CZCS Electronic Ringing Mask
The CZCS instrument normally delivers a voltage response to the light intensity in a scene in a linear manner, making it useful for scientific work. However, if it encounters a very bright source, such as cloud tops and some land features, the detectors will not give accurate measures of the far less intense down-scan features. This ringing needs to be either corrected or masked from processing.
Several masking techniques used in past processings were examined. They fall into 2 categories:
- Mueller's CZCS ringing mask. The Mueller algorithm uses the excess brightness in the previous bright pixels to determine how many of the following samples need to be masked. It produced good results for clouds in the ocean, although sometimes it was conservative. Coastal areas were significantly affected, although imagery frequently showed a much decreased ringing problem.
- Evans and Gordon ringing mask. The mask that Evans and Gordon (94) used for the ringing mask was based more on the resultant water-leaving radiances in 520 nm. It seemed to consistantly mask a large number of samples after a bright target, regardless of it's intensity. After very bright targets, it did not mask enough.
Based on the tests of both methods, it was decided to use the Mueller ringing mask in CZCS processing. In the process of examining the methods, several problems were also found in the previous algorithmic implementations.
As was mentioned before, the Mueller mask works very well with bright clouds but over-corrects frequently off of coastal sites. The technique was modified to apply the mask based on the amount of saturation in the other visible bands. This modification was found to mask ringing well while retaining more samples in coastal regions. This was the masking algorithm adopted for the processing.