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INTRODUCTION

The Russians kept the Doppler-effect in mind with

the launch of Sputnik I in 1957 .To keep radio

contact with a moving object, you have to

keep changing your frequency.

The Americans discovered how to invert this in 1959

with the start of the TRANSIT navigation

system.(Navy Navigational satellite system). If youknow the position of the satellite, you can

determine your relative position to it

BEE0002213,BEE0004613,BEE0004713,BEE0005113,BEE0005213


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The Transit navigation system could only stay

for 2 years after it conception because it had

its limitations. Listed below are few of itslimitations.

Low orbit + few satellites = bad coverage

Receiver needs a continuous signal

Receiver has to wait for satellite to pass

overhead

Only up to 500/25 meter accuracy

Assumes sea level altitude



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NAVSTAR GPS(navigation satellite and

timing and ranging)

The concept of Navistar was to improve the

inaccuracies of transit navigation system.

Concept

Six orbital planes

(plane = orbit containing multiple satellites)

21 active satellites, plus 3 sparesFour per plane



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How it works

A receiver receives a signal from a GPS

satellite

It calculates the difference from the current

time and the time sent by the satellite

It now knows how far away the satellite is

Because we know that radio signals travel atthe speed of light, we can calculate this



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How it works(cont)

There are two possible locations

One is practically impossible, so it can be ruled

out

Too far away from Earth (too high)

Velocity is not realistic

Still, we need a fourth satellite

Confirm this location

Improve accuracy



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NAVSTAR GPS

Accuracy improved with the forth satellite



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THE GPS

The Global positioning system is a marvel of

satellites, a system of supporting ground

stations and coded radio transmissions all to

do one simple thing, tell you exactly where

you are. It was designed, built, operated and

maintained by Department of Defense which

serves the initial purpose of trackingsubmarines carrying nuclear missiles.



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THE GPS (CONT)

The development of the GPS first came about

in the year 1973 with the first four satellites

launched in 1978.However full operational

capacity was reached on 17 July 1995 which

cost the U.S Defense about 12 million dollars.



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HOW GPS WORKS

The GPS receivers collect the signals transmittedfrom the system of 31 satellites. The GPS worksunder five(5) conditions;

Satellite triangulation set basis of the system

Satellites ranging measures distance from asatellite

Satellite positioning locate where a satellite is inspace

Correcting errors does correction forionosphere and troposphere delays



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COMPONENTS OF GPS

The GPS comprises of three (3) segments.

The space segment

The ground segment(the ground controllednetwork)

The user segment(GPS receivers and their

users)



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GPS SPACE SEGMENT

GPS Constellation 31 satellites (Space Vehicles or SVs)

With three on standby

20,200km altitude (12 hour orbitperiod)

6 orbital planes (55

inclination) 4 satellites in each plane

GPS Satellite Details Manufactured by Rockwell

International, later by LockheedM&S

7000 miles per hour 2.2m body, 7m with solar panels

Average of 21 years expectedlifetime



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GPS CONTROL SEGMENT

The control segment consist of five

monitoring stations

Diego Garcia

Ascension Island

Hawaii

Kwajalein Island andColorado



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GPS CONTROL SEGMENT(CONT)

The control segment performs three main

tasks

Monitoring the health of the satellites

Determine their orbits and the behavior of

their atomic clocks

Inject the broadcast message into the

satellites



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GPS USER SEGMENT

The user segment consist ofthe user and all earth basedGPS receivers. The typicalreceiver is composed of,

Antenna Preamplifier

Radio signal microprocessor

Control and display device

Data recording unit and Power supply



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GPS RECEIVER RF FRONT END



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GPS SIGNAL

GPS SIGNAL

C/A code and P(Y) code

All SVs transmit at1575.42MHz

Each SV modulates using aunique 1023-bitpseudorandom (PRN) codesent at 1.023Mcps (chips persecond)

PRN allows spread-spectrumCDMA management of GPStransmit frequency

Receivers distance to the SVcan be determined bymeasuring the PRN time skew

between the transmitted andreceived signals

GPS system data (ephemeris,clock, and atmosphericparameters) are transmittedby further modulating the PRN

code at 50bps

1023 chips (1ms)

PRN

cod

e

PRN PRN PRN PRN PRN

0

01 10 0Data

Bits*

50bps (20ms/bit)

*PRN code inverts to signify bit transition (0/1)

1 chip



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ERRORS IN GPS

Errors in GPS takes place because of the

following reasons

Atmospheric delays

Satellite clock

Orbital

Multipath andReceiver measurement noise



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ERROR CORRECTION

DGPS is Differential GPS is used to correct thevarious inaccuracies in the GPS system, pushingits inaccuracies ever further.

Error is reduced by determining the error at aknown location, then subtracting that error fromthe solution at an unknown location.

DGPS consist of two receivers,

Stationary and a

Moving around making position measurements.



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GPS APPLICATIONS

GPS can be applied in the following areas:Agriculture

Marine

AviationRoad and Highways

Environment

Rail

Surveying and Mapping

Public Safety and Disaster Relief



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LIMITATION OF GPS

Solar activities can distort the GPS signals asthey pass through the earths ionospherecausing accuracy error.

The solar storm that occurred in early March2012 disrupted satellite communication andforce airline to reroute some flights.

More solar storm are likely to occur through2013-2014 as the sun reaches its solar maxperiod.



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RECOMMENDATION

We recommend that Africa should come up

with its own system for the interest of the

continent.



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CONCLUSION

Programs such as Earthscope is planning to

inculcate GPS to help understand why earth

quake and volcanic eruption occurs.

The GPS is without doubt the most successful

dual use(civilian and military) system

developed by US.



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REFERENCES

http://www.gpsy.com/gpsinfo/

Robert Sweet, GPS for mariners,

International Marine Publishing co., 2nd

Edition.

Alison.M.Smith,Law Enforcement use of GPS

device to monitor motor vehicle,

congressional research service, February 28,

2011.



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REFRENCES (CONT)

WWW.gps.gov/applications/



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THANK YOU

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