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GPS Multipath and its Relation to Near-Surface Reflectivity

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Natasha WhitneyHarvard University

National Geodetic Survey/NOSMentor: Andria Bilich

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Forecast

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Can we use reflected GPS satellite signals (multipath errors) to detect nearby precipitation events and soil

moisture content?

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Outline

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• Background

• Methodology

• Results

• Further work

• Summary

• Acknowledgement

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Background

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• Signal to noise ratio (SNR)

• Ground reflectivity & soil moisture content

• Geometric optics: (i) Specular reflection(ii) Ray tracing

• d=22hcosθdθ dt dt

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Methodology

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Large metal experiment:Simplify various parameters

• Effective penetration depth (d)

• Polarization (all Left Hand Circular)

• Reflect signal amplitude attenuation

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Methodology

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• Signal Amplitude (A)Antenna gain• Effective reflector height (h)Phase center offset• Governing equationSNRMP =AMcos(+o)

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Results

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Observations:-non-sinusoidal @ low -noisy @ midrange -suppressed at high Relevant range of ?

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Results

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Fast Fourier Transform (FFT):Eff. reflector height? Max amp?

Wavelet Analysis:Phase center offset changes?

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Results

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Initial modeling attempts Role of surface roughness

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Results

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0 2 4 6 8 10-42

-40

-38

-36

-34

-32

-30

-28

Elevation Angle, deg.

Po

we

r, d

B

U10

= 1 m/s

U10

= 5 m/s

U10

= 10 m/s

Sensitivity of interference fringes to sea surface roughness for L2 signal

Second modeling attempt

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Results

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Predicted Antenna GainExperimental results: Effect of 180 rotation

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Results

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Antenna tilted 90 Effect of tilt on raw SNR signal

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Further Work

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1. Reduce edge effects: reduce discontinuity of induced electric field of conducting plane

2. Continue tomographical scan: use azimuthal, boresite, and tilt rotations to assess amplitude, phase center, and effective reflector height

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Summary

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• Hypothesis v. Results: How useful was the large metal experiment?

• Calibration potential:Which parameters can provide real, physical information?

• Bigger picture:In-situ soil moisture network

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Acknowledgements

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• Thank you NGS and ESRL team at Skaggs:Valery Zavorotny Felipe Nadinsky [TKTK]Kristine LarsonAndria BilichKyle [TKTK] • Resources used: VALERY PRES.1.K. M. Larson, E. E Small, E. Gutmann, A. Bilich, J. Braun, and V. Zavorotny, “Use of GPS receivers as a soil moisture network for water cycle studies,” Geophys. Res. Lett. vol.. 35, no. DOI:10.1029/2008GL036013, p. L24405, 2008.2.K. M. Larson, J. J. Braun, E. E. Small, V. U. Zavorotny, E. Gutmann, and A. L. Bilich, “GPS multipath and its relation to near surface soil moisture content ﾓﾊ IEEE J. Sel. Topics Appl. Earth Obs. Remote Sens.” vol. 3 no. 1, Mar. 2010.3.V. U. Zavorotny, K. M. Larson, J. J. Braun, E. E. Small, E. D. Gutmann, and A. L. Bilich, “A Physical model for GPS Multipath Caused by Land Reflections: Toward Bare Soil Moisture Retrieval”, IEEE ﾊ J. Sel. Topics Appl. Earth Obs. Remote Sens.Vol. 3, no. 1, Mar. 2010.4.Bilich, A. L., "Improving the Precision and Accuracy of Geodetic GPS: Applications to Multipath and Seismology". Ph.D. dissertation, University of Colorado, 2006.

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Backup Slides

Team survey topography at Table Mountain Experimental Site