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Detection of GPS station position errors due to instrumentals and environmental effects

Detection of GPS station position errors due to instrumentals and environmental effects. Thomas Herring, tah@mit.edu , Department of Earth Atmospheric and Planetary Sciences, MIT. Overview.

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Detection of GPS station position errors due to instrumentals and environmental effects

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  1. Detection of GPS station position errors due to instrumentals and environmental effects Thomas Herring, tah@mit.edu, Department of Earth Atmospheric and Planetary Sciences, MIT

  2. Overview • Examine origin and diagnostics of systematic deviations in GPS time series due to instrumental and environmental effects (not monument stability) • Cases: • Antenna failures • Environmental effects: Snow but also growing vegetation. • Diagnostics: • TEQC MP1 and MP2: Estimates of pseudorange multipath. • Signal to noise ratio • Phase residual RMS Scatter IGS ACWS TAH Errors

  3. Diagnostics • Pseudorange multipath at L1 and L2: MP1 and MP2 • Difference between L1 and L2 pseudorange computed from L1 and L2 phase measurements • Values generated by TEQC using an algorithm that accounts for cycle slips and offset between phase and range. • Values shown are an RMS over the duration of the rinex file (24-hours) • Phase RMS scatter • RMS scatter of the ionospheric free phase residuals from post-processing. • Signal-to-noise ratio • Commonly available now in rinex files although quality is variable • Here we use an “average” value for the day • MP1/2 and SNR can be used before processing data. IGS ACWS TAH Errors

  4. Multipath diagnostic • Example of antenna failure: • Site NDAP (Nucleus site) • Site had strong annual signal in north but not in height (rather strange if loading effect) • Effect of antenna change on position and multipath values • Example a LTUT (no MP1 in this case: Should be available but is not?) IGS ACWS TAH Errors

  5. Site NDAP Site had shown annual signal in North for a long period of time In July 2006, antenna replaced. IGS ACWS TAH Errors

  6. IGS ACWS TAH Errors

  7. NDAP North/MP1 The MP1 values before antenna switch are correlated with temperature. Old antenna had failed patch element. IGS ACWS TAH Errors

  8. LTUT (another example) Antenna change IGS ACWS TAH Errors

  9. SNR analysis • Many RINEX files now contain S1 and S2 and the units are now better defined • Analysis here: • Fit elevation angle dependent model to SNR at L1 and L2 • Extract the fitted values at zenith and 20 degrees elevation • Example on following slides IGS ACWS TAH Errors

  10. P708 near Yellowstone Site notes indicate that so much snow during visit that they could not get to monument IGS ACWS TAH Errors

  11. IGS ACWS TAH Errors

  12. P708 L1 SNR (no snow) IGS ACWS TAH Errors

  13. Note: Low elevations not that effected by snow P708 L1 SNR (snow) IGS ACWS TAH Errors

  14. P708 L2 (no snow) IGS ACWS TAH Errors

  15. As average: Use fit value at zenith P708 L2 SNR (snow) IGS ACWS TAH Errors

  16. P708 Height and SNR IGS ACWS TAH Errors

  17. P708 Height/MP1 overlay Generated online from interactive plots:http://pboweb.unavco.org/plotting/src/public/gps_qc_soh_page.php?station=P708 East effects here IGS ACWS TAH Errors

  18. P708 East MP1 Jump in east detected in multipath RMS IGS ACWS TAH Errors

  19. Phase RMS also catches early event but others also. P708 Height/Phase RMS IGS ACWS TAH Errors

  20. Summary for snow events • Both SNR and MP1/2 work well as indicators of snow • Low elevation angle SNR may indicate horizontal position error (not tested) • Phase noise RMS has problems in that atmospheric “noise” that might not effect position very much can increase noise • SNR and MP1/2 can be computed before data analysis as a quality check. IGS ACWS TAH Errors

  21. Other examples • Cases generated using on-line tools at http://pboweb.unavco.org • Most of these are probably snow events (cases based on site visit notes and height of site) IGS ACWS TAH Errors

  22. P665 IGS ACWS TAH Errors

  23. P665 North MP1 IGS ACWS TAH Errors

  24. Alaska Volcano (AV04) IGS ACWS TAH Errors

  25. AV04 time series IGS ACWS TAH Errors

  26. AV04 Height and MP MP1/2 indicates that most height signal due to snow coverSome interesting hysteresis after snow melt IGS ACWS TAH Errors

  27. Summary • Assessment of data quality through use of pseudorange multipath measures and SNR analysis looks promising. (Available for PBO sites at UNAVCO: current UNAVCO summary files do not have enough significant digits.) • Detection of antenna problems and snow effects possible. Vegetation effects also probably can be detected. • Phase RMS scatter is probably not a good detector in that it can be disturbed by effects that don’t greatly change the station positions. • Not discussed but needed: Detection of sites with corrupt data and how these should be treated in archives. Important in re-processing so that station numbers are not wasted in bad sites (i.e., most ACs use fixed number of stations). IGS ACWS TAH Errors

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