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

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Primarily low elevation data

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ResultsResults

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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

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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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