1. Polarization analysis in MODIS Gerhard Meister, Ewa Kwiatkowska, Bryan Franz, Chuck McClain Ocean Biology Processing Group 18 June 2008 Polarization Technology and Resources for Earth Science Missions

2. Quantifying Phytoplankton from Space: Chl-a Algorithm CH 3 Chlorophyll-a Marine Spectral Reflectance vs. Chlorophyll-a Chlorophyll Algorithm: Statistical “Band-Ratio” Regression SeaWiFS From chlorophyll absorption to chlorophyll concentration via optics [Chl] mg/m 3 10 0.01 Clear Water Turbid Water Reflectance Ratio log Reflectance Wavelength (nm) Wavelengths used for Chl-a

3. MODIS Optics System Page 3 +/-55deg scan angles

4. MODIS Prelaunch Characterization PSA Issue: Homogeneity

5. MODIS Prelaunch Characterization PSA Issue: 4-cycle effect Solid=2 cycle, dashed=4 cycle

6. MODIS Prelaunch Characterization Documentation Issues MODIS requirements were for polarization amplitude and accuracy The characterization team (SBRS) did not know that the phase was important, orientation of test equipment relative to MODIS not documented Incorrect interpretation of setup lead to 2+ years of production of erroneous ocean color data

7. Effect of misinterpretation on global water-leaving radiance

8. This picture (based on Gene Waluschka’s notes and inquiries) cleared it up: Lesson: Document setup well, words are not enough Note: Direction of PSA rotation still not 100% resolved, implemented based on best agreement with SeaWiFS and most-likely SBRS setup

9. MODIS-Terra cross-calibration for ocean color bands Ewa Kwiatkowska Bryan Franz, Gerhard Meister Ocean Biology Processing Group 13 May 2008 MODIS Science Team Meeting

10. MODIS calibration changes since launch band 8 412nm SD door permanently opened B side electronics A side electronics pre-launch damage to mirror coating, MS2 Terra SD – Solar Diffuser frame 979, mirror AOI 50.3 0 SV – Space View frame 23, mirror AOI 11.4 0 MS1 – Mirror Side 1 MS2 – Mirror Side 2

11. MODIS trends in ocean color products Terra/Aqua ratios normalized water-leaving radiances L wn L wn (412nm) band 8 L wn (443nm) band 9 L wn (488nm) band 10 L wn (531nm) band 11 L wn (551nm) band 12 SD door permanently opened

12. MODIS-Terra ocean color RVS band 8 412nm Terra response versus scan angle (RVS) in terms of normalized water-leaving radiances L wn L wn (412nm) band 8 Mirror side 1 MS1 Mirror side 2 MS2 -10% +10% per detector

13. Issues and limitations with MODIS-Terra On-board calibration (lunar or solar) CANNOT assess –changes in RVS “shape” –changes in polarization sensitivities Temporal and scan-dependent trend in L wn of MODIS-Terra suggests –instrument RVS may be in error –polarization sensitivity may be changing –on-board calibration capabilities may be degrading (SD, SDSM) OBPG developed a vicarious approach for on-orbit characterization of Terra –RVS, and –polarization sensitivity

14. Surface Effects Sun Glint White Caps Corrections based on statistical models (wind & geometry)

15. L wn (λ’) gasglint whitecap air aerosol L t (λ) = [ L r (λ) + L a (λ) + tL f (λ) + TL g (λ) + t d (λ)L w (λ) ] · t g (λ) water SeaWiFS 9-day mean from MODIS NIR assumes MCST NIR band characterization λ’ → λ fit based on bio-optical models Modeling of TOA Stokes vector over oceans LtQtUt0LtQtUt0

16. Vicarious TOA MODIS total signal L m (λ) = M 11 L t (λ) + M 12 Q t (λ) + M 13 U t (λ) = · LmQmUm0LmQmUm0 M 11 M 12 M 13 M 14 M 21 … M 24 M 31 … M 34 M 41 M 42 M 43 M 44 LtQtUt0LtQtUt0

17. TOA sensor cross-calibration L m (λ) – M 11 L t (λ) + M 12 Q t (λ) + M 13 U t (λ) minimize over global distribution of path geometries find best M 11, M 12, M 13 per band, detector, and mirror-side M 11, M 12, M 13 = f (mirror AOI) do this for one day per month over the mission lifespan MODIS measured TOA radiance, polarized m

18. Polarization of the atmosphere degree of atmospheric polarization d p air molecule (Rayleigh) and glint scattering polarization correction f p pre-launch MODIS characterization √Q t 2 + U t 2 LtLt d p = MODIS Terra swath 412nm band 8 LmLm LtLt f p =

19. Blue band temporal trends Mirror side 2 Detector 4 Mirror side 1 Space View (lunar) frame Nadir frame Solar Diffuser frame RVS = 1/M 11 412 443 488

20. Verification of the vicarious cross-calibration with lunar measurements band 8 412nm Terra detector ratios detector 10 / detector 1 lunar measurements — vicarious cross-cal 412nm band 8 Mirror side 1 MS1 Mirror side 2 MS2

21. Blue band temporal trends Mirror side 2 Detector 4 Mirror side 1 M12 Space View (lunar) frame Nadir frame Solar Diffuser frame 412 443 488

22. M13 fixed to pre-launch 412 443 488 Detector 4 Blue band RVS & polarization sensitivity MS1

23. Pre-launch M13 RVS M12 M13

24. Blue band RVS & polarization sensitivity MS2 M13 fixed to pre-launch 412 443 488 Detector 4

25. MODIS-Terra vicarious characterization RVSM12M13 412 531 678 RVSM12M13 443 488 551 667 MS1, Detector 4MS2, Detector 4

26. MODIS Terra and Aqua comparison RVSM12M13 412 531 678 RVSM12M13 443 488 551 667 Terra MS2, Detector 4Aqua MS2, Detector 4

27. Comparison of MODIS-Terra and MODIS-Aqua L wn After Vicarious Characterization Before Vicarious Characterization L wn (412nm) band 8 L wn (443nm) band 9 L wn (488nm) band 10 L wn (531nm) band 11 L wn (551nm) band 12

28. MODIS-Terra residual RVS in L wn 412443 After Vicarious Characterization 488 412443488 Before Vicarious Characterization MS1 MS2

29. MODIS-Terra temporal anomalies After Vicarious Characterization Before Vicarious Characterization L wn (488nm) band 10 chlor_a L wn (488nm) band 10 chlor_a

30. Summary MODIS-Terra shows evidence of residual error in instrument response versus scan-angle and on-orbit changes in polarization sensitivity (polarization amplitude of more than 30%). The OBPG developed a vicarious on-orbit characterization for visible band polarization and RVS changes over time, using SeaWiFS L wn. Results show significant improvement in agreement between MODIS- Terra and MODIS-Aqua L wn over the combined mission lifetime. Future work will focus on vicarious characterization of the NIR. Franz, B.A., E.J. Kwiatkowska, G. Meister, and C.R. McClain (2008). Moderate Resolution Imaging Spectroradiometer on Terra: limitations for ocean color applications. J. Appl. Rem. Sens., in press.