Presented By: Scott Rodgers UNC Chapel Hill Engineering Information Services

1. Types of GPS Receivers Presented By: Scott Rodgers UNC Chapel Hill Engineering Information Services

2. Topics • Accuracy vs. Precision • Grades of GPS Receivers • Field Experiment • Sources of GPS Error • Planning a GPS Survey

3. Definitions • Precision: • Repeatability of a measurement • Accuracy: • The proximity to the TRUE value, or accepted value of a measurement

4. Example In general, GPS receivers are precise but not accurate.

5. Grades of GPS Receivers • Consumer • Garmin Street Pilot • Garmin eTrex • Mapping • Trimble Pro-XR/XRS • Trimble GeoXH/XT • Geodetic/Survey • Trimble 5800 • Trimble R7/R8

6. Consumer GPS Receivers • Track the L1 code frequency only • L1 Frequency: 1575.42MHz GPS carrier frequency which contains the C/A-Code, the encrypted P-Code (or Y-Code) and the Navigation Message. • 12 parallel channel GPS antenna • Capable of simultaneously tracking 12 satellites. • Track all visible satellites • Accuracy: • 15-30 meters

7. Mapping Grade GPS Receivers • Track the L1 code and carrier frequencies • Some models capable of tracking L2 carrier frequency • Some models now track GLONASS satellites simultaneously. • Built-in software to resist multi-path error • User definable PDOP and Elevation Masks.

8. Mapping Grade (cont.) • Differential GPS (DGPS) Capable • WAAS – Wide Area Augmentation System • CORS – Continuously Operating Reference Station • OmniStar or other Satellite based DGPS source • 12 parallel channel GPS antenna • Accuracy: • Real – time: sub-meter • Postprocessed: 1-50 cm

9. Geodetic GPS Receivers • Track the L1 carrier/code frequencies and the L2 carrier frequency. • 24 parallel channel GPS antenna • WAAS Support • Built-in software to resist multi-path error • Accuracy: • 5-30mm Horizontal/Vertical

10. Differential GPS (DGPS) • Definition: • an enhancement to the Global Positioning System that uses a network of fixed ground based reference stations to broadcast the difference between the positions indicated by the satellite systems and the known fixed positions.

11. Differential GPS Sources • WAAS – Wide Area Augmentation System • Developed by the FAA • Worst case accuracy is 7.6m at 95% • This type of system is considered a Satellite Based Augmentation System (SBAS)

12. Differential GPS Sources

13. Differential GPS Sources • Other SBAS sources: • StarFire – developed by John Deere engineers to aid in crop yield calculations • OmniStar – Mapping and surveying applications • EGNOS – European Geostationary Navigation Overlay Service

14. Differential GPS Sources • CORS – Continually Operating Reference Station • National network of GPS sites that collect and distribute GPS data • Sampling rates vary; 1, 5, 10, 15 and 30-seconds • Data available in ReceiverINdependent Exchange (RINEX) format • Some stations also broadcast a Real-time Differential correction signal

15. Comparison of different GPS receivers and processing methods • Trimble Pro-XR • Terra Sync set to “Production” • Terra Sync set to “Precision” • Real Time DGPS • Post Processed DGPS

16. ‘Production’ vs. ‘Precision’

17. Real-time vs. Post-Processed

18. All positions Post-Processed

19. GeoXH and GeoXT • GeoXH • H-Star Technology • GeoXT • Sub-Meter GPS • Both units: • Sub-Meter GPS • Post-Process Files • External DGPS Beacon • ArcMap or TerraSync • Microsoft Window Mobile

20. GeoXH and GeoXT Uncorrected Positions

21. GeoXH and GeoXT Corrected Positions

22. Comparison of different GPS receivers and processing methods • Trimble 5700 Geodetic Receiver • GPS Data Collected for 15 minutes • Comparison of ‘Raw’ and Post-processed positions

23. Geodetic vs. Mapping Grade

24. Conclusions?

25. GPS Error Sources • Satellite Geometry • Measures of satellite geometry are called Dilutions of Precision or DOP’s • GDOP –Overall Accuracy (3-D, Time) • PDOP – Positional Accuracy (3D) • HDOP – Positional Accuracy (2D) • VDOP – Vertical Accuracy (Height) • TDOP – Time accuracy

26. GPS Error Sources • Multi-path • The reflection of GPS signals off buildings, cars, trees, etc.

27. GPS Error Sources • Atmospheric Effects • Ionosphere • Electromagnetic particles slow and distort the GPS signal • Troposphere • Water vapor slows and distorts the GPS signal

28. GPS Error Sources • Clock Inaccuracies and Rounding Errors • Relativistic Effects • Errors in Satellite Orbits

29. GPS Error Sources • Summary: • Ionospheric/Tropospheric Effects: +/- 5.5m • Shifts in Satellite Orbits: +/- 2.5m • Clock Errors: +/- 2m • Multipath Effects: +/- 1m • Calculation and Rounding Errors: +/- 1m

30. Methods to Eliminate Error • DGPS – Real-time broadcast solutions • PDOP – Setting receivers so data cannot be collected when PDOP is greater than 6. (High PDOP means large position error.) • SNR – Signal to Noise Ratio; set to minimum of 4. • Elevation Mask – Setting receivers to track only satellites 15 degrees above the horizon.

31. Methods to Eliminate Error • Software and antennas designed to resist multi-path interference • Avoid using high-powered CB radios

32. Planning a GPS Survey • Tools • Trimble’s ‘QuickPlan’ Utility • Satellite Visibility • PDOP Chart • Site Investigation • Site Visit • Getting the lay of the land • Google Earth

33. Planning a GPS Survey15 Degree Elevation Mask

34. Planning a GPS Survey5 Degree Elevation Mask

35. Planning a GPS Survey15 Degree Elevation Mask

36. Planning a GPS Survey5 Degree Elevation Mask

37. Five Degrees or Fifteen degrees?

38. Questions?