1 Analysis of Airborne Microwave Polarimetric Radiometer Measurements in the Presence of Dynamic Platform Attitude Errors Jean Yves Kabore Central Florida Remote Sensing Laboratory University of Central Florida

2 Presentation Outline Instrument Description and characteristics Analysis of Attitude Errors on Measurement Geometry Experimental validation Conclusions

3 C-STAR Conically Scanning Two Look Airborne Radiometer Total power microwave radiometer Two dual polarized antenna beams Conically scanning Frequency37 Ghz Antenna2 Horns & mirrors PolarizationH, V, + 45, - 45 Scan rate/period6 rpm (10 s period) Delta T0.15 K

4 C-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

5 Aircraft Attitude Definitions Yaw

6 Dependence of Incidence Angle on Aircraft Attitude 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

7 Dependence of Incidence Angle on Aircraft Roll and Pitch Cont’d Incidence angle is a function of aircraft pitch and roll, and C-STAR azimuth look direction

8 Incidence Angle Simulation - Pitch and Roll

9 Brightness Temperature Normalization Equivalent T b (incidence angle corrected) is: where  i is the instantaneous incidence angle  o is the mean incidence angle dTb/d  is change in T b with respect to incidence angle (from radiative transfer model) T b correction { Measured T b V-pol or H-pol {

10 Experimental Verification of Incidence Angle Normalized T b, (5 scans) Corrected Uncorrected Forward LookAft Look

11 Polarization Rotation Caused by Mis-alignment of Spin Axis 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: Where V H P Q V’ H’

12 Tbv & Tbh models Neglecting Pol-Rotation where: Model assumes Tb surface is isotropic but allows for anisotropic Tb atmos (clouds)

13 Observed 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

14 Prior to Time Bias Adjustment

15 Tbv & 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

16 Tbv model verification, Low winds flight

17 Tbh model verification, Low winds flight

18 Tbv model verification, High winds flight

19 Tbh model verification, High winds flight

20 Tbp & Tbq model verification Where:

21 Tbp model verification, Low winds flight

22 Tbq model verification, Low winds flight

23 Tbp model verification, High winds flight

24 Tbq model verification, High winds flight

25 Conclusions 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