Global Positioning System.pdf - How GPS Works by bestt571

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									                Global Positioning System (GPS)                                                                 Global Positioning System (GPS)

                an all-weather, space-based, radio-navigation                                                   initiated in 1973 to reduce the proliferation
                system                                                                                          of navigation aids.
                designed by the U.S. Department of                                                              accuracy improvements over earlier ground
                Defense (DOD)                                                                                   based radio navigation systems and can be
                                                                                                                used world-wide.
                known as the Navstar Global Positioning
                                                                                                                downing of Korean Airlines flight 007 over
                System (NAVigation Satellite Timing And
                                                                                                                the Soviet Union.
                Ranging)



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                Global Positioning System (GPS)                                                                 How GPS Works

                available to the world free of charge.                                                          man-made constellation of stars.
                its accuracy has been degraded for general                                                      improvement on celestial navigation.
                user.
                                                                                                                instead of measuring imprecise angles to stars
                degradation is called Selective Availability                                                    of known angular position, measurement to
                or SA                                                                                           precise distances to artificial stars with known
                affects stand alone navigation.                                                                 3-D position.
                completely removed by using two or more
                receivers differentially.


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                How GPS Works                                                                                   How GPS Works

                21 primary satellite constellation with 3                                                       satellites orbit the earth every 12 hours in
                operational spares - total of 24 satellite                                                      circular orbit at about 20,200 kilometres.
                constellation
                                                                                                                6 orbital planes with 4 satellites per plane.
                space commander's mandate is to provide 21
                working satellites at all times.                                                                2 of the orbital planes inclined at 55
                                                                                                                degrees to the equator and spaced equally
                visibility of 5 to 9 satellites world-wide 24
                                                                                                                at 60 degrees around the equator.
                hours a day.



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                How GPS Works                                                                                   How GPS Works

                monitor and uplink stations are located                                                         continually monitored at Falcon Air Force
                around the world.                                                                               Base and on Hawaii, Ascension Island in the
                controlled by the master control station at                                                     Atlantic Ocean, Diego Atoll in the Indian
                Falcon Air Force Base in Colorado Springs,                                                      Ocean, and Kwajalein Island in the South
                Colorado.                                                                                       Pacific Ocean.




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                How GPS Works                                                                                    Advantages Of GPS

                Updated ephemeris (health and orbit)                                                             allows extremely accurate positioning
                information is periodically uplinked to the                                                      worldwide in all kinds of weather.
                satellites for re-broadcast in the satellite                                                     no need to be able to see stars or landmarks to
                message.                                                                                         accurately position a vessel, vehicle, person or
                User’s receiver calculate the positions of the                                                   survey mark.
                satellites and by applying the ranges                                                            allows surveyor to perform surveys to
                (distances) it measures to the satellites, the                                                   accuracy levels impossible conventionally.
                receiver can compute its own position.


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                Advantages Of GPS                                                                                Limitations Of GPS

                quick and economically.                                                                          must have clear line of sight to the satellites.
                suited to bringing in plane coordinate control                                                   signal is very low in tracking in foliage.
                for projects of all types -- boundaries, well                                                    signal would not be discernible at all from
                locations, route alignments, mapping control.                                                    the background noise without the presence
                                                                                                                 of the pseudo-random noise (PRN) code
                                                                                                                 modulated on the carrier frequency.




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                Limitations Of GPS                                                                               Applications Of GPS

                Canyons (urban or natural) are areas of very                                                     used in many areas of navigation and
                limited usability.                                                                               surveying and its application list is growing
                mountains, buildings and trees pose                                                              daily.
                visibility problems.                                                                             used to establish position for many things,
                limited to surface surveys and obstructions                                                      from single receiver positioning of airlines in
                to satellite visibility should be as few as                                                      flight and ships at sea to the measurement of
                possible.                                                                                        minute movements in the earth's surface.




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                Applications Of GPS                                                                              Applications Of GPS

                rent autos in several test market areas with                                                     control aerial photogrammetry from the air
                GPS sensors and map display screens which                                                        and not the ground, especially with attitude
                will graphically show your present location                                                      determining GPS systems.
                and the location you are travelling to.
                                                                                                                 3-D position control of dredges, and road-
                Surveyors are currently using GPS for any                                                        graders is possible by plugging a digital
                kind of location survey where all points                                                         terrain model CD-ROM into the road grader's
                cannot be seen from one place.                                                                   computer and automatically blading out a
                                                                                                                 road to grade.


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                Applications Of GPS                                                                              Applications Of GPS

                worldwide remote control of vehicles to                                                          Vehicle tracking
                submeter accuracy is possible.                                                                       • Trucks, trains. buses. maintenance
                General navigation                                                                                     vehicles
                   •   Extremely accurate Ocean wide positions                                                       • ambulances, police cars, stolen cars
                   •   Recreational sailing and racing
                   •   Bay and coastal navigation
                                                                                                                     • airplanes, ships, cargo containers
                   •   Return to great fishing holes without leaving                                                 • taxis, rental cars
                       a buoy
                                                                                                                     • just-in-time manufacturing

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                Applications Of GPS                                                                              Applications Of GPS

                Hydrographic location                                                                            Facilities location
                    • Platform location, seismic vessels,                                                            • Utility poles, manholes, water and gas
                      charting vessels                                                                                 valves
                    • Dredging. channel navigation,                                                                  • Maintenance problems. road hazards.
                      inclement weather positioning                                                                    Accidents
                    • Obstruction locations, collision                                                               • Controlling stereo video camera
                      avoidance positioning                                                                            positions for road inventory
                                                                                                                     • Signs, bridges and culverts
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                Applications Of GPS                                                                              Applications Of GPS

                Surveying                                                                                        Agriculture and biology
                    • Aerial photo control. topographic surveys                                                     • Tractor location for smart application
                    • Statewide high precision geodetic networks                                                      of chemicals
                    • City and county networks
                    • Large boundary surveys
                                                                                                                    • Drainage surveys and watershed
                    • section and township breakdowns                                                                       modelling
                    • Subsidence surveys (ground water. oil. mining)
                                                                                                                    • Wetlands mapping. habitat mapping
                    • Dam deformation surveys, Core hole locations
                    • Utility, pipeline and highway routes
                    • Plate tectonics
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                Applications Of GPS                                                                              GPS Coordinate System

                Archaeology                                                                                      based on the Earth Centred Earth Fixed
                    • Site location, site relationships                                                          (ECEF) Cartesian coordinate system.
                    • Returning to sites without leaving physical                                                origin is the centre of the mass of the earth.
                      marks
                                                                                                                 X axis is aligned with the Greenwich
                Environmental surveys                                                                            Meridian.
                    • Spill and accident locations
                                                                                                                 In the plane of equator. the Y axis is aligned
                    • Sample locations                                                                           90 degrees to the X axis
                    • Tracking hazardous shipments

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                GPS Coordinate System                                                                                                       GPS Coordinate System

                    M e r i d ia n t h r o u g h A
                                                        N o rth p o le
                                                                                        E llip so id                                        Z axis runs approx. through the poles.
                                                                                                                                            all of the positioning and vector
                                               N o rm a l                       G re e n w ic h
                                                                                                                                            measurement mathematics is performed in
                                                                                m e rid ia n
                                    A
                                                                                                                                            this system.
                                         φΑ                                                                                                 an ellipsoid has been defined that best fits
                                                             λΑ
                                                                                                                                            the shape the earth.
                                                            P o la r a x i s
                                                                         E q u a to ria l p la n e
                                                                                                                                            WGS 84 ellipsoid.


                                              S o u th p o le
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                GPS Coordinate System                                                                                                       GPS Coordinate System

                latitude, longitude, and height are specified                                                                               done in the post processing stage or in real-
                in relation to this ellipsoidal surface.                                                                                    time for some receiver.
                receiver computes its position in X, Y and Z.                                                                               next step: computing rectangular grid
                                                                                                                                            coordinates on a mapping projection such as
                calculates and displays it position in latitude,
                                                                                                                                            UTM or Hong Kong Grid Coordinates.
                longitude and height.
                                                                                                                                            grid coordinates are used for mapping, GIS
                other positional systems must be translated,
                                                                                                                                            positioning. survey plans. and ‘right-of-way’
                rotated and scaled from the WGS 84
                                                                                                                                            plans.
                positions using transformation parameters.

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                GPS Coordinate System                                                                            GPS Accuracy

                rectangular system: provide simple                                                               accuracies ranges from 100 m to mm.
                coordinate geometry computations and                                                             single receiver using C/A code - 100 meters
                consistent azimuths, unlike the curved                                                           2-D rms (150 meters vertical) 95% of the
                geodetic surface.                                                                                time with SA turned on.
                                                                                                                 two receivers using dual frequency carrier,
                                                                                                                 phase measurements - millimetres over large
                                                                                                                 distances approaching one part in 109.



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                GPS Accuracy                                                                                     GPS Accuracy

                for accuracy better than 100 meters need at                                                      accuracy depends on the observables use
                least 2 receivers;                                                                               and the time span of the observations.
                Then                                                                                             For navigation and real-time applications:
                apply differential range corrections for code
                                                                                                                 instantaneous position accuracy is highly
                phase measurements; or
                                                                                                                 influenced by satellite geometry.
                difference out common errors for carrier
                phase measurements.



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                GPS Accuracy                                                                                         GPS Accuracy

                For longer surveying sessions,                                                                       Satellite geometry
                effect of poor geometry is not vital because                                                         Dilution of Precision (DOP) calculations
                it can be averaged with periods of good                                                              = ratio of the positioning accuracy to the
                geometry.                                                                                            measurement accuracy, or σ = DOP σO
                good satellite geometry is very important in                                                         σO = the measurement accuracy (standard
                getting good positions.                                                                              deviation),
                                                                                                                     σ = positioning accuracy (e.g., standard
                                                                                                                     deviation in one coordinate).

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                GPS Accuracy                                                                                         Position Computation

                unitless numbers indicating the geometric strength                                                   receiver computes its position by measuring
                of the figure formed by the user and the satellites.                                                 the distance (range) to satellites of known
                the smaller the DOP value, the stronger the                                                          position.
                geometry of the figure.
                                                                                                                     a stopwatch measurement; range = speed of
                   •       GDOP (three position coordinates plus clock offset in
                           the solution)
                                                                                                                     light multiplied by the time of signal
                   •       PDOP (three coordinates)                                                                  transmission
                   •       HDOP (two horizontal coordinates)
                                                                                                                     In racing, speed of the racer = distance
                   •       VDOP (height only)
                                                                                                                     travelled x time taken to complete the race
                   •       TDOP (clock offset only), and
                   •       HTDOP (horizontal position and time)

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                Position Computation                                                                              Position Computation

                In GPS, the speed of the signal (the speed of                                                     amount of shift between the two signals
                light) is known.                                                                                  equals the time of transmission.
                by measuring the time, the signal takes to get to                                                                Code Correlation
                our receiver, we can compute the distance it
                travelled.
                receiver measures the time it takes the signal to
                reach the receiver from the satellite by correlating
                a code sent by the satellite to a replica code
                generated in the receiver.                                                                        dT
                                                                                                                             dT = Time of transmission = Range
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                Position Computation                                                                              Position Computation

                Timing is the vital component in the process                                                      In EDM, one clock is used to measure the time
                                                                                                                  of the reflecting light waves.
                a nanosecond (a billionth of a second) =
                about 3 decimetres (one foot) of range                                                            In GPS, two clocks, the satellite clock and the
                                                                                                                  receiver clock, are used.
                a microsecond (millionth of a second) =                                                           If use only one clock, we need only three
                about 300 meters (980 feet)                                                                       satellites to compute a good 3-dimensional
                a millisecond (a thousandth of a second) =                                                        position.
                about 300 kilometres (186 miles)                                                                  For two clocks , fourth measurement is required
                                                                                                                  to solve for four unknowns - X, Y, Z and T (our
                                                                                                                  receiver clock offset).
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                Position Computation                                                                                     Position Computation
                Clock offset results in large errors in position computation                                             time is distance.
                because the receiver clock is simply a quartz oscillator - slightly
                better than your wristwatch (remember a millionth of a second
                is 300 meters).
                                                                                                                         small time errors can lead to large range
                                                                                                                         errors.
                                                                                                                         receiver measures the time interval by
                                      Y                                                                                  correlating a code.
                                                 Z
                              X                           T
                                                                                                                         How exactly is this done?
                                                                                                                         What is this code that the receiver uses to
                                                                                                                         measure the time interval?
                                            Receiver Clock Offset

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                Position Computation                                                                                     Position Computation
                                                                                                                         C/A code is 1024 bits long, and is repeated every millisecond.
                two ranging codes available.
                                                                                                                         32 unique codes reserved which allow to identify individual
                for single frequency receivers, only one                                                                 satellites and measure ranges to them.

                code is used.                                                                                                                      Carriers


                                                                                                                                L2
                this code is the C/A or Coarse Acquisition
                code which contains no information.
                                                                                                                                L1
                a unique pattern of bits, on or off, that is
                                                                                                                                                 Modulated Carrier
                repeated over and over.
                created with bi-phase modulation of the
                                                                                                                                                 Ranging Code
                carrier frequency.
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                Position Computation                                                                              Position Computation

                satellites modulate own C/A code on the                                                           ranges are measured by comparing the time
                same Ll (1575.42 MHz) frequency.                                                                  shift between the receiver generated and the
                                                                                                                  satellite generated codes.
                each receiver channel generates replica
                codes, cycling through the available codes                                                        once at least four satellites have valid ranges
                until it finds a match.                                                                           and the receiver can determine where the
                                                                                                                  satellites are, the receiver can start to
                once a channel locks on to the signals from                                                       compute its position.
                a satellite, the receiver clock can be
                synchronised to GPS time from the satellite.

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                Position Computation                                                                              Position Computation

                How does the receiver know where the                                                              the broadcast message is also modulated on
                satellites are so that it can calculate its                                                       the ranging code.
                position?                                                                                         every 20 repetitions of the C/A code (every
                How do we get this information from a                                                             20 milliseconds) marks a bit boundary in the
                code that is simply a pattern containing no                                                       broadcast message.
                information?                                                                                      to change the value of the bit, the ranging
                                                                                                                  code is inverted.
                                                                                                                  message is broadcast at 50 bits per second.

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                Position Computation                                                                                  Position Computation

                message contains clock corrections, satellite                                                         orbital parameters allow the receiver to
                health information and orbital parameters,                                                            compute the satellite positions at any instant.
                among other things.
                                                                                                                      selective availability is only accurate to
                                                                                                                      about 100 meters horizontally and 150
                               19        20                  1
                                                                                                                      meters vertically - 95 percent of the time.
                                                      Code is inverted
                                                                                                                      99 percent of the time, it is accurate to
                                                      Message bit changes
                                                                                                                      within 300 meters horizontally.


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                Better Positioning Accuracy                                                                           Better Positioning Accuracy

                100 meters is “bull's eye” accurate for:                                                               one receiver is on a known position, range
                                                                                                                       corrections can be calculated and then passed
                a transcontinental flight,                                                                             to any number of remote receivers.
                driving across a trackless desert or                                                                                                      X,Y,Z
                                                                                                                                     X,Y,Z

                sailing in the middle of the ocean.

                For increased accuracy, at least two receivers
                are required in order to remove common errors
                by differencing.
                                                                                                                                  X,Y,Z



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                Better Positioning Accuracy                                                                       Better Positioning Accuracy

                simple inverse between known receiver                                                             differential correction can be applied in real-
                position and known satellite positions (from                                                      time (with communication links between
                the broadcast ephemeris) to establish the                                                         receivers) or in post processing.
                correct range.
                                                                                                                  using C/A code, a position computed using
                ranges are then subtracted from measured                                                          the differentially corrected ranges is
                pseudo-ranges to compute the range                                                                typically within 2 to 5 meters of its true
                corrections.                                                                                      position.

                applied to remote receiver's measured                                                             using P code, corrected position is submeter,
                ranges before the position is computed.                                                           usually around 3 decimetres.
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                Better Positioning Accuracy                                                                       Better Positioning Accuracy

                P code contains no information and is a                                                           P code allows comparing of the differing
                more complex code with a faster chipping                                                          delays in the signals and compute the true
                rate (10 MHz as opposed to 1 MHz) that                                                            ionospheric delay.
                allows resolving the range more accurately.
                                                                                                                  ionospheric delay is one of the largest errors.
                also modulated on the L2 (1227.60 MHz)
                carrier frequency and on the Ll carrier                                                           How do we get centimetres or even
                frequency.                                                                                        millimetres out of our measurements ?
                                                                                                                  method of measurement is changed from
                                                                                                                  stopwatch to a ruler.

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                Better Positioning Accuracy                                                                             Better Positioning Accuracy

                switch to measuring the phase of the carrier                                                            introduce ambiguity into measurement.
                wavelength.
                                                                                                                        as Ll wavelength is very short (about 19
                                                                                                                        centimetres) and the ambiguity is the whole
           The elements of carrier phase ranging                                                                        number of wavelengths associated with our
                                                                                                                        phase measurement.
                Fractional phase
                measured                                     Unknown number of
                                                                                                                        receiver cannot determine the number of
                                                             whole cycles associates with                               whole wavelength associated with the first
                                                             the first measurement, i.e. the
                                                             initial range ... determined                               measurement.
                                                             in postprocessing.


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                Better Positioning Accuracy                                                                             Better Positioning Accuracy

                receiver can track the change in range                                                                  previously this processing was done in the
                throughout the observation.                                                                             office or hotel room after observations.
                through post processing techniques the                                                                  possible to do this in real-time (using a
                initial phase ambiguity, or integer bias can                                                            communications link to combine the data
                be determined (initial ranges to satellites)                                                            from two receivers) now.
                requires the processing of simultaneously                                                               provides capability to do setting out.
                measured data from two or more receivers.



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                Carrier Phase Differencing                                                                        Carrier Phase Differencing

                range corrections are not computed and                                                            there are many errors which are not under
                passed.                                                                                           our control; and cannot be measure
                                                                                                                  accurately. By differencing, any common
                differencing the actual phase                                                                     errors cancel out in the equations.
                measurements and removing common
                errors as satellite ranges and survey
                vectors are solved.
                purpose of differencing measurements is
                to remove common errors.

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