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A

SEMINAR REPORT

ON

GLOBAL POSITIONING SYSTEM

In

ELECTRONICS AND COMMUNICATION ENGINEERING

By

RAMAKRISHNA RAJU.M

08D15A0409

DEPARTMENT OF ELECTRONICS AND COMMUNICATION

ENGINEERING

TRR ENGINEERING COLLEGE

(AFFILATED TO JNTU HYDERABAD),HYDARABAD-500072.

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ABSTRACT

INTRODUCTION

HISTORY

ELEMENTS OF GPS

THE FREQUENCIES USED

RECEIVERS(USER SEGMENT)

NAVIGATION SIGNALS(CONTROL SEGMENT)

CALCULATING POSITIONS

ELEVATION

GPS APPLICATIONS

CONCLUSION

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GLOBAL POSITIONING SYSTEM(GPS)

ABSTRACT

The ability to locate oneself on the earth and in its vicinity and navigate over its

surface is knowledge that has fascinated humankind for these millennia. All the

techniques available that implement the above mentioned core theme use,

GLOBAL POSITIONING SYSTEM (GPS), which has become a vital global

utility, indispensable for modern navigation on land, sea, and air around the world,as well as an important tool for Map-Making and Land Surveying.

Global Positioning System (GPS) technology is a great boon to

anyone who has the need to navigate either great or small distances. This

wonderful navigation technology was actually first available for government use

back in the late 1970s. In the past ten or so years, It has been made available to the

general public in the form of handheld receivers that use this satellite technology

provided by the U.S. government.

Through the use of these handheld

receivers, one can navigate back to a starting point or other predetermined

locations without the use of maps or any other equipment. In conjunction withaccurate maps like ones provided by the USGS, and other basic tools like a

compass and Lat/Long or UTM scales, one can navigate to identified locations on

maps or take readings from a location that they are at or have been at and plot

those locations on a map.

Global Positioning System (GPS) is the only fully functional

Global Navigation Satellite System (GNSS). The GPS uses a constellation of

between 24 and 32 Medium Earth Orbit satellites that transmit precise microwave

signals that enable GPS receivers to determine their location, speed, direction, andtime. GPS was developed by the United States Department of Defenses. Its official

name is NAVSTAR-GPS

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4This paper provides description of all the technical aspects of

GPS along with techniques to improve the accuracy of the system. It also deals

with some of very important GPS applications like vehicle tracking, mobile

tracking, in military, in geology etc towards the end.

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5INTRODUCTION

GPS is a space-based radio positioning system that provides 24*7 three-

dimensional positions, velocity and time information to suitably equipped users

anywhere on or near the surface of the earth and sometimes off the earth.

It consists of a network of twenty four orbiting satellites that are

orbiting in space eleven thousand miles from earth and a GPS receiver includes an

antenna adapted to receive GPS satellite signals from each of the GPS satellites.

There is navigation solution determining circuitry coupled to the antenna that

receives the GPS satellite signals and performs navigation and fault detection and

exclusion functions. The GPS satellites are orbited high enough to avoid problems

associated with land based systems, yet can provide accurate positioning twenty

four hours a day, anywhere in the world.

HISTORY:

GPS was developed by the United States Department of Defense,

for its tremendous application as a military locating utility.

A team led by Richard B. Kershner discovered that, because of the

Doppler Effect, the frequency of the signal being transmitted by satellite Sputnik

was higher as it approached, and lower as it continued away from them. This is

because; they knew their exact location on the globe that they could pinpoint where

the satellite was along its orbit by measuring the Doppler distortion. This shows

that if the satellite's position was known, they could identify their own position on

earth and thus came the first satellite navigation system, Transit. Later US

Department of Defense developed NAVSTAR GPS (navigation signal

timing and ranging GPS), and launched the first experimental satellite.

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6Elements of GPS:

GPS has three parts:

1) Space segment: The space segment consists of a constellation of 24satellites (and about six "spares"), each in its own orbit 11,000 nautical miles

above Earth.

2) User segment: The user segment consists of receivers, which you canhold in your hand or mount in a vehicle, like your car.

3) Control segment: The control segment consists of ground stations (sixof them, located around the world) that make sure the satellites are working

properly.

Elements of GPS:?

GPS Constellation (Space segment) :

There are at least twenty four operational GPS satellites at all

times which orbit with a time period of twelve hours. Ground stations are used to

precisely track each satellite's orbit. Each GPS satellite transmits data that indicates

its location and the current time. All GPS satellites synchronize operations so that

these repeating signals are transmitted at the same instant. The receiver calculates

position of the sa dimensions if the distance to at least four GPS satellites is

known. The 24 satellites that make up the GPS space segment are orbiting the earthabout 12,000 miles above us. They are constantly moving, making two complete

orbits in less than 24 hours. These satellites are travelling at speeds of roughly

7,000 miles an hour.

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7The signal travels to the ground at the speed of light. Even at this

speed, the signal takes a measurable amount of time to reach the receiver. The

difference between the time when the signal is received and the time when it was

sent, multiplied by the speed of light, enables the receiver to calculate the distance

to the satellite. To make this measurement as accurate as possible, the GPS

navigation signals are specially designed to make it easy for GPS receivers to

measure the time of arrival and to allow all the satellites to operate on the same

frequency without interfering with each other.

The Frequencies Used:

The following figure shows the GPS constellation with all the 24 satellites rev

The frequencies that make

up the GPS electromagnetic spectrum are L1 (1575.42 MHz), L2 (1227.60 MHz),

L3 (1381.05 MHz) with two new signals L4 (1841.40 MHz), L5 (1176.45 MHz)

also being studied.

Receivers(User segment):

GPS receivers vary widely in accuracy because of the expense of

adding more radio receivers needed to tune in more satellites. More expensive

units, known as "dual-frequency receivers", tune in the L2 signals in order to

correct for ionospheric delays.

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8Another major factor in the accuracy of a GPS fix is the amount of

processing applied to the received

signals. GPS receivers may include an input for differential corrections, using the

RTCM SC-104 format. Receivers with internal DGPS receivers can outperform

those using external RTCM data. Many GPS receivers can relay position data to a

PC or other device using the NMEA 0183 protocol. Receivers can interface with

external devices via a number of means, such as a serial connection, a USB

connection or even a wireless connection like the BLUETOOTH technology.

Navigation Signals(control segment):

GPS satellites transmit signals to equipment on the ground. GPS

receivers passively receive satellite signals but do not transmit. GPS receivers

require an unobstructed view of the sky, so they are used only outdoors and they

often do not perform well within forested areas or near tall buildings.

GPS satellites broadcast three different types of data in the primary

navigation signals. The first is the almanac which sends coarse time information

with second precision along with status information about the satellites.

The second is the ephemeris, which contains orbital information that

allows the receiver to calculate the position of the satellite at any point in time.

These bits of data are folded into the 37,500 bit Navigation Message.

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9The satellites also broadcast two forms of accurate clock information,

the Coarse Acquisition code and the precise code. In normal operation, the so-

called "anti-spoofing mode", the P code is first encrypted into the Y-code.

The following figure gives an overview of how the GPS works. It

includes the spacecraft constellation, master control system and the signal structure

Ionosphere and troposphere delays

the satellite signal slows as it passes through the atmosphere. The GPS system uses

a built-in model that calculates an average amount of delay to partially correct for

this type of error.

Signal multipath This occurs when the GPS signal is reflected off objects such

as tall buildings or large rock surfaces before it reaches the receiver. This increases

the travel time of the signal, thereby causing errors.

Receiver clock errors A receiver's built-in clock is not as accurate as the atomic

clocks onboard the GPS satellites. Therefore, it may have very slight timing errors.

Orbital errors Also known as ephemeris errors, these are inaccuracies of the

satellite's reported location.

Number of satellites visible The more satellites a GPS receiver can "see," the

better the accuracy. Buildings, terrain, electronic interference, or sometimes even

dense foliage can block signal reception, causing position errors or possibly no

position reading at all. GPS units typically will not work indoors, underwater or

underground.

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10Satellite geometry/shading this refers to the relative position of the satellites at

any given time. Ideal satellite geometry exists when the satellites are located at

wide angles relative to each other. Poor geometry results when the satellites are

located in a line or in a tight grouping.

Calculating Positions:

In a nutshell, GPS is based on satellite ranging - calculating the

distances between the receiver and the position of satellites. The receivers measure

the time delay between when the signal is sent and the local time when the signal is

received. This delay, multiplied by the speed of light, gives the distance to that

satellite.

The receiver now has an accurate estimate of the position and the

distance of the satellite. This tells the receiver that it lies on the surface of an

imaginary sphere whose radius is that distance. To calculate the precise position, at

least four such measurements are taken simultaneously. This places the receiver at

the intersection of the four imaginary spheres as shown in the figure below.

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11Mathematics in GPS:

Specific mathematics is used by the receivers on the ground to calculate location

on earth, using the two signals (location and timing) emanating from the satellites.

Although the actual calculations that GPS makes involve much more sophisticated

mathematics, we can boil down the essence of the mathematics to the following

description:

1. Each satellite sends two signals:1. The location/3-dimensional coordinates of the

o satellite in space (based on on-board computer models and updateddata from NORAD)

Signal 1: (xs, ys, zs)

o 2. A timing signal, based on a &quotpseudo-random code" (in ourclass, we consider the signal to contain the precise moment the signal

is sent, which can then be compared to when the signal is received)

Signal 2: time sent

2. The receiver then:o Subtracts the timing signals to find the total time it took the signal to

reach it

ttotal = trcvd - tsent

Multiplies this total by the speed of light to get the distance away that

each satellite is

d = rt(distance = rate x time)

Rate of the signal= c = the speed of light = 186,282.396 miles per second

dto satellite = c * ttotal

o Since the satellites are in motion, the GPS receivers must then takeinto account Doppler data, which represents the relative speeds

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12between the satellites and the receiver. However, for the purposes of

this consider the system to be stationary.

o The receiver can now establish equations of spheres that surroundeach satellite:

(x - xs)2 + (y - ys)2 + (z - zs)2 = d2to satellite(s=s1,s2,s2,s4)

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Elevation

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Elevation

The accuracy of position calculated by a GPS receiver relies on three

accurate measurements: the current time, the position of the satellite, and the time

delay for the signal. However, several "real world" effects intrude and degrade the

accuracy of the system as shown in the table.

Source Effect

Ionospheric effects 5 meter

Ephemeris errors 2.5 meter

Satellite clock errors 2 meter

Multipath distortion 1 meter

Tropospheric effects 0.5 meter

Numerical errors 1 meter or less

Techniques to Improve Accuracy:

There are various methods available which improve the accuracy to a

great extent.

Differential GPS used by receivers which can improve the normal

GPS accuracy of 4-20 meters to 1-3 meters.

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15Wide Area Augmentation System is used in case of ionospheric

delays. This system uses a series of ground reference stations to calculate GPS

correction messages, which are uploaded to a series of additional satellites in

geosynchronous orbit.

A Local Area Augmentation System is similar to WAAS but in this case, the

correction data are transmitted from a local source, typically at an airport or

another location where accurate positioning is needed. These correction data are

typically useful for only about a thirty to fifty kilometer radius around the

transmitter.

A Carrier-Phase Enhancement utilizes the 1.575 GHz L1 carrier wave

to act as a sort of additional clock signal, resolving ambiguity caused by variations

in the location of the pulse transition (logic 1-0 or 0-1) of the C/A signal.

Wide Area GPS Enhancement which provides more accurate satellite

clock and ephemeris data to specially-equipped receivers.

GPS APPLICATIONS

GPS can provide any point on earth with a unique address and hence

finds its greatest utility in the field of Geographic Information Systems (GIS).

However GPS is not just confined to GIS but is widely being used in many other

areas as well.

Vehicle Tracking:

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16A GPS tracking system uses GPS to determine the location of a vehicle, person, or

pet and to record the position at regular intervals in order to create a track file or

log of activities. The recorded data can be stored within the tracking unit, or it may

be transmitted to a central location, or Internet-connected computer, using a

cellular modem, 2-way radio or satellite. This allows the data to be reported in

real-time; using either web browser based tools or customized software.

Mobile Phone Tracking:

This is ideal for business especially if its mobile, what you need is

mobile phone tracking which is simple and easy to use and can be set up in

minutes. Unlike vehicle tracking, mobile tracking also means one can continue to

track, even when the employee has left their car or van. This allows you to monitor

the work force every minute, any time, day or night to improve business efficiency.

Interestingly, mobile phone tracking can be extended to track where a

person is, by triangulating his position between cells, track which cell he is

entering by knowing the cell from which he enters.

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17GPS is a great system to use, especially with the event of GPS

receivers becoming smaller and smaller and better GPS software coming out and

providing more accurate tracking for the user who needs to track more frequently.

The following suggest a clear description of the whole process of tracking that

takes place. Navigation:GPS is used by people around the world as a navigation aid in cars,

airplanes, and ships. Hand-held GPS receivers can be used by mountain climbers

and hikers. Glider pilots use the logged signal to verify their arrival at turn points

in competitions.

Earthquake Detection:

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18The satellites in the constellation are arranged so that several of them are "visible"

from any point on the surface of the earth at any time. The GPS network will

continuously measure movements of the earth's crust with a precision of one

millimeter per year, which will show us where strain is building up and the

receivers that will continuously measure the constant, yet physically imperceptible,

movements of earthquake faults. Scientists face lot of difficulty to rapidly assess

the size of large earthquakes but, GPS-based detection offers the most advantages

even when an earthquake has a magnitude of 8.5.

Not only earthquakes but, scientists can now estimate an earthquakes

potential for causing a tsunami using deep ocean buoy readings and seismometer

data.

A new tsunami-detection technique that uses GPS data could potentially save many

lives by helping alert people in time to escape.

Military:

GPS allows accurate targeting of various military weapons including

cruise missiles and precision-guided munitions, as well as improved command and

control of forces through improved location awareness. The satellites also carry

nuclear detonation detectors, which form a major portion of the United States

Nuclear Detonation Detection System. Most effective use of GPS in military can

be seen in jamming of any radio navigation system.

Geophysics and Geology:

High precision measurements of earths crustal strain can be made

with GPS by finding the relative displacement between GPS sites, one of which is

assumed to be stationary. Multiple stations situated around an actively deforming

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19area (such as a volcano or fault zone) can be used to find strain and site velocities

relative to a stable reference site. For example, measurements of ground

deformation around a volcano can be used to interpret the source and causea

dike, sill, or other body beneath the surface.

Surveying:

More costly and precise receivers are used by land surveyors to locate

boundaries, structures, and survey markers, and for road construction. There is also

a growing demand for Automatic Grade Control systems that use GPS positions

and 3D site plans to automatically control the blades and buckets of constructionequipment.

Entertainment:

The availability and cost of hand-held GPS receivers has led to

recreational applications including location-based games like the popular game

Geocaching

CONCLUSION

GPS is the product of a long and complicated history and has brought

many changes in the society since then. It has increasingly become more evident in

society through its implementation into items such as vehicles, mobile phones and

pet collars. The economics of GPS make the measurement technology readily

available and accessible to all users. Many issues surround GPS when it comes to

its future. The most certain aspect of the future of GPS is its increased usage and

its expansion into new areas of application.

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20We have seen that GPS setup could be just the one we may need for

the PDA devices which are considered to be using the latest technology as on

today. Thus, what one can conclude is that GPS will continue to intrude into many

other fields and will be an evergreen technology for many more years to come