the receiver
This is not the true range due to clock errors -what is obtained is know as the pseudo-range
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GPS Position By knowing how far one is from
three satellites one can ideally find
their 3D coordinatesTo correct for clock errors one
needs to receive four satellites
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GPS: How does it work?Typical receiver: one channel C/A code on L1
During the acquisition time you are
receiving the navigation message also on L1The receiver then reads the timing
information and computes the pseudo-ranges
The pseudo-ranges are then corrected
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GPS: How does it work?
Corrected ranges are used tocompute the position
This is a very complicated iterativenonlinear equation
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Navigation Message
To compute your position one must knowthe position of the satellite
Navigation Message - transmitted on bothL1 and L2 at 50 bits/s for 30 s
Navigation message consists of two parts:
- satellite almanac
- clock bias
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Differential GPS
Used to improve accuracy
Put a satellite on the ground at a
precise position Differential signal is not
transmitted on standard satellite
frequencies
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Uses of GPS
Airplane and Boat Navigation
Continental Drift
Surveying
Precise Timing
Iceberg Tracking Archaeological Expeditions
Mobile Multimedia
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Conclusion
GPS will find more civilian uses
DOD has promised to eliminate SA
Russia has a system known asGLONASS
The EU is discussing deploying itsown system