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.