1. Making good from bad: Can we use GPS multipath to measure soil moisture? Kristine M. Larson Department of Aerospace Engineering Sciences University of Colorado

2. 2 Outline  GPS basics  Multipath research at CU  If we can’t model multipath, can we extract soil moisture estimates from it?  Preliminary results

3. 3 GPS Basics  Ranges (  from 4 or more satellites are measured.  The position of the GPS antenna and time of the GPS receiver’s clock are estimated.

4. 4 Challenges  We don’t control the GPS satellites.  We can’t directly calibrate the satellites.  We don’t design the receivers we use.  Scientific market share for GPS is <<<< commercial market share.

5. 5 Opportunities  New constellation in Europe (Galileo)  Expanded Russian constellation (Glonass)  Expansion of ground networks and orbiting GPS receivers.

6. 6  Multipath introduces a (frequency dependent) range error  Antenna gain pattern, RHCP/LHCP  for a simple horizontal reflector  multi =2h*(sin  Impacts phase, pseudorange and SNR How does multipath impact gps?

7. 7 Assume reflectance of 10%, L1 frequency

8. 8 Multipath error frequency (for a given receiver/satellite

9. 9 Salar de Uyuni, Boliva  Large salt flat  Sub-decimeter topography; used to calibrate ICESat  Experimental setup:  3 Ashtech Z-12 receivers  2 flush with ground  1 tripod (~1.4m height) Bilich, 2006

10. 10 FIT SNR data Primarily low elevation data

11. 11Results

12. 12 Multipath Assessment: Power Spectral Maps  Idea: frequency and power content of SNR  multipath environment  Method:  Power spectra of small data sections  Assign to satellite azimuth/elevation  Plot all points on a grid

13. 13 MKEA Power Maps 10-30s 30-60s 60-90s 90-130s Bilich, 2006

14. 14 Mauna Kea (MKEA)

15. 15 Conclusions I  Multipath frequencies are controlled by obstructions near the antenna.  The ground is the major source of multipath.  Far reflectors are also important, particularly for high-rate applications.  Low elevation data (< 30 degrees) are most corrupted by multipath.  If SNR data accurately map multipath geometries, can they be used to infer multipath reflectivity as well?

16. 16 GPS as a soil moisture instrument  Ratio of reflected to direct GPS signal is proportional to reflectivity.  Reflectivity in turn is proportional to the dielectric constant of the surface.  L-band sensitive to ~ few cm depth.  Wet ground reflects GPS signals better than dry ground. Is it measurable?

17. 17 Why measure Soil moisture?  Important measurement for climate system studies.  Needed for improving models and ground truth for remote sensing studies.  Current instrumentation is sparse (and data are costly to collect). SCAN

18. 18 Past work on GPS reflections Masters, 2004 Reflections from specular locations

19. 19 Limitations  Specialized equipment  Antennas looking down!

20. 20 day of year 160 210260 What does soil moisture look like?

21. 21 Southern California

22. 22 GPS SNR data issues  Negatives  Quality questionable (digitation, L1-L2)  Not much of a published literature.  Positives  No cycle slips  No dependence on orbit quality  Independent for each satellite  Differences with previous reflection work  Use existing GPS instrumentation  No installation costs - but weaker signal.

23. 23 TASH (Tashkent)

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25. 25 What do the SNR data look like?

26. 26 Separate the Arcs

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30. 30 Issues to resolve  Direct signal power  Temperature  Orbital (ground-track)  Vegetation  Comparisons with in situ sensors

31. 31 Known Temperature effects on pseudorange

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33. 33 http://dragon.larc.nasa.gov/ Katzberg, Torres et al. Vegetation

34. 34 Temperature Effects on Dielectric http://www.lsbu.ac.uk/water/microwave.html

35. 35 Soil moisture model  Ethan Gutmann and Eric Small.  Model description: Chen and Dudhia [2001]  Modification of land surface hydrology model MM5 developed by PSU and NCAR.  Make assumptions as to soil type (sand).  We have no in situ radiation data.

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38. 38 Broader Impacts

39. 39 Conclusions  GPS multipath clearly tracks rain events via multipath amplitude changes.  At this time, we cannot explain all the seasonal signal.  We need to work on temperature, power, and vegetation effects.  The “unsolved” GPS error source, multipath, may provide useful information about soil moisture.

40. 40 Acknowledgements  A. Bilich, D. Agnew, P. Axelrad,  IGS, JPL, UNAVCO, NGS, CORS, SOPAC, CDDIS  NASA, CU faculty fellowship.

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