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Analysis of Airborne Microwave Analysis of Airborne Microwave Polarimetric Radiometer Polarimetric Radiometer

Measurements in the Presence of Measurements in the Presence of Dynamic Platform Attitude ErrorsDynamic Platform Attitude Errors

Jean Yves KaboreJean Yves KaboreCentral Florida Remote Sensing LaboratoryCentral Florida Remote Sensing Laboratory

University of Central FloridaUniversity of Central Florida

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Presentation OutlinePresentation Outline

• Instrument Description and characteristics• Analysis of Attitude Errors on Measurement

Geometry • Experimental validation• Conclusions

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C-STARC-STAR Conically Scanning Two Look Airborne RadiometerConically Scanning Two Look Airborne Radiometer

• Total power microwave radiometer

• Two dual polarized antenna beams

• Conically scanning

Frequency 37 Ghz

Antenna 2 Horns & mirrors

Polarization H, V, + 45, - 45

Scan rate/period 6 rpm (10 s period)

Delta T 0.15 K

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C-STAR Scan GeometryC-STAR Scan Geometry

• Scans clockwise over 360° in azimuth

• Planar reflectors are used to produce required incidence angle

• Cold calibration load is outside ambient air

• Hot calibration load is temperature controlled

Spin Axis(aligned to

Nadir)Horn Antenna

Reflector

Scan Circle

Horns

Planar Reflectors

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Aircraft Attitude DefinitionsAircraft Attitude Definitions

Yaw

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Dependence of Incidence Angle on Aircraft Dependence of Incidence Angle on Aircraft AttitudeAttitude

• Ideal conical scan, spin axis points to nadir

• Time-varying aircraft roll and pitch misalign spin axis causing incidence angle variations at each azimuth position

• Yaw and altitude have negligible effects

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Dependence of Incidence Angle on Aircraft Roll Dependence of Incidence Angle on Aircraft Roll and Pitch Cont’dand Pitch Cont’d

• Incidence angle is a function of aircraft pitch and roll, and C-STAR azimuth look direction

AzrollAzpitch

where

Azrollpitchi

sin*cos*

AngleIncidence Nominal

:

,,

0

0

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Incidence Angle Simulation - Pitch and RollIncidence Angle Simulation - Pitch and Roll

Incidence angle, Pitch = 1

50.00

50.50

51.00

51.50

52.00

52.50

53.00

53.50

54.00

54.50

55.00

0

20 40 60 80

100

120

140

160

180

200

220

240

260

280

300

320

340

360

Azimuth, deg

Inci

den

ce a

ng

le, d

eg Roll= 0 deg

Roll= 0.5 dge

Roll = 1 deg

Roll=-0.5 deg

Roll = -1 deg

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Brightness Temperature NormalizationBrightness Temperature Normalization

• Equivalent Tb (incidence angle corrected) is:

where i is the instantaneous incidence angle

o is the mean incidence angle

dTb/d is change in Tb with respect to

incidence angle (from radiative transfer model)

oib

bnormb d

dii

*

Tb correction

{Measured Tb

V-pol or H-pol

{

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Experimental Verification of Incidence Angle Experimental Verification of Incidence Angle Normalized TNormalized Tb b ,, (5 scans)(5 scans)

Corrected

Uncorrected

Forward Look Aft Look

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Polarization Rotation Caused by Mis-alignment Polarization Rotation Caused by Mis-alignment of Spin Axisof Spin Axis

)sin(*)cos(* AzpitchAzroll

• Alignment error caused by time varying A/C roll and pitch• Small effect (< 1 K) on V & H pols• Dominant effect for p & q-pols

Modelled by:

)(sin*)(cos* 22 surfacesurface TbVTbVTbV

)(cos*)(sin* 22 surfacesurface TbHTbHTbH

Where

V

H

P

Q

V’

H’

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Tbv & Tbh models Neglecting Pol-RotationTbv & Tbh models Neglecting Pol-Rotation

isurfaceivatmosAtmos d

dTbvTbvAzTbvAzTbvTbv

***)(*)()(

ii

SurfaceihAtmosAtmos

TbhTbhAzTbhAzTbhTbh

*)(

)(**)(*)()(

v

vnormAtmos

SSTAzTbvAzTbv

*1

)1(*)()(

where:

h

hnormAtmos

SSTAzTbhAzTbh

*1

)1(*)()(

Model assumes Tbsurface is isotropic but allows for anisotropic Tbatmos (clouds)

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Observed Problems with Tbv & Tbh modelObserved Problems with Tbv & Tbh model

• Could not obtain a consistent match between measured and modeled Tb’s

• Investigated possible causes– Cloud contamination

• Model updated to include this effect

– Instrument calibration drift– Antenna pattern interactions with AC fuselage

• Tb biases with azimuth position

– Improper sampling of AC roll & pitch• AC data smoothed and resampled

• Observed that the measured Tb’s and aircraft attitude were not synchronized

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Prior to Time Bias AdjustmentPrior to Time Bias Adjustment

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Tbv & Tbh model verificationTbv & Tbh model verification

• Isotropic surface brightness temperatures were assumed

• Anisotropic atmospheric Tb hypothesis:– Clouds can occur over portions of the scan; therefore,

Tb would be anisotropic

• Used differences between measured and modeled Tb’s to determine atmospheric contributions versus azimuth

• Checked reasonableness of clouds against radiative transfer model results– Similarity of V- and H-pol results

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Tbv model verification, Low winds flightTbv model verification, Low winds flight

0 20 40 60 80 100 120 140 160229

230

231

232

233

234

235

236

237

238

239

240

Time series

TbV

,K

Modelled and Measured, Scan 173-180

Measured

Modelled

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Tbh model verification, Low winds flightTbh model verification, Low winds flight

0 20 40 60 80 100 120 140 160176

177

178

179

180

181

182

Time series

TbH

,K

Modelled and Measured, Scan 1525-1532

Measured

Modelled

0 20 40 60 80 100 120 140 160176

177

178

179

180

181

182

Time series

TbH

,KModelled and Measured, Scan 173-180

Measured

Modelled

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Tbv model verification, High winds flightTbv model verification, High winds flight

0 20 40 60 80 100 120 140 160225

230

235

240

245

250

Time series

TbV

,KModelled and Measured, Scan 1525-1532

Measured

Modelled

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Tbh model verification, High winds flightTbh model verification, High winds flight

0 20 40 60 80 100 120 140 160180

185

190

195

200

205

210

215

220

225

Time series

TbH

,KModelled and Measured, Scan 1525-1532

Measured

Modelled

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Tbp & Tbq model verificationTbp & Tbq model verification

)(sin*)()(cos*)( 452

452

ii TbVTbHTbp

)(sin*)()(cos*)( 452

452

ii TbVTbHTbq

)45(p

)45(q

**)(*)()()( surfaceivatmosAtmosi TbvAzTbvAzTbvTbV

**)(*)()()( surfaceihatmosAtmosi TbhAzTbhAzTbhTbH Where:

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Tbp model verification, Low winds flightTbp model verification, Low winds flight

0 20 40 60 80 100 120 140 160200

202

204

206

208

210

212

Time series

TbP

,K

Modelled and Measured, Scan 173-180

Measured

Modelled

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Tbq model verification, Low winds flightTbq model verification, Low winds flight

0 20 40 60 80 100 120 140 160200

202

204

206

208

210

212

Time series

TbQ

,K Modelled and Measured, Scan 173-180

Measured

Modelled

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Tbp model verification, High winds flightTbp model verification, High winds flight

0 20 40 60 80 100 120 140 160200

205

210

215

220

225

230

235

Time series

TbP

,K Modelled and Measured,Scan 1525-1532

Measured

Modelled

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Tbq model verification, High winds flightTbq model verification, High winds flight

0 20 40 60 80 100 120 140 160200

205

210

215

220

225

230

235

Time series

TbQ

,K Modelled and Measured,Scan 1525-1532

Measured

Modelled

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ConclusionsConclusions

• Effectively characterized effects of a/c attitude variations on measured brightness

• Made adjustments to remove inconsistencies in A/C and radiometer data sets

• Accurately modeled TbV, TbH, TbP and TbQ using A/C roll and pitch