mapping the gps multipath environment using the signal-to- noise ratio (snr) andria bilich*,...

Mapping the GPS Multipath Environment Using the Signal-to-Noise Ratio (SNR)

Andria Bilich*, Kristine M. Larson+

* Geosciences Research Division, National Geodetic Survey+ Department of Aerospace Engineering Sciences,

University of Colorado, Boulder

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Overview

GPS system basics Motivation: multipath with GPS signals

Why do we care?What do we know?

Measurement: SNR Technique: power spectral maps

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Global Positioning System (GPS)

Radio navigation system

L-band 1575.42 MHz (L1) 1227.60 MHz (L2)

28+ satellites CDMA Global coverage 4-10 in view at

any instant

courtesy of Dept. of Defense

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Positioning with GPS

Trilateration using distance to satellites

Must have accurate satellite-receiver range

ArA

B

rB

C

rC

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Multipath with GPS

Multipath Range error =

positioning error Systematic

(quasi-sinusoidal) Large magnitude Site-specific

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Signal-to-Noise Ratio (SNR)

SNR = strength of composite signal

Phase relationship changes with satellite motion

direct

multipath

composite

cos222

2

mdmd

md

AAAA

AASNR

Multipath strength

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Multipath Oscillations in SNR

Parameters affecting multipath frequency:

Reflector distance h Reflection angle GPS wavelength

cos2

222

md

md

AA

AASNR

dt

dh

dt

d

cos22

Multipath frequency

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Ground Distance vs. Multipath Period

dt

dh

dt

d

cos22

Fast MP = far awaySlow MP = nearby

For a fixed reflector, satellite motion generates time-varying signature

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Multipath and SNR:putting it all together

Frequency = distance to reflector Amplitude = multipath strength Satellite position:

Azimuth/elevation = location of reflections relative to antenna

Rate of elevation change = impact on frequency and height

National Geodetic SurveyNational Oceanic and Atmospheric Administration

SNR Data

Total SNR = direct plus reflected signal(s)

Direct amplitude = dominant trend

Multipath signal = superimposed on direct

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Power Spectral Maps

Wavelet spectra of detrended SNR

Assign frequency and power to satellite azimuth & elevation

Plot all points on a grid (sky map)

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Multipath from Nearby Structure: TRO1

Antenna on a mast:4.09 m above

ground surface1.3 m above flat

tar-paper roof Roof to S of

antenna

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TRO1 Power Maps

High power at long periods = close-in reflector

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Multipath from Distant Topography: MKEA

Mauna Kea (MKEA), Hawaii

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MKEA Power Maps

Frequency (distance to reflector) changes with satellite position

High power returns from cinder cones

60-90s 30-60s 10-30s

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Combined Multipath Environments: KYVW Standard GPS

monument ~ 1.8m above ground

Nearfield: sandy, flat ground

Farfield: gentle hillsides to NW and E

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KYVW Power Maps

Long periods (L1)

Short periods (L2)

Ground reflections

Reflections from hillsides

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Summary

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Acknowledgements

Tools: Torrence and Compo wavelet toolbox:

http://paos.colorado.edu/research/wavelets/ Generic Mapping Tools (GMT)

IGS, CORS, SOPAC, UNAVCO, JPL NSF grants and fellowships

Bilich, A., K.M. Larson (2007) Mapping the GPS multipath environment using the signal-to-noise ratio (SNR), Radio Science, 42, RS6003.

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Multipath Assessment:Power Spectral Maps

Idea: frequency and power content of SNR multipath environment

Method:Wavelet spectra of

detrended SNRAssign frequency

and power to satellite azimuth/elevation

Plot all points on a grid (sky map)

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Mauna Kea (MKEA)

National Geodetic SurveyNational Oceanic and Atmospheric Administration

What do these equations tell us?

Oscillations in SNR, phase MP, and pseudorange MP all have common frequency

MP frequencyKey to determining Function of

• Reflector distance h• Reflection angle• GPS wavelength

dt

dh

dt

d

cos22

cos2

222

md

md

AA

AASNR

cos

sin)tan(

md

m

AA

A

cos1

cos

MP

Fast MP = far awaySlow MP = nearby

For a fixed reflector, satellite motion generates time-varying signature

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Time-evolving Multipath

dt

dh

dt

d cos2

2

cos2

2222

md

mdc

AA

AAASNR

cos

sin)tan(

md

m

AA

A

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Understanding Multipath:Power Spectral Maps

Idea: frequency and power content of SNR multipath environment

Method: Wavelet spectra of

detrended SNR Assign frequency

and power to satellite azimuth/elevation

Plot all points on a grid (sky map)

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Dual-FrequencyPower Spectral Maps

S1 S2

Reflection from distant object (building?)

Reflection from nearby object (rock outcrops?)

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Simplified Multipath Model and SNR Recorded SNR =

direct + multipath signal

Carrier phase error:

Code (pseudorange) error (short delay):

cos2

2222

md

mdc

AA

AAASNR

cos

sin)tan(

md

m

AA

A

d

mMP A

A

,

cos1

cos

multipath

direct

composite

MP

direct

multipath

composite

National Geodetic SurveyNational Oceanic and Atmospheric Administration

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Take-home lessons:Environmental Imaging Assess multipath environment

Frequency: distance to object Amplitude: magnitude of errors due to object Consider position errors at different

frequencies (think high-rate GPS positioning) No new equipment

SNR routinely recorded … but need precise and accurate SNR

related to multipath model (not always possible)

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Summary

Existing CGPS networks extended to unforeseen science applications Sensing soil moisture Understanding reflections and potential

sources of error Measuring displacements from short-period,

transient phenomena

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Space Segment

24+ satellites Orbit

26K km radius12 hour periodStationary ground

tracks6 orbital planes

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GPS signal (1)Receiver takes in… 4-12 satellites (in view) 2 L-band (1-2 GHz) frequencies

L1 = 1572.42 MHz L2 = 1227.60 MHz

Signal components Carrier (sinusoidal signal) PRN code (data bits for satellite ID and ranging) Navigation message (satellite position/velocity info)

Timing information

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GPS signal (2)PRN codes

C/A “Coarse

acquisition” code Civil use Chip = 1s = 300m

wavelength Range +/-30m

P(Y) “Precision” code Military use Chip = 0.1 s = 30m

wavelength Range +/-3m Encrypted (Y code) to

limit access = anti-spoofing

3 separate signals:•On L1 = C/A and P(Y)•On L2 = P(Y) only

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GNSSGlobal Navigation Satellite Systems

System Who runs it # Satellites(design/in use)

When

GPS US DoD 24 / ~30 now

GLONASS Russia & India

24 / ~10 usable

2011

Galileo EU & ESA 30 / 1 test 2012All are L-band radio systems (~ 1100 -1600 MHz)

Mostly free signals

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What is multipath?

Multipath introduces range error -> position error

Why is multipath such an issue? Difficult to prevent Difficult to model Systematic error Problematic for high-rate

applications How can we understand,

characterize, or remove multipath?

National Geodetic SurveyNational Oceanic and Atmospheric Administration

Multipath Geometry

amplitudes angles

Ad direct signal amplitude

Am multipath signal amplitude

h reflector distance angle of reflection

satellite elevation angle

path delay