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OBTAINING THE BEST PROJECT ACCURACIES- STATIC GNSS

FWS 2011 GNSS TRAINING CORBIN, VA APRIL 5-7, 2011. OBTAINING THE BEST PROJECT ACCURACIES- STATIC GNSS. Bill Henning Geodesist, PLS. CONSTRAINTS (OR NOT). B ≥ 4 H & V, KNOWN & TRUSTED POINTS? B LOCALIZATION RESIDUALS-OUTLIERS? B DO ANY PASSIVE MARKS NEED TO BE HELD?

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OBTAINING THE BEST PROJECT ACCURACIES- STATIC GNSS

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  1. FWS 2011 GNSS TRAINING CORBIN, VA APRIL 5-7, 2011 OBTAINING THE BEST PROJECT ACCURACIES- STATIC GNSS Bill Henning Geodesist, PLS.

  2. CONSTRAINTS (OR NOT) • B ≥ 4 H & V, KNOWN & TRUSTED POINTS? • B LOCALIZATION RESIDUALS-OUTLIERS? • B DO ANY PASSIVE MARKS NEED TO BE HELD? • RT BASE WITHIN CALIBRATION (QUALITY TIE TO NEAREST CALIBRATION POINT)? • B SAME OFFICE & FIELD CALIBRATION USED? FYI: GNSS CAN PROVIDE GOOD RELATIVE POSITIONS IN A PROJECT WHILE STILL NOT CHECKING TO KNOWNS IN AN ABSOLUTE SENSE TWO POINT CALIBRATION

  3. RT DERIVED ORTHO HEIGHTS - LOCALIZE OR NOT? • PASSIVE MARKS ARE A SNAP SHOT OF WHEN THEY WERE LEVELED OR DERIVED FROM GPS • IF YOU BUILD FROM A MONUMENTED BM AND THE DESIGN WAS DONE REFERENCED TO IT, IT IS “THE TRUTH”, UNLESS IN GROSS ERROR. • CONSTRAINING TO PASSIVE BMs IS A GOOD WAY TO NOT ONLY LOCK TO THE SURROUNDING PASSIVE MARKS, BUT ALSO TO EVALUATE HOW THE CONTROL FITS TOGETHER. • HOW GOOD IS THE NGS HYBRID GEOID MODEL IN YOUR AREA? (SIDE NOTE: GEOID 09 IS THE CURRENT MODEL USED BY OPUS)

  4. CALIBRATIONS/VERTICAL LOCALIZATIONS

  5. HOW FAR CAN I GO BEFORE MY LEVELED DIFFERENCES ARE DIFFERENT FROM MY ORTHOMETRIC HEIGHT DIFFERENCES?

  6. LVL_DH Program ROD & INSTRUMENT CORRECTIONS ORTHOMETRIC HEIGHT CORRECTION Program LVL_DH converts the published orthometric height difference between two NAVD 88 bench marks into a leveled height difference by removing the orthometric correction from the published relative height. This process requires the exact gravity values at the two bench marks used in the NAVD 88 adjustment. These gravity values are maintained by NGS in its data base.

  7. EXAMPLE

  8. LVL_DH OUT

  9. GNSS DERIVED HEIGHTS Summary of expected orthometric height precisions/accuracies- 95% ConfidenceREMEMBER REDUNDANCY AND A CHECK ON KNOWN POINTS CORS = 0.05 m OPUS-S = 0.05 m OPUS-RS = 0.05 m NGS 58/59 = 0.02 m local, 0.05 m to NSRS SINGLE BASE REAL TIME = 0.02 m ≤ 10 Km, remember GIGO RTN = 0.05- 0.08 m,

  10. ELLIPSOID, GEOID & ORTHO HEIGHTS H88 = h83 – N03 NAD 83 (HARN) USE GEOID O3 NAD 83 (CORS 96) USE GEOID 09 ITRF USE SCIENTIFIC GEOID (USGG)

  11. USING GNSS MANUFACTURER’S SOFTWARE- TGO

  12. Available “On-Line” at the NGS Web Site: www.ngs.noaa.gov SEARCH: “NGS 58”

  13. Network / Local Accuracy NSRS

  14. GPS ELLIPSOID HEIGHT HIERARCHY HARN/Control Stations (75 km) Primary Base (40 km) Secondary Base (15 km) Local Network Stations (7 to 10 km)

  15. DATA COLLECTION PARAMETERS • VDOP < 6 for 90% or longer of 30 minute session • Shorter session lengths stay < 6 always • Schedule travel during periods of higher VDOP • Session lengths for baselines ≤ 10 KM = 30 minutes & collect at 5 second data interval • Session lengths for baselines 10 – 15 KM = 1 hour & collect at 15 second data interval • Track satellites down to 10° elevation angle

  16. REDUNDANCY More measurements should be included than the minimum - needed to determine the origin and possibly the orientation and scale of the survey If extra measurements are included then a least squares adjustment will provide a check on the accuracy of control point coordinates and can also be used to identify bad observations. Redundant measurements taken with different satellites and satellite geometry provide a mitigation for multipath effects. Each local station must have at least two acceptable baselines to its closest neighbor

  17. Two Days/ Same Time 20.660 > 20.661 20.662 Difference = -0.2 cm “Truth” = 20.615 Difference = 4.6 cm Two Days/ Different Times 20.660 > 20.637 20.614 Difference = 4.6 cm “Truth” = 20.615 Difference = 2.3 cm

  18. BASELINE PROCESSING • “MULTI-STATION” PROCESSING MODE • DOUBLE DIFFERENCING (ELIMINATES SAT/RECEIVER CLOCK, HARDWARE BIASES, REDUCES NOISE PARAMETERS) • PRECISE EPHEMERIS (14 DAYS LATENCY) • 15° CUT OFF • FIX ALL INTEGERS FOR BASELINES LESS THAN 40 KM • USE A TROPO MODEL RATHER THAN FIELD MET DATA UNLESS PROVEN BETTER • USE RELATIVE TROPO SCALE PARAMETER FOR STATIONS OVER 15 KM AND FOR LARGE INTERSTATION RELIEF • BASELINE RMS ≤ 1.5 CM • REDUNDANT BASELINES DIFFER BY ≤ 2.0 CM IN ELLIPSOID HEIGHT

  19. ADJUSTMENT OF PRIMARY NETWORK STATIONS FROM CONTROL Horizontal Adjustment (Latitude, Longitude, Ellipsoid Heights) • Minimum Constrained [One fixed station] • Fix latitude, longitude and ellipsoid height at one station • Resolve all blunders and large residuals • Determine which Control and known Primary Base Station coordinates should be fixed • Constrained [All suitable stations fixed] • Fix latitude, longitude, and ellipsoid heights at Control and known Primary Base Stations • Make sure the constraints did not distort the project NOTE - Geoid model NOT applied at this time

  20. ADJUSTMENT OF LOCAL NETWORK STATIONS Horizontal Adjustment (Latitude, Longitude, Ellipsoid Heights) • Minimum Constrained [One fixed station] • Fix latitude, longitude and ellipsoid height at one station • Resolve all blunders and large residuals • Evaluate coordinates at Control and Primary Base Station • should not be greatly affected by Local Station baselines (similar to NAD 83 NSRS 2007 VS. CORS 96) • Constrained [All suitable stations fixed] • Fix latitude, longitude, and ellipsoid heights at Control and Primary Base Stations • Make sure the constraints did not distort the project NOTE - Geoid model NOT applied at this time

  21. Summary-Vector Processing Accomplished • Elevation Mask - 15 degrees • Ephemeris - Precise (typ. 14 days latency) • Tropospheric Correction Model • Iono Corrections - All baselines longer than 5 km. • Fix IntegersBaselines less than 5 km: L1 fixed solutionBaselines greater than 5 km: Iono free (L3) solution • Baselines must have RMS values ≤ 1.5 cm • Baselines must have difference in “up” ellipsoid height ≤ 2.0 cm

  22. Station pairs with large repeat base line differences also result in large residuals. NGS guidelines for estimating GPS-derived ellipsoid heights require user to re-observe these base lines.

  23. Table 1. -- Summary of Guidelines

  24. Table 1. -- Summary of Guidelines (continued)

  25. Guidelines for Establishing GPS-Derived Orthometric Heights (Standards: 2 cm and 5 cm) http://www.ngs.noaa.gov/ SEARCH: “NGS 59”

  26. A Guide for Establishing GPS-Derived Orthometric Heights(Standards: 2 cm and 5 cm) 3 BASIC RULES: • USE NOS-NGS 58 – GPS DERIVED ELLIPSOID HEIGHTS • USE PUBLISHED NAVD 88 CONTROL • USE CURRENT HYBRID GEOID MODEL

  27. ESTIMATING GPS-DERIVED ORTHOMETRIC HEIGHTS -FOUR BASIC CONTROL REQUIREMENTS 1-Occupy stations with known NAVD 88 orthometric heights (Stations should be evenly distributed throughout project) 2-Project areas less than 20 km on a side, surround project with NAVD 88 bench marks, i.e., minimum number of stations is four; one in each corner of project 3-Project areas greater than 20 km on a side, keep distances between GPS-occupied NAVD 88 bench marks to less than 20 km 4-Projects located in mountainous regions, occupy bench marks at base and summit of mountains, even if distance is less than 20 km

  28. COMBINED NETWORK VERTICAL ADJUSTMENT 3-D Vertical Adjustment (Orthometric Heights) • Minimum Constrained [One fixed station] • Fix latitude, longitude, and orthometric height at one station • Resolve all blunders and large residuals • Compare orthometric heights from adjustment with published NAVD 88 • Determine which NAVD 88 bench marks should be fixed • Constrained [All suitable orthometric heights fixed] • Fix latitude, longitude at one station • Fix orthometric heights at all suitable stations • Make sure the constraints did not distort the project

  29. Combined Network Horizontal Adjustment • Perform combined adjustment • Control and Primary Base network along with local network • Latitude, longitude, and ellipsoid height • Use GEOID model to obtain geoid heights • Make sure combined adjustment did not distort the project

  30. SUMMARY • Mistakes (blunders) and systematic errors must be removed before the adjustment • A least squares adjustment handles random errors and provides a single solution (Try to eliminate all systematic errors) • The Minimally Constrained adjustment checks the internal consistency of the network • The Constrained adjustment checks the existing control and references the network to the datum • The vertical adjustment estimates GPS-derived Orthometric heights- Approaching 3rd order leveling accuracies • OPUS with redundant observations can produce 5 cm orthometric heights in areas of high accuracy hybrid geoid coverage

  31. Identified as Height Mod survey station Elevation published to centimeters Orthometric height determined by GPS

  32. BIG PICTURE ISSUES IN RT POSITIONING • PASSIVE / ACTIVE – WHAT IS ‘TRUTH’? • GEOID + ELLIPSOID / LOCALIZE – QUALITY OF GEOID MODELS LOCALLY. ORTHOMETRIC HEIGHTS ON CORS? • GRID / GROUND – LOW DISTORTION PROJECTIONS- SHOULD NGS PLAY? • ACCURACY / PRECISION-IMPORTANCE OF METADATA • SINGLE SHOT / REDUNDANCY • RTK / RTN • NATIONAL DATUMS / LOCAL DATUMS / ADJUSTMENTS- DIFFERENT WAYS RTN GET THEIR COORDINATES-VARIOUS OPUS, OPUS-DB, CORS ADJUSTED, PASSIVE MARKS.VELOCITIES - NEW DATUMS, “4 -D” POSITIONS • GNSS / GPS

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