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MODIS is a major instrument for the NASA EOS Terra and Aqua missions, launched in December 1999 and May 2002
respectively. MODIS has 16 thermal emissive bands and they are calibrated using an onboard blackbody (BB) based on
a nonlinear second order relationship. While the gains of the MODIS thermal bands are calibrated on a scan-by-scan
basis, the offset and non-linear terms are determined either from prelaunch or on-orbit measurements during scheduled
BB warm-up and cool-down cycles. A major concern on determination of the offset and non-linear terms from on-orbit
BB measurements is that the controlled BB temperature range is relative small compared to the temperature range used
in prelaunch tests, which could have impacts on the retrieval of brightness temperatures (BT) well outside the calibration
range. In this study, the stability of offset and non-linear terms obtained from BB warm-up/cool-down cycles is
presented. Several approaches to derive the on-orbit offset and non-linear terms are used and their impacts on the Earth
scene BT estimates are examined. By comparison with BT derived using prelaunch offset and nonlinear terms under the
same electronic configuration, it is shown that the current approach of deriving on-orbit offset and nonlinear terms
applied in L1B radiance products causes positive BT biases of exceeding 1K at low temperatures for middle- to longwave
IR bands. Comparison of MODIS and AIRS (The Atmospheric Infrared Sounder), both on-board Aqua spacecraft,
for cold temperature scenes at Antarctica for two long-wave IR bands also indicates that there are temperature-dependent
positive BT biases for about the same magnitudes. Results of this study have a significant impact on improving the current approach of setting a0 and a2 used to produce MODIS L1B data products.
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