NASA Ocean Biology Processing Group September 2006
CZCS data in the original CRTT (Calibrated Radiance and Temperature Tape) file
format contains up to a maximum of 970 lines (approximately 2 minutes)
of CZCS data, which was extracted from the periods of the orbit where the
CZCS was turned on. The files created this way from the mission
frequently overlap 2 or more times in coverage and quality. Thus,
merging the data into orbit-sized files has the advantage of having fewer
files to deal with, the elimination of duplicate data and the reduction
of poorer data in the overalp regions.
The method for merging the files tries to follow 2 rules: keep contiguous
data ranges from the contributing CRTT files, and merge the data so that
the fewest dropped lines or bad data quality. The first rule is followed
because individual CRTT files can be made with slightly different navigation
and the mixing of lines from different CRTT files may introduce more
navigation jumps than if contiguous data is used. The second rule ensures
that the final merged file will contain the fewest number of missing or
low quality lines.
The algorithm starts with a list of CRTT files for one orbit. It then orders
the files in temporal order and for the first file, it constructs a
line-by-line list of the time tag for each line, the existance of data for
the line and any quality problem for the line. Any line gaps (seen as
time gaps of more than 124 msec) are considered to be missing lines.
Each subsequent file is then processed in the same way and the results
are placed in comparison storage areas for the same time tag. If there
is an overlap that is entirely contained in the previous data, the total
number of missing or bad quality lines are compared for both stre4tches
and the best stretch of lines is used. If the overlap stretches beyond the
previous data, a break is determined where the fewest missing and bad quality
lines remain and the lines from the new file are used beyond this break. This
process goes on for all the files. In the end, a list exists of the lines
to take from each file to make the merged file. The input files are
opened and the determined lines are written into the output merged file.
The CRTT data contains several quality values but only a handfull were found
to indicate data problems. If any of the following conditions existed, the
line was assigned a bad quality:
The parameter presence global attribute (name parm_presence) indicated that
any of the first 5 bands (443, 520, 550, 670, or 750 nm) are not present
in the file.
The calibration summary (name cal_sum) values 3 and 4 (0 origin) are set
to 1, indicating bad values. These seemed to indicate that the navigation
was not valid for the line.
The calibration scan quality (name cal_scan) is set to 1 (missing data)
for the first 5 bands. This indicated that data for the band was not
included on the line.
