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Making good from bad: Can Making good from bad: Can we use GPS multipath to we use GPS multipath to measure soil moisture?measure soil moisture?
Kristine M. LarsonDepartment of Aerospace
Engineering Sciences
University of Colorado
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OutlineOutline
GPS basics
Multipath research at CU
If we can’t model multipath, can we extract
soil moisture estimates from it?
Preliminary results
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GPS BasicsGPS 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.
€
Rrs = ρ r
s − cδ s + cδr + ρ trop + ρ iono + ρ multi +ε
€
rs (tr ) =
r X s (t s ) −
r X r (tr )
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Challenges 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.
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OpportunitiesOpportunities
New constellation in Europe (Galileo)Expanded Russian constellation
(Glonass)Expansion of ground networks and
orbiting GPS receivers.
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Multipath introduces a (frequency dependent) range error
Antenna gain pattern, RHCP/LHCP
for a simple horizontal reflector
multi=2h*(sinImpacts phase, pseudorange and SNR
How does multipath How does multipath impact gps? impact gps?
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Assume reflectance of 10%, L1 frequency
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Multipath error frequency (for a Multipath error frequency (for a given receiver/satellitegiven receiver/satellite
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Salar de Uyuni, Salar de Uyuni, BolivaBoliva
Large salt flatSub-decimeter
topography; used to calibrate ICESat
Experimental setup:3 Ashtech Z-12
receivers 2 flush with ground 1 tripod (~1.4m height)
Bilich, 2006
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FIT SNR dataFIT SNR data
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
Primarily low elevation data
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ResultsResults
QuickTime™ and aTIFF (LZW) decompressor
are needed to see this picture.
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Multipath Assessment:Multipath Assessment:Power Spectral MapsPower Spectral Maps Idea: frequency
and power content of SNR multipath environment
Method:Power spectra of
small data sectionsAssign to satellite
azimuth/elevationPlot all points on a
grid
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MKEA Power MapsMKEA Power Maps10-30s 30-60s
60-90s 90-130sBilich, 2006
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Mauna Kea (MKEA)Mauna Kea (MKEA)
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Conclusions IConclusions 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?
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GPS as a soil GPS as a soil moisture instrumentmoisture 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?
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Why measure Soil Why measure Soil moisture?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
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Past work on GPS reflectionsPast work on GPS reflections
Masters, 2004Reflections from specular locations
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LimitationsLimitations
Specialized equipment Antennas looking down!
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day of year
QuickTime™ and a decompressor
are needed to see this picture.
160 210 260
What does soil moisture look like?
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Southern CaliforniaSouthern California
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GPS SNR data issuesGPS SNR data issues
NegativesQuality questionable (digitation, L1-L2)Not much of a published literature.
PositivesNo cycle slipsNo dependence on orbit qualityIndependent for each satellite
Differences with previous reflection workUse existing GPS instrumentationNo installation costs - but weaker signal.
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TASH (Tashkent)
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What do the What do the SNR data look like?SNR data look like?
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Separate the ArcsSeparate the Arcs
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Issues to resolveIssues to resolve
Direct signal powerTemperatureOrbital (ground-track)VegetationComparisons with in situ sensors
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KnownKnownTemperature effectsTemperature effects
on pseudorangeon pseudorange
KnownKnownTemperature effectsTemperature effects
on pseudorangeon pseudorange
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33http://dragon.larc.nasa.gov/
Katzberg, Torres et al.
Vegetation
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Temperature Effects on Dielectric
http://www.lsbu.ac.uk/water/microwave.html
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Soil moisture modelSoil 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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Broader Impacts
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ConclusionsConclusions
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.
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AcknowledgementsAcknowledgements
A. Bilich, D. Agnew, P. Axelrad, IGS, JPL, UNAVCO, NGS, CORS, SOPAC, CDDIS NASA, CU faculty fellowship.
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