AIRS Level 1B Visible/Near-Infrared Channels ATBD - NASA's Earth ...

AIRS Level 1b ATBD Part 2: Vis/Near IR Channels
Figure 2.4: Cross-calibration of AIRS channel 3 (a and b) and 4 (c and d) using MODIS data. As
in Fig. 2.3, calculations were made with 3 Atmospheric profiles, 5 solar zenith angles, 3 aerosol
types with 3 visibilities each, cloud optical thickness 0, 1 and 2 with 4 effective radii at 5 heights.
This implies that only a pure desert area, like White Sands, New Mexico, can be used as a
ground target for AIRS 2
and AIRS 3
cross-calibration. However, desert and semi-desert areas
can be used as ground targets for AIRS 1
and AIRS 4
cross-calibration. The acceptable ground
targets for the cross-calibration of AIRS 1
and AIRS 4
are therefore more abundant. It can also
be seen that only the surface albedo has a significant effect on the accuracy of the crosscalibration.
The influence of the atmospheric profile, aerosol type, visibility, and layer of thin
clouds on the cross-calibration is negligible. This means that, unlike the ground-based
calibration, no local meteorological or surface condition data are required for the crosscalibration.
Therefore, the cross-calibration can be performed as frequently as required.
After 5 years in orbit, the spectral response of AIRS 4
is expected to degrade as shown in Fig.
1.2. Figure 2.5 shows the AIRS 4
radiance as a function of linear combinations of MODIS
radiance data, similar to Fig 2.4, but for expected beginning and end of life conditions. Note
that as the AIRS 4
response changes, the signal from a given ground target will change and
the MODIS cross-calibration coefficients in Table 2.2 will change as well. If desired, the
end-of-life AIRS 4
data can be restored to beginning-of-life equivalent values by assuming the
linear relation
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