ممتاز!!!!!Gps basics
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 ممتاز!!!!!Gps basics ممتاز!!!!!Gps basics Presentation Transcript

  • GPS Basic Course
  • GPS Basic Course Page 1-1
  • GPS Basic CourseSection 2 • Definition • The Real Earth (The Geoid) • The Ellipsoid • World Geodetic System (WGS84) • Height System Page 1-2
  • GPS Basic CourseDefinition Geodesy :- Geo - Earth desy - The study of the earth Geodesy is the science of the measurement and mapping of the earth’s surface (F.R Helmert (1880 Page 1-3
  • GPS Basic Course(The Real Earth (The Geoid • Equipotential surface that best equates to mean sea Topography level • Physical Definition that is a complicated surface N. America Europe • Described by an infinite number of parameters • Can be sensed by instruments S. America Africa Page 1-4
  • GPS Basic CourseThe Ellipsoid • An ellipse is a mathematical figure which is defined by a – Semi-Major Axis (a) – Semi-Minor Axis (b) • It is a simple geometrical b surface • Cannot be sensed by instruments a Page 1-5
  • GPS Basic CourseThe Ellipsoid and Geoid N Topography N. America Europe O1 S. America Africa Page 1-6
  • GPS Basic CourseThe Ellipsoid and Geoid • Which ellipsoid to choose ? N N Topography N. America Europe O1 O2 S. America Africa Page 1-7
  • GPS Basic CourseThe Ellipsoid and Geoid • The World Geodetic System N Topography – WGS 1984 • The best mean fit to the Earth N. America Europe S. America Africa Page 1-8
  • GPS Basic Course(World Geodetic System (WGS84 • Origin coincides with Earth’s center of mass • X and Y axis are perpendicular to Z each other in the equatorial plane • Z axis is at right angles to the X,Y P plane and coincides with the Earth’s rotational axis h γ Y ϕ λ X Page 1-9
  • GPS Basic Course(World Geodetic System (WGS84 • The prime orientation (X) is Greenwich Meridian • Positions and Coordinate Z differences are obtained in the WGS 84 Coordinate System P – Latitude, Longitude Ellipsoid height h – Geocentric X,Y,Z coordinates γ Y ϕ λ X Page 1-10
  • GPS Basic CourseHeighting • Heights determined using GPS are referenced to the WGS 84 Ellipsoid – Ellipsoid Heights are heights above the ellipsoid P Topography h Ellipsoidal heighth = Ellipsoid Page 1-11
  • GPS Basic CourseHeighting • The Geoid is that equipotential surface (equal gravity) that best equates to Mean Sea Level • The geoid undulates due to the effects of Topography P – Topology, geology etc. h • Orthometric heights are H referenced to a Datum which is typically M.S.L Geoid • M.S.L approximates the Geoid H = Height above Geoid (Orthometric Height)~ Ellipsoid Page 1-12
  • GPS Basic CourseHeighting • The height difference between ellipsoid and geoid is called the geoidal undulation • To obtain orthometric heights, the P Topography geoidal undulation must be h accounted for H Geoid N N = Geoidal Separation Ellipsoid Page 1-13
  • GPS Basic CourseHeighting • The geoidal undulation may be positive or negative. Ellipsoidal heighth = P Topography H = Height above Geoid h (Orthometric Height)~ H Geoid N = Geoidal Separation N h=H+N h=H+N Ellipsoid Page 1-14
  • GPS Basic CourseProjections – The Basics•A projection is a flatrepresentation of a 3-Dsurface.•The secret of a goodprojection is to minimizethe distortion. A distanceor angle measured on theprojection should be veryclose to the same distanceor angle measured in thereal world.•Users should specify theirprojection. Page 1-15
  • GPS Basic Course Projections - Types Mercator Transverse Lambert – 1 Lambert – 2 Mercator Cylindrical Conical Conical Cylindrical Great for Great for east- Great for east-east-to-west Great for north- to-west areas to-west areasareas around to-south areas. along a Covers a widerthe equator. common The projection north-to-south latitude for UTM range with less distortion Page 1-16
  • GPS Basic Course Projections - DistortionA good projection minimizes the “distortion” of distances and angles when measured in the real world and on your chart. Page 1-17
  • GPS Basic CourseSecondly: GPS Basic Theory Page 2-1
  • GPS Basic CourseIndex - Section 1 • Traditionally • Initial Phase Ambiguity • Why GPS ? • Resolving the Ambiguity • GPS General Characteristics • Range Determination form • Phase Observations GPS System Components • Selective Availability • Outline Principle : Range • Error Sources • Outline Principle : Position • Dilution of Precision • GPS Signal Structure • Errors Reduction • Range Determination form • Differencing Techniques Code Observations • Linear Combinations Page2-2
  • GPS Basic CourseTraditionally • GPS has many advantages over Traditional Terrestrial Surveying Techniques • These traditional techniques rely on the visibility between the survey instrument and a target – If an obstructions exists, it must be traversed around • Typically distance measurement is limited to 5 Km • Weather can limit operations, e.g. fog, rain etc Page 2-3
  • GPS Basic Course?Why GPS•Weather Independent•Does not require line of sights•Gives high Geodetic Accuracy•Can be operated day and night•Quicker and requires lessManpower –Economical advantages•Common Coordinate System•Wide Range of Applications•Competitively Priced Line of sight is not necessary Page 2-3
  • GPS Basic CourseGPS General Characteristics • Developed by the US Department of Defense • Provides – Accurate Navigation • 5 - 15 m – Worldwide Coverage – 24 hour access – Common Coordinate System • Designed to replace existing navigation systems • Accessible by Civil and Military Page 2-5
  • GPS Basic CourseGPS SystemComponents Space Segment Space Segment NAVSTAR : :NAVigation NAVSTAR NAVigation Satellite Time and Ranging Satellite Time and Ranging Satellites 24 Satellites 24 Km 20200 Km 20200 Control Segment Control Segment User Segment Master Station 11 Master Station User Segment Monitoring Stations 55 ` Receive Satellite Signal Receive Satellite Signal Monitoring Stations Page 2-6
  • GPS Basic CourseControl Segment • Master Control Station – Responsible for collecting tracking data from the monitoring stations and calculating satellite orbits and clock parameters • 5 Monitoring Stations – Responsible for measuring pseudorange data. This orbital tracking network is used to determine the broadcast ephemeris and satellite clock modeling – Ground Control Stations – Responsible for upload of information to the satellites Page 2-7
  • GPS Basic CourseSpace Segment • 24 Satellites • 12 Hourly orbits – 4 satellites in 6 Orbital – In view for 4-5 hours Planes inclined at 55 • Designed to last 7.5 years Degrees • Different Classifications • 20200 Km above the Earth – Block 1, 2, 2A, 2R & 2 F 55 Equator Page 2-8
  • GPS Basic CourseUser Segment • The most visible segment • GPS receivers are found in many locations and applications Page 2-9
  • Knowing Where the Satellites Are - Ephemeris Space Segment Current ephemeris is transmitted to usersMonitor stations• Diego Garcia• Ascension Island• Kwajalein• Hawaii GPS Control Colorado Springs Page 2-3
  • Trilateration From Satellites• By measuring distance from several satellites you can calculate your positionSatellite RangingMeasuring the distance from a satellite• Done by measuring travel time of radio signals• Done by measuring the phase of radio signalsMeasure how long it takes the GPS signal to get to us• Multiply that time by 300,000 km/sec – Time (sec) x 300,000 = km• Multiply that phase by the carrier wavelngth. Page 2-3
  • GPS Basic CourseOutline Principle : Range lX l X ll lll Xll Xl lV V Vl ll Vl l Vl Page 2-10
  • GPS Basic CourseOutline Principle : Range lX l X ll lll Xll Xl lV V Vl ll Vl l Vl Page 2-11
  • GPS Basic CourseOutline Principle : Range lX l X ll lll Xll Xl lV V Vl ll Vl l Vl Page 2-12
  • GPS Basic CourseOutline Principle : Range lX l X ll lll Xll Xl lV V Vl ll Vl l Vl Range = Time Taken x Speed of Light Page 2-13
  • GPS Basic CourseOutline Principle : Position R1 We are somewhere on a sphere of radius, R1 Page 2-14
  • GPS Basic CourseOutline Principle : Position R1 R2 Spheres intersect as a circle 2 Page 2-15
  • GPS Basic CourseOutline Principle : Position R1 R3 R2 Spheres intersect at a point 3 Ranges to resolve for Latitude, Longitude and Height 3 Page 2-16
  • GPS Basic CourseOutline Principle : Position • The satellites are like “Orbiting Control Stations” Stations • Ranges (distances) are measured to each satellite using time dependent codes • Typically GPS receivers use inexpensive clocks. They are much less accurate than the clocks on board the satellites • A radio wave travels at the speed of light • (Distance = Velocity x Time) – Consider an error in the receiver clock • 1/10 second error = 30,000 Km error second error = 300 m error 1/1,000,000 • Page 2-17
  • GPS Basic CourseOutline Principle : Position Ranges to resolve for Latitude, Longitude, Height & Time 4 It is similar in principle to a resection problem Page 2-18
  • GPS Basic CourseGPS Signal Structure Each GPS satellite transmits a number of signals • The signal comprises two carrier waves (L1 and L2) and two • codes (C/A on L1 and P or Y on both L1 and L2) as well as a satellite orbit message Fundamental Fundamental Frequency Frequency 10.23 MHz 10.23 MHz 10 ÷ L1 L1 C/A Code C/A Code P (Y)-Code P (Y)-Code x 154 1575.42 MHz 1.023 MHz 1575.42 MHz 1.023 MHz 10.23 MHz 10.23 MHz x 120 L2 P (Y)-Code L2 P (Y)-Code 1227.60 MHz 1227.60 MHz 10.23 MHz 10.23 MHz BPS 50 BPS 50 (Satellite Message (Almanac & Ephemeris (Satellite Message (Almanac & Ephemeris Page 2-20
  • GPS Basic CourseRange Determination from Code Observations • Pseudoranges (Code) Received Code – Each satellite sends a unique signal from Satellite which repeats itself approx. 1 msec – Receiver compares self generated signal with received signal Generated – From the time difference (dT) a range Code from observation can be determined Receiver – Receiver clock needs to be synchronized with the satellite clock ∆T (D = V (∆T ρ i (t ( = R(t ( + c(dt − dT ( + λ i N i − Ii (t ( + T + ε ϕ Page 2-21
  • GPS Basic CourseRange Determination from Phase Observations • Phase Observations – Wavelength of the signal is 19 cm on L1 Received Satellite and 24 cm on L2 Phase – Receiver compares self-generated phase with received phase – Number of wavelengths is not known at the Generated time the receiver is switched on (carrier Phase from phase ambiguity) Receiver – As long as you track the satellite, the change in distance can be observed (the carrier phase ambiguity remains constant) ∆T D = c ∆T + λNλ iϕ i (t ( = R(t ( + c(dt − dT ( + λ i N i − Ii (t ( + T + ε ϕ Page 2-24
  • GPS Basic CourseAutonomous Navigation Accuracy 5 - 20 m A receiver in autonomous mode provides navigation and positioning accuracy of about 5 to 20m Page 2-25
  • GPS Basic CourseAutonomous Navigation Accuracy 5 - 20m A receiver in autonomous mode provides navigation and positioning accuracy of about 5 to 20m Page 2-26
  • GPS Basic CoursePoint Positioning Ranges to resolve for Latitude, Longitude, Height & Time 4 It is similar in principle to a resection problem Page 2-27
  • GPS Basic Course(Selective Availability (SA 100m 30m • In theory a point position can be accurate to 5 - 20m based on the C/A Code P P = True Position Page 2-28
  • GPS Basic Course(Selective Availability (SA 100m 30m • In theory a point position can be accurate to 5 - 20m based on the C/A Code • The USDoD degrades the accuracy of the broadcast information P – Dither the Satellite Clocks – Satellite Orbital Information • This is known as Selective Availability (100m (95%+/- • Positional accuracy 100m (95%) P = True Position • Nowadays, for competition & political reasons SA is off Page 2-29
  • GPS Basic CourseAutonomous NavigationUnder S.A. Accuracy 10- 100 m A receiver in autonomous mode provides navigation and positioning accuracy of about 10 to 100 m due to the effects of Selective Availability Page 2-30
  • GPS Basic CourseError Sources • Satellite errors • Observation errors – Orbit uncertainty – Ionospheric Delay – Satellite Clock Model – Tropspheric Delay • Receiver errors • Station errors – Receiver Clock – Station Coordinates – Receiver noise – Multipath Page 2-31
  • Metres 100 200 300 400 0 GPS Basic Course Satellite Clock Recvr Noise Multipath User Equivalent Range Errors TroposphericPage 2-32 Ephemeris Ionospheric Recvr Clock
  • GPS Basic CourseHow Does One Can Reduce The Errors of GPS? Page 2-33
  • GPS Basic CourseFirstly: Differencing Techniques B A Page 3-16
  • Single-Difference Observation• Remove the effect of the satellite clock offset.• Reduce the effect of the satellite orbital error depending on the distance between stations.• The atmospheric delay is significantly reduced especially with short baselines and can be neglected.∆ (roh( j AB = ∆ R (t ( + c(dt − dt ( + ∆ I (t ( + ∆ T + ε j AB B A j AB AB j j ∆ ϕ AB Page 2-34
  • Double-Difference Observation• Remove the effect of the receiver & satellite clock offset.• Remove the correlated part of satellite orbital error.• Remove the correlated part of the atmospheric delay.λ∇ ∆ϕ AB (t ( = ∇ ∆R AB (t ( + λ ∇ ∆N AB − ∇ ∆I AB (t ( + ∇ ∆T AB + ε ∇jk∆ϕ jk jk jk jk jk AB Page2-35
  • Trible - Difference Observation• Remove the effect of the clock offsets.• Remove the ambiguity bias.• Remove the correlated part of satellite orbital error.• Remove the correlated part of the atmospheric delay. λδ ∇ ∆ ϕ AB ( t 12 ) δ ∇ ∆ R AB (t 12 ( − δ ∇ ∆ I AB ( t12 ) + δ ∇ ∆ T AB ( t12 ) + δ ∇ ∆ ε δjk ∆ ϕ ( t12 ) jk jk jk jk ∇ AB Page 2-36
  • GPS Basic Course:SecondlyLinear Combinations of GPS Observables Page 2-25
  • Linear Combinations • The actual GPS observables are the carrier phases and the code observations. Some other artificial observations can be created from the actual observation by linearly combining them. The main applied linear combinations formula is described by: φ,b a = φ+ φ a 1 b 2• The corresponding frequency is: f a ,b = f 1 + f 2 a b• The corresponding wavelength is: λ a ,b = a 1 b + λ 1 λ 2 Page 2-37
  • Linear Combinations• The frequency-dependent biases such as the ionospheric delay and the multipath will be affected by these combinations. The linear combinations have no effect on the frequency- independent biases such as the tropospheric delay, the clock and the ephemeris errors.• The linear combinations will alter the ionospheric delay by a ratio depending on the integers a, b. The ionospheric delay can be written as: a f 1∆ 1 +b f 2 ∆ Ion Ion2 ∆Iona ,b = a f 1 +b f 2• The ratio between the ionospheric delay in the linear combination and in L1 observations will be: f 1 [b f 1 + f 2 ] a ψ = f 2 [a f 1 + f 2 ] ion b Page 2-38
  • Linear CombinationsThe most common linear combinations are summarisedin the following table:Signal a b λ a,b (m) ψ ion• L1 1 0 0.190 1.00• L2 0 1 0.244 1.65• Wide-lane 1 -1 0.862 -1.28• Narrow-lane 1 1 0.107 1.28• Ionosphere-free 77 -60 0.006 0.00• low iono. effect 5 -4 0.101 -0.07• Very long wavelength -7 9 14.65 350.35 Page 2-39
  • GPS Basic Course Ionosphere TroposphereOrbit Thirdly: Modeling GPS Biases Page 2-3
  • Ionospere Delay: Atmospheric Corrections• The ionosphere is extending from Ionosphere about 50 to 1000 kilometres. Troposphere• the suns radiation ionises gas molecules which then lose an electron.• These free electrons influence the propagation of microwave signals.• The refractive index of Microwaves is a function of frequency f and the density of free electrons Ne• The sign will depend on whether the range (+) or the phase (–) refractive index is required. A.N e n =1 ±• the "phase velocity" is actually f2 increased, or "advanced", and the & c ranging codes is decreased (the so- v = called "group velocity") . n Page 2-3
  • Ionoospheric Delay IonosphereThe factors influence the Tropospheremagnitude of the TEC including:•the latitude of the receiver,•the season,•the time of day•the level of solar activity .MAGNITUDE:•Extreme at zenith 30m.•Extreme at horizon 3 times zenith value.•Extreme in day 5-10 times night value.•Use IONOSPHERE PREDICTION MODELS -- TECbroadcast model generally <50% accuracy, may d ion = 40.28 * 2be useful for point positioning users. f•Use DUAL-FREQUENCY receivers -- form"ionosphere-free" L1/L2 data combination Page 2-3
  • Tropospheric Delay• The tropoosphere is extending from Ionosphere the earth to 50 kilometres. Troposphere• It is a function of the satellite elevation angle and the altitude of the receiver, and is dependent on the atmospheric pressure, temperature, and water vapour pressure .• The tropospheric refractivity can be partitioned into the two components, one for the dry part of the atmosphere and the other for the wet part• About 90% of the magnitude of the d trop = d dry + d wet tropospheric delay arises from the dry component, and the remaining d tro = MFd d dtro (90  ( + MFw d w (90  ( tro 10% from the wet component.• There are several mapping functions Page 2-3
  • Tropospheric Delay• The magnitude of the tropospheric Ionosphere delay is the same for both L1 and L2 Troposphere observations, and for pseudo-range.• The tropospheric delay can be predicted using values of temperature, pressure, and humidity.• Such models can account for approximately 90% of the delay (corresponding mainly to the dry part), however the remaining 10% (largely due to the wet part)• Neglecting to apply tropospheric refraction results in an absolute scale error. (1m leads to 0.4 ppm scale effect) d trop = d dry + d wet• Any uncertainty in modelling the differential tropospheric refraction bias results mostly in a degradation of the height component in the solution.
  • Orbit & Sat. clock biases can be fixed by IGSThe total IGS tracking network by late 2000 (248 stations) http://igscb.jpl.nasa.gov/products/
  • Orbit & Sat. clock biases can be fixed by IGS False position True position•Predicted Orbit 0.5 m (Real-Time) &Satellite clock 150 nanosecond.•UltraRapid 0.25 m (Real-Time) &Satellite clock 5 nanosecond•Rapid 0.05 m (17 hours later) &Satellite clock 0.2 nanosecond•Final < 0.05 m (13 days) &Satellite clock 0.1 nanosecond http://igscb.jpl.nasa.gov/products/ Page 2-3
  • GPS Basic Course(Dilution of Precision (DOP • A description of purely geometrical contribution to the uncertainty in a position fix • It is an indicator as to the geometrical strength of the satellites being tracked at the time of measurement Good GDOP – GDOP (Geometrical) • Includes Lat, Lon, Height & Time – PDOP (Positional) • Includes Lat, Lon & Height – HDOP (Horizontal) • Includes Lat & Lon – VDOP (Vertical) • Includes Height only Page 2-40
  • GPS Basic Course(Dilution of Precision (DOP • A description of purely geometrical contribution to the uncertainty in a position fix • It is an indicator as to the geometrical strength of the satellites being tracked at the time of measurement Poor DOP – GDOP (Geometrical) • Includes Lat, Lon, Height & Time – PDOP (Positional) • Includes Lat, Lon & Height – HDOP (Horizontal) • Includes Lat & Lon – VDOP (Vertical) • Includes Height only Page 2-41
  • GPS Basic Course Different GPS Operation Types and Applications Page 3-1
  • GPS Basic CourseInitial Phase Ambiguity • Initial phase Ambiguity must be determined to use carrier phase data as distance measurements over time (Time (0 (Time (i Ambiguity Ambiguity Phase Measurement Counted Cycles Phase Measurement Page 2-22
  • GPS Basic CourseResolving the Ambiguity • The effect of resolving the ambiguity is shown below • Note that once the ambiguities are resolved, the accuracy of the measurement does not significantly improve with time (Accuracy (m 1.00 Ambiguities Not resolved 0.10 Ambiguities Resolved 0.01 (Time (mins 120 0 Static 5 2 0 Rapid Static Page 2-23
  • GPS Basic CourseUsing GPS for Surveying• All GPS Surveying is carried out using differential techniques. That is to say a baseline is measured from a fixed point, (a reference station) to a unknown point (a rover station).• This is undertaken using one of two methods• Post Processing – The raw GPS data from the satellites is recorded and processed in the office using software• Real Time – The processing of the data is carried out as you work giving an instantaneous and accurate position Page 3-2
  • GPS Basic Course(Static (STS• The classical method for long lines and the highest accuracy 5mm + 0.1ppm baseline r.m.s – Classical GPS baseline measurement, where each line is observed for at least one hour – The observation time is proportional to the length of the line – Standard method for lines over 20 Km• Applications – Geodetic control over large areas – National and continental networks – Monitoring tectonic movement – Network adjustments for highest accuracy Page 3-3
  • GPS Basic Course(Rapid Static (STS• Short observation time for baselines up to 20 km. Accuracy 5-10mm +0.1ppm• Applications – Control Surveys, GIS city inventories, detail surveys. Replace traversing and local triangulation. Any job where many points have to be surveyed Advantages – Easy, quick, efficient – Ideal for short range survey Page 3-4
  • GPS Basic Course(Kinematic (KIS• Stop Mode – The rover must first initialize Page 3-8
  • GPS Basic Course(Kinematic (KIS• Moving Mode – The rover must first initialize – Once enough data is collected to resolve the ambiguities the user can now move the receiver – Lock must be maintained on a minimum of 4 satellites at all times – Rover records data at a specific time interval – If lock is lost, the system must re-initialized 28 2 20 20 2 : 27: 18: 1 : 18: 2 : 24: 14: 2 26: 16: 30: 8: 22: 1 : 12: 1 : 14: 1 : 16: 12: 0 8 4 6 0 2 : :1 :1 10 10 10 1 10 10 10 1 10 10 1 10 10 1 10 10 10 10 10 0: 0 0 0: 0 0 0: 0: 0: 0 : : : 3 : 23 : : 23 :2 :2 :2 : : : : : : :2 : : 23 2 23 2 23 23 2 23 23 2 23 2 23 23 23 23 3 3 3 3 3 3 3 3 3 3 Page 3-9
  • GPS Basic Course(Ambiguity Resolution On The Fly (KOF• Moving Mode – This technique does not require a static initialization – While moving, once the rover is continuously tracking a minimum of 5 satellites on the L1 & L2 for a period of time the ambiguities can be resolved 12: 10 : 23 Page 3-10
  • GPS Basic Course(Ambiguity Resolution On The Fly (KOF• Moving Mode – This technique does not require a static initialization; – While moving, once the rover is continuously tracking a minimum of 5 satellites on the L1 & L2 for a period of time the ambiguities can be resolved. 1 : 14: 1 : 16: 1 : 12: 10 10 10 0: 0 0: : :2 : : 23 23 3 3 3 3 Page 3-11
  • GPS Basic Course(Ambiguity Resolution On The Fly (KOF• Moving Mode – If traveling under an obstruction and loss of lock occurs, 1 : 18: 1 : 14: 1 : 16: 1 : 12: 10 10 10 10 0 0: 0 0: : 23 : :2 : : 23 23 3 3 3 3 Page 3-12
  • GPS Basic Course(Ambiguity Resolution On The Fly (KOF• Moving Mode – If traveling under an obstruction and loss of lock occurs, – Ambiguity resolution will re-occur once 5 satellites on L1 & L2 are acquired and tracking is consistent for a short period of time 1 : 18: 24: 22: 1 : 14: 1 : 16: 1 : 12: 10 10 10 10 10 10 0 0: 0 0: : 23 : : : :2 : : 23 23 23 23 3 3 3 3 Page 3-13
  • GPS Basic Course(Ambiguity Resolution On The Fly (KOF• Moving Mode – If traveling under an obstruction and loss of lock occurs, – Ambiguity resolution will re-occur once 5 satellites on L1 & L2 are acquired and tracking is consistent for a short period of time – This technique allows positions to be determined up to the point that the min. satellites were re-acquired 20 27: 1 : 18: 24: 26: 22: 1 : 14: 1 : 16: 1 : 12: 0 :1 10 10 10 10 10 10 10 10 0 0: 0 0: 0 0: : 23 : 23 :2 : : : :2 : : 23 23 23 23 23 3 3 3 3 3 3 Page 3-14
  • GPS Basic Course(Ambiguity Resolution On The Fly (KOF• Moving Mode – If traveling under an obstruction and loss of lock occurs, – Ambiguity resolution will re-occur once 5 satellites on L1 & L2 are acquired and tracking is consistent for a short period of time – This technique allows positions to be determined up to the point that the min. satellites were re-acquired 28 2 : 27: 1 : 18: 2 : 24: 2 26: 30: 8: 22: 1 : 14: 1 : 16: 1 : 12: 0 2 : 10 10 10 10 1 10 1 10 1 10 10 10 10 0: 0 0 0: 0 0: : : 3 : 23 : 23 :2 :2 : : : : : : :2 : : 23 2 23 23 2 23 23 23 3 3 3 3 3 3 3 Page 3-15
  • GPS Basic Course Real-Time Differential GPS Concept
  • GPS Basic CourseConcept of Real Time• Real Time Code, Real Time Phase – No post processing required – Results are instantly available – Can operate in two modes • RTK • RT-DGPS B A Page 3-16
  • REAL-TIME Code DGPS REAL-TIME Code DGPS RoverReference Page 3-17
  • GPS Basic CourseDifferential Positioning • It is possible to determine the position of Rover ‘B’ in relation to Reference ‘A’ provided – The coordinates of the Reference Station (A) are known – Satellites are tracked simultaneously • Differential Positioning – eliminates errors in the Baseline Vector sat. and receiver clocks A B – minimizes atmospheric delays – Accuracy 0.5 cm - 5 m Page 3-18
  • DGPS in PhotogrammetryDifferential Positioning • If using the Code only part of the signal, accuracy’s in the range of 0.5 - 5 m can be achieved • This is typically referred to as DGPS r to ec eV el in Bas Page 3-19
  • REAL-TIME KINEMATIC “RTK” SURVEYINGDisadvantages: - Needs a radio modem (data link(; - Radio contact can be interrupted by obstructions such as hills, valleys buildings etc.Advantages: •Coordinates in real time in the field (WGS84 or local coordinates(; •Quality control - you know in the field that the ambiguities are resolved and that the results are correct; •No post processing; •One person system; •Several rovers can use one reference station; •All high precision applications (Land, Marine& Aviation(. Page 3-19
  • GPS Basic CourseDifferential Positioning • If using Phase or Code & Phase accuracy is in the order of 5 - 10 mm + 1ppm Baseline Vector A B Page 3-20
  • GPS Basic CourseDifferential Positioning • If using the Code and Phase part of the signal, accuracy’s in the order of 5 - 10 mm + 1ppm can be achieved r to ec eV el in Bas Page 2-3
  • GPS Basic CourseSummary of GPS Positioning • Point Positioning Methods using stand alone receivers provide 10 - 100 m accuracy – Dependent on SA – 1 Epoch solution • Differential Positioning Methods using 2 receivers, simultaneously tracking a minimum of 4 satellites (preferably 5) will yield 0.5 cm to 5 m accuracy with respect to a Reference Station • Remember – Differential Techniques using Code will give meter accuracy – Differential Techniques using Phase will give centimeter accuracy Page 3-21
  • Real Time GPSReal Time Phase (RTK)• The Action takes Initialization Moving Part Rapid Static Move continuously to determine trajectories, place at the Rover SR9400, SR399, SR9500 recording automatically at predetermined intervals. Unit. The System can be initialized. This normally takes about 1 min. SR9400,Point SR9500 Known SR399, Initializing on a known point takes Move quickly from point to point 15 sec.• Then the moving On the fly part can begin. SR399, SR9500 This is where points and assoc. information can be Positions to centimeter level accuracy recorded Page 2-3
  • Real Time GPSQuality Assurance • Blunder Detection – Base Station Coordinates – Heights of Instrumentation – Transformation (if used) – Vector determination • Vector Confirmation Page 2-3
  • Real Time GPSDilution of Precision (DOP) • Operating in RTK mode – Recommended to use a minimum of 5 Satellites and a GDOP of 8 or less. (Once integers are fixed) • Operating in Code only mode – Recommended to use a minimum of 5 satellites with a GDOP of 6 or less. Page 2-3
  • Real Time GPSUnderstanding the CQ • The coordinate Quality Indicator (CQ) is a 3-Dimensional estimator of the accuracy of a point derived in Real Time. • GPS results normally yield Horizontal accuracys 2-3 times better than the Vertical • Coordinate Quality indicator………. 0.03 m • Vertical accuracy……………………. 0.03 m • Horizontal accuracy………………… 0.01 m Page 2-3
  • Real Time GPSRecording Quality Control Data • Mode of Operation • Navigation (0), Differential Code (1), • Differential Float (3), Differential Phase (4) • GDOP, PDOP, HDOP, VDOP • Antenna Height • Number of satellites used in solution • Number of epochs on a point • Length of interval between epochs • Receiver Serial Type and Serial Number Page 2-3
  • Real Time GPSFeatures…..Stakeout • Orientation. – To North, to a point, last point, to a line Target Position •Stakeout a point by Azimuth and Distance Offset Present Position Distance Target Position ce •Stakeout a point in orthogonal Azimuth Di s tan mode (Distance and Offset) Present Position Page 2-3
  • Real Time GPS SurveyingFeatures…..Stakeout Line to be Staked • Stakeout Lines – Auto Increment and Defined Line Offset O ffs • Stakeout Hidden Points et – East/North/Height t en em n io cr at In • Minimum Key Strokes St Start Point Page 2-3
  • Real Time GPSFeatures…..Applications • COGO (Coordinate Geometry) – Inverse, Traverse, Intersections – Arcs, Station Offset Well Head Crossline Distance Source Line Inline Distance Page 2-3
  • Real Time GPSFeatures…..Auto Record • Continuous Recording – By Distance – By Time – Targeting available during continuous tracking 23 23 23 23 23 23 :1 :1 :1 :1 : :1 10 0: 0: 0: 0: 0: :1 2 20 18 16 14 22 Page 2-3
  • Real Time GPSFeatures…..Coordinate Conversion • Getting into Local Coordinate Z Systems • Choose from one of two methods • Classical – Compute it in the field – Download it from the appropriate software. Y X Page 2-3
  • Real Time GPSLimitations • GPS is not always the correct tool – Obstructions • Multipath • Loss of lock Page 2-3
  • GPS Applications (Photogrammetry)Use of GPS in Airborne Surveying • Ground Control Determination • Flight Management and Camera Control • Determination of Perspective Centers • Orientation of non-imaging sensors Page 3-22
  • GPS Applications (Photogrammetry)Ground Control Determination • Ground Control – Survey techniques identical to cadastral or other applications – High Accuracy – Faster than conventional techniques – No direct line of sight necessary – 3-D Ground Control Points – Ground units may be used as reference receivers for airborne operations Page 3-23
  • GPS Applications (Photogrammetry)Flight Navigation and Camera Control • Flight Navigation – Guidance from airport to project area – Optimum flight path between flight lines • Camera Control – Shutter release at predefined positions – Annotation of position on the image – Side lap control Page 3-24
  • GPS Applications (Photogrammetry)Positions at Camera Events Camera • GPS measurements are taken at regular intervals Event or epochs – for kinematic applications, usually 1 sec • The camera is triggered as required – Camera firing is not coincidental with GPS epoch – Controlled by operator or flight management GPS system Position • Camera positions are interpolated from GPS results – Correct time of camera event is critical – Must be accurate relative to GPS measurement time – Use the same time source for both GPS and trapping the camera event. This is an important role of the ‘event catcher’ Page 3-25
  • GPS Applications (Photogrammetry)Aircraft System Components Antenna Pilot Display ACU30 28v DC Operator Terminal NSF3 RC30 Page 3-26
  • GPS Applications (Photogrammetry)Data Processing Flow Airborne Transfer Reference Data and Data Airborne Data to PC Reference Process GPS data to generate epoch positions Data Compute camera positions from GPS epochs and actual times of camera events Camera Event Epoch Positions Positions Local AeroTransform to Local Coordinates TriangulationCoordinate System Page 3-27