1. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu Detection of GPS station position errors due to instrumentals and environmental effects Thomas Herring, tah@mit.edu,tah@mit.edu Department of Earth Atmospheric and Planetary Sciences, MIT

2. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 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 06/04/08IGS ACWS TAH Errors2

3. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 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. 06/04/08IGS ACWS TAH Errors3

4. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 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?) 06/04/08IGS ACWS TAH Errors4

5. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 06/04/08IGS ACWS TAH Errors5 Site NDAP Site had shown annual signal in North for a long period of time In July 2006, antenna replaced.

6. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 06/04/08IGS ACWS TAH Errors6

7. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu NDAP North/MP1 06/04/08IGS ACWS TAH Errors7 The MP1 values before antenna switch are correlated with temperature. Old antenna had failed patch element.

8. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu LTUT (another example) 06/04/08IGS ACWS TAH Errors8 Antenna change

9. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 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 06/04/08IGS ACWS TAH Errors9

10. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 near Yellowstone 06/04/08IGS ACWS TAH Errors10 Site notes indicate that so much snow during visit that they could not get to monument

11. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 06/04/08IGS ACWS TAH Errors11

12. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 L1 SNR (no snow) 06/04/08IGS ACWS TAH Errors12

13. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 L1 SNR (snow) 06/04/08IGS ACWS TAH Errors13 Note: Low elevations not that effected by snow

14. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 L2 (no snow) 06/04/08IGS ACWS TAH Errors14

15. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 L2 SNR (snow) 06/04/08IGS ACWS TAH Errors15 As average: Use fit value at zenith

16. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 Height and SNR 06/04/08IGS ACWS TAH Errors16

17. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 Height/MP1 overlay 06/04/08IGS ACWS TAH Errors17 Generated online from interactive plots: http://pboweb.unavco.org/plotting/src/public/gps_qc_soh_page.php?station=P708 East effects here

18. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 East MP1 06/04/08IGS ACWS TAH Errors18 Jump in east detected in multipath RMS

19. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P708 Height/Phase RMS 06/04/08IGS ACWS TAH Errors19 Phase RMS also catches early event but others also.

20. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 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. 06/04/08IGS ACWS TAH Errors20

21. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu Other examples Cases generated using on-line tools at http://pboweb.unavco.org http://pboweb.unavco.org Most of these are probably snow events (cases based on site visit notes and height of site) 06/04/08IGS ACWS TAH Errors21

22. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P665 06/04/08IGS ACWS TAH Errors22

23. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu P665 North MP1 06/04/08IGS ACWS TAH Errors23

24. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu Alaska Volcano (AV04) 06/04/08IGS ACWS TAH Errors24

25. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu AV04 time series 06/04/08IGS ACWS TAH Errors25

26. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu AV04 Height and MP 06/04/08IGS ACWS TAH Errors26 MP1/2 indicates that most height signal due to snow cover Some interesting hysteresis after snow melt

27. Earth, Atmospheric and Planetary Sciences Massachusetts Institute of Technology 77 Massachusetts Avenue | Cambridge MA 02139 V 617.253.2127 F 617.253.8292 | eapsinfo@mit.edu 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). 06/04/08IGS ACWS TAH Errors27