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GNSS Reflectometry for Earth Remote Sensing
James L. GarrisonPurdue University, West Lafayette, USA
Stanford PNT SymposiumInvited Presentation
Stanford, CAOctober 30, 2019
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Outline
• Basic Principles of Reflectometry• Early oceanographic applications
• Spaceborne reflectometry• Research directions: Data assimilation
• Other variables• Future work – beyond GNSS
• Conclusions
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GNSS Basic Principles
cτ1 = (X − X1)2 + (Y −Y1)
2 + (Z − Z1)2 +ctb
cτ 2 = (X − X2 )2 + (Y −Y2 )
2 + (Z − Z2 )2 +ctb
cτ 3 = (X − X3)2 + (Y −Y3)
2 + (Z − Z3)2 +ctb
cτ 4 = (X − X4 )2 + (Y −Y4 )
2 + (Z − Z4 )2 +ctb4+ Observations
4 Unknowns
τ1
τ 2
τ 3τ 4
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• Ideal navigation signal – infinitely-long sequence of random pulses (in reality – Pseudorandom noise (PRN)
• Received and local signals not aligned
• Received and local signals partially aligned
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GNSS Basic Principles
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• Cross-correlation of ( -long) sequence of random pulses – large values for narrow range of delay
-Tc Tc
1
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GNSS Basic Principles
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GNSS-R Basic Principles
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Receiver
glistening zone
glistening zone
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Rough Surface Scattering
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(From Chapron and Ruffini, 2003 GNSS-R workshop, Barcelona. Photo taken at Le Conquet, Brittany)
Phenomenon observed in visible light at sunset: (water is calm inside the red ellipse)
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Outline
• Basic Principles of Reflectometry• Early oceanographic applications• Spaceborne reflectometry• Research directions: Data assimilation• Other variables• Future work – beyond GNSS• Conclusions
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Early Airborne Experiments
Measured Waveform
Surface Wind Speed (Assumed wave spectrum model)
[Garrison, et al. GRSL 2011]
Best Fit of Model Waveform
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[Garrison, et al TGARS 2002]
Brightness Temperature
Ca. 1997
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Outline
• Basic Principles of Reflectometry• Early oceanographic applications• Spaceborne reflectometry• Research directions: Data assimilation• Other variables• Future work – beyond GNSS• Conclusions
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Why GNSS-R from Space ?
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C. S. Ruf, et al., Bull. Am. Met. Soc., V. 97, N. 3, pp. 385–395, 2016. DOI: 10.1175/BAMS-D-14-00218.1
Simulated Coverage of Hurricane Frances: ASCAT vs. CYGNSS
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Going to Space …Maximum Delay for 25 km resolution
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• Spaceborne observation – 2D “delay-Doppler Map” (DDM)• 25 km resolution requirement: 3 X 5 pixels used in L2 retrieval • Downloaded to the ground: 17 x 11 pixels around specular point
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Going to Space …
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CYGNSS
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• NASA Earth Ventures Mission: Launched Dec. 2016• 8 satellite GNSS-R constellation
• Improved forecast of tropical cyclone intensification:
1. Better penetration of rain (L-band vs K-band)
2. Higher revisit rate (2.8 H med. vs 11-35 H)
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CYGNSS
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https://www.youtube.com/watch?time_continue=1&v=rRBqn6JPtv8
Cyclone GNSS (CYGNSS) Launch 12-Dec-2016[Ruf, et al. JSTARS, 2018, DOI:10.1109/JSTARS.2018.2833075]
[University of Michigan]
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Outline
• Basic Principles of Reflectometry• Early oceanographic applications• Spaceborne reflectometry• Research directions: Data assimilation• Other variables• Future work – beyond GNSS• Conclusions
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Let’s return to this picture …
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Data Assimilation
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Data Assimilation
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Integralover surface
Cross-section: Proportional to
Probability of slope scattering
in correct direction
Path lossTrans-surface
Path lossSurface-receiverMasking of
Surface by delay-Doppler mapping
Masking ofSurface by
Receiver antenna
Delay-Doppler Map (DDM)
As an equation …
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Wind speed
Waves
AntennaPattern, S/C AttitudeGPS EIRP …
Observable
Wind Speed Retrieval (point)
True Wind field Observed DDM
Wind direction
(Data is just for illustration – not actual observations)
Data Assimilation
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Wind speed
Waves
AntennaPattern, S/C AttitudeGPS EIRP …
Diff
True Wind field
(Data is just for illustration – not actual observations)
Wind direction
Observed DDM
Model Wind field Model DDM
ForwardModel
Adjustment
Data Assimilation
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• Experience from GNSS Radio-occultation (GNSS-RO):
Water vapor -> Refractivity -> bending angle
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• Direct inversion (bending angle -> refractivity) assumes uniform propertiesover area covered by the integral
• Alternative: assimilate bending angle directly into weather models.
Data Assimilation
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DDM forward model
• Sensitivity of DDM sample to surface wind speed
DDM bins
2 4 6 8 10Doppler
2
4
6
8
10
12
14
16
Delay
304.5 305 305.5Longitude
16.5
17
17.5
18
Latitude
Sensitivity to surface grid points
DDM bins
2 4 6 8 10Doppler
2
4
6
8
10
12
14
16
Delay
304.5 305 305.5Longitude
16.5
17
17.5
18
Latitude
Sensitivity to surface grid points
Delay-Doppler ambiguity
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Synthetic Aperture Radar (SAR) (Monostatic) [history.nasa.gov]
Bistatic Reflectometry[Zavorotny & Voronovich, 2000]
DDM forward model
“Delay-Doppler Ambiguity” – Have you heard this before ?
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Outline
• Basic Principles of Reflectometry• Early oceanographic applications
• Spaceborne reflectometry• Research directions: Data assimilation
• Other variables• Future work – beyond GNSS
• Conclusions
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Other Variables
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[1] Remote Sens. of Envi. DOI: 10.1016/j.rse.2017.06.020[2] Scientific Reports DOI: 10.1038/s41598-018-27127-4
Mapping inland waterways under dense biomass [1]
Post-Hurricane Flood Inundation [1]Soil Moisture [1]
Freeze-Thaw State [2]
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Outline
• Basic Principles of Reflectometry• Early oceanographic applications• Spaceborne reflectometry• Research directions: Data assimilation• Other variables• Future work – beyond GNSS• Conclusions
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Why GNSS ?
• Continuous global coverage• L-band good penetration of atmosphere, vegetation,
rain, etc … (but not good penetrating soil)• Pseudorandom noise (PRN) code – designed for ranging• My career began with GNSS! (not radiometry, radar or communications)
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What about other signals ?
• Approximately 400 communication satellites in GEO
• High-powered (~30 dB above GNSS) signals
• Allocations in most bands used for remote sensing:
L,S, C, Ku/Ka
• Designed for data transmission – Not ranging!• Assumption: Compression & Encryption are very efficient
at filling available spectrum– Data is nearly random
– Direct signal can be used as reference
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Signals of Opportunity (SoOp)
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Self-Ambiguity Function(Defined in: [Baker, et al., 2005, DOI: 10.1049/ip-rsn:20045083])
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ScienceNav/CommShared
Why would SoOp be useful ?
NTIA Spectrum Allocations: Very Valuable “Real Estate”
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Two New Applications
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P-band: Long wavelength, Better Penetration (RZSM & SWE)
Ku/K-band: Wide bandwidth “For free” (coastal altimetry)
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Two New Applications
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P-band: Long wavelength, Better Penetration (RZSM & SWE)
Ku/K-band: Wide bandwidth “For free” (coastal altimetry)
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Root Zone Soil Moisture (RZSM)• Water in top ~1m of soil • Essential for understanding water cycle & agricultural forecast.• Global RZSM from model assimilation (e.g. SMAP L4)
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P-bandI-band L-band
5 cm
100 cm
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Wideband SoOp Altimetry
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• Improved spatial-temporal sampling & coverage from SoOp Altimetry
(Example: 9 days, Monterey Bay)
SSH Signature Altimeter (e.g. Jason-2/3) 10-day repeat
8-satellite W-SoOpconstellation
[Data from R. Shah, JPL]
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The “Virtual Swath”
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Future Mission ConceptR
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SoOp (Bistatic)
Soil Tg
Vegetation canopy
PT PRa GPTTB=eTg
Passive
Conservation of Energy: G=1-e
Active (Monostatic)
PTPra s0PT
SoOp as the “Third Way”
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Conclusions
• Spaceborne GNSS-R wind retrievals successfully demonstrated under some conditions.• Perhaps wind speed is not the best variable ?
• Wind/Wave coupling• Direct DDM Assimilation
• Other geophysical variables can be retrieved• GNSS-R just the beginning - many other signals are out there!
• Diversity of frequencies in a crowded spectrum• High EIRP (+30 dB Vs. GNSS)• Broadly – “Signals of Opportunity” may introduce a
“third way” of microwave remote sensing.
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Data Sources• CYGNSS (at the JPL PO.DAAC): https://podaac.jpl.nasa.gov/CYGNSS• UK TDS-1:http://merrbys.co.uk/
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