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Department of Electronics and Communications U.I.E.T. PanjabUniversity

SUBMITTED TO:

Mr. Vishal Sharma

GUIDE:

Mr. Jaget Singh

SPEAKER:

Ankush Dhiman

ECE- 7th Sem

UE85015

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What Is GPS?

The Global Positioning System (GPS)

is a navigation and precise-positioning

tool.

Developed by the Department of

Defense in 1973.

GPS was originally designed to assist soldiersand military vehicles, planes, and ships in

accurately determining their locations world-wide.

Today, the uses of GPS have extended to includeboth the commercial and scientific worlds.

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Before GPS ground based navigational

systems like LORAN and DECCA

NAVIGATOR were used to assist

military forces in world war 2.

OMEGA NAVIGATION SYSTEM (ONS) was

developed in 1970 based on phase

comparison transmitted signal and

pairs of stations.

ONS was the first world wide radio

navigation system.

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Lack of accuracy in ground basednavigation systems.

Need of precise positioning of

military vehicles during cold war.To assist nuclear triad of SLBMs

ICBMs and USAF strategic bombers.

To assist commercial carriers ,

aircrafts and civilian vehiclesaround the globe.

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GPS system is composed of

three segments.

The segments are :

1. THE SPACE SEGMENT

2. THE GROUND SEGMENT

3. THE CONTROL SEGMENT

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The GPS Operational

Constellation consists of 24

satellites that orbit the Earth

in very precise orbits twice a

day. GPS satellites emitcontinuous navigation signals.

They orbit earth in six orbits

with 4 satellites in each orbit

at an altitude of 20200km.

These orbits are inclined at an

angle of 55 deg. to the equator

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Control segment or Datacenter is an integral partof GPS system . The roleof the data center is twofold. It both monitors andcontrols the global GPS

stations, and it usesautomated computersystems to retrieve andanalyze data from thereceivers at thosestations. Once processed,the data , along with theoriginal raw data, is madeavailable to scientistsaround the world for usein a variety ofapplications.

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A GPS receiver calculates its position byprecisely timing signal sent by GPSsatellite.

Position is calculated with help oftriangulation

Each transmitted message include

1. the time it was transmitted

2. precise orbital information (ephemeris)3. the general system health and roughorbitals of other satellites (almanac)

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Position is calculatedfrom distancemeasurement

The intersection of three

spheres with satellites attheir center and thedistance of satellite fromreceiver being radius

Mathematically we needfour satellites but threeare sufficient by rejectingthe ridiculous answer

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Distance to a satellite is determined by

measuring how long a radio signal takes to

reach us from the satellite

Assuming the satellite and receiver clocksare sync. The delay of the code in the

receiver multiplied by the speed of light

gives us the distance

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The GPS receiver and satellite generate

the same pseudo-random code at exactly

the same time. When the code arrives

from the satellite, the time difference iscompared to the same code generated by

the receiver. This difference is multiplied

by the speed of light (186,000 miles per

second) to determine the distance to thesatellite

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If the clocks are perfectly sync the satellite

range will intersect at a single point.

But if imperfect the four satellite will not

intersect at the same point. The receiver looks for a common correction

that will make all the satellite intersect at

the same point

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95% due to hardware ,environment and

atmosphere

Intentional signal degradation

Selective availability Anti spoofing

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Two components

Dither :

manipulation of the satellite clock freq

Epsilon:

errors imposed within the ephemeris data sent in

the broadcast message

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Here the P code is made un gettable by

converting it into the Y code.

This problem is over come by cross

correlation

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Satellite errors Errors in modeling clock offset

Errors in Keplerian representation of ephemeris

Atmospheric propagation errors Through the ionosphere , carrier experiences

phase advance and the code experiences groupdelay

Dependent on Geomagnetic latitude Time of the day

Elevation of the satellite

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Atmospheric errors can be removed by

Dual freq measurement

low freq get refracted more than high freq

thus by comparing delays of L1 and L2 errorscan be eliminated

Troposphere causes delays in code and

carrier

But they arent freq dependentBut the errors are successfully modeled

Errors due to Multipath

Receiver noise

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Forces on the GPS satellite

Earth is not a perfect sphere and hence uneven

gravitational potential distribution

Other heavenly bodies attract the satellite ,but these are very well modeled

Not a perfect vacuum hence drag but it is

negligible at GPS orbits

Solar radiation effects which depends on thesurface reflectivity , luminosity of the sun ,

distance of to the sun. this error is the largest

unknown errors source

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GLONASS : RUSSIAN GLOBAL NAVIGATION

SATELLITE SYSTEM became available for

civilian use in 2007.

Planned Chinese COMPASS NAVIGATIONSYSTEM.

EUROPEAN UNIONS GALILEO POSITIONING

SYSTEM.

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GPS is a boon for man kind , as it assists military

forces in fight against terrorists and other evils.

It has changed the way we travel today , as car

GPS receivers on a press of a button display our

position , travel route , distance to the

destination and even estimate time at spont.

speed

GPS play a vital role in navigation for cargo ships

instead of relying on conventional magneticcompass

Finally, it can be said one can not be lost with

GPS assisting his path.

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