Modified time dependency of radiometric calibration
The radiometric degradation of the MODIS Aqua ocean color bands
can be well described by an exponential function for the early part of the mission.
Recently, it has become clear that this exponential model is not applicable to more
recent calibration measurements from the on-board solar diffuser and the moon. A new
model was developed that consists of an exponential degradation in the early part of
the mission and a linear change in the later part of the mission. The transition date from
exponential to linear varies by band, see
this PDF file, where
the old (blue line) and the new (red line)
radiometric calibration coefficients (m1, evaluated at the SD view angle (pixel 983) and at the lunar view angle (pixel 22), averaged over all 10 detectors of each band) are compared as a function of time. The normalized values are shown on the left, the ratios are shown on the right. The changes are
usually less than 0.2% before January 2005, but higher than 1% in band 8 (412nm) for
April 2005. For the NIR bands (15 and 16), the new m1 are about 0.3% lower in April 2005.
Except for band 13, the sensitivities for all bands
decrease
in the early part
of the mission (which corresponds to an increase in the m1). In the later part of the mission, the linear change can be either positive
or negative. Especially in the atmospheric correction bands, the sensitivities of
several detectors have been
increasing recently. In band 8, the
rate of
degradation has
increased recently. None of these developments in the later
part of the mission were captured by the old model assumption of a continous
exponential decrease of sensitivity.
A mission test was performed to evaluate the impact of the change in instrument calibration
on the derived ocean products. As shown in the plot below, the change in global-averaged, deep-water,
water-leaving radiances is less than 2% before January 2005. In April 2005, the Reprocessing 1.1
averages are about 10% higher than those of Reprocessing 1 for band 8 (412nm, green line),
and about 2% higher for band 10 (488nm, blue line). The changes in April 2005
of bands 9 (443nm, red line) and 12 (551nm, pink line) are about 3% and 2%,
respectively.
The complete analysis is available
here.
