i Detection of Smoke and Dust Aerosols Using Multi-sensor Satellite ...

different spectral bands are slightly mismatching, it will lead to undesired effects when
they are combined together and consequently less accurate data products.
7.2.1 Impact on the L1B measurements
7.2.1.1 Uncertainty from SZA correction
Solar Zenith Angle (SZA) is the angle between the local zenith and the line of sight
to the sun. As one of important parameters in MODIS Level 1A (L1A) geo-location
measurement (MOD03 for Terra/MODIS and MYD03 for Aqua/MODIS), SZA is usually
used for L1B response (radiance or reflectance) correction of RSBs. The SZA is the
function of latitude and longitude, not identical in scan and track directions. Fig. 7.5
provides a case to describe the model of the SZA change, in which the first, middle, and
last row/column are selected as representative for scan/track direction. In the Fig., the
SZA changes non-linearly with the frame in scan direction because of bow-tie effect
(Wolfe et al., 2002); rapidly at both ends while slowly in the middle frame. On the other
hand, the SZA change shows a proximately linear model for all rows in track direction.
The correction is executed by dividing the cosine of SZA with original L1B
measurement for corresponding pixel in one granule (Barbieri et al., 1997), given in Eq.
(7.3)
R R / cos( SZA)
(7.3)
where R and R’are the response of given RSB before and after correction respectively,
and SZA’is the s corresponding SZA stored in MODIS L1A dataset. However, SZA’is a
incorrect angle due to the mis-registration. The real response R ’should be computed
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