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NASA’s operational approach for the vicarious calibration of

NASA’s operational approach for the vicarious calibration of on-orbit ocean color satellites (using Gordon and Wang 1994) Bryan Franz, Sean Bailey, the OBPG, and Jeremy Werdell MODIS Science Team Meeting, 1 Nov 2006. The vicarious calibration …

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NASA’s operational approach for the vicarious calibration of

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  1. NASA’s operational approach for the vicarious calibration of on-orbit ocean color satellites (using Gordon and Wang 1994) Bryan Franz, Sean Bailey, the OBPG, and Jeremy Werdell MODIS Science Team Meeting, 1 Nov 2006

  2. The vicarious calibration … … makes use of a single set of fractional gains, where unity indicates no correction … (Lt = counts * Krad * Ktime * Kx,y,z… * g … (minimize difference between satellite Lw and ground-truth Lw) Vicarious Calibration, NASA OBPG, 1 Nov 2006

  3. The vicarious calibration … … makes use of a single set of fractional gains, where unity indicates no correction … (Lt = counts * Krad * Ktime * Kx,y,z… * g … (minimize difference between satellite Lw and ground-truth Lw) … modifies the integrated instrument-atmospheric correction system … (effectively accounts for undetermined post-launch instrument changes … (and atmospheric correction biases) Vicarious Calibration, NASA OBPG, 1 Nov 2006

  4. The vicarious calibration … … makes use of a single set of fractional gains, where unity indicates no correction … (Lt = counts * Krad * Ktime * Kx,y,z… * g … (minimize difference between satellite Lw and ground-truth Lw) … modifies the integrated instrument-atmospheric correction system … (effectively accounts for undetermined post-launch instrument changes … (and atmospheric correction biases) … assumes that temporal trends are independently removed Vicarious Calibration, NASA OBPG, 1 Nov 2006

  5. The vicarious calibration … … makes use of a single set of fractional gains, where unity indicates no correction … (Lt = counts * Krad * Ktime * Kx,y,z… * g … (minimize difference between satellite Lw and ground-truth Lw) … modifies the integrated instrument-atmospheric correction system … (effectively accounts for undetermined post-launch instrument changes … (and atmospheric correction biases) … assumes that temporal trends are independently removed … was updated for SeaWiFS Reprocessing 4 (July 2002) Vicarious Calibration, NASA OBPG, 1 Nov 2006

  6. Lt = Lr,g,wc,… + La + td Lw Vicarious Calibration, NASA OBPG, 1 Nov 2006

  7. Lt = Lr,g,wc,… + La + td Lw pre-Reprocessing 4 (launch – July 2002) gain calculated using Lw g =Lwtarget / Lw assign g and iterate until mean radiance ratio reaches unity Vicarious Calibration, NASA OBPG, 1 Nov 2006

  8. Lt = Lr,g,wc,… + La + td Lw pre-Reprocessing 4 (launch – July 2002) gain calculated using Lw g =Lwtarget / Lw assign g and iterate until mean radiance ratio reaches unity post-Reprocessing 4 (July 2002 – present) gain calculated at Lt g = Lttarget / Lt satellite atmospheric parameters used to propagate Lwtarget to TOA * no iteration (less computationally expensive) * permits exploration of gain offset * requires disabling correction for non-negligible Lw(NIR) methods agree to within 0.04% Vicarious Calibration, NASA OBPG, 1 Nov 2006

  9. Lt = Lr,g,wc,… + La + td Lw Lttarget = Lr,g,wc,… + La + td Lwtarget post-Reprocessing 4 (July 2002 – present) gain calculated at Lt g = Lttarget / Lt satellite atmospheric parameters used to propagate Lwtarget to TOA * no iteration (less computationally expensive) * permits exploration of gain offset * requires disabling correction for non-negligible Lw(NIR) methods agree to within 0.04% all terms are computed for the full relative spectral response of each sensor band Vicarious Calibration, NASA OBPG, 1 Nov 2006

  10. SUN Lwn = Lw / ( cos(sun) tdFdist ) TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  11. SUN Lwn(qsun≠0, qsens =0, frel =0 ) Lwn TARGET f/Q from Morel et al. 2002 Vicarious Calibration, NASA OBPG, 1 Nov 2006

  12. SUN SUN Lwn(qsun=0, qsens =0, frel =0 ) Lwn Lwnex TARGET f/Q from Morel et al. 2002 Vicarious Calibration, NASA OBPG, 1 Nov 2006

  13. SUN SUN SUN SATELLITE Lwn(qsun≠0, qsens ≠0, frel ≠0 ) Lwn Lwnex Lwn TARGET f/Q from Morel et al. 2002 Vicarious Calibration, NASA OBPG, 1 Nov 2006

  14. SATELLITE Lttarget TOP OF ATMOSPHERE from the satellite + Lr , td , … TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  15. our operational targets … Vicarious Calibration, NASA OBPG, 1 Nov 2006

  16. Lt(NIR) = Lr,g,wc,…(NIR) + La(NIR) + td Lw(NIR) assumptions: SATELLITE Lttarget TOA from the satellite + Lr , td , … Lwtarget TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  17. Lt(NIR) = Lr,g,wc,…(NIR) + La(NIR)+ td Lw(NIR) assumptions: (1) target sites exist where aerosol type is known (1) and Lw(NIR) is negligible 0 M90 SATELLITE Lttarget TOA from the satellite + Lr , td , … TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  18. Lt(NIR) = Lr,g,wc,…(NIR) + La(NIR)+ td Lw(NIR) assumptions: (1) target sites exist where aerosol type is known (1) and Lw(NIR) is negligible (2) 865-nm perfectly calibrated, (2) such that g(865) = 1.0 0 M90 SATELLITE Lttarget TOA from the satellite + Lr , td , … TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  19. Lt(NIR) = Lr,g,wc,…(NIR) + La(NIR)+ td Lw(NIR) assumptions: (1) target sites exist where aerosol type is known (1) and Lw(NIR) is negligible (2) 865-nm perfectly calibrated, (2) such that g(865) = 1.0 implementation: knowledge of the aerosol type and La(865) permits the estimation of La(765) once La(765)target known, calculate Lt(765)target 0 M90 SATELLITE Lttarget TOA from the satellite + Lr , td , … TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  20. the NIR calibration is completed prior to the VIS calibration … the g(NIR) are now fixed the OBPG use data from the Marine Optical Buoy (MOBY) for the VIS calibration MOBY provides hyperspectral Lwn(VIS) measured during the satellite overpass Vicarious Calibration, NASA OBPG, 1 Nov 2006

  21. Lt(VIS) = Lr,g,wc,…(VIS) + La(VIS) + td Lw(VIS) implementation: calibrated NIR bands used to determine local aerosol type and concentration, which provides La(VIS)target Lttarget TOA from the satellite + Lr , td , … Lwtarget TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  22. Lt(VIS) = Lr,g,wc,…(VIS) + La(VIS) + td Lw(VIS) implementation: calibrated NIR bands used to determine local aerosol type and concentration, which provides La(VIS)target calculate Lt(VIS)target using La(VIS)target , Lw(VIS)target , and satellite td , … Lttarget TOA from the satellite + Lr , td , … Lwtarget TARGET Vicarious Calibration, NASA OBPG, 1 Nov 2006

  23. various complications: (1) f/Q requires an estimation of Ca (2) non-hyperspectral targets (conceptually) require adjustment to the (2) satellite spectral bandpass (3) non-clear water targets require an estimation of Lw(NIR) (4) polarization can be problematic as the correction depends on the (4) observed radiances Vicarious Calibration, NASA OBPG, 1 Nov 2006

  24. locate L1A files extract 101x101 pixel box process to L2 target data extract 5x5 box Vicarious Calibration, NASA OBPG, 1 Nov 2006

  25. locate L1A files extract 101x101 pixel box process to L2 target data extract 5x5 box limit to scenes with average values: < 0.20 Ca < 0.15 (865) < 60 sensor zenith < 75 solar zenith identify flagged pixels: LAND, CLDICE, HILT, HIGLINT, ATMFAIL, STRAYLIGHT, LOWLW require 25 valid pixels calculate gpixel for each pixel in semi-interquartile range; then: gscene = gpixel / npixel • calculate gains for each matchup Vicarious Calibration, NASA OBPG, 1 Nov 2006

  26. locate L1A files extract 101x101 pixel box process to L2 target data extract 5x5 box limit to scenes with average values: < 0.20 Ca < 0.15 (865) < 60 sensor zenith < 75 solar zenith identify flagged pixels: LAND, CLDICE, HILT, HIGLINT, ATMFAIL, STRAYLIGHT, LOWLW require 25 valid pixels calculate gpixel for each pixel in semi-interquartile range; then: gscene = gpixel / npixel • calculate gains for each matchup • calculate final, average gain limit to gscene within semi-interquartile range visually inspect all scenes g = gscene / nscene Vicarious Calibration, NASA OBPG, 1 Nov 2006

  27. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  28. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  29. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  30. g remains stable as a function of time, both long-term and seasonally * verifies temporal calibration * suggests consistency in MOBY deployments * scatter (5%) underscores need for independent temporal calibration * (SeaWiFS 443-nm has degraded 2% since launch) gconsistent with both solar and sensor zenith angles variations with geometry might suggest problems with * the atmospheric correction * the f/Q correction * the in situ determination of Lw under certain sky conditions Vicarious Calibration, NASA OBPG, 1 Nov 2006

  31. input the calibration scenes into validation system … satellite-to-in situ biases and mean ratios approach zero and unity, respectively Vicarious Calibration, NASA OBPG, 1 Nov 2006

  32. input the calibration scenes into validation system … satellite-to-in situ biases and mean ratios approach zero and unity, respectively RMSs and absolute MPDs not entirely negligible compare with MPD of 13% for deep water validation set at 443-nm Vicarious Calibration, NASA OBPG, 1 Nov 2006

  33. changes in g with increasing sample size … standard error of g decreases to 0.2% overall variability (min vs. max g) approaches 0.5% provides insight into temporal calibration, statistical choices Vicarious Calibration, NASA OBPG, 1 Nov 2006

  34. future ruminations … … statistical and visual exclusion criteria influence g only slightly, yet … they reduce the standard deviations … can uncertainties be quantified … for the assigned thresholds? … how do the uncertainties of the embedded models (e.g., f / Q, the NIR- … correction, etc.) propagate into the calibration? … what are the uncertainties associated with Lwtarget? Vicarious Calibration, NASA OBPG, 1 Nov 2006

  35. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  36. 1993 SBRC / 1997 NIST Vicarious Calibration, NASA OBPG, 1 Nov 2006

  37. SeaWiFS validation for the deep water subset: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  38. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  39. MODIS-Aqua validation for the deep water subset: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  40. Backup slides … Vicarious Calibration, NASA OBPG, 1 Nov 2006

  41. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  42. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  43. Vicarious Calibration, NASA OBPG, 1 Nov 2006

  44. SeaWiFS validation for the global data set: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  45. MODIS-Aqua validation for the global data set: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  46. SeaWiFS validation statistics: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  47. MODIS-Aqua validation statistics: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  48. SeaWiFS validation statistics: Vicarious Calibration, NASA OBPG, 1 Nov 2006

  49. MODIS-Aqua validation statistics: Vicarious Calibration, NASA OBPG, 1 Nov 2006

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