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Correction of subframe striping in high resolution MODIS ocean color products

Correction of subframe striping in high resolution MODIS ocean color products. Gerhard Meister a , Chunhui Pan b , Frederick S. Patt a , Bryan Franz a , Jack Xiong b , Charles McClain a

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Correction of subframe striping in high resolution MODIS ocean color products

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  1. Correction of subframe striping in high resolution MODIS ocean color products Gerhard Meistera, Chunhui Panb, Frederick S. Patta, Bryan Franza, Jack Xiongb, Charles McClaina a: OBPG (Ocean Biology Processing Group; Futuretech, SAIC, or NASA) b: MCST (MODIS Characterization Support Team; SSAI or NASA) NASA Goddard Space Flight Center August 27, 2007

  2. Background • Ocean color products (e.g. chlorophyll) are derived from water-leaving radiances • Ocean color remote sensing imposes very stringent requirements on radiometric precision (typically only 10% or less of the TOA signal is of interest) • Very small calibration differences between adjacent pixels can result in noticeable artifacts (e.g. striping) in the product images

  3. Example of subframe striping Coastal scene of nLw at 645nm Cutout from coastal scene

  4. MODIS Optical Path Page 4

  5. MODIS Focal Plane Design

  6. Subsampling for a 500m band • a) Detectors on the focal plane • b) IFOV without subsampling • c) IFOV with subsampling

  7. Subsampling used for bands 1-7

  8. Center wavelengths for bands 1-7 Oceancolor applications: Band 1: Water-leaving radiances => sediments Bands 3,4: Water-leaving radiances => chlorophyll Bands 5-7: Atmospheric correction (band 6 problematic on Aqua)

  9. First correction method: Analysis of prelaunch data • System level test in thermal vacuum • 100cm diameter SIS (spherical inte-grating source), scan mirror rotating • 38 different radiance levels, from 0.3 Ltyp to 0.9 Lmax • Significant variations of the subframe ratios as a function of radiance • No significant variation as a function of temperature (251K, 268K, 281K)

  10. First correction method: Analysis of prelaunch data • Prelaunch results in dn • On-orbit L1B data will be normalized to 1.0 at the solar diffuser radiance LSD • Therefore, prelaunch results will be normalized to 1.0 at LSD as well • In most cases, these normalization factors agree well with subframe calibration factor (m1) ratios

  11. First correction method: Analysis of prelaunch data No normalization: measured (stars) and fitted (solid line) Normalization to LSD : measured (diamonds) and fitted (dashed line) Calibration factor (m1) ratios: SF1/SF4: 1.0002, SF2/SF4: 1.0052, SF3/SF4: 1.0003

  12. Second correction method: Analysis of on-orbit data • TOA radiance median of a single ocean granule: Dashed line: Dashed line: Dashed line: Subframe 1 Subframe 1 Subframe 2

  13. Second correction method: Analysis of on-orbit data • Time series of on-orbit m1 (solar diffuser calibration coefficient) ratios for band 1: -m1 Ratios for non-striping subframes are close to 1 -Band 1 has significant subframe 4 detector variation (detectors 38-40 out-of-family)

  14. Second correction method: Analysis of on-orbit data • Time series of band 2 on-orbit solar diffuser calibration coefficient ratios: - No on-orbit temporal trend for either band at solar diffuser radiance level - Only band 1 has a significant detector variation

  15. Analysis of on-orbit data: Method • For each band, 60 radiance brackets were defined • 10+ granules were selected to calculate average subframe ratio • For each pixel, the first subframe (or first 3 subframes for bands 1 and 2) was divided by the last subframe • The ratio was averaged for each radiance bracket • If a granule had more than 10000 ratios for a radiance bracket, the average ratio was used in the averaging over granules • Third order polynomials (in some cases two for different radiance ranges) were fitted to the data

  16. Analysis results: Band 1 • SF1/SF4: strong variation for low radiances, good agreement for both on-orbit and prelaunch analysis • SF2/SF4 and SF3/SF4: no variation in on-orbit results, decrease of SF4 in prelaunch results • Adjusting for SF4 would improve SF1 agreement even more

  17. Analysis results: Band 2 • SF1/SF4:flat in both cases, but offset in on-orbit results • SF2/SF4: increase for low radiances • SF3/SF4: prelaunch flat, on-orbit slight increase with radiance

  18. Analysis results: Bands 3-7

  19. Results per granule Symbols: subframe ratios from 3 granules Solid line: On-orbit and prelaunch correction Variation with granule larger than variation between on-orbit and prelaunch analysis ! Related to overall radiance level ?

  20. Striping evaluation: L1B radiances • Difference between adjacent subframes in scan direction:DeltaS(i,j) = | L(i,j) – L(i,j-1) | / ( L(i,j)+L(i,j-1) ) • Average difference < DeltaS> calculated for all pixels (i,j) in a granule, and separated by radiance range: Units: MODIS radiance units

  21. Striping evaluation: L1B radiances (granule average) Prelaunch (diamonds): better for high radiances in most bands On-orbit (stars): better for low radiances in bands 1-4,7

  22. Striping evaluation: L1B radiances (per granule) • Comparison of improvement for low radiance range (solid line) and high radiance range (dashed line) for prelaunch and on-orbit correction

  23. Striping evaluation: water-leaving radiances at 645nm Cutout from coastal scene No correction: Corrected with on-orbit correction:

  24. Summary • Linear MODIS Aqua subframe calibration needs adjustment by up to 1.5% at very low radiances • Correction based on subframe radiance level alone may not be sufficient; further analysis necessary • Corrections derived from on-orbit and prelaunch data agree qualitatively • On-orbit correction performs better for low radiances in bands 1-4 and 7, implemented into SeaDAS for ocean color processing

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