MODIS/Aqua Straylight Flagging and Masking
The stray light of an instrument is characterized by its Point Spread Function (PSF). The PSF of MODIS Aqua has not been measured pre-launch. A Line Spread Function (LSF) has been measured in scan direction. From this and other information from SBRS, we are currently trying to derive a PSF. Our current version of the PSF for band 16, detector 1, is shown in the figure below on the left. The shape of the PSF is strongly influenced by the position of the detector on the focal plane. The central pixel typically measures only 66% of the correct intensity. A 3×3 window around the central pixel already captures about 0.9973% of the intensity, a 5×5 window about 0.9982%, etc., see plot on the right below. A correction for straylight using the PSF consumes a large amount of computing time and in the most important cases (e.g. clouds) requires a knowledge of the approximate radiance measured by saturated bands. Important details on the final form of the PSF and the implementation of a correction are still being worked out.
The MODIS PSF will result in stray light contamination from adjacent pixels. The largest impact is to low radiance ocean observations which are within a few kilometers of bright sources such as clouds, coastlines, or sun glitter. Until a correction can be developed, the near-term solution which is proposed for this reprocessing is to simply mask all observations within a few pixels of any cloud or saturation-flagged pixels. The model estimates of the point-spread function for MODIS suggest that the majority of the contamination can be eliminated with a 7×5-pixel masking (±3 pixels along scan, ±2 pixels along track) around these high-radiance pixels. This represents a considerable data loss at Level-2, but confidence in the remaining data is substantially enhanced.
A direct impact of this additional masking can be seen in the global deep-water aerosol optical thickness (AOT) comparisons with SeaWiFS. The straylight masking brings MODIS mean AOT retrievals down by more than 50%, and the results are now in good agreement with SeaWiFS. Note that the standard SeaWiFS processing already includes masking and correction for straylight contamination.
Last updated 19 January 2005, B. Franz & G. Meister.