Initial Results on the Cross-Calibration of QuikSCAT and Oceansat-2 Scatterometers

David G. Long

Department of Electrical and Computer EngineeringBrigham Young University

May 2011

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ParameterParameter QuikSCATQuikSCAT OceanSat-2 ScatterometerOceanSat-2 Scatterometer

Sponsoring Organization NASA/JPL ISRO

Operational Frequency 13.402 GHz 13.515 GHz

Polarization (Inner/Outer) HH/VV HH/VV

Antenna Diameter 1 m 1 m

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

QuikSCAT vs OSCAT

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OSCAT & QuikSCAT Hi Res•QuikSCAT and OceanSat-2 Scatterometer (OSCAT) have comparable spatial sampling and resolution

•Similar image enhancement possible

– Can use QuikSCAT algorithms for OSCAT

OSCATGRD

QuikSCATGRD

QuikSCATSIR

OSCATSIR

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Enhanced Resolution* OSCAT Image

JD 309, 2009

* Preliminary 2.225 km/pixel

Conventional resolution 25 km/pixel

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QuikSCAT/OSCAT Image Comparison

• 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

OSCAT H QuikSCAT H

OSCAT V QuikSCAT V

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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

ASCAT Amazon Rain Forest Example

ASCAT

• Can also use for egg/slice incidence angle correction

ASCAT

(dB

)

(dB

)

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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 withclimate changes if not accounted for

• Do not expect azimuth variations over the Amazon

ASCAT Wilkes Land Example

ASCAT (C-Band)

ASCAT (C-Band) V-pol

40 inc

(dB

)

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QuikSCAT Anisotropy• QuikSCAT has fixed incidence

angles but high diversity in azimuth angle observations– Similar anisotropy observed

QuikSCAT H-pol (Ku-Band)

QuikSCAT V-pol (Ku-Band)

QuikSCAT (Ku-Band)

54 inc

46 inc

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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

Locations of OSCATsigma-0 measurements

within study regionJD 309-327, 2009

OSCAT slice measurements: 13,099

QuikSCAT slice measurements: 14,118

OSCAT 310-311, 2009

25 k

m

25 km

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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

QuikSCAT

OSCAT

H-pol V-pol

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OSCAT Slice Sigma-0 vs Incidence Angle

(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

V-pol bias 0.0 dB

H-pol bias 0.7 dB

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Comparison of Azimuth Corrected Sigma-0 Distributions in the Antarctic Test Region

V-pol bias 0.2 dB

H-pol bias 0.6 dB

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Amazon Study Region

Select region that both QuikSCAT and OSCAT sigma-0 fall within narrow range

• 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• 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

V-pol bias 0.25 dB

H-pol bias 0.05 dB

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Comparison of Corrected Sigma-0 Distribution in Amazon Study Region

V-pol bias 0.25 dB

H-pol bias 0 dB

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OSCAT Local Time of Day Analysis

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

Scatterplot of LTD vs UTC in the Northern Hemisphere for different longitude bins (a) OSCAT, (b) Seawinds, (c) QuikSCAT

OSCAT

SeaWinds

QuikSCAT

LTD (hours) = UTC + Local_Longitude / 15

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Comparison of Southern Hemisphere Local Time of Day Observations

Scatterplot of LTD vs UTC in the Southern Hemisphere for different longitude bins (a) OSCAT, (b) Seawinds, (c) QuikSCAT

OSCAT

SeaWinds

QuikSCAT

LTD (hours) = UTC + Local_Longitude / 15

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Diagram of LTD Divisions for Four Scatterometers

LTD (hrs) = UTC + Local_Longitude / 15

24 hours 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