Detector-dependent radiometric correction
Each 1-km MODIS bands is compised of 10 independent detectors, and each detector must
be independently calibrated. The primary mechanism to remove relative differences between
detectors is by way of the Solar Diffuser (SD) calibration. However, the SD calibration
seems to leave some residual differences between detectors, resulting in low-level striping
which is evident in the derived product images (e.g., nLw_412).
MODIS/Aqua detector-to-detector relative calibration differences were quantified
by MCST using the Moon as a reference point.
For every lunar observation, integrated lunar irradiance was computed for each band
and detector. Then, its value was normalized by the band averaged lunar irradiance.
The residuals from this normalization were detector dependent and exhibited noisy
behavior through time. Averaging of the residuals through time provided the
detector-to-detector relative calibration factors.
The Ocean Biology Processing Group (OBPG) independently confirmed the
MCST results. For each lunar calibration,
integrated lunar reflectance was calculated for each band and detector. The nearest
in time set of solar diffuser calibration factors, m1's, was used to convert the
detector counts to reflectance. The focal plane temperature corrections were also
applied to the data. The relative response of each detector was obtained by
normalizing the reflectance for each detector by the band-averaged reflectance. The
detector-dependent result was then averaged over the 29 lunar calibrations acquired to
date over the Aqua mission.
Unfortunatley, the detector-to-detector analysis using lunar observation can only be performed
for MODIS reflective solar bands which do not saturate when viewing the Moon. The
non-saturating 1km bands are 8-12 and 17-19. Detector dependence in red through near-infrared
ocean color bands, 13-16, cannot be evaluated using the lunar measurements. An alternative approach
to detector-to-detector relative calibration was therefore investigated which applies to all ocean
color bands and whose initial results for bands 8-12 agree relatively well with the MCST and OBPG
numbers based on lunar observations. The figure below shows the comparison of detector relative
calibration change obtained by MCST (red stars), OBPG (blue stars), and the alternative analysis
based on sensor's top-of-the-atmosphere (TOA) radiances (circles, filled circles for the mirror
side 1 and unfilled circles for the mirror side 2).
MODIS detector striping was alternatively quantified using sensor's
TOA radiances (Ewa Kwiatkowska). For the
analysis, runs of consecutive 20 pixels in the flight
direction were extracted from processed level 2 granules. The 20-pixel runs passed through strict
exclusion criteria, where
the following flags were used as masks: ATMFAIL, LAND, HIGLINT, HILT, STRAYLIGHT, CLDICE,
COCCOLITH, LOWLW, CHLFAIL, CHLWARN, NAVWARN, ABSAER, MAXAERITER, ATMWARN, HISOLZEN, HISATZEN, NAVFAIL,
FILTER, SSTWARN, SSTFAIL, TURBIDW
chlorophyll-a concentration within each run was limited to 0.15 mg/m^3
and a very clear atmosphere was selected with aerosol optical thickness at the 869 nm band below
0.05
For the extracted runs, TOA radiances at all MODIS ocean color bands were investigated and their
statistical dependence on the detector and mirror side were quantified.
Because MODIS consecutive detectors view the ocean using slightly different geometry, some detector
dependence in TOA radiances is expected. The problem therefore reduced to separating instrument artifacts
from atmospheric and ocean variabilities caused by viewing path differences. To apply the viewing-path
correction, for each 20-pixel run additional information was extracted, including molecular (Rayleigh)
radiances, aerosol radiances, and water-leaving radiances multiplied by the diffuse transmittance through
the atmosphere. The contribution of these radiances to the total TOA radiance was corrected for the
detector-to-detector variability using the estimated change for each geometry derived from spatially flat
fields of molecular atmosphere, aerosol optical thickness, and normalized water-leaving radiances.
The striping analysis was performed using MODIS granules from a single day, 14 August 2002. The striping
correction derived from this single day is applied in the current calibration table
used in the MODIS-Aqua reprocessing 1.1. More intensive
analyses are planned for the future when the striping code is implemented in the ocean color
processing environment. Large global datasets will then be used and possible temporal changes in the detector and mirror side behavior will be
traced over the mission lifetime.
The images below illustrate the impact on the chlorophyll-a product from the forward stream. The image on top was acquired on June 10th, 2005, and processed with
the uncorrected calibration, the image below was acquired on June 13th, 2005 and processed with the corrected calibration. The same color bar is used for both
images. Although some stripes are still apparent, their frequency and intensity has been reduced.
Processed without striping correction:
Processed with striping correction:
