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MODIS Aqua PSF (Point Spread Function)

MODIS Aqua PSF (Point Spread Function). Gerhard Meister, OBPG NASA Goddard Space Flight Center (Futuretech Corp.) January 4, 2006 . Background. Ocean color remote sensing imposes very stringent requirements on radiometric accuracy of bands relative to each other.

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MODIS Aqua PSF (Point Spread Function)

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  1. MODIS Aqua PSF (Point Spread Function) Gerhard Meister, OBPG NASA Goddard Space Flight Center (Futuretech Corp.) January 4, 2006

  2. Background • Ocean color remote sensing imposes very stringent requirements on radiometric accuracy of bands relative to each other. • Stray light effects are very large for bright clouds next to dark ocean water. • Theoretical solution: correction with point-spread function (PSF). • Problems: PSF not well characterized, light source intensity sometimes unknown, computationally very expensive.

  3. PSF definition: Lm (i0,j0) = Sij PSF(i- i0,j- j0)*LT(i,j) Lm = Measured radiance LT = True radiance PSF = Point Spread Function i = line index j = column index PSF kernel 25x25: i= i0-12, i0 -11,..., i0+11, i0+12 j= j0-12, j0 -11,..., j0+11, j0+12

  4. Available from SBRS: • LSF (Line Spread Functions): smaller slit, 1 pixel from either side of slit, dynamic range 1-10-3, scan and (simulated) track direction • NFR (Near Field Response) measurements: scans of 1x10 slit (slit in track direction), up to 100 pixels to either side of peak, dynamic range 1-10-7 • NFR modeling: scatter model, certain shape parameters, no absolute scale for PSF

  5. Creation of PSF: Assume that PSF of adjacent pixel in scan direction is given by 0.125/0.75 (theoretical value from Geolocation ATBD) of center value, adjacent pixel in track direction is 5% (from LSF) Adjust center value and scale parameter from SBRS model (Harvey-Shack) so that NFR measure- ments are reproduced

  6. NFR measurements and model: Band 11

  7. NFR measurements and model: All bands

  8. Paper by Qiu et al. on Terra PSF:

  9. Aqua and Terra: Band 11 with Lcloud/Ltypical=20

  10. Aqua: all bands with Lcloud/Ltypical=20 (PSF size:512x512)

  11. Aqua: all bands with Lcloud/Ltypical from 10-40

  12. Same as before, but 10x10 pixel cloud

  13. 10x10 pixel cloud, band 11 only, different PSF sizes

  14. 100x100 pixel cloud, band 11 only

  15. Summary • Created Aqua PSF based on SBRS model and NFR measurements • For huge clouds, TOA radiances 50 pixels away from cloud will be contaminated up to 1% (even after correcting with 101x101 PSF) • For 100x100 clouds, radiances will be up to 0.5% too high after correction with 25x25 PSF (0.2% for 51x51) • For small clouds, radiances can be adequately corrected with 25x25 PSF as close as 2 pixels to the cloud

  16. Outlook: • Still need to check the model in track direction • Refinement needed for adjacent pixel • Validate with real data (nLw should ‘look better’, but more noise is expected) • If successful, implement into SeaDAS as an option • Work with MCST on Terra PSF derivation/validation

  17. Backup slides:

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