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Lewis Lapine South Carolina Geodetic Survey

South Carolina Virtual Reference Station Network - Centimeter Positional Accuracy in Real Time for South Carolina. DMT ‘07 May 22, 2007. Lewis Lapine South Carolina Geodetic Survey. Technology. Miniaturization of Components. The Global Positioning System. Three Segments.

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Lewis Lapine South Carolina Geodetic Survey

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  1. South Carolina Virtual Reference Station Network - Centimeter Positional Accuracy in Real Time for South Carolina DMT ‘07 May 22, 2007 Lewis Lapine South Carolina Geodetic Survey

  2. Technology

  3. Miniaturization of Components

  4. The Global Positioning System Three Segments 2.) Space Segment Control Segment Monitor Stations Diego Garcia Ascension Is. Kwajalein Hawaii 3.) User Segment 1.) Control Segment Master Control Station Colorado Springs

  5. Space Segment • 29 Satellites • 6 orbital planes inclined 55 degrees to equator • 4 or 5 SVs in each plane • 20,200 Km high orbits (12,550 Miles) • Orbit twice daily (11 hr 58 min)

  6. Control Segment Schriever AFB

  7. Typical Future Geologist At WorkUser Segment

  8. One measurement narrows down our position to the surface of a sphere 4 unknowns : Latitude Longitude Height Time Need 4 equations 12,000 miles We are somewhere on the surface of this sphere. Calculate your position

  9. Intersection of two spheres is a circle Calculate your position, cont’d Second measurement narrows it down to intersection of two spheres

  10. Third measurement narrows to just two points Intersection of three spheres is only two points Calculate your position, cont’d In practice 3 measurements are enough to determine a position. We can usually discard one point because it will be a ridiculous answer, either out in space or moving at high speed.

  11. Calculate your position, cont’d Fourth measurement will decide between the two points Fourth measurement will only go through one of the two points The fourth measurement allows us to solve for the receiver clock bias.

  12. Dl Integer Ambiguity Resolution Yields Survey Quality Baselines Dl = First Partial Wavelength N N = Integer Ambiguity D Distance D = Nl + Dl

  13. 12 9 3 6 Initialization From The Phase Measurements Dl Continuous Carrier Phase Measurement

  14. Good Satellite Geometry - PDOP

  15. Multipath

  16. GPS Signals • Variable signal paths and piercing points

  17. GPS Post Processed or RTK Uses Baselines From A Known Position Determined 33o 23’ 30.195065” N 079o 02’ 33.948394” W -12.445 m Known 33o 23’ 28.607434” N 079o 02’ 41.161474” W -12.637 m

  18. Real Time Kinematic - RTK

  19. Ambiguity Resolution for RTK l

  20. Limitations of Classical RTK Survey • Limited range from single reference station • Potential gross error in establishing reference station • No integrity monitoring • Dependency on single reference station • Productivity loss • Security • Communications • Power supply

  21. RTK PPM Error vs. Baseline Length

  22. What is a CORS?

  23. Reference Stations…defined • Located at a precisely known position • Records GPS data for later use • Post Processing • Generates GPS corrections for immediate use • Real - time, broadcast or dial - in • Results degrade with distance from Reference Station • Useable range can be from 10 km to 500 km (RTK vs DGPS)

  24. Components of a CORS site Monumentation Hardware Communications Photo Courtesy UNAVCO Software

  25. Progress?

  26. Virtual Reference Stations Infrastructure - The future for your industry

  27. Component Monitoring: Server • Server: Central Data Management

  28. RTKNet VRS-Data Flow Reference Station data streams back to server through LAN or Internet

  29. RTKNet VRS-Data Flow Roving receiver sends an NMEA string back to server using cellular modem - VRS position is established VRS NMEA - GGA

  30. RTKNet VRS-Data Flow Server uses VRS position to create corrected observables and broadcasts to rover VRS RTCM or CMR+

  31. RTKNet VRS-Data Flow Rover surveys as in normal RTK survey but getting data as if coming from the VRS VRS

  32. VRS-How does it work? • Network Processes • Integrity monitoring • Geometric correction • Correction for tropospheric errors • Correction for ionospheric errors • Ambiguity resolution • Consistency check • Delivery of VRS corrections for each mobile‘s location

  33. Network Multiple Base Station Solution Let B1, B2 B3 be base stations, R be the rover B1 B2 R B3 The indexes r= 1,2,3 are for bases, r=0 stands for the rover • Four RTK engines run simultaneously: • (1,2) • (2,3) • (3,1) • (1,0)+(2.0)+(3,0)

  34. Field Test Results Probability of Certainty Time (secs)

  35. Ionosphere Modeling gradients of the ionosphere Changes of satellite geometry lead to changes in mapping functionChanges of piercing coordinates and dynamical changes in ionosphere lead to time dependence of expansion parameters Gradients Reference Stations

  36. Modeling gradients of the ionosphere Changes of satellite geometry lead to changes in mapping functionChanges of piercing coordinates and dynamical changes in ionosphere lead to time dependence of expansion parameters Gradients Ionosphere Reference Stations

  37. Observation, measurement measurements from N stations Statevector N ambiguitiesN stations N multipathsN stations 3 parameters 1 ionosphere Designmatrix mapping function relative distance to reference piercepoint Kalman filter – part 1: observation equation

  38. multipath (centimeters) ambiguity (meters) ionosphere parameters 5 2.5 6.0 1.0 4 2.0 5.8 3 0.5 1.5 5.6 2 1 0 1.0 5.4 0.0 0.5 5.2 -1 -2 0 5.0 -0.5 -3 Japan, day 1Japan, day 2 (shifted by 4 minutes) -0.5 4.8 -4 -1.0 15:00 6000 uncorrelated between stations 8000 10000 12000 14000 16000 activity of ionosphere about to decline 12:30 15:00 12:30 15:00 12:30 correlated between days differences between ambiguities are constant Use double differences together with code measurement and tropo model to resolve the ambiguity (FAMCAR) Components of the state vector

  39. Advantages of VRS • Eliminates dependency on single reference station • Graceful degradation should a R.S. fail • Uses established communications • Establishes a single coordinate system

  40. The South Carolina Virtual Reference StationNetwork

  41. SC Prototype VRS Network ~100km ~120km

  42. Test Results from the SC Original VRS Prototype Network 0.150 ft = ~5 cm

  43. Scatter Gram of DifferencesWith Logical Rejection of 5 Points2 – Five Minute Sessions RMSE(2-D) = 0.020m @95% 0.05 ft = 1.5 cm Axis Units in Meters

  44. Computation of Horizontal Positional Accuracy RMSE (2-D) @95% Local Network Accuracy (NGS) = 0.020m (equivalent relative accuracy = 1:400,000) Centering Error = 0.003m Contract Specification for VRS = 0.012m Combined RMSE = (0.0202 + 0.0032 + 0.0122)1/2 = 0.024m

  45. Scatter Gram of All Differences2 – One Minute Sessions RMSE(2-D) = 0.024m @95% Axis Units in Meters

  46. RS Data Data for Users Data for Users Data for Users Data for Users Quality Control RS Data USERS BAD! Quality Control

  47. SC VRS Network Construction

  48. Questions?

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