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Navigation Post-Processing for Ellipsoid-Referenced Surveys

Navigation Post-Processing for Ellipsoid-Referenced Surveys. Susan Sebastian Naval Oceanographic Office Navigation Department 28 April 2011.

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Navigation Post-Processing for Ellipsoid-Referenced Surveys

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  1. Navigation Post-Processing for Ellipsoid-Referenced Surveys Susan Sebastian Naval Oceanographic Office Navigation Department 28 April 2011 The inclusion of names of any specific commercial product, commodity or service in this presentation does not imply endorsement by the U. S. Navy or NAVOCEANO. 1

  2. What is ERS? • Alternative to conventional water level corrections (tide+draft+squat+etc…) • Vertical component of GPS (height from ellipsoid) for water level all inclusive of correctors. • Upgraded navigation equipment and software allows use of GPS heights from ellipsoid for water level corrections. • Termed Ellipsoidal Referenced Survey (ERS). Why ERS? • Reduces the dependency on shore-based tide gauges. • Reduces dependency on difficult to measure parameters, such as dynamic draft, settlement, squat. • Provides a more seamless vertical datum than conventional tidal zoning.

  3. ERS Components • GPS measurement of the vertical height of the survey vessel relative to the Ellipsoid • Two levels of GPS accuracy: • NavComRTG data available real-time; meets Order 1 minimally • PPP (precise point positioning) can be generated after 24 hours through post-processing; will be required once process is in place. Requires re-merge of nav data prior to applying GPS water level correctors. • SEP (or separation) – distance from the Ellipsoid to chart datum • Water Level Correction – SEP minus GPS height @MRP

  4. Reference Ellipsoid h SEP Water Surface Za Heave Datum MRP D heave T Zt RS Chart Datum CS Bottom ERS Corrected Sounding Calculation • h: Antenna height above ellipsoid • RS: Raw sounding corrected for heave, • re: master reference point (MRP) • Za: Antenna Z offset on boat • Zt: Transducer Z offset on boat • T: Tide • SEP: Chart datum to Ellipsoid separation • CS: Corrected sounding, re: chart datum • D: Draft (static, loading, and S&S) Conventional Water Level Reduction: CS = RS + D – T ERS Water Level Reduction: CS = RS + SEP – h + Za From: RTK Tide Basics – Hydro International, December 2003, Pat Sanders

  5. Data Types Needed for ERS • Raw GPS observables from a dual-frequency GPS receiver • NAVCOM raw GPS data @ 10Hz (primary GPS data source) • POSMV raw GPS data @5 Hz (backup GPS data source) • AND • c) Heave at 100Hz • GPS buoy and/or GPS observations at tidal benchmark

  6. GPS Observations at Tidal Benchmark Provides SEP: Ellipsoid to Chart Datum Separation Ellipsoid X Ell. HT Geodetic BM Y Elev. BM to Gauge Tide Gauge SEP Z Chart Datum SEP = X+Y+Z X obtained from 24-hour GPS observation over BM Y obtained by level line Z obtained from 30 days of tide data

  7. Data Collection Recommendations • Verify the NAVCOM output settings using a laptop and the Starfire utility program at the beginning and end of the survey. • Verify the POSMV output settings (frequency and telegram groups) at the beginning, middle, and end of the survey, using the POSMV controller program.

  8. Comparing GSP Tide vs. Tides To compare GPS WL correctors to conventional tide correctors, subtract WLZ from the GPS tide Example: |GPS Tide Corr.| - WLZ = |Tide Corr.| |-1.05| - 0.82 = |-0.23|, ~ |-0.22|

  9. PR Tides vs. GPSz Correctors A B A Bathy X-section, GPSz corrected B Residual Diff.

  10. Tide vs. GPSz Excellent vertical alignment desired for area- based editing. Predicted Tide Corrected GPSz Corrected

  11. Tide vs. GPSz Excellent vertical alignment desired for area- based editing. 11

  12. Area-Based Editing Tide vs. GPSz Predicted Tides GPSz, Smoother surface for automated processing.

  13. DATA PROCESSING FOR ERS Note: Nomenclature for GPS water level corrections varies in the applications and documentation. GPS water level corrections = GPS tides = GPSz Apply delayed heave Apply GPS water level corrections Review GPS water level correctors and ellipsoid heights in GSF file Process and merge nav as needed

  14. Navigation Processing Field processing navigation: Simple if no problems, complexity increases as degradations move upstream. Final dataset, apply all final PPP. Simple: RTG solution good Complex; need PPP

  15. Apply GPS Tide correctors w/EGM2008 gravitational model Need 2 files: 1. EGM2008 1-minute grid AND 2. Uncertainty for that surface RTTIDE DTC EGM Model and uncertainty

  16. Inspect GPS Tides and Ellipsoid Heights with Profile Viewer

  17. Use View Settings Menu to Configure Display Note Ant. Height offset NAVCOM POSMV GSF Example: GSF E-HT bad, POSMV and NAVCOM OK

  18. Exammb Using the Cursor Information Menu on TSView, the time is shown to be around 08:18. This proves to be true as can be seen in the image below of exammb, showing that at 08:18:05, the ellipsoid heights jump from 28.68m to 3.49m.

  19. Processing Strategy A Note: GPS Tides must always be re-applied after correcting the ellipsoid heights Increasing Complexity B C D E

  20. Profile Interpretation A: GPS tides and ellipsoid heights in GSF file are good. No action needed. B: GPS tide data is corrupted, and ellipsoid heights in GSF file are good. After re-applying GPS Tides, the ellipsoid heights were corrected. Example A: All good.

  21. Example B: UNCORRECTED Example B: CORRECTED by re-applying GPS Tides

  22. Profile Interpretation C: GPS tide data AND ellipsoid heights in GSF file corrupted To correct this if POSMV ellipsoid heights OK, merge this data with the GSF file in SABER.

  23. Profile Interpretation D: GPS tide data AND ellipsoid heights in GSF file AND POSMV ellipsoid heights corrupted GSF POSMV To correct this if NAVCOM ellipsoid heights OK, merge this data with the GSF file in SABER. Remember NAVCOM needs the vessel file.

  24. Profile Interpretation E: GPS tide data AND ellipsoid heights in GSF file AND POSMV ellipsoid heights AND NAVCOM ellipsoid heights corrupted NAVCOM Data To correct this, post-process NAVCOM with GraphNav and merge PPP with the GSF file in SABER. Remember NAVCOM needs the vessel file.

  25. Profile Interpretation • Example E corrected by merge with PPP

  26. MERGE NAV INTO MULTIBEAM • NOTE: • POSMV is lever-arm corrected; no vessel file needed • NAVCOM is uncorrected; need a vessel file

  27. MERGENAV • Process->Navigation->Merge Navigation. • Select the GSF file or a list of files. • Select the corresponding POSMV, NAVCOM, or PPP. • NAVCOM data: select the vessel file. • POSMV data: do not apply vessel offsets.

  28. Navigation Processing Log • Fill out the Navigation Processing Log, a worksheet of the GSF Processing Log, Version 3.1. • Use the orange section: RTG • Blue Section: PPP

  29. Navigation Processing Log Worksheet of the GSF Processing Log, Version 3.1

  30. Accuracy Requirements • IHO Order 1 allowable vertical error from the tide component is 30cm or better at 2 SD • Data analysis to date has demonstrated that GPS PPP solution heights and a well defined SEP value give accuracies of 20 -25 cm or better • GPS RTG solution heights accuracies with a well defined SEP are generally 30-35cm accuracy; RTG solution heights are an acceptable field solution until automated procedures for field processing of PPP data can be developed • A well defined SEP requires the use of a GPS buoy or GPS observations over a tidal benchmark

  31. More on SEP Methods to define SEP: 1) GPS observations over a tidal benchmark 2) GPS Buoy 3) EGM 2008 Model – 1min x 1min model of geoid height relative to the WGS84 Ellipsoid; use of EGM 2008 will not meet IHO Order 1 accuracy requirements in shallow water Survey areas with large geoid undulations with respect to the Ellipsoid will require multiple SEP measurements and might require conventional tide data if IHO Order 1 is required

  32. Example Area of Large Geoid Undulations Change in geoid-ellipse height of 2m across test area ERS data accuracy (using EGM 2008) was 50 cm as compared to tide corrected data

  33. RTG v.s. PPP Uncertainty

  34. Precise Point Positioning SOP Resource intensive; SOP executed in-house. More resources needed to accomplish in the field.

  35. Nagging Offset in 2 Receivers “intuition suggests that there is a systematic offset that weshould be able to correct, we need to keep the keep in mind that theobserved difference between the two height solutions is well within theexpected overall uncertainty value”Shannon Byrne, SAIC

  36. Summary Each ship SURVOP is collecting data needed for ERS, even if we’re not completely there yet. For ERS water level corrections, Navigation data must be inspected and sometimes processed. The issue in the field is to get the best vertical alignment for area-based editing purposes, which is GPS Tides in most cases. GPS Tides does increase the vertical alignment of the data so for building PFMs, it is advantageous, regardless of depth. Need dedicated Nav Lead to accomplish PPP in field and the last few days will always be done in-house.

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