Initial Results on the CrossCalibration of QuikSCAT and
Oceansat-2 Scatterometers
David G. Long
Department of Electrical and Computer Engineering
Brigham Young University
May 2011
QuikSCAT vs OSCAT
Parameter
QuikSCAT
OceanSat-2 Scatterometer
Sponsoring Organization
NASA/JPL
ISRO
Operational Frequency
13.402 GHz
13.515 GHz
Polarization (Inner/Outer)
HH/VV
HH/VV
Antenna Diameter
1m
1m
Altitude at Equator
803 km
720 km
Orbit Near Repeat Cycle
4 days
2 days
Local time at asc/desc node
6:00 a.m. at asc node
noon at desc node
HH 3dB footprint (Az x El)
24 x 31 km
26.8 x 45.1 km
VV 3 dB footprint (Az x El)
26 x 36 km
29.7 x 68.5 km
Incidence Angle (Inner)
46 deg
49 deg
Incidence Angle (Outer)
54 deg
57 deg
Swath Diameter (Inner)
1400 km
1400 km
Swath Diameter (Outer)
1800 km
1836 km
Pointing Accuracy
+/- 0.05 deg
+/- 0.15 deg
Elevation Pointing (w/ attitude error)
+/- 0.05 deg
+/- 0.25 deg
Begin / End Date
201-1999/ 327-2009
309-2009/ present
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OSCAT & QuikSCAT Hi Res
QuikSCAT
GRD
OSCAT
GRD
QuikSCAT
SIR
OSCAT
SIR
•QuikSCAT and
OceanSat-2
Scatterometer
(OSCAT) have
comparable
spatial sampling
and resolution
•Similar image
enhancement
possible
– Can use
QuikSCAT
algorithms for
OSCAT
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Conventional resolution 25 km/pixel
JD 309, 2009
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* Preliminary 2.225 km/pixel
Enhanced Resolution*
OSCAT Image
4
QuikSCAT/OSCAT Image Comparison
OSCAT H
OSCAT V
QuikSCAT H
QuikSCAT V
• Comparison of one day
BYU backscatter images
– OSCAT and QuikSCAT
have nearly identical
characteristics
– Differences due to
azimuth and local time of
day
– Similar variances and
means
– Similar spatial
enhancement possible
• OSCAT can contribute to
the multi-decade
scatterometer climate
record of land and ice
observations
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OSCAT/QuikSCAT Differences for the
Land/Ice Scatterometer Climate Record
• Nominal incidence angle differs
– H: QuikSCAT=46°, OSCAT=48°
– V: QuikSCAT=54°, OSCAT=56°
• Orbit geometry differs
– OSCAT has better coverage near poles (smaller holes)
– Time of orbit ascending node differ
• QuikSCAT=6:30 am OSCAT=noon
• Local time of measurements vary (location dependent)
– Orbit revisit time (Q=4 day repeat, O=2 day repeat)
– Azimuth angle distributions differ and vary
• Need to apply azimuth angle corrections
• Improved sigma-0 cross-calibration needed
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Linear model for sigma-0 vs incidence angle
• Simplified model
• Can also use for egg/slice incidence angle correction
(dB)
(dB)
ASCAT Amazon Rain Forest Example
ASCAT
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ASCAT
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Backscatter Anisotropy
• Due to sastrugi and topography, some polar regions exhibit
anisotropic backscatter response
- Differences in azimuth geometry can be confused with
climate changes if not accounted for
• Do not expect azimuth variations over the Amazon
ASCAT Wilkes Land Example
ASCAT (C-Band) V-pol
(dB)
40 inc
ASCAT (C-Band)
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QuikSCAT Anisotropy
• QuikSCAT has fixed incidence
angles but high diversity in
azimuth angle observations
54 inc
– Similar anisotropy observed
QuikSCAT V-pol (Ku-Band)
46 inc
QuikSCAT (Ku-Band)
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QuikSCAT H-pol (Ku-Band)
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QuikSCAT / ASCAT Comparison
• Different frequencies (5.4 GHz vs 13.5 GHz) and
incidence angles (40 V vs 46 H & 54 V)
– Consistent with dominant sastrugi scattering
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OSCAT Azimuth Modulation Analysis
OSCAT 310-311, 2009
OSCAT slice measurements: 13,099
QuikSCAT slice measurements: 14,118
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Locations of OSCAT
sigma-0 measurements
within study region
JD 309-327, 2009
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Slice Sigma-0 vs Azimuth Angle
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Azimuth Corrected
Slice Sigma-0 vs Azimuth Angle
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Comparison of OSCAT and QuikSCAT
modulation for the study region
• OSCAT azimuth modulation does not match
QuikSCAT azimuth modulation
– Improved processing is expected to resolve this
H-pol
V-pol
OSCAT
QuikSCAT
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OSCAT Slice Sigma-0 vs Incidence Angle
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(narrow incidence angle range)
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Azimuth Corrected Slice Sigma-0 vs Incidence Angle
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Comparison of Sigma-0 Distributions
in the Antarctic Study Region
H-pol bias 0.7 dB
V-pol bias 0.0 dB
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Comparison of Azimuth Corrected Sigma-0 Distributions
in the Antarctic Test Region
H-pol bias 0.6 dB
V-pol bias 0.2 dB
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Amazon Study Region
• Rain forest is a good
calibration target
(anisotropic), but exhibits
spatial inhomogeneity
– Select homogenous
region
• Time-of-day variation
– Sigma-0 varies with time
of day as moisture moves
up/down in canopy
– Several tenths of a dB
effect
• OSCAT and QuikSCAT
observe at different local
times
– No azimuth variation
expected
Select region that both QuikSCAT and
OSCAT sigma-0 fall within narrow range
• Different incidence angles
– Small mean differences
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Egg Sigma-0 vs Incidence Angle
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Incidence-Corrected Egg Sigma-0 vs Incidence Angle
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Egg Sigma-0 vs Azimuth Angle
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Azimuth-Corrected Egg Sigma-0 vs Azimuth Angle
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Comparison of Egg Sigma-0 distribution in
Amazon Study Region
H-pol bias 0.05 dB
V-pol bias 0.25 dB
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Comparison of Corrected Sigma-0 Distribution
in Amazon Study Region
H-pol bias 0 dB
V-pol bias 0.25 dB
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OSCAT Local Time of Day Analysis
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Time in minutes from start
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OSCAT Local Time of Day Analysis
Arctic
Antarctic
Equi-latitude strips used for measurement extraction in LTD analysis
superimposed upon OSCAT gridded sigma-0 images of the polar regions
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Comparison of Northern Hemisphere
Local Time of Day Observations
OSCAT
SeaWinds
QuikSCAT
LTD (hours) = UTC + Local_Longitude / 15
Scatterplot of LTD vs UTC in the Northern Hemisphere for different
longitude bins (a) OSCAT, (b) Seawinds, (c) QuikSCAT
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Comparison of Southern Hemisphere
Local Time of Day Observations
OSCAT
SeaWinds
QuikSCAT
LTD (hours) = UTC + Local_Longitude / 15
Scatterplot of LTD vs UTC in the Southern Hemisphere for different
longitude bins (a) OSCAT, (b) Seawinds, (c) QuikSCAT
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Diagram of LTD Divisions for Four
Scatterometers
LTD (hrs) = UTC + Local_Longitude / 15
24 hours
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24 hours
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Conclusion
• QuikSCAT and OSCAT sensors very similar
– Calibrated OSCAT products will be similar to QuikSCAT products
• Validated QuikSCAT land/ice SCP products
– Daily Antarctic iceberg products (operational)
– Daily sea ice extent and mapping (operational, widely distributed)
– Daily FY/MY ice classification (relatively new)
• Can be averaged to longer time scales
• Post wind mission (PWM) QuikSCAT data supports OSCAT
calibration
– PWM QuikSCAT Coverage is too limited for less than monthly maps,
aliasing an issue for ice movement for monthly maps
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