A Civil Engineer S Guide To GPS And GNSS

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Geospatial Engineering Panel Briefing SheetA Civil Engineer’s Guide to GPSand GNSSAbstractThe usage of the Global Positioning System (GPS)and other developing Global Navigation SatelliteSystems (GNSS), for example the RussianGLONASS system and the emerging EuropeanGalileo and Chinese BeiDou systems is expandingexponentially. Current and future utilization,research, and application developments arebecoming key to most walks of life and henceprevalent within government, commercial industry,research communities as well as for the citizen. Theability to accurately locate and relocate, to navigateand track and to synchronize data streams hasmade this a technology that we all utilise, whetherwe are aware of it or not.Current GNSS drivers within the engineeringcommunity are to improve efficiency, to providebetter customer service, to enhance health andsafety provision and as an aid to meet industryregulation. Augmenting GNSS devices with otherpositioning tools (whether they be groundpenetrating radar, inertial devices, lasers,tachographs, etc) can provide a total solution whenthey are appropriately combined together.This paper aims to provide some backgroundinformation on GNSS through concentrating on thebasic technology, issues and applications of GPS. Aview to the future of GNSS will be given at the end.Overview of GPSGPS is a constellation of orbiting satellites thatprovides navigation data to military and civilianusers all over the world. The system is operatedand controlled by the 50th Space Wing of the U.S.Air Force located at Schriever Air Force Base,Colorado, USA. GPS is a global 24-hour, allweather, navigation system which can provideextremely accurate three-dimensional locationinformation (latitude, longitude and altitude), velocityand timing.GPS was designed as a dual-use military / civiliansystem, but its primary purpose was to enhance theeffectiveness of U.S. and allied military forces. GPShas its origins in the 1970s and it became fullyoperational in 1995. Since that period however,GPS has rapidly become integral to the globalinformation economy and hence civilian users andcommercial communities have gained muchinfluence over the development of GPS. This wasoutlined by the U.S. Policy Statement RegardingGPS Availability in March 2003; “The United StatesGovernment recognizes that GPS plays a key rolearound the world as part of the global informationinfrastructure and takes seriously the responsibilityto provide the best possible service to civil andcommercial users worldwide. This is as true in timesof conflict as it is in times of peace.”Currently (2014) 31 GPS satellites orbit the earth inone of six orbital planes at an inclination of 55degrees to the equator, there are also around fourresidual spare satellites in orbit. A GPS orbit has analtitude of around 20,200km above the Earth. As aresult it takes about 11 hours and 56 minutes for asatellite to complete one orbit.Briefing sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken toensure information is correct, however readers are advised to consult source documents for authoritative information.Institution of Civil Engineers, One Great George Street, Westminster, London SW1P 3AA Registered Charity No. 210252

The Growth in GNSSOne straightforward way of depicting the growth inthe use of GNSS is through looking at the predictedrevenue from GNSS receiver sales over the next 15years. In 2001 this stood at 15 billion, by 2020 thisis predicted to be 150 billion. The mass marketswill be in personal navigation and telematics,followed by transport, emergency services and thesurveying / engineering / asset managementindustries. The accurate timing that GNSS alsoprovides is exploited to enable the synchronizationof communications networks, electric powerdistribution and banking.This current popularity and future explosion in theuse of GNSS can partly be traced to the comingtogether of a variety of factors; the advances andincreased use of telecommunication technologiesand geographic information systems, together withthe availability of geospatial information. This hascombined with the overall decrease in cost, sizeand power consumption of satellite navigationreceivers.This measurement is called a pseudo-rangebecause not all the errors in the measurement aretaken into account at this stage.The receiver knows ‘when and where to look’ in thesky for the signal from the satellite because orbitalinformation is transmitted from the satellites andstored in the receiver memory. Each satellitegenerates a different coded signal and ifmeasurements are made to enough satellites aposition for the receiver can be computed. Thereare four unknowns (latitude, longitude, height andtime offset) so ranges to four, or more, satellites atonce have to be observed to determine a position.Raw plan positional accuracy with a single receiver,for civilian users, is around 5m to 10m, 95% of thetime. Height accuracy in GPS is generally two tothree times worse than plan whatever technique isndused. Before the 2 May 2000, the accuracy wasfar worse (100m, 95%) because during the 1990sthe U.S. DoD deliberately degraded the GPSsignals to limit the real-time accuracy to civilianusers. Military users had, and still have, access toa more accurate coded signal from the satellites.The Basic GPS MeasurementsThe basic measurement in GPS positioning is thecalculation of the receiver to satellite distance. Themost fundamental method to do this uses veryaccurate satellite atomic clocks to generate aunique coded signal (Coarse Acquisition (C/A)Code) for each satellite. This signal is sent on oneof the two signal frequencies broadcast by thesatellites, the L1 frequency (1575.42 MHz).The positional accuracy is affected by a variety ofdifferent sources of error.These include theimprecise knowledge of the GPS satellite orbits,small timing errors in the satellite & receiver clocks,the atmosphere (slowing down the signal and soproducing a range that is too long), receiver biasesand reflected indirect signals arriving at the antenna(multipath).C/A Code sequence:Improving Raw GPS PositioningToobtainSummary ofMagnitudeimprovedGPS Error(approx) maccuracy,theSourceserrors must be Orbit Errors3calculatedor Clocks1.5modelled.ForIonosphere6.0basic L1 process is knownMultipath1asDifferentialCode GPS or dGPS. DGPS involves thecomputation of corrections to the code-basedpseudo-ranges.The receiver on the ground generates the samecoded signal, at the same time, and compares thereceived code with the one being generated. Thetime offset between the two codes gives the time oftravel of the signal between the satellite andreceiver. The distance can then be calculated by:(pseudo-)range to satellite signal travel time xspeed of lightThe corrections counteract the effect of the majorerrors in the GPS position (orbit, clocks andatmosphere). These corrections are then combinedwith GPS signals from the receiver to improve thecomputed position. Multipath errors are not reducedby dGPS, but are generally reduced by good sitingof the GPS antenna, software in the receiver orantenna design which either detects the reflectedsignals and ignores them or reduces their effect.Briefing sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken toensure information is correct, however readers are advised to consult source documents for authoritative information.Institution of Civil Engineers, One Great George Street, Westminster, London SW1P 3AA Registered Charity No. 210252

The dGPS corrections are generally computed byusing another GPS receiver at a known point. Thisreceiver compares the measured and computedpseudo-ranges and uses the differences to computethe corrections to the GPS signal. The dGPScorrections can be combined with the user’sreceiver either in real-time through delivering themover a communications link or after the fact in postprocessing software. Accuracy from this techniqueis typically around 50cm-5m, depending on theconditions and equipment used.Two examples of free dGPS services are theGeneral Lighthouse Authority (GLA) medium waveradio broadcast service aimed at mariners (but alsoavailable throughout mainland United Kingdom) andthe European Geostationary Navigation OverlayService (EGNOS) which transmits corrections tousers via satellites. These two systems shouldprovide 2-5m positional accuracy. There are also anumber of commercial systems available which canproduce better accuracy.Carrier Phase PositioningUp until now we have just been dealing with themeasurements of the satellite-receiver rangethrough using the timing codes on the L1 frequency.It is however possible to measure the range moreaccurately through using measurements of thephase of the carrier waves on both the GPSfrequencies; L1 and L2.Carrier Phase:The 19cm or 24cm wavelength carrier waves canbe used as an accurate ruler to measure the rangeto the satellites. In simple terms, by counting thenumber of whole cycles between the satellite andthe receiver and multiplying by the wavelength (plusthe additional fraction of a wavelength) gives therange. This gives a more accurate range than acode derived pseudo-range. Determining the wholenumber of wavelengths between the receiver andthe satellite is where the problem lies however aseach wavelength looks just like the last (unlike inthe L1 C/A Code which is a very complex sequenceand therefore it is relatively easy to determinespecific points along its series). If a code position isused as a starting point to determine position, thenumber of satellite-receiver wavelengths is thenknown to probably /- 20 wavelengths. This ‘carrierphase ambiguity’ is then easier to calculate todetermine a final end user position.A carrier phase derived range has all of the sameerrors as a code-based pseudo-range. Either realtime or post-processed differential techniquestherefore have to be performed to obtain a highlyaccurate position. This again generally involves areceiver at a known point (base station) collectingdata at the same time as a user’s receiver, but thistime instead of deriving a correction, a receiver toreceiver baseline is computed, either in real-time orafter the fact during post-processing. The techniqueof real-time carrier phase positioning is also calledReal Time Kinematic (RTK) if the user is moving.It is possible to obtain extremely high levels ofpositional accuracy by using code and L1/L2 carrierphase GPS positioning. It is easily possible for anRTK user to obtain 2cm level positioning in plan,and even better if they are careful. In RTK however,the distance from the reference station affects thefinal positional accuracy that can be achieved.If a receiver is left at a location for many months, itis possible to determine positions down to the fewmillimetre level – ideal for bridge monitoring or platetectonic studies for example.A technique known as Networked RTK hasdeveloped which links together data from a numberof dual frequency GPS receivers in real-time –providing a regional dGPS or RTK correctionsolution. The main advantages of Networked RTKare cost savings, as there is no need to set up alocal base station and the accuracy is notdependant on the base – rover receiver distance.Examples of a Networked RTK systems are thecommercial services built on the Ordnance Survey OS Net national GPS infrastructure.Carrier-smoothed code positioning provides a halfway house between just code and dual frequencycarrier phase positioning. Plan accuracy levels areat the 20cm-80cm level.CoordinatesGPS provides coordinates in the World GeodeticSystem 1984 (WGS84). WGS84 consists of a threedimensional cartesian coordinate system and anassociated ellipsoid whose origin is at the centre ofthe Earth’s mass. It is ideally suited for positioninganywhere on the Earth. The coordinate systemdefined and adopted for Europe however is theEuropean Terrestrial Reference System 1989(ETRS89) which is basically WGS84 at its’ positionston January 1 1989. ETRS89 is used acrossEurope as the primary coordinate system instead ofWGS84 because WGS84 is fixed to the earth’splates and so moves over time. In fact thedivergenceofETRS89andWGS84isapproximately 2.5cm / year. This may not soundvery much, but over the last 24 years this hasamounted to 90cm. This becomes important whenNational Mapping Agencies and others wish toprovide a coordinate transformation between theirGPS derived positions and mapping / plans.If WGS84 was used, these transformations wouldhave to change every few years to take into accountthe movement of WGS84. ETRS89 is thereforeused as standard for high accuracy GPS workacross the United Kingdom.Briefing sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken toensure information is correct, however readers are advised to consult source documents for authoritative information.Institution of Civil Engineers, One Great George Street, Westminster, London SW1P 3AA Registered Charity No. 210252

The most accurate transformation available in GreatBritain from ETRS89 coordinates to the Nationalmapping coordinate system (OSGB36 National Grid) is OSTN02 . For height this is the ETRS89 toheight above sea level correction surface, currently OSGM02 .Alternatively, if a local grid is being used, a localtransformation model needs to be established. Todo this at least 4 points with known localcoordinates should be observed with GPS acrossthe site. The GPS to local grid transformationparameters can then be established.User ApplicationsGNSS has a plethora of possible applications; fromroad (road user charging and navigation), rail(safety and positioning), offshore (navigation) andair (routing), to construction and engineeringpositioning to the location and relocation of utilityassets.The figure below illustrates a water industryapplication example. Raw stand-alone GPSpositioning could be used to navigate an engineerto the correct road where a leak has occurred.Basic or high quality dGPS could be used toposition assets into a database and RTK to locateor relocate a specific valve or pipe within a network.TechniqueSummary of GPS Plan Approximate OneAccuracy (m)Receiver CostRaw GPSDGPSPhase smoothedcode5-100.8-30.2-0.8 70 90 1500 RTKLong period staticcarrier phase0.01-0.050.001-0.01 6000 5000 Broader civil engineering GNSS applications arenow widespread across the industry and havedeveloped a long way over the last 10 years. This isespecially so with the deployment of RTK forsetting-out, profiling, as well as for the automaticguidance of plant for piling, grading, cutting andfilling applications.The FutureThe future for GNSS is very exciting over the nextten years where we could see more than 120positioning satellites available to be used. GPS isslowly being modernised as new satellites arelaunched with new signals. This will mean GPSpositions will become increasingly more accurateand quicker to determine. The Russian GLONASSnavigation system was under funded for manyyears but was returned to a full constellation of 24satellites by the end of 2012. A full modernisationprogramme is also planned for GLONASS over thecoming years. Many survey grade receivers arenow operating with both GPS and GLONASS, andindeed the Network RTK services based on OS Net now provide corrections for both GPS andGLONASS satellites. The European satellitenavigation system Galileo will consist of 30satellites and will provide a range of open (free) andrestricted services. Four Galileo validation satellitesare now in orbit with the remainder due to belaunched from 2014 onwards with a fullconstellation expected by 2020. The ChineseBeiDou system became operational over SouthEast Asia in 2012 and should be available as aglobal system also around 2020.There are techniques that now enable very fastacquisition (sub-second) of the GPS satellites(Assisted GPS or AGPS). After first power, thereceiver is sent orbital and timing information via amobile phone - enabling the receiver to very quicklytrack the satellites. It is also possible to obtain aposition, quite inaccurately currently, whilst indoors.High Sensitivity GPS works by detecting the verylow strength GPS signals that permeate indoors –typically 20dB and 30dB lower. If an inertial systemis coupled with the GPS receiver, positioning ispossible in places where the receiver cannot seethe four satellites it needs (in tunnels for example).Map matching techniques can be used whichexploit the intelligence within digital mapping toallign a user’s GNSS track onto the right road –useful in vehicle tracking applications.The acceptance and use of GNSS within civilengineering, often when combined with othermeasurement or sensor technology, will continue toflourish. GNSS is now used to control plant acrossthe site, leading to the potential for it to becometruly autonomous.Briefing sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken toensure information is correct, however readers are advised to consult source documents for authoritative information.Institution of Civil Engineers, One Great George Street, Westminster, London SW1P 3AA Registered Charity No. 210252

Reference Material:There are numerous text books and websitesrelated to GPS and GNSS, including online tutorialmaterial.About the Authors:The first edition of this guide was written by Dr PaulCruddace and James Brayshaw of the OrdnanceSurvey. It was updated in 2013 by Professor TerryMoore of the University of NottinghamProfessor Terry Moore is Director of theNottingham Geospatial Institute (NGI) at theUniversity of Nottingham; where he is the Professorof Satellite Navigation and also currently anAssociate Dean within the Faculty of Engineering.He holds a BSc degree in Civil Engineering andPhD degree in Space Geodesy, both from theUniversity of Nottingham. He has over 30 years ofresearch experience in surveying, positioning andnavigation technologies and is a consultant andadviser to European and UK governmentorganisations and industry. He is a Fellow and aMember of Council of both the Institute ofNavigation and of the Royal Institute of Navigation.He is also a Fellow of the Chartered Institution ofCivil Engineering Surveyors and a Fellow of theRoyal Astronomical Society.Briefing sheets are provided free of charge to help increase knowledge and awareness. They may be freely copied. Care is taken toensure information is correct, however readers are advised to consult source documents for authoritative information.Institution of Civil Engineers, One Great George Street, Westminster, London SW1P 3AA Registered Charity No. 210252

A Civil Engineer’s Guide to GPS and GNSS Abstract The usage of the Global Positioning System (GPS) and other developing Global Navigation Satellite Systems (GNSS), for example the Russian GLONASS system and the emerging European Galileo and Chinese BeiDou systems is expanding exponentially. Current and future utilization,

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