Gps Technical Seminnar

8/3/2019 Gps Technical Seminnar

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CHAPTER- 1

INTRODUCTION

Today, the development of the science and technology is rapid. To compete with this, an

individual must utilize innovative intelligence. Now the whole world is running to discover latest

techniques for each and every field. The Global Positioning System (GPS) is an application of

computer and microcontroller technology. Today’s fast growing life standards demands such a

navigation system. The main purpose of GPS is the military navigation and tracking. GPS satellites

are orbited high enough to avoid the problems associated with land based systems, yet can provide

accurate positioning 24 hours a day, anywhere in the world. Uncorrected positions determined from

GPS satellite signals produce accuracies in the range of 50 to 100 meters. When using a technique

called differential correction, users can get positions accurate to within 5 meters or less.

The Global Positioning System (GPS) is a satellite-based navigation system made up of a

network of 24 satellites placed into orbit by the U.S. Department of Defense. GPS was originally

intended for military applications, but now available for civilian use also. GPS is changing the

lifestyle of new generation and having its influence in almost all areas such as military, agriculture,

aerospace, research, industrial etc. The Global Positioning System (GPS) is a location system based

on a constellation of about 24 satellites orbiting the earth at altitudes of approximately 11,000

miles. GPS was developed by the United States Department of Defense (DOD), for its tremendous

application as a military locating utility. The DOD's investment in GPS is immense. Billions and

billions of dollars have been invested in creating this technology for military uses. However, over the past several years, GPS has proven to be a useful tool in non-military mapping applications as

well.It is now widely used for navigation using cell phones and in vehicles. Also to track the path,

GPS is used.Advances in technology and new demands on the existing system have now led to

efforts to modernize the GPS system and implement the next generation of GPS III satellites and

Next Generation Operational Control System.

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Main inspiration for the GPS came when the Soviet Union launched the first Sputnik in 1957.

A team of U.S. scientists led by Dr. Richard B. Kershner were monitoring Sputnik's radio

transmissions. They discovered that, because of the Doppler Effect, the frequency of the signal

being transmitted by Sputnik was higher as the satellite approached, and lower as it continued away

from them. They realized that since they knew their exact location on the globe, they could pinpoint

where the satellite was along its orbit by measuring the Doppler distortion.

1.1 HISTORY OF GPS

• 1940 - LORAN & DECCA – Radio Navigation( Ground based )

• 1960 - Transit - U S Navy- five satellites

• 1967 - Timation -U.S.Navy- for time info

• 1970 - Omega Navigation System-1st satellite based radio navigation

• 1983 - First version of GPS tested

• 1989 - GPS satellites setting to 1993

• 1993 - GPS is ready to use only for military

• 1997 - GPS is ready to use for public also , but not clear signals

• 2000 - GPS is available to civilians in clear form.

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1.2 WORKING OF GPS

24 GPS satellites circle the earth twice a day in a very precise orbit and transmit signal

information to earth. GPS receivers take this information and use triangulation to calculate the

user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a

satellite with the time it was received. The time difference tells the GPS receiver how far away the

satellite is. Now, with distance measurements from a few more satellites, the receiver can determine

the user's position and display it on the unit's electronic map.

Figure:-1.1 24 SATELLITES REVOLVING EARTH

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Figure:-1.2 SCHEMATIC DIAGRAM OF GPS WORKING MODEL

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D

position (latitude and longitude) and track movement. With four or more satellites in view, the

receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's

position has been determined, the GPS unit can calculate other information, such as speed, bearing,

track, trip distance, distance to destination, sunrise and sunset time and more.

Although four satellites are required for normal operation, fewer apply in special cases. If

one variable is already known (for example, a ship or plane may have known elevation), a receiver

can determine its position using only three satellites. Some GPS receivers may use additional clues

or assumptions (such as reusing the last known altitude, dead reckoning, inertial navigation, or

including information from the vehicle computer) to give a degraded position when fewer than four

satellites are visible.

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1.3 PRINCIPLE OF GPS

A GPS receiver calculates its position by precisely timing the signals sent by the GPS satellites

high above the Earth. Each satellite continually transmits messages containing the time the message

was sent, precise orbital information. It might seem three satellites are enough to solve for position,

since space has three dimensions. However a very small clock error multiplied by the very large

speed of light the speed at which satellite signals propagate results in a large positional error.

The receiver uses a fourth satellite to solve for x, y, z, and t While most GPS applications use

the computed location only and effectively hide the very accurately computed time, it is used in a

few specialized GPS applications such as time transfer and traffic signal timing.

Using messages received from a minimum of four visible satellites, a GPS receiver is able to

determine the satellite positions and time sent. The x, y, and z components of position and the time

sent are designated as xi,yi,zi,ti ; Knowing the indicated time the message was received tr, the GPS

receiver can compute the indicated transit time, tr-ti. of the message. Assuming the message

traveled at the speed of light, c, the distance traveled, can be computed as . (tr-ti)c . GPS receiver is

on the surface of a sphere centered at the position of a satellite. Thus we know that the indicated

position of the GPS receiver is at the intersection of the surfaces of four spheres (If no error)

A circle and sphere surface in most cases of practical interest intersects at two points,

although it is conceivable that they could intersect at one point—or not at all. Another figure,

Surface of Sphere Intersecting a Circle at Two Points, shows this intersection. The two intersections

are marked with dots. Again trilateration clearly shows this mathematically.

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The correct position of the GPS receiver is the intersection that is closest to the surface of the

earth for automobiles and other near-Earth vehicles. The correct position of the GPS receiver is also

the intersection which is closest to the surface of the sphere corresponding to the fourth satellite.

(The two intersections are symmetrical with respect to the plane containing the three satellites. If

the three satellites are not in the same orbital plane, the plane containing the three satellites will not

be a vertical plane passing through the center of the Earth. In this case one of the intersections will

be closer to the earth than the other. The near-Earth intersection will be the correct position for the

case of a near-Earth vehicle. The intersection which is farthest from Earth may be the correct

position for space vehicles.)

The method of calculating position for the case of no errors has been explained. One of the

most significant error sources is the GPS receiver's clock. Because of the very large value of the

speed of light, c, the estimated distances from the GPS receiver to the satellites, the pseudo ranges,

are very sensitive to errors in the GPS receiver clock. This suggests that an extremely accurate and

expensive clock is required for the GPS receiver to work. On the other hand, manufacturers prefer

to build inexpensive GPS receivers for mass markets. The solution for this dilemma is based on the

way sphere surfaces intersect in the GPS problem.

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CHAPTER-2

SYSTEM DETAILS

There are 3 main segments in a GPS system. They are

1. Space segment (SS).

2. Control Segment (CS).

3. User Segment (US).

2.1 Space Segment (SS)

Figure:-2.1 SCHEMATIC DIAGRAM OF GPS SYSTEM

The space segment (SS) comprises the orbiting GPS satellites or Space Vehicles (SV) in

GPS parlance. The GPS design originally called for 24 SVs, eight each in three circular orbital

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planes, but this was modified to six planes with four satellites each. The orbital planes are centered

on the Earth, not rotating with respect to the distant stars. The six planes have approximately 55°

inclination (tilt relative to Earth's equator) and are separated by 60° right ascension of the ascending

node (angle along the equator from a reference point to the orbit's intersection). The orbits are

arranged so that at least six satellites are always within line of sight from almost everywhere on

Earth's surface.

Figure:-2.2 A GPS SATELLITE- SPACE SEGMENT

Orbiting at an altitude of approximately 20,200 kilometers about 10 satellites are visible

within sight (12,600 miles or 10,900 nautical miles; orbital radius of 26,600 km (16,500 mi or

14,400 NM)), each SV makes two complete orbits each sidereal day. The ground track of each

satellite therefore repeats each day. This was very helpful during development, since even with just

four satellites, correct alignment means all four are visible from one spot for a few hours each day.

For military operations, the ground track repeat can be used to ensure good coverage in combat

zones.

As of March 2008, there are 31 actively broadcasting satellites in the GPS constellation. The

additional satellites improve the precision of GPS receiver calculations by providing redundant

measurements. With the increased number of satellites, the constellation was changed to a no

uniform arrangement. Such an arrangement was shown to improve reliability and availability of the

system, relative to a uniform system, when multiple satellites fail.

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2.2 CONTROL SEGMENT

Figure:-2.3 GPS SATELLITE SENDING SIGNAL TO CONTROL STATION

The flight paths of the satellites are tracked by US Air Force monitoring stations in Hawaii,

Kwajalein, Ascension Island, Diego Garcia, and Colorado Springs, Colorado, along with monitor

stations operated by the National Geospatial-Intelligence Agency (NGA). The tracking information

is sent to the Air Force Space Command's master control station at Schriever Air Force Base in

Colorado Springs, which is operated by the 2nd Space Operations Squadron (2 SOPS) of the United

States Air Force (USAF).

There are 3 stations for the control segment. They are

• A)Master Control Stations (MCS)

• B)Monitor Stations(MS)

• C)Ground Antennas(GA)

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Master Control Stations

Its located near Colorado Springs in US. They receive the signals from Monitor Stations and

pass it to antennas. They are the intermediate between the Monitor Stations and Antennas. The

Demodulation and transmission of signals are done here.

Monitor Stations

There are 6 Monitor Stations all over world. They receive signals from satellites & pass to

Master Control Stations. They can receive the modulated signal. They are the one for the

maintenance of the Satellite and for the control.

Ground Antennas

They receive demodulated signals from MCS and transmit to air and receivers receive those

signals. Thus the desired result will be shown in the display.

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2.3 USER SEGMENT

The user's GPS receiver is the user segment (US) of the GPS. In general, GPS receivers are

composed of an antenna, tuned to the frequencies transmitted by the satellites, receiver-processors,

and a highly-stable clock (often a crystal oscillator).

They may also include a display for providing location and speed information to the user. A

receiver is often described by its number of channels: this signifies how many satellites it can

monitor simultaneously. Originally limited to four or five, this has progressively increased over the

years so that, as of 2007, receivers typically have between 12 and 20 channels

GPS RECEIVERS

Figure:-2.4 GPS RECEIVER DEVICES

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2.4 GPS SIGNALS

Each GPS satellite continuously broadcasts a Navigation Message at 50 bit/s giving the time-

of-week, GPS week number and satellite health information (all transmitted in the first part of the

message), an ephemeris (transmitted in the second part of the message) and an almanac (later part

of the message). The messages are sent in frames, each taking 30 seconds to transmit 1500 bits.

Transmission of each 30 second frame begins precisely on the minute and half minute as

indicated by the satellite's atomic clock according to Satellite message format. Each frame contains

5 sub frames of length 6 seconds and with 300 bits. Each sub frame contains 10 words of 30 bits

with length 0.6 seconds each.Words 1 and 2 of every sub frame have the same type of data. The

first word is the telemetry word which indicates the beginning of a sub frame and is used by the

receiver to synch with the navigation message. The second word is the HOW or handover word and

it contains timing information which enables the receiver to identify the sub frame and provides the

time the next sub frame was sent.

Words 3 through 10 of sub frame 1 contain data describing the satellite clock and itsrelationship to GPS time. Words 3 through 10 of sub frames 2 and 3, contain the ephemeris data,

giving the satellite's own precise orbit. The ephemeris is updated every 2 hours and is generally

valid for 4 hours, with provisions for updates every 6 hours or longer in non-nominal conditions.

The time needed to acquire the ephemeris is becoming a significant element of the delay to first

position fix, because, as the hardware becomes more capable, the time to lock onto the satellite

signals shrinks, but the ephemeris data requires 30 seconds (worst case) before it is received, due to

the low data transmission rate.

The almanac consists of coarse orbit and status information for each satellite in the

constellation, an ionosphere model, and information to relate GPS derived time to Coordinated

Universal Time (UTC). Words 3 through 10 of sub frames 4 and 5 contain a new part of the

almanac. Each frame contains 1/25th of the almanac, so 12.5 minutes are required to receive the

entire almanac from a single satellite. [23] The almanac serves several purposes. The first is toassist in the acquisition of satellites at power-up by allowing the receiver to generate a list of visible

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satellites based on stored position and time, while an ephemeris from each satellite is needed to

compute position fixes using that satellite.

All satellites broadcast at the same two frequencies, 1.57542 GHz (L1 signal) and 1.2276

GHz (L2 signal). The receiver can distinguish the signals from different satellites because GPS uses

a code division multiple access (CDMA) spread-spectrum technique where the low-bit rate message

data is encoded with a high-rate pseudo-random (PRN) sequence that is different for each satellite.

The receiver knows the PRN codes for each satellite and can use this to reconstruct the actual

message data. The message data is transmitted at 50 bits per second. Two distinct CDMA

encodings are used: the coarse/acquisition (C/A) code (a so-called Gold code) at 1.023 millionchips per second, and the precise (P) code at 10.23 million chips per second. The L1 carrier is

modulated by both the C/A and P codes, while the L2 carrier is only modulated by the P code.[24]

The C/A code is public and used by civilian GPS receivers, while the P code can be encrypted as a

so-called P(Y) code which is only available to military equipment with a proper decryption key.

Both the C/A and P(Y) codes impart the precise time-of-day to the user.

Figure:-2.5 SCHEMATIC OF GPS SIGNALS

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2.5 FREQUENCY RANGE OF SATELLITES

L1 (1575.42 MHz): Mix of Navigation Message, coarse-acquisition (C/A) code and

encrypted precision P(Y) code, plus the new L1C on future Block III satellites.

L2 (1227.60 MHz): P(Y) code, plus the new L2C code on the Block IIR-M and newer

satellites.

L3 (1381.05 MHz): Used by the Nuclear Detonation (NUDET) Detection System Payload

(NDS) to signal detection of nuclear detonations and other high-energy infrared events. Used to

enforce nuclear test ban treaties.

L4 (1379.913 MHz): Being studied for additional ionospheric correction.

L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (Sol) signal (see GPS

modernization). This frequency falls into an internationally protected range for aeronautical

navigation, promising little or no interference under all circumstances. The first Block IIF satellite

that would provide this signal is set to be launched in 2009

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CHAPTER -3

SOURCES OF ERRORS

User equivalent range errors (UERE) are shown in the table. There is also a numerical error

with an estimated value, of about 1 meter. The standard deviations, , for the coarse/acquisition and

precise codes are also shown in the table. These standard deviations are computed by taking the

square root of the sum of the squares of the individual components (i.e. RSS for root sum squares).

To get the standard deviation of receiver position estimate, these range errors must be multiplied by

the appropriate dilution of precision terms and then RSS'ed with the numerical error.The main

sources are

1. Signal arrival time measurement

2. Atmospheric effects

3. Multipath effects

4. Ephemeris and clock errors

5. Geometric dilution of precision computation (DOP)

Source Effect (m)

Signal arrival C/A ±3

Signal arrival P(Y) ±0.3

Ionospheric effects ±5

Ephemeris errors ±2.5

Satellite clock errors ±2

Multipath distortion ±1

Tropospheric effects ±0.5

C/A ±6.7

P(Y) ±6.0

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USER EQUIVALENT RANGE ERRORS (UERE)

CHAPTER-4

SOURCES OF INTERFERENCE

There are mainly two sources.

1. Natural sources

2. Artificial sources

Natural sources

Since GPS signals at terrestrial receivers tend to be relatively weak, natural radio signals or

scattering of the GPS signals can desensitize the receiver, making acquiring and tracking the

satellite signals difficult or impossible.

Artificial sources

In automotive GPS receivers, metallic features in windshields, such as defrosters, or car window

tinting film scan act as a Faraday cage, degrading reception just inside the car.

Man-made EMI (electromagnetic interference) can also disrupt, or jam, GPS signals. In one well

documented case, the entire harbor of Moss Landing, Calif. was unable to receive GPS signals due

to unintentional jamming caused by malfunctioning TV antenna preamplifiers. Intentional jamming

is also possible. Generally, stronger signals can interfere with GPS receivers when they are within

radio range

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CHAPTER-5

DIFFERENTIAL GPS

The idea behind all differential positioning is to correct bias errors at one location with

measured bias errors at a known position. A reference receiver, or base station, computes

corrections for each satellite signal.

Because individual pseudo-ranges must be corrected prior to the formation of a

navigation solution, DGPS implementations require software in the reference receiver that can track

all SVs in view and form individual pseudo-range corrections for each SV. These corrections are

passed to the remote, or rover, receiver which must be capable of applying these individual pseudo-

range corrections to each SV used in the navigation solution. Applying a simple position correction

from the reference receiver to the remote receiver has limited effect at useful ranges because both

receivers would have to be using the same set of SVs in their navigation solutions and have

identical GDOP terms (not possible at different locations) to be identically affected by bias errors.

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CHAPTR-6

APPLICATIONS OF GPS

• Agriculture (proper soil selection , Fertilizer application)

• Aviation (to view roots and air ports , pads etc)

• Environment (mapping , weather forecast )

• Transportation (vehicle tracking, personal )

• Marine (Root navigation, prediction of risks)

• Military (target navigation,mapping,routing)

• Rail (passage navign,building up of new tracks)

• Space (Space research,forecasting)

• Surveying ( for survey purposes)

• Timing (for accurate time calculation)

•

Industrial ( for selecting plant locations , transportation , etc. )

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CHAPTER-7

CONCLUSION

The GPS is now changing the way we live , standards and very useful. There

should be more and more applications for GPS in the future years. Also its available in any part of

the world. GPS functionality has now started to move into mobile phones en masse. The first

handsets with integrated GPS were launched already in the late 1990’s, and were available for

broader consumer availability on networks such as those run by Nextel, Sprint and Verizon in 2002

in response to US FCC mandates for handset positioning in emergency calls. Capabilities for access

by third party software developers to these features were slower in coming, with Nextel opening up

those APIs upon launch to any developer, Sprint following in 2006, and Verizon soon thereafter.

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CHAPTER-8

REFERENCE

1. 1999 Federal Radio navigation Plan, February 2000

Washington, US Dept.of Deffence.

2. GPS Std Positioning Service Specification, 2nd Edn

June1995. onlinefrom U.S Coast Guard Navign Center

3. GPS Joint Program Office. 1997. ICD-GPS-200:

GPS Interface Control Document. ARINC Research

4. Hoffmann-Wellenhof, B. H. Lichtenegger, & Collins.

1994. GPS: Theory and Practice. 3rd ed.New York.

5. Leick, Alfred. 1995. GPS Satellite Surveying. 2nd. ed. New

6. http://www.nasa.com/globalpositioningsystem

7. http://www.wikipedia.org/search#gps#articles.php

8. http://www.google.com/imagesearch/gps

9. http://www.navstar.com/cgi/goto_m.pl?icao/en

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