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Dgps
- 1. 1 DGPS Concepts Learning Objectives After completing this lecture you will be able to: – Explain errors in point positioning – List accuracy expected from various GPS measurement techniques – Explain the process of Differential GPS – Describe how errors are minimised by using DGPS Lecture Outline • Introduction – context • Accuracy Issues • SA • Dilution of Precision (DOP) • Error sources • Satellite Visibility • Differential Correction – Post-processed – Real-time • Conclusion/Summary
- 2. 2 Introduction • 3 Levels of accuracy: – Point Positioning (Codes) – Differential Positioning (Codes) – GPS Surveying (also a differential technique but uses carrier phase) • Point positioning uses the code observable • Point positioning is subject to many error sources • Differential techniques (DGPS) can minimise these errors A Typical GPS Position •Many organizations use GPS these days •To query a position we may get the following: –Latitude - 41.342565555432134 –Longitude – 114.7841234532983 •This Is Accurate……. Right? Physical Data Collection Method •Lat - 41.342565555432134 •Long – 114.78412345329836 When we lose knowledge on how data was created, we lose its accuracy ‘context’ 16 decimal places ≠ high accuracy data
- 3. 3 Primary GPS Error Sources •Wave path errors (ionospheric and tropospheric) •Satellite orbit errors •Multipath •Satellite Geometry (High PDOP) •Satellite Constellation Changes Meters S/A (now turned off) Atmospheric Ephemeris Satellite Clocks 0 20 40 60 80 100 Point Positioning Errors • System-wide errors - DGPS Correctable Selective Availability (S/A) • Turned of May 02 2000 • Government may again degrade the accuracy • To prevent hostile forces from using GPS to full accuracy • By introducing intentional errors of timing signals and/or satellite ephemeris • Reduced using DGPS
- 4. 4 -200 -180 -160 -140 -120 -100 -80 -60 -40 -20 0 20 40 60 80 100 120 140 160 0 1 2 3 4 5 6 7 8 9 10 Time of Day (Hours UTC) InstantaneousError(meters) Horizontal Error (meters) Vertical Error (meters) 2 May 2000Colorado Springs, Colorado ANALYSIS NOTES - Data taken from Overlook PAN Monitor Station, equipped with Trimble SVeeSix Receiver - Single Frequency Civil Receiver - Four Satellite Position Solution at Surveyed Benchmark - Data presented is raw, no smoothing or editing SPS CEP AFTER TRANSITION: 2.8 meters SPS SEP AFTER TRANSITION: 4.6 meters May 2000 Satellite Orbit Errors •Satellite isn’t really where it says it is •2-3 meters of error X2 Y2 Z2 X Y Z Ephem eris (X Y Z) X Constellation Changes • Position P3 computed using ranges to satellites 1, 2, 3, 4, 5 and 6 • Position P4 computed using satellites 4, 5, 6, 7 and 8 P3 S1 S2 S3 S4 S5 S6 S7 S8 X XP1 P2 XP4
- 5. 5 Atmospheric Errors • Due to the ionosphere and troposphere, measured range to satellite is longer True Range to SatelliteMeasured Range Ionosphere Troposphere • Contributes around 4-5 meters of range error Atmospheric Delay • GPS signals are delayed as they pass through the atmosphere Ionosphere Baseline not too long Troposphere Base Rover Receiver/Location Errors DGPS won’t help: • Multipath – Use a ground plane on antenna – Move away from multipath source • Receiver Channel noise – Use a different GPS receiver • 2-D fix with a bad altitude – Use Manual 3D when collecting data • High PDOP – Wait for the geometry to change
- 6. 6 Multipath • GPS computes position by measuring ranges to satellites • Receiving a signal reflected off anything, will result in an incorrect range • This results in an incorrect position computation Dilution of Precision (DOP) • A measure of Satellite geometry • Indicates the quality of position fix • Can be expressed in different dimensions – for example: PDOP, HDOP, VDOP, TDOP • Generally not reduced by DGPS idealized situation 0.04 sec 0.06 sec Dilution of Precision (DOP) • Relative position of satellites can affect error
- 7. 7 Dilution of Precision (DOP) • Real situation - fuzzy circles Point representing position is really a box 0.04 ‘ish sec 0.06 ‘ish sec uncertainty uncertainty Area of uncertainty becomes larger as satellites get closer together Dilution of Precision (DOP) • Even worse at some angles Satellite Visibility and PDOP • Use an almanac from a GPS receiver to calculate best times of day for GPS.
- 8. 8 The Proximity Factor Activity - Quiz Which of the following errors do you think can be reduced using DGPS? • Multipath • Receiver Channel noise • High DOPs • Satellite clock errors • Atmospheric corrections • Poor ephemeris data Differential Correction • Base Station generates corrections for all satellites in view • Roving GPS receiver uses corrections to reduce errors • Differential correction can be performed in either real-time or post-processed mode
- 9. 9 BASE Time, t t + 1GPS Positions Absolute Reference Position Differential Correction (Simplified) • If you collect data at one location, there will be errors • Each of these errors are tagged with GPS time ROVER Time, t t + 1 ? Differential Correction (Cont.) • At the same time, the errors occurring at one location are occurring everywhere within the same vicinity Satellites Seen 1 2 3 4 5 6 7 8 Satellites Used 1 2 3 4 1 3 5 6 Any Combination of Base satellites ROVER Time, t t + 1 ? BASE Time, t t + 1 Differential Correction (Cont.)
- 10. 10 Base Receiver Known Coordinates Shift 10m South 30m West Block Shift Correction Remote Receiver Measured Coordinates Apply same Block Shift Measured Coordinates Final Coordinates Base Receiver Range Correction Range Correction Remote Receiver Range Correction Example of DGPS CorrectionsExample of DGPS Corrections Uncorrected Corrected
- 11. 11 DGPSDGPS –– BenefitsBenefits Red = Uncorrected GPS Green = GPS after differential correction Code GPS Errors and DGPS •Wave path errors (ionospheric and troposperic) – corrected by DGPS •Satellite orbit errors - corrected by DGPS •Multipath •Satellite Geometry (High PDOP) •Satellite Constellation Changes – corrected by DGPS DDDD DDDD ffff ffff f Activity - Quiz • List different accuracies you might expect from: – Point positioning using C/A code – Post-processed DGPS – Real-time DGPS • Explain two general processes used to correct positions and minimise errors using DGPS
- 12. 12 Base Station Site Requirements • Clear view to satellites • Known coordinates • Clear of transmitters (TV, radar) • Line of site to rover is not necessary Base Station: 15° Rover: 15° Poor Situation – Satellite visible to Rover only. Base Station is not tracking it. Possible Problem... • setting the elevation masks 1°per 100 km distance between base and rover Base Station: 10° Rover: 15° Ideal Situation – Satellite is being tracked by both Rover and Base Station. Solution... • Set base elevation mask less than rover
- 13. 13 Sources of Base Data for Post-Processed DGPS Where does one get differential GPS base station data? • Community Base Stations (CBS) – government, commercial or public • Internet Access • Set up your own – GPS Base Station – Virtual Reference Stations (VRS) – Rover units used as a base RTCM Corrections Reference Station at a known location Real-Time Differential GPS Sources of Real-Time Corrections Where does one get real-time differential GPS base station data? • AMSA or other Beacons • Satellite Services (OmniStar) • Set up your own real-time DGPS – Required Source of DGPS correction in RTCM-SC-104 format Data link, for example, a data radio
- 14. 14 EGNOS • European Geostationary Overlay System • EGNOS V1 (Advanced Operation) technically qualified in June 2005 • EGNOS measured performance is excellent (e.g. 1-3m HNSE; >99% APV availability) • EGNOS Evolution Roadmap covering 2005-10 being defined (EGNOS V2 and EGNOS V3 concept) – EGNOS Service extension, non geo- stationary broadcast, provision of new services, GPS modernisation, introduction of Galileo….. IOR-W EGNOS Geo-stationary satellite coverage Mean Horizontal AccuracyMean Horizontal AccuracyMean Horizontal AccuracyMean Horizontal Accuracy (95% of time)(95% of time)(95% of time)(95% of time) GPSGPSGPSGPS GPS & EGNOSGPS & EGNOSGPS & EGNOSGPS & EGNOS GPS & EGNOS + GALILEOGPS & EGNOS + GALILEOGPS & EGNOS + GALILEOGPS & EGNOS + GALILEO Note: single frequency user, error in meters.
- 15. 15 EGNOSWAAS MSAS EGNOS is an integral part of 3 inter-regional systems SBAS: A worldwide Initiative Today and … EGNOSWAAS MSAS MEDA EAST EUROPE & RUSSIA MIDAN CHINA SOUTH AMERICA BRASIL AFRICA APEC GAGAN … in the mid-term future Mexico Canada VRS Sydnet Vicpos Many Accuracies Can Be Achieved with GPS These depend on some variables: • Design of receiver • Relative positions of satellites • Time spent on measurements • Use of Differential techniques
- 16. 16 < 10 m SPS (C/A-Code) RMS or 1 sigma < 6 - 8 m PPS P(Y) Code GPS < 1- 5m Base Station Mapping Grade Receiver-DGPS Post-Processed is slightly better than real-time
- 17. 17 < 0.5 - 1mBase Station High Quality Mapping Receiver SPS-DGPS < 1 – 2 cm RTK Base Station Real-Time Kinematic Surveying Not DGPS <1/2 cm Base Station Survey Grade GPS Not DGPS Post Processed
- 18. 18 10m 2m 1m 6m 1/2cm 1cm Where Do You Need to Be? Real-Time or Postprocessed? Accuracy Requirements • Code Phase GPS – 40 m - Navigation (air, sea, land) – 10 m - Navigation to rural property corners – 2 m - Rural GIS data capture – 1 m - Municipality GIS data collection • Carrier Phase GPS – 1 cm - Survey stake out – ½ cm - Control Surveys Key Points on Accuracy •Knowledge of how data was collected impacts understanding of accuracy •Not all GPS errors are fixed through differential correction •If accuracy is critical, you must use a GPS system that includes Post-Processing
- 19. 19 Conclusion You can now: – Explain errors in point positioning – List accuracy expected from various GPS measurement techniques – Explain the process of Differential GPS – Describe how errors are minimised by using DGPS Self Study • Read relevant module in study books • Do self assessment quiz Review Questions
- 20. 20 Questions?