global positioning system.pdf

7/29/2019 Global Positioning System.pdf

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Prepared By:

Er. Suresh Pd Sah


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Introduction Official name of GPS is NAVigational Satellite Timing

And Ranging Global Positioning System (NAVSTARGPS)

Global Positioning Systems (GPS) is a form of GlobalNavigation Satellite System (GNSS)

GPS is funded and controlled by the U. S. Department

of Defense (DOD). While there are many thousands ofcivil users of GPS world-wide, the system was designedfor and is operated by the U. S. military.


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Introduction (contd) The GPS receivers convert the satellite's signals intoposition, velocity, and time estimates for navigation,positioning, or geodesy.

Four GPS satellite signals are used to computepositions in three dimensions and the time offset inthe receiver clock.

GPS units are becoming smaller and less expensive,

there are an expanding number of applications forGPS. In transportation applications, GPS assists pilotsand drivers in pinpointing their locations and avoidingcollisions.


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Introduction (contd) GPS can provide accurate positioning 24 hours a day,

anywhere in the world. Uncorrected positionsdetermined from GPS satellite signals produce

accuracies in the range of50 to 100 meters. Whenusing a technique called differential correction, userscan get positions accurate to within 5 meters or less.

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 auseful tool in non-military mapping applications aswell.


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Introduction (contd)History of the GPS

1969Defense Navigation Satellite System (DNSS)formed

1973NAVSTAR Global Positioning Systemdeveloped

1978first 4 satellites launched

Delta rocket launch


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Introduction (contd)History of the GPS 199324th satellite

launched; initial

operational capability 1995full operational

capability

May2000Militaryaccuracy available to allusers


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GPS system block diagram The GPS system can be divided into three basic segments.

Space Segment (SS)

Control Segment (CS)

User Segment (US)


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Control Segment

Space Segment

User Segment

The three Segments of the GPS

Monitor Stations

Ground

Antennas

Master Station


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Theory of operation

To help us understand GPS theory of operation letsdiscuss the three parts of the systemthe satellites,the receivers, and the ground stationsand then look

more closely at how GPS works.


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Space Segment The Global Positioning System (GPS) is a location

system , based on a constellation of about 24 satellites(also there are 6 satellites spare) in 6 orbital planes

with 4 satellites in each plane .

Orbital planes are centered on the Earth.

GPS satellites fly at an altitude of20,200 km and with

a period of12 hours. The ascending nodes of the orbital planes are

separated by60 degrees .


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Space Segment(contd)


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Space Segment(contd)

and the planes are inclined 55 degrees relative toEarth's equator in order to cover the polar regions.

GPS satellites powered by solar cells, the satellitescontinuously orient themselves to point their solarpanels toward the sun and their antenna toward theearth.


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Space Segment (Continued) The satellites orbit the earth with a speed of3.9 km

per second and have a circulation time of 12 h siderealtime, corresponding to 11 h 58 min earth time.

This means that the same satellite reaches a certainposition about 4 minutes earlier each day.

the height of the orbits is about 20,200 km. Orbits inthis height are referred to as MEO medium earth

orbit. Orbits are designed so that at the very least, six

satellites are always within line of sight from anylocation on the planet.


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Space Segment (Continued) There are currently30 actively broadcasting satellites

in the GPS constellation.

Redundancy is used by the additional satellites toimprove the precision of GPS receiver calculations.

A non-uniform arrangement improves the reliabilityand availability of the system over that of a uniform

system, when multiple satellites fail.


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Control Segment The CS consists of3 entities:

Master Control System

Monitor Stations

Ground Antennas


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Master Control Station The master control station, located at Falcon Air Force

Base in Colorado Springs, Colorado, is responsible foroverall management of the remote monitoring and

transmission sites.

Here the data from the monitor stations are processed24 h a day in real time. As results, information aboutorbits and clocks of the satellites are obtained. Doingthis, possible malfunctions can quickly be detected.


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Master Control Station(contd)Additionally, from the raw data new ephemeresis data

are calculated. Once to twice a day, theses data andother commands are sent back to the satellites via the

transmitting antennas .

Satellites are capable of exchanging data with othersatellites and can correct their orbit data on their own.

In theory they only need a contact to a ground stationevery180 days.


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Monitor Stations Six monitor stations are located at Falcon Air Force

Base in Colorado, Cape Canaveral, Florida, Hawaii,Ascension Island in the Atlantic Ocean, Diego Garcia

Atoll in the Indian Ocean, and Kwajalein Island in theSouth Pacific Ocean.

Each of the monitor stations checks the exact altitude,position, speed, and overall health of the orbitingsatellites.


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Monitor Stations (continued) The control segment uses measurements collected by

the monitor stations to predict the behavior of eachsatellite's orbit and clock.

The prediction data is up-linked, or transmitted, to thesatellites for transmission back to the users.

The control segment also ensures that the GPS satellite

orbits and clocks remain within acceptable limits. Astation can track up to 11 satellites at a time.


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Monitor Stations (continued) This "check-up" is performed twice a day, by each

station, as the satellites complete their journeysaround the earth.

Variations such as those caused by the gravity of themoon, sun and the pressure of solar radiation, arepassed along to the master control station.


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Ground Antennas Ground antennas monitor and track the satellites from

horizon to horizon.

They also transmit correction information toindividual satellites.


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User Segment The user's GPS receiver is the US of the GPS system.

GPS receivers are generally composed of an antenna,tuned to the frequencies transmitted by the satellites,receiver-processors, and a highly-stable clock,commonly a crystal oscillator).

They can also include a display for showing locationand speed information to the user.

A receiver is often described by its number of channelsthis signifies how many satellites it can monitorsimultaneously. As of recent, receivers usually havebetween twelve and twenty channels.


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User Segment (continued)

Using the RTCM SC-104 format, GPS receivers mayinclude an input for differential corrections.

This is typically in the form of a RS-232 port at 4,800bps speed. Data is actually sent at a much lower rate,

which limits the accuracy of the signal sent using RTCM.

Receivers with internal DGPS receivers are able tooutclass those using external RTCM data.


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Navigational Systems GPS satellites broadcast three different types of data in

the primary navigation signal.

Almanac sends time and status information about thesatellites.

Ephemeris has orbital information that allows thereceiver to calculate the position of the satellite.

This data is included in the 37,500 bit Navigation Message,

which takes 12.5 minutes to send at 50 bps.


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Navigational Systems (contd) Satellites broadcast two forms of clock information

Coarse / Acquisition code (C/A) - freely available to thepublic. The C/A code is a 1,023 bit long pseudo-random

code broadcast at 1.023 MHz, repeating everymillisecond.

Restricted Precise code (P-code) - reserved for militaryusage. The P-code is a similar code broadcast at 10.23

MHz, but it repeats only once a week. In normaloperation, the anti-spoofing mode, the P code is firstencrypted into the Y-code, or P(Y), which can only bedecrypted by users have a valid key.


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GPS Transmitted Signal

Two signals are transmitted on carriers:

L1 = 1575.42 MHz

L2 = 1227.60 MHzThese 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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GPS Signals


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GPS Clock Signals Two types of clock signals are transmitted

C/A Code - Coarse/Acquisition Code available forcivilian use on L1 provides 300 m resolution

P Code - Precise Code on L1 and L2 used by themilitary provides 3m resolution


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Spread Spectrum Spread Spectrum is used because

- resistance to jamming

- masks the transmissions- resist multipath effects

- multiple access

All 24 GPS satellites transmit on the same two

frequencies BUT use a different ID sequence


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GPS Signals The satellites transmit as part of their unique Spread

Spectrum signal a clock or timing signal

The range or distance to the satellite is obtained bymeasuring how long it takes for the transmitted signalto 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 coordinates

To correct for clock errors one needs to receive foursatellites


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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 thenavigation message also on L1

The receiver then reads the timing information andcomputes the pseudo-ranges

The pseudo-ranges are then corrected


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GPS: How does it work? Corrected ranges are used to compute the position

This is a very complicated iterative nonlinear equation


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Navigation Message To compute your position one must know the position

of the satellite

Navigation Message - transmitted on both L1 and L2 at50 bits/s for 30 s

Navigation message consists of two parts:

- satellite almanac

- clock bias


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Why Do I Need

To See 4 Satellites? The problem is that the clock signal from the satellite

is corrupted by atmospheric refraction

Another major problem is that the receivers clock isnot very accurate

For a 2D fix 3 satellites


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Why Do I Need

To See 4 Satellites?


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Denial of Accuracy (DOA) The US military uses two approaches to prohibit use of

the full resolution of the system

Selective Availability (SA) - noise is added to the clocksignal and the navigation message has lies in it

Anti-Spoofing (AS) - P-code is encrypted

The military sometimes turns off both DOA

techniques


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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 standardsatellite frequencies


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GPS Clock Rollover GPS System Time rolled over at midnight 21-22 August

1999, 132 days before the Year 2000

On 22 August 1999, unless repaired, many GPSreceivers claimed that it is 6 January 1980


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GPS Frequencies L1 (1575.42 MHz) - Mix of Navigation Message,

coarse/acquisition (C/A) code and encrypted precisionP(Y) code.

L2 (1227.60 MHz) - P(Y) code, plus the new L2C codeon the Block IIR-M and newer satellites.

L3 (1381.05 MHz) - Used by the Defense SupportProgram to signal detection of missile launches,nuclear detonations, and other applications.


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GPS Proposed Frequencies L4 (1379.913 MHz) - Being studied for additional

correction to the part of the atmosphere that is ionizedby solar radiation.

L5 (1176.45 MHz) To be used as a civilian safety-of-life (SOL) signal.

Internationally protected range for aeronauticalnavigation.

The first satellite that using this signal to be launched in2008.


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Position Calculation The coordinates are calculated according to the World

Geodetic System WGS84 coordinate system.

The satellites are equipped with atomic clocks

Receiver uses an internal crystal oscillator-based clockthat is continually updated using the signals from thesatellites.

Receiver identifies each satellite's signal by its distinctC/A code pattern, then measures the time delay foreach satellite.


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Position Calculation (contd) The receiver emits an identical C/A sequence using the

same seed number the satellite used.

By aligning the two sequences, the receiver canmeasure the delay and calculate the distance to thesatellite, called the pseudorange.

Orbital position data from the Navigation Message isused to calculate the satellite's precise position.Knowing the position and the distance of a satelliteindicates that the receiver is located somewhere on thesurface of an imaginary sphere centered on thatsatellite and whose radius is the distance to it.


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Position Calculation (contd)When four satellites are measured at the same time,

the point where the four imaginary spheres meet isrecorded as the location of the receiver.

Earth-based users can substitute the sphere of theplanet for one satellite by using their altitude. Often,these spheres will overlap slightly instead of meetingat one point, so the receiver will yield a mathematically

most-probable position.


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Performance AnalysisGPS Accuracy

The U.S. government is committed to providing GPS tothe civilian community at the performance levels

specified in the GPS Standard Positioning Service (SPS)Performance Standard. For example, the GPS signal inspace will provide a "worst case" pseudo range accuracy of7.8 meters at a 95% confidence level.

The actual accuracy users attain depends on factorsoutside the government's control, including atmosphericeffects and receiver quality. Real-world data collected bythe FAA show that some high-quality GPS SPS receivers

currently provide better than 3 meter horizontal accuracy.


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Performance AnalysisGPS Accuracy


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Performance Analysis

GPS Accuracy Higher accuracy is available today by using GPS in

combination with augmentation systems. Theseenable real-time positioning to within a fewcentimeters, and post-mission measurements at themillimeter level.

The U.S. government is committed to modernizing the

GPS constellation to enable higher civilian accuracywithout augmentations. The first of many next-generation GPS satellites was fielded in 2005.


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Advantages and Limitations

Issues that affect accuracy

Changing atmospheric conditions change the speed ofthe GPS signals as they pass through the Earth's

atmosphere and ionosphere. Effect is minimized when the satellite is directly

overhead.

Becomes greater for satellites nearer the horizon, sincethe signal is affected for a longer time.

Once the receiver's approximate location is known, amathematical model can be used to estimate andcompensate for these errors.


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Issues that affect accuracy(contd) Clock Errors can occur when, for example, a GPS

satellite is boosted back into a proper orbit.

The receiver's calculation of the satellite's position willbe incorrect until it receives another ephemeris update.

Onboard clocks are accurate, but they suffer from partialclock drift.


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Issues that affect accuracy(contd)

GPS Jamming can be used to limit the effectiveness ofthe GPS signal

For example, it is believed GPS guided missiles havebeen misled to attack non-target locations in the war in

Afghanistan.

The stronger the jamming signal, the more interference

can be caused to the GPS signal.


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Issues that affect accuracy(contd) GPS signals can also be affected by multipath issues

Radio signals reflect off surrounding objects at a

location. These delayed signals can cause inaccuracy. Less severe in moving vehicles. When the GPS antenna

is moving, the false solutions using reflected signalsquickly fail to converge and only the direct signals resultin stable solutions.


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Methods of improving accuracy Precision monitoring

Dual Frequency Monitoring

Refers to systems that can compare two or more signals These two frequencies are affected in two different ways. How

they are affected can be predicted however

After monitoring these signals, its possible to calculate whatthe error is and eliminate it.

Receivers that have the correct decryption key can decode theP(Y)-code transmitted on signals to measure the error.


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Methods of improving accuracy (contd) Carrier-Phase Enhancement (CPGPS)

CPGPS uses the L1 carrier wave, which has a period 1000times smaller than that of the C/A bit period, to act as anadditional clock signal and resolve uncertainty.

The phase difference error in the normal GPS amounts tobetween 2 and 3 meters (6 to 10 ft) of ambiguity.

CPGPS works to within 1% of perfect transition to reduce the

error to 3 centimeters (1 inch) of ambiguity. By eliminating this source of error, CPGPS coupled with DGPS

normally realizes between 20 and 30 centimeters (8 to 12inches) of absolute accuracy.


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Methods of improving accuracy (contd) Relative Kinematic Positioning (RKP)

Determination of range signal can be resolved to an accuracyof less than 10 centimeters (4 in).

Resolves the number of cycles in which the signal istransmitted and received by the receiver.

Accomplished by using a combination of DGPS correctiondata, transmitting GPS signal phase information and

ambiguity resolution techniques via statistical tests possiblywith processing in real-time.


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Methods of improving accuracy(contd)Augmentation

Relies on external information being integrated into thecalculation process.

Some augmentation systems transmit additionalinformation about sources of error.

Some provide direct measurements of how much thesignal was off in the past.

Another group could provide additional navigational orvehicle information to be integrated in the calculationprocess.


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Applications Military Military GPS user equipment has been integrated into

fighters, bombers, tankers, helicopters, ships,submarines, tanks, jeeps, and soldiers' equipment.

In addition to basic navigation activities, militaryapplications of GPS include target designation ofcruise missiles and precision-guided weapons andclose air support.

To prevent GPS interception by the enemy, thegovernment controls GPS receiver exports.

GPS satellites also can contain nuclear detonation

detectors.


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Applications CivilianAutomobiles are often equipped GPS receivers.

They show moving maps and information about yourposition on the map, speed you are traveling, buildings,

highways, exits etc. Some of the market leaders in this technology are

Garmin and TomTom, not to mention the built in GPSnavigational systems from automotive manufacturers.


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ApplicationsCivilian (contd) For aircraft, GPS provides

Continuous, reliable, and accurate positioninginformation for all phases of flight on a global basis,

freely available to all. Safe, flexible, and fuel-efficient routes for airspace

service providers and airspace users.

Potential decommissioning and reduction of expensive

ground based navigation facilities, systems, andservices.

Increased safety for surface movement operations madepossible by situational awareness.


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ApplicationsCivilian (contd)Agriculture

GPS provides precision soil sampling, data collection,and data analysis, enable localized variation of chemical

applications and planting density to suit specific areas ofthe field.

Ability to work through low visibility field conditionssuch as rain, dust, fog and darkness increases

productivity. Accurately monitored yield data enables future site-

specific field preparation.


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ApplicationsCivilian (contd) Disaster Relief

Deliver disaster relief to impacted areas faster, savinglives.

Provide position information for mapping of disasterregions where little or no mapping information isavailable.

Example, using the precise position information

provided by GPS, scientists can study how strain buildsup slowly over time in an attempt to characterize andpossibly anticipate earthquakes in the future.


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ApplicationsCivilian (contd) Marine applications

GPS allows access to fast and accurate position, course,and speed information, saving navigators time and fuel

through more efficient traffic routing. Provides precise navigation information to boaters.

Enhances efficiency and economy for containermanagement in port facilities.


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ApplicationsCivilian (contd) Other Applications not mentioned here include

Railroad systems

Recreational activities (returning to the same fishing

spot) Heading information replacing compasses now that

the poles are shifting

Weather Prediction

Skydiving taking into account winds, plane and drop

zone location

Many more!


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Summary

A GPS receiver can tell its own position by using theposition data of itself, and compares that data with 3 ormore GPS satellites.

To get the distance to each satellite, the GPS transmits asignal to each satellite. The signal travels at a known speed.

The system measures the time delay between the signaltransmission and signal reception of the GPS signal.

The signals carry information about the satellites location. Determines the position of, and distance to, at least three

satellites, to reduce error.

The receiver computes position using trilateration.


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Functions of various segments of GPS

Segment Input Function Output

Space Navigation message Generate andTransmit code andcarrier phases andnavigation message

P-CodeC/A codeL1,L2 carrier Navigationmessage

Control P-Code ObservationsTime

Produce GPS timepredict ephemerismanage space vehicles

Navigation message

User Code observationCarrier phaseObservation Navigationmessage

Navigation solutionSurveying solution

Position , velocity ,time


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Operational Overview Video NASA produced short film that sumarizes GPS http://www.youtube.com/watch?v=wi_3XwkA8cQ
http://www.youtube.com/watch?v=wi_3XwkA8cQhttp://www.youtube.com/watch?v=wi_3XwkA8cQ


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Thank You

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