Introduction to gps

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Introduction to GPS

Aksh

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CE 211T Advanced Survey Unit III Photogrammetry PDPU-Gandhinagar

Content

� Introduction

� Segments of GPS

� Principles of GPS

� Types of GPS

� GPS Survey Methods

� Errors in GPS

� Application of GPS in Civil Engineering

� Other Satellite Navigation System

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Introduction

� The Global Positioning System (GPS) is a satellite-based navigation system that was developed by theU.S. Department of Defense (DoD) in the early 1970s.

� Initially, GPS was developed as a military system tofulfill U.S. military needs. However, it was latermade available to civilians, and is now a dual-usesystem that can be accessed by both military andcivilian users

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� GPS provides continuous positioning and timinginformation, anywhere in the world under anyweather conditions.

� Because it serves an unlimited number of users aswell as being used for security reasons, GPS is a one-way-ranging (passive) system. That is, users canonly receive the satellite signals.

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Segments of GPS

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1. Space Segment

� GPS consists, nominally, of a constellation of 24 operationalsatellites.

� To ensure continuous worldwide coverage, GPS satellites arearranged so that four satellites are placed in each of six orbitalplanes (Figure). With this constellation geometry, four to tenGPS satellites will be visible anywhere in the world, if anelevation angle of 10° is considered.

� Only four satellites are needed to provide the positioning, orlocation, information.

� GPS satellite orbits are nearly circular (an elliptical shape witha maximum eccentricity is about 0.01), with an inclination ofabout 55° to the equator. The semi-major axis of a GPS orbit isabout 26,560 km (i.e., the satellite altitude of about 20,200 kmabove the Earth’s surface). The corresponding GPS orbitalperiod is about 12 sidereal hours (~11 hours, 58 minutes).

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GPS Constellation

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GPS Satellite Single

� Each GPS satellite transmits a signal, which has a numberof components: two sine waves (also known as carrierfrequencies), two digital codes, and a navigationmessage.

� The codes and the navigation message are added to thecarriers as binary biphase modulations.

� The carriers and the codes are used mainly to determinethe distance from the user’s receiver to the GPS satellites.

� The navigation message contains, along with otherinformation, the coordinates (the location) of thesatellites as a function of time. The transmitted signalsare controlled by highly accurate atomic clocks onboardthe satellites.

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2. Control Segment

� The control segment of the GPS system consists of aworldwide network of tracking stations, with amaster control station (MCS) located in the UnitedStates at Colorado Springs, Colorado.

� The primary task of the operational control segmentis tracking the GPS satellites in order to determineand predict satellite locations, system integrity,behavior of the satellite atomic clocks, atmosphericdata, the satellite almanac, and other considerations.

� This information is then packed and uploaded intothe GPS satellites through the S-band link.

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3. User Segment

� The user segment includes all military and civilianusers.

� With a GPS receiver connected to a GPS antenna, auser can receive the GPS signals, which can be usedto determine his or her position anywhere in theworld.

� GPS is currently available to all users worldwide atno direct charge.

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Principle of GPS

� The idea behind GPS is rather simple. If thedistances from a point on the Earth (a GPS receiver)to three GPS satellites are known along with thesatellite locations, then the location of the point (orreceiver) can be determined by simply applying thewell-known concept of resection.

� How can we get the distances to the satellites aswell as the satellite locations?

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Answer

� Each GPS satellite continuously transmits amicrowave radio signal composed of two carriers,two codes, and a navigation message.

� When a GPS receiver is switched on, it will pick upthe GPS signal through the receiver antenna. Oncethe receiver acquires the GPS signal, it will process itusing its built-in software.

� The partial outcome of the signal processing consistsof the distances to the GPS satellites through thedigital codes (known as the pseudoranges) and thesatellite coordinates through the navigationmessage.

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� Theoretically, only three distances to threesimultaneously tracked satellites are needed.

� In this case, the receiver would be located at theintersection of three spheres; each has a radius of onereceiver-satellite distance and is centered on thatparticular satellite (Figure).

� From the practical point of view, however, a fourthsatellite is needed to account for the receiver clockoffset.

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Principle of GPS

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� The accuracy obtained with the method describedearlier was limited to 100m for the horizontalcomponent, 156m for the vertical component, and340 ns for the time component, all at the 95%probability level. This low accuracy level was due tothe effect of the so-called selective availability, atechnique used to intentionally degrade theautonomous real-time positioning accuracy tounauthorized users.

� With the decision of terminating the selectiveavailability, the obtained horizontal accuracy isabout 22m (95% probability level).

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� To further improve the GPS positioning accuracy,the so-called differential method, which employstwo receivers simultaneously tracking the same GPSsatellites, is used. In this case, positioning accuracylevel of the order of a sub-centimeter to a few meterscan be obtained.

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Some GPS Linear Combination

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GPS Data

� GPS receiver gives coordinate of earth’s surface using GPS

satellites in space as reference points.

� Precision of this location coordinate depends on many factors

such as type of GPS, survey method employed, knowledge of

surveyor, atmospheric and instrumental error, etc.

� GPS data include the horizontal location based on a

geographic or projected coordinate system and the elevation of

the point location in relation to reference ellipsoid.

� A collection of GPS position along a line can determine a line

feature, and a series of lines measured by GPS can determine

an area feature.

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Types of GPS

� Based on channels:- Single Channel Receivers- Multi-Channel Receivers: 4, 12, 36

� Based on application:- Hand-held / Military Receivers- Surveying & Mapping Receivers- Geodetic Receivers- Timing Receivers- Atomic Clock GPS Receivers- Real-time DGPS Receivers

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Types of GPS

� Based on frequencies

* Single frequency Receivers

* Dual Frequency Receivers

� Range

- Cost: Rs. 15000 to Rs 10,00,000- Size: 3 mm to few metres- Accuracy: mm-level to 100m-level

GEODETIC TYPE ANTENNA AND RECEIVER

MAPPING TYPE ANTENNA AND RECEIVER HAND HELD RECEIVER

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Types of GPS

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GPS Data Collection

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GPS Data Collection Process

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GPS Survey Methods

1. Point Positioning

2. Relative Positioning

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Point Positioning

� GPS point positioning employs one GPS receiverthat measures the code pseudoranges to determinethe user’s position instantaneously, as long as four ormore satellites are visible at the receiver.

� The expected horizontal positioning accuracy fromthe civilian C/A-code receivers has gone down fromabout 100m when selective availability was on, toabout 22m in the absence of selective availability.

� GPS point positioning is used mainly when arelatively low accuracy is required. This includesrecreation applications and low accuracy navigation.

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Principle of GPS Point Positioning

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Relative Positioning

� GPS relative positioning employs two GPS receiverssimultaneously tracking the same satellites.

� If both receivers track at least four commonsatellites, a positioning accuracy level of the order ofa sub-centimeter to a few meters can be obtained.

� Of the two receivers, one is selected as a reference, orbase, which remains stationary at a site withprecisely known coordinates. The other receiver,known as the rover or remote receiver, has itscoordinates unknown.

� The rover receiver may or may not be stationary,depending on the type of the GPS operation..

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Principle of Relative Positioning

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Types of Relative Positioning

1. Static GPS Surveying

2. Fast (Rapid) Static Surveying

3. Stop-and-Go GPS Surveying

4. RTK GPS Surveying

5. Real-time Differential GPS Surveying

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1. Static GPS Survey

� Static GPS surveying is a relative positioning techniquethat depends on the carrier-phase measurements.

� It employs two (or more) stationary receiverssimultaneously tracking the same satellites (Figure).

� One receiver, the base receiver, is set up over a pointwith precisely known coordinates such as a surveymonument (sometimes referred to as the known point).The other receiver, the remote receiver, is set up over apoint whose coordinates are sought (sometimes referredto as the unknown point).

� The base receiver can support any number of remotereceivers, as long as a minimum of four commonsatellites is visible at both the base and the remote sites.

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Static GPS Survey

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2. Fast (Rapid) Static Survey� Fast, or rapid, static surveying is a carrier-phase based relative

positioning� technique similar to static GPS surveying. That is, it employs

two or more receivers simultaneously tracking the samesatellites.

� However, with rapid static surveying, only the base receiverremains stationary over the known point during the entireobservation session (Figure ). The rover receiver remainsstationary over the unknown point for a short period of timeonly, and then moves to another point whose coordinates aresought

� Similar to the static GPS surveying, the base receiver cansupport any number of rovers.

� This method is This method is suitable when the surveyinvolves a number of unknown points located in the vicinity(i.e., within up to about 15 km) of a known point.

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� The survey starts by setting up the base receiver over theknown point, while setting up the rover receiver over thefirst unknown point .

� The base receiver remains stationary and collects datacontinuously. The rover receiver collects data for aperiod of about 2 to 10 minutes, depending on thedistance to the base as well as the satellite geometry.

� Once the rover receiver has collected the data, the usermoves to the following point with unknown coordinatesand repeats the procedures. It should be pointed out that,while moving, the rover receiver may be turned off. Dueto the relatively short occupation time for the roverreceiver, the recording interval is reduced to 5 seconds.

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Fast Static Survey

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3. Stop-and-go GPS surveying

� Stop-and-go surveying is also carrier-phase-basedrelative positioning technique. It also employs two ormore GPS receivers simultaneously tracking the samesatellites (Figure): a base receiver that remains stationary

� over the known point and one or more rover receivers.The rover receiver travels between the unknown points,and makes a brief stop at each point to collect the GPSdata.

� The data is usually collected at a 1- to 2-second recordingrate for a period of about 30 seconds per each stop.

� Similar to the previous methods, the base receiver cansupport any number of rovers. This method is suitablewhen the survey involves a large number of unknownpoints located in the vicinity (i.e., within up to 10-15 km)of a known point.

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Stop-and-Go GPS Survey

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RTK GPS Survey

� RTK surveying is a carrier phase based relativepositioning technique that, like the previous methods,employs two (or more) receivers simultaneously trackingthe same satellites.

� This method is suitable when:� (1) the survey involves a large number of unknown points

located in the vicinity (i.e., within up to about 10 -15 km) of aknown point;

� (2) the coordinates of the unknown points are required inreal time; and

� (3) the line of sight, the propagation path, is relativelyunobstructed.

� Because of its ease of use as well as its capability todetermine the coordinates in real time, this method is thepreferred method by many users.

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RTK GPS Survey

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Real-time differential GPS

� Real-time differential GPS (DGPS) is a code-basedrelative positioning technique that employs two ormore receivers simultaneously tracking the samesatellites. It is used when a real-time meter-levelaccuracy is enough.

� The method is based on the fact that the GPS errorsin the measured pseudoranges are essentially thesame at both the base and the rover, as long as thebaseline length is within a few hundred kilometers.

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Real-time differential GPS (DGPS)

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Errors in GPS

� GPS pseudorange and carrier-phase measurementsare both affected by several types of random errorsand biases (systematic errors).

� These errors may be classified as those originating atthe satellites, those originating at the receiver, andthose that are due to signal propagation(atmospheric refraction).

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Error in GPS� The errors originating at the satellites include ephemeris, or

orbital, errors, satellite clock errors, and the effect of selectiveavailability.

� The errors originating at the receiver include receiver clockerrors, multipath error, receiver noise, and antenna phasecenter variations.

� The signal propagation errors include the delays of the GPSsignal as it passes through the ionospheric and troposphericlayers of the atmosphere.

� In fact, it is only in a vacuum (free space) that the GPS signaltravels, or propagates, at the speed of light.

� In addition to the effect of these errors, the accuracy of thecomputed GPS position is also affected by the geometriclocations of the GPS satellites as seen by the receiver. The morespread out the satellites are in the sky, the better the obtainedaccuracy.

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GPS ephemeris error

� Satellite positions as a function of time, which areincluded in the broadcast satellite navigation message,are predicted from previous GPS observations at theground control stations.

� Typically, overlapping 4-hour GPS data spans are usedby the operational control system to predict fresh satelliteorbital elements for each 1-hour period.

� As might be expected, modeling the forces acting on theGPS satellites will not in general be perfect, which causessome errors in the estimated satellite positions, known asephemeris errors.

� Nominally, an ephemeris error is usually in the order of2m to 5m, and can reach up to 50m under selectiveavailability

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Selective Availability

� GPS was originally designed so that real-timeautonomous positioning and navigation with thecivilian C/A code receivers would be less precisethan military P-code receivers.

� Surprisingly, the obtained accuracy was almost thesame from both receivers. To ensure nationalsecurity, the U.S. DoD implemented the so-calledselective availability (SA) on Block II GPS satellitesto deny accurate real-time autonomous positioningto unauthorized users. SA was officially activated onMarch 25, 1990.

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Satellite and receiver clock errors

� Each GPS Block II and Block IIA satellite contains four atomicclocks, two cesium and two rubidium.

� The newer generation Block IIR satellites carry rubidiumclocks only.

� One of the onboard clocks, primarily a cesium for Block II andIIA, is selected to provide the frequency and the timingrequirements for generating the GPS signals. The others arebackups.

� The GPS satellite clocks, although highly accurate, are notperfect.

� Their stability is about 1 to 2 parts in 1013 over a period of oneday. This means that the satellite clock error is about 8.64 to17.28 ns per day. The corresponding range error is 2.59m to5.18m, which can be easily calculated by multiplying the clockerror by the speed of light (i.e., 299,729,458 m/s).

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Multipath error

� Multipath is a major error source for both the carrier-phase and pseudorange measurements.

� Multipath error occurs when the GPS signal arrivesat the receiver antenna through different paths.

� These paths can be the direct line of sight signal andreflected signals from objects surrounding thereceiver antenna.

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Multipath Error

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Antenna-phase-center variation

� A GPS antenna receives the incoming satellite signaland then converts its energy into an electric current,which can be handled by the GPS receiver.

� The point at which the GPS signal is received iscalled the antenna phase center. Generally, theantenna phase center does not coincide with thephysical (geometrical) center of the antenna. It variesdepending on the elevation and the azimuth of theGPS satellite as well as the intensity of the observedsignal. As a result, additional range error can beexpected.

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Receiver measurement error

� The receiver measurement noise results from thelimitations of the receiver’s electronics.

� A good GPS system should have a minimum noiselevel.

� Generally, a GPS receiver performs a self-test whenthe user turns it on. However, for high-cost preciseGPS systems, it might be important for the user toperform the system evaluation. Two tests can beperformed for evaluating a GPS receiver (system):zero baseline and short baseline tests.

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Ionospheric delay

� At the uppermost part of the earth’s atmosphere,ultraviolet and X-ray radiations coming from the suninteract with the gas molecules and atoms.

� These interactions result in gas ionization: a largenumber of free negatively charged electrons andpositively charged atoms and molecules.

� Such a region of the atmosphere where gasionization takes place is called the ionosphere. Itextends from an altitude of approximately 50 km toabout 1,000 km or even more. In fact, the upper limitof the ionospheric region is not clearly defined.

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Tropospheric Error

� The troposphere is the electrically neutral atmosphericregion that extends up to about 50 km from the surface ofthe earth.

� The troposphere is a nondispersive medium for radiofrequencies below 15 GHz. As a result, it delays the GPScarriers and codes identically. That is, the measuredsatellite-to-receiver range will be longer than the actualgeometric range, whichmeans that a distance betweentwo receivers will be longer than the actual distance.Unlike the ionospheric delay, the tropospheric delaycannot be removed by combining the L1 and the L2observations. This is mainly because the troposphericdelay is frequency independent.

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Satellite Geometry

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GPS in Civil Engineering

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GPS in Civil Engineering� Civil Engineering works often involve complexity and

unfriendliness of environment which makes it difficult forengineer to operate efficiently.

� With ability to provide real-time sub meter and centimeter-level accuracy in a very cost effective manner, GPS has beenused by many construction companies in India due to itsapplication in road construction, Earth work, disastermanagement, etc.

� In road construction and Earth work, GPS along with wirelesscommunication and computer systems, is installed onboardthe Earthmoving machine. Surface information, in a digitalformat (spatial format), is uploaded into the system. With thehelp of the computer display and the real-time GPS positioninformation, the operator can view whether the correct gradehas been reached or not. In situations in which millimeter-level elevation is needed, GPS can be integrated with rotatedbeam lasers.

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GPS in Civil Engineering

� The GPS technology (combination of GPS, wirelesscommunications, and computers) is also used forfoundation works (e.g., pile positioning) and precisestructural placement (e.g., prefabricated bridgesections and coastal structures). In theseapplications, the operators are guided through theonboard computer displays, eliminating the need forconventional methods.

� GPS is also used to track the location and usage ofequipment at different sites. By sending thisinformation to a central location, GPS enables

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Cadastral Planning

� Cadastral surveys establish property corners, boundaries,and areas of land parcels.

� Conventional methods have the drawback that extensivetraversing is required. Moreover, extensive clear cuttingand intervening private properties might be required aswell. GPS overcomes these conventional-methoddrawbacks.

� Any of the GPS surveying methods, such as kinematicGPS or RTK GPS, can be used depending on the projectrequirements, location, and other factors. The RTKsurveying, however, seems to be the most suitablemethod, especially in unobstructed areas. This is mainlybecause of its ease of use and the availability of theresults while in the field. Inaccessible locations orobstructed areas can be surveyed with integratedsystems such as GPS/LRF or GPS/total station.

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Cadastral Planning� There are several advantages of using GPS for cadastral

surveying.� The most important one is that inter visibility between the

points is not required with GPS. This means that extensivetraversing is eliminated, clear-cutting is not required, andintervening private properties is avoided.

� Other advantages include the fact that GPS provides user-defined coordinates in a digital format, which can be easilyexported to any GIS system for further analysis.

� The accuracy obtained with GPS is consistent over the entirenetwork; such accuracy is lacked by conventional surveyingmethods.

� Also, with GPS, one reference station can support an unlimitednumber of rover receivers. A number of governmental andprivate organizations have reported that the use of GPS incadastral surveying is cost-effective.

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Monitoring Bridge Deformation

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Monitoring Structural Deformations

� GPS has been used successfully in monitoring thestability of structures, an application that requires thehighest possible accuracy.

� Typical examples include monitoring the deformation ofdams, bridges, and TV towers.

� Monitoring ground subsidence of oil fields and miningareas are other examples where GPS has been usedsuccessfully.

� In some cases, GPS may be supplemented by othersystems such as total stations to work more efficiently.

� Deformation monitoring is done by taking GPSmeasurements over the same area at different timeintervals.

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� Slow-deforming structures such as dams require submillimeter- to millimeter-level accuracy to monitortheir displacement. Although this accuracy levelmay be achieved with GPS alone under certainconditions, it is not a cost-effective method. Toeffectively monitor such structures, GPS should besupplemented with geotechnical sensors and specialtypes of total stations.

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� Bridges, in contrast, are subjected to vibrationscaused by dynamic traffic loads. To effectivelymonitor such cyclic deforming structures, dual GPSreceivers should be located at several points withmaximum amplitude of cyclic deformation.

� For example, in monitoring the world’s longestsuspension bridge (Akashi Bridge, Japan), a GPSreceiver is installed at the midpoint of the bridgewhile two others are installed at the main towers.The Ashtech Z12 dual-frequency receiver is used formonitoring bridge deformation.

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Land Seismic Survey

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GPS in marine seismic survey

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Utilities Industry

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Utilities Industry

� Accurate and up-to-date maps of utilities areessential for utility companies. The availability ofsuch maps helps electric, gas, and water utilitycompanies to plan, build, and maintain their assets.

� The GPS/GIS system provides a cost-effective,efficient, and accurate tool for creating utility maps.With the help of GPS, locations of features such asgas lines can be accurately collected, along with theirattributes (such as their conditions and whether ornot a repair is needed). The collected informationcan then be used by a GIS system to create updatedutility maps.

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Utilities Industry� In situations of poor GPS reception, such as in urban canyons,

it might be useful to use integrated GPS and LRF systems. Thisintegrated system is an efficient tool for rapid utility mapping.A GPS receiver remains in the open for the best signalreception, while the LRF measures the offset information(range and azimuth) to the utility assets such as light poles.The processing software should be able to combine both theGPS and the LRF information.

� Buried utilities such as electric cables or water pipes can alsobe mapped efficiently using GPS. With the help of apipe/cable locator attached to the second port of the GPShandheld controller, accurate information on the location andthe depth of the buried utility can be collected. This is a verycost-effective and efficient tool, as no ground marking isrequired.

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GPS

Advantages

� It helps to survey with many times greater Precision.

� It helps to complete a Survey with lesser time and thus helpsto cut down the Completion Period.

� It Reduces the Difficulty of taking manual measurements togreat extent.

� With GPS there is a very less chances of error. And thiserror may come only due to the Instrument malfunction.

Disadvantages

� The main Disadvantage is that, it requires high initialinvestments.

� To conduct such High End Survey works and to operate suchElectronic Equipments much skilled persons are required.

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Other Satellite Navigation System

� GLONASS – Russia's global navigation system. Fullyoperational worldwide.

� Galileo – a global system being developed bythe European Union and other partner countries,planned to be operational by 2016 (and fully deployed by2020)

� Beidou– People's Republic of China's regional system,currently limited to Asia and the West Pacific

� COMPASS – People's Republic of China's global system,planned to be operational by 2020

� IRNSS – India's regional navigation system, coveringIndia and Northern Indian Ocean

� QZSS – Japanese regional system covering Asiaand Oceania

Thanks

Introduction to gps

Engineering

Transcript of Introduction to gps