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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

MODIS Aqua PSF (Point Spread Function)

Gerhard Meister, OBPG

NASA Goddard Space Flight Center

(Futuretech Corp.)

January 4, 2006

background
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.
psf definition
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

available from sbrs
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
creation of psf
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

summary
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
outlook
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