mapping the gps multipath environment using the signal-to- noise ratio (snr) andria bilich*,...
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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
National Geodetic SurveyNational Oceanic and Atmospheric Administration
Positioning with GPS
Trilateration using distance to satellites
Must have accurate satellite-receiver range
ArA
B
rB
C
rC
National Geodetic SurveyNational Oceanic and Atmospheric Administration
Multipath with GPS
Multipath Range error =
positioning error Systematic
(quasi-sinusoidal) Large magnitude Site-specific
National Geodetic SurveyNational Oceanic and Atmospheric Administration
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
National Geodetic SurveyNational Oceanic and Atmospheric Administration
TRO1 Power Maps
High power at long periods = close-in reflector
National Geodetic SurveyNational Oceanic and Atmospheric Administration
Multipath from Distant Topography: MKEA
Mauna Kea (MKEA), Hawaii
National Geodetic SurveyNational Oceanic and Atmospheric Administration
MKEA Power Maps
Frequency (distance to reflector) changes with satellite position
High power returns from cinder cones
60-90s 30-60s 10-30s
National Geodetic SurveyNational Oceanic and Atmospheric Administration
Combined Multipath Environments: KYVW Standard GPS
monument ~ 1.8m above ground
Nearfield: sandy, flat ground
Farfield: gentle hillsides to NW and E
National Geodetic SurveyNational Oceanic and Atmospheric Administration
KYVW Power Maps
Long periods (L1)
Short periods (L2)
Ground reflections
Reflections from hillsides
National Geodetic SurveyNational Oceanic and Atmospheric Administration
Summary
National Geodetic SurveyNational Oceanic and Atmospheric Administration
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
National Geodetic SurveyNational Oceanic and Atmospheric Administration
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
National Geodetic SurveyNational Oceanic and Atmospheric Administration
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
National Geodetic SurveyNational Oceanic and Atmospheric Administration
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