Initial Results on the Cross- Calibration of QuikSCAT and Oceansat-2 Scatterometers David G. Long Department of Electrical and Computer Engineering Brigham Young University May 2011

DGL May ParameterQuikSCAT OceanSat-2 Scatterometer Sponsoring OrganizationNASA/JPLISRO Operational Frequency GHz GHz Polarization (Inner/Outer)HH/VV Antenna Diameter1 m Altitude at Equator803 km720 km Orbit Near Repeat Cycle4 days2 days Local time at asc/desc node6:00 a.m. at asc nodenoon at desc node HH 3dB footprint (Az x El)24 x 31 km26.8 x 45.1 km VV 3 dB footprint (Az x El)26 x 36 km29.7 x 68.5 km Incidence Angle (Inner)46 deg49 deg Incidence Angle (Outer)54 deg57 deg Swath Diameter (Inner)1400 km Swath Diameter (Outer)1800 km1836 km Pointing Accuracy+/ deg+/ deg Elevation Pointing (w/ attitude error)+/ deg+/ deg Begin / End Date / / present QuikSCAT vs OSCAT

DGL May 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 OSCAT GRD QuikSCAT GRD QuikSCAT SIR OSCAT SIR

DGL May Enhanced Resolution* OSCAT Image JD 309, 2009 * Preliminary km/pixel Conventional resolution 25 km/pixel

DGL May 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 HQuikSCAT H OSCAT VQuikSCAT V

DGL May 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

DGL May 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)

DGL May 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) ASCAT (C-Band) V-pol 40  inc (dB)

DGL May 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

DGL May 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

DGL May OSCAT Azimuth Modulation Analysis Locations of OSCAT sigma-0 measurements within study region JD , 2009 OSCAT slice measurements: 13,099 QuikSCAT slice measurements: 14,118 OSCAT , km

DGL May Slice Sigma-0 vs Azimuth Angle

DGL May Azimuth Corrected Slice Sigma-0 vs Azimuth Angle

DGL May 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-polV-pol

DGL May OSCAT Slice Sigma-0 vs Incidence Angle (narrow incidence angle range)

DGL May Azimuth Corrected Slice Sigma-0 vs Incidence Angle

DGL May Comparison of Sigma-0 Distributions in the Antarctic Study Region V-pol bias 0.0 dB H-pol bias 0.7 dB

DGL May Comparison of Azimuth Corrected Sigma-0 Distributions in the Antarctic Test Region V-pol bias 0.2 dB H-pol bias 0.6 dB

DGL May 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

DGL May Egg Sigma-0 vs Incidence Angle

DGL May Incidence-Corrected Egg Sigma-0 vs Incidence Angle

DGL May Egg Sigma-0 vs Azimuth Angle

DGL May Azimuth-Corrected Egg Sigma-0 vs Azimuth Angle

DGL May Comparison of Egg Sigma-0 distribution in Amazon Study Region V-pol bias 0.25 dB H-pol bias 0.05 dB

DGL May Comparison of Corrected Sigma-0 Distribution in Amazon Study Region V-pol bias 0.25 dB H-pol bias 0 dB

DGL May OSCAT Local Time of Day Analysis Time in minutes from start

DGL May OSCAT Local Time of Day Analysis ArcticAntarctic Equi-latitude strips used for measurement extraction in LTD analysis superimposed upon OSCAT gridded sigma-0 images of the polar regions

DGL May 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

DGL May 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

DGL May Diagram of LTD Divisions for Four Scatterometers LTD (hrs) = UTC + Local_Longitude / hours

DGL May 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