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Figure 3.7 – (Left) Bowtie effect due to the growth of pixel size in the cross track
direction of a swath (Right) Image from Terra MODIS band 17, extracted from an area
with a scan angle raging from 47˚ to 55˚. Bowtie effect appears as overlapping fields of
view.
geometrical features in successive scans. This is clearly visible along land/ocean transitions
(figure 3.7b). The OFOV method then exploits the information redundancy to equalize
the detectors responses. In the bowtie region (areas corresponding to scan angles higher
than ±25˚), some of MODIS detectors are viewing exactly the same scene and, therefore
thus deviations in the detectors reponses are only due to stripe noise. The OFOV algorithm
is divided in two steps. Following the classification of the detectors as in-family and outof
family detectors, a reference detector is selected and equalization functions are derived
using only bow-tie measurements. The equalization curves are then applied to the detectors
over the entire swath. To assess the accuracy of relative inter-detectors calibration, a Bowtie
Based Detector Distance (BTBDD) between detectors i and j is defined as :
d i,j =
∑ Ni,j
n=1 |I i(n) − I j (n)| .W (n)
∑ Ni,j
n=1 W (n) (3.11)
where N i,j is the number of OFOVs of detectors i and j, with an overlapping percentage
above η, a value fixed to 65%. I i (n) (respectively I j (n)) is the radiance measured by
detector i (respectively j) on the n th OFOV and W (n) is the overlapping percentage. In
addition to the BTBDD distances, the similarity between detectors ECDFs is computed
using the Kolmogorov-Smirnov distance :
D i,j = 1
N obs
N obs
∑
|P i (r(n)) − P j (r(n))| (3.12)
n