global positioning satellite system (gps)

8/14/2019 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

global positioning satellite system (gps)

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