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On-Orbit Polarization Sensitivity of MODIS Detector: Insights from Characterization

This study delves into the assessment of the polarization sensitivity of MODIS detector through prelaunch and on-orbit characterization. Detailed analysis includes comparisons with SeaWiFS data and ideal measurements, addressing issues with homogeneity and calibration coefficients. Results show discrepancies in detector dependencies and propose constant offsets for Terra and Aqua MODIS calibration.

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On-Orbit Polarization Sensitivity of MODIS Detector: Insights from Characterization

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  1. Detector dependency of MODIS Polarization Sensitivity derived from on-orbit characterization Gerhard Meistera,b, Bryan A. Franzb,c, Ewa J. Kwiatkoskad, Robert E. Epleee,b, Charles R. McClainb,c a: Futuretech Corp. b: OBPG (Ocean Biology Processing Group) c: NASA Goddard Space Flight Center, Code 614.2 d: TEC-EEP, ESTEC, ESA e: SAIC 8/03/09 Earth Observing Systems XIV, SPIE Optics and Photonics 2009, San Diego

  2. Overview • MODIS prelaunch polarization characterization • MODIS on-orbit polarization characterization using SeaWiFS • Comparison of results • MODIS solar diffuser on-orbit calibration • MODIS on-orbit characterization using SeaWiFS and moon • Comparison of results

  3. MODIS Prelaunch Polarization Characterization • MODIS prelaunch characterization was done by Raytheon (SBRS), using PSA (Polarization Source Assembly) • An Ahrens prism is rotated, light is collimated to MODIS entrance aperture

  4. Ideal Polarization Characterization • Ideal measurements show two sinusoidal curves when rotating the polarization plane by 360o • In test below (NASA radiometer, data provided by Eugene Waluschka), polarization plane was rotated by 720o • Ideal response can be described by amplitude and phase (or the two second order Fourier coefficients)

  5. MODIS Prelaunch Polarization Characterization • Strong non-two-cycle effects in data: increase with beta angle, no clear two cycle, sometimes four-cycle • Polarization amplitude pa and phase delta derived fromIm(beta)= It(1+pa*cos(2beta-2delta)

  6. MODIS Prelaunch Characterization PSA Issue: Homogeneity

  7. MODIS Prelaunch Characterization PSA Issue: Four-cycle

  8. Vicarious calibration of MODIS to SeaWiFS: • Rayleigh, foam, glint can be modeled • Aerosol radiances from NIR bands using standard calibration • Water-leaving radiances from SeaWiFS level 3 data Where do we get the true radiance from ? LTOA = [ Lrayleigh + Laerosol + tLfoam + TLglint + tdLw ] · tg (T, t, td, tg: transmittances )

  9. Optimization process Lm: MODIS measured TOA radiance (without polarization corr.) LTOA: modeled (nLw from SeaWiFS, aerosols from MODIS NIR bands) Q, U: Stokes vector parameters (modeled from Rayleigh) Lm = M11LTOA + M12Q + M13U • find best M11, M12, M13 per band, detector, and mirror-side • M11, M12, M13 = f (scan angle), cubic (M11) and linear (M12) • do this for one day per month over the mission lifespan • optimize over global distribution of path geometries

  10. M11: MODIS Terra blue band temporal trends Mirror side 1 Mirror side 2 RVS = 1/M11 412 Detector 4 443 Space View (lunar) frame Nadir frame Solar Diffuser frame 488

  11. M12: MODIS Terrablue band temporal trends Mirror side 1 Mirror side 2 M12 acts on the Q component 412 Detector 4 443 Space View (lunar) frame Nadir frame Solar Diffuser frame 488

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

  13. MODIS Terra and Aqua comparison Terra MS2, Detector 4 Aqua MS2, Detector 4 1/M11 M12 M13 1/M11 M12 M13 412 443 488 531 551 667 678

  14. M12 as a function of time from on-orbit characterization

  15. Detector polarization dependency prelaunch and on-orbit: band 12

  16. Detector polarization dependency prelaunch and on-orbit: band 8

  17. MODIS SD Measurement Setup (Waluschka et al., 2004)

  18. M11 as a function of time from on-orbit characterization

  19. Comparison of Terra M11 from vicarious calibration to SeaWiFS for 3 view angles lunar detector residuals and Aqua operational corr. Black, blue, red line: Terra M11 for -45deg,0deg,+45deg Black (green) triangles: lunar residuals for Terra (Aqua) Green diamonds: operational Aqua detector corrections

  20. Aqua M11 from vicarious calibration to SeaWiFS for 3 view angles Black, blue, red line: Terra M11 for -45deg,0deg,+45deg Dashed lines: +/- 0.1%

  21. Conclusions • Detector dependency of MODIS prelaunch polarization characterization is suspect • Vicarious calibration of MODIS to SeaWiFS shows very small variation of polarization sensitivity for detectors, opposite to prelaunch results for bands 10-13 (~1% variation) • For bands 8 and 9, there is good agreement • For the first four years on-orbit, MODIS Terra and Aqua need constant offsets to calibration coefficients m1 (detector 1 too high, detector 10 too low, ~1% variation) • These offsets do not vary with scan angle

  22. Backup slides

  23. MODIS Prelaunch Characterization PSA Issue: Homogeneity

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

  25. M12 average and standard deviation

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