The instrument calibration makes use of indirect solar observations (via an onboard solar diffuser) and direct lunar observations to track and correct for the long-term temporal change in the MODIS sensor optics and electronics. A detailed description of the MODIS onboard calibration (OBC) capabilities is available here. The long-term temporal change has been modeled by MCST as a logarithmic function, with parameters optimized to best fit the solar diffuser observations. The current operational LUT is known as MCST version 4.3.1.5S, which was derived from OBC measurements obtained through March of 2004. Some extrapolation of the trends beyond the last OBC measurements is assumed, but the error in that extrapolation has become quite large, as can be seen in the deep-water comparisons with SeaWiFS.
The proposed LUT update is described in this draft report. It is based on the MCST LUT V5.0.1, which incorporates OBC measurements through November 2004. MCST fitted a combination of a logarithmic function and a second order polynomial to the solar calibration measurements. The OBPG proposes to use a modified version of this MCST-developed LUT. Analysis of the solar diffuser measurements has revealed a significant correlation between the variation in Sun beta-angle on the diffuser and periodicities observed in the solar-diffuser gains. The effects are band and detector dependent, and the evidence suggest that the likely cause is uncorrected vignetting of the diffuser screen (which is only used for the ocean bands, to prevent saturation). The OBPG removed this effect from the diffuser trends by statistical correlation, and refit the corrected diffuser trends to a simple exponential function to derive the new LUT used for the reflective solar bands 8-16. The differences between the MCST approach and the new approach are relatively small for those periods of time where calibration measurements are available (in this case, up to November 2004), but large differences can occur for the extrapolation of the calibration into the future. The report linked above provides details on the analysis of the solar diffuser measurements and the modifications to the LUT described in this paragraph.
The plot below shows the MODIS to SeaWiFS Deep-Water Ratio from a test processing using the proposed LUT. The differences between this plot and the one shown above include other changes which effect seasonal and absolute differences between the sensors, but the general reduction in long-term relative change can be traced to the LUT update.
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The MCST LUT V5.0.1 is compatible with collection 5 L1B code. This code was not available at the time of this writing, so the new OBPG LUT has the version number V4.3.1.11_OCh, which is compatible with the currently available collection 4 L1B code.
Last updated 19 January 2005, G. Meister & B. Franz
