ANISOTROPY IN OCEAN
SCATTERING OF BISTATIC
RADAR USING SIGNALS OF
OPPORTUNITY
Rashmi Shah*, Dr. James Garrison*, Dr. Michael Grant**
*School of Aeronautics and Astronautics,
Purdue University
**NASA Langley Research Center
IGARSS – Vancouver, Canada - July 24-29, 2011
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Outline
• Background
– Previous Work
– Overview of XM Radio System
– Motivation
• Objective
• Airborne Experiment
• Data Processing
• Results
• Summary and Future Work
IGARSS – Vancouver, Canada - July 24-29, 2011
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Previous Work
• Global Navigation Satellite System Reflectometry (GNSS-R)
– First demonstration of remote sensing with “signals of
opportunity”
– Retrieval of ocean surface roughness, wind speed, soil
moisture, and ice
– 15 years of development: airborne and space (UK-DMC)
– GNSS-R enabled by use of known pseudo-random noise
(PRN) code
• Digital Communication Signals Reflectometry
– Expand methods to other “signals of opportunity”
– Demonstrated with XM digital radio
• Commercial satellite radio system in the US
IGARSS – Vancouver, Canada - July 24-29, 2011
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Motivation
• Why Digital Communication Signals?
– Multiple frequency bands: Different sensitivity
– Higher transmitted power: Better accuracy
– Located in geostationary orbit: Fixed geometry
• Challenge:
– No a priori knowledge of data bits
• Hypothesis:
– Data bits are long, random, uncorrelated stream
• Hypothesis verified using XM digital radio signals (S-band)
[Shah, et al, IGARSS 2010]
IGARSS – Vancouver, Canada - July 24-29, 2011
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XM System
• Two active geostationary satellites
o
– Rhythm (115 W)
• Elevation: 31.3o
• Azimuth: 234.4o
o
– Blues (85 W)
• Elevation: 41.6o
• Azimuth: 196.4o
IGARSS – Vancouver, Canada - July 24-29, 2011
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Objective
• Quantify retrieval anisotropy between two satellites at
different azimuth
• Examine potential to retrieve wind direction from this effect
IGARSS – Vancouver, Canada - July 24-29, 2011
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Airborne Experiment
IGARSS – Vancouver, Canada - July 24-29, 2011
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Waveforms
• Altitude of the aircraft: 3,471 meters (11,400 feet)
• Cross-correlation of direct and reflected signals
Flight Result, Fs = 8MHz, Altitude = 3174 meters
X: 12.13
Y: 1
1
0.9
X: 17.13
Y: 1
Satellite "Blues"
Satellite "Rhythm"
0.8
|Y(k,0)|
2
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
5
10
15
Lags[sec]
20
• Sampling Frequency
− 8MHz
• Coherent Integration
− 10ms
• Incoherent Integration
− 1sec
IGARSS – Vancouver, Canada - July 24-29, 2011
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Data Processing
• Two-step estimation process [Garrison 2003]
• First step:
– Isotropic normal PDF assumed
– Nonlinear least squares estimation
𝐽𝑆𝑉 𝑿𝑺𝑽 =
𝑌𝑆𝑉 𝜏𝑘 , 0
2
− 𝑌𝑀 𝜏𝑘, , 0; 𝑿𝑺𝑽
2 2
𝑘
𝑿𝑺𝑽 = 𝜎 2 𝐼𝑆𝑂𝑆𝑉 , 𝑆𝑆𝑉 , 𝜏0𝑆𝑉
𝜎 2 𝐼𝑆𝑂𝑆𝑉 = Mean Square Slope (MSS)
𝑆𝑆𝑉 = Scale factor (remove variation in signal power)
𝜏0𝑆𝑉 = Delay offset (adjust small uncertainties in delay)
– Independent estimate for each satellite
IGARSS – Vancouver, Canada - July 24-29, 2011
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Data Processing
• Chesapeake Lighthouse (CHLV2): 7.5 m/s (MSS = 0.0010)
Flight Estimation, Elevation = 46.3o, Azimuth = 196.4o
Flight Estimation, Elevation = 31.3o, Azimuth = 234.4o
0.8
1
0.7
0.8
0.6
0.6
k
|Y(  ,0)|2
k
|Y(  ,0)|2
0.5
0.4
0.3
0.2
0.4
0.2
0.1
0
0
12
13
14
Lags[sec]
15
16
Rhythm: 0.0076 (6 m/s)
17
16
17
18
19
20
Lags[sec]
21
22
23
Blues: 0.0098 (7.2 m/s)
IGARSS – Vancouver, Canada - July 24-29, 2011
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Data Processing
• Second Step:
– 2-D normal slope PDF
– Model fit to both satellites simultaneously
– Isotropic MSS su2+sc2 fixed to average from first step
– Cross-/up-wind ratio (sc/su) fixed to 0.85
– Principal axis α of PDF varied to minimize residuals
𝐽 𝜶 =
𝑌𝑆𝑉 𝜏𝑘 , 0
𝑆𝑉
𝑘
𝑌𝑀 𝜏𝑘, , 0; 𝜶
2
2
− 𝑌𝑀 𝜏𝑘, , 0; 𝜶
2 2
= 𝑆 𝑌 2 𝜏𝑘 − 𝜏0 , 0; 𝜎 2 𝑢 , 𝜎 2 𝑐 , 𝛼
– Wind direction with respect to North
𝜃𝑊 = 𝑎𝑧1,2 − 𝛼
IGARSS – Vancouver, Canada - July 24-29, 2011
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Results
• Minimum residual found at θW = 57o
4.2815
x 10
Sum of residuals from the two satellites
8
4.281
4.2805
Residuals
4.28
4.2795
4.279
4.2785
4.278
4.2775
4.277
0
20
40
60
80
100
120
140
160
180
Wind Direction from the North, in degrees
IGARSS – Vancouver, Canada - July 24-29, 2011
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Results
• CHLV2 reported that the wind direction was 28o
• Discrepancy can be due to:
– CHLV2 & measurement location separated by 80km
• Closest Buoy (44014) not reporting data
– Azimuth separation between satellites only 38o - may
give reduced sensitivity
IGARSS – Vancouver, Canada - July 24-29, 2011
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Summary and Future Work
• Summary
– Anisotropy gives equivalent wind speed difference of
1.2m/s
– Wind direction may contribute to this effect
– Difference between principal axis of wind speed was 29o
• Future Work
– Single model fit to both satellites data
– Find better surface truth values:
• Model runs at measurement location
IGARSS – Vancouver, Canada - July 24-29, 2011
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Thank You!
IGARSS – Vancouver, Canada - July 24-29, 2011
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References
J.L. Garrison, A. Komjathy, V.U. Zavorotny, and S.J. Katzberg, “Wind speed
measurement using forward scattered GPS signals,” IEEE Transactions
on Geoscience and Remote Sensing, vol. 40(1), pp. 50–65, 2002.
J.L. Garrison, S.J. Katzberg, and M.I. Hill, “Effect of sea roughness on
bistatically scattered range coded signals from the Global Positioning
System,” Geophys. Res. Lett, vol. 25(13), pp. 2257–2260, 1998.
E. Cardellach and A. Rius, “A new technique to sense non-Gaussian features
of the sea surface from L-band bi-static GNSS reflections,” Remote
Sensing of Environment, vol. 112, no. 6, pp. 2927 – 2937, 2008.
R. Shah, J.L. Garrison, M.S. Grant, and S.J. Katzberg, Analysis of correlation
properties of digital satellite signals and their applicability in bistatic
remote sensing,” Proceedings of the 2010 IEEE International Geoscience
and Remote Sensing Symposium, pp. 4114–4117, July 2010.
J.L. Garrison, “Anisotropy in reflected GPS measurements of ocean winds,”
Proceedings of the 2003 IEEE International Geoscience and Remote
Sensing Symposium, pp. 4480–4482, July 2003.
IGARSS – Vancouver, Canada - July 24-29, 2011
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BACKUP AND EXTRAS
IGARSS – Vancouver, Canada - July 24-29, 2011
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Previous Work
• Waveform generated by cross-correlating ocean-reflected
signals with locally generated pseudorandom code was
used
• Methods used:
– Matched Filters (Garrison, et al 1998)
– Nonlinear least squares parameter estimation (Garrison, et al 2002)
– Discretized approximation of the full-slope PDF (Caedellach, et al
2008)
IGARSS – Vancouver, Canada - July 24-29, 2011
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Glistening Zone
• Rhythm: Semi-major = 4.76km, Semi-minor = 2.47km
• Blues: Semi-major = 5.40km, Semi-minor = 2.80km
Glistening Zone
3
Rhythm
Blues
2
Y [km]
1
0
-1
-2
-3
0
2
4
6
8
10
12
14
X [km]
IGARSS – Vancouver, CANADA- July 21-24, 2010
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Airborne Experiment: Geometry
• Experiment conducted: 02-July-2010 in Piper Navajo
• Experiment time period: 07:51AM EDT - 09:19AM EDT
IGARSS – Vancouver, Canada - July 24-29, 2011
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