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Quality Flags for QuikSCAT/RapidScat/OSCAT/SeaWinds

PostPosted: Mon Apr 16, 2018 11:51 am
by dmoroni
The following is a general policy statement for all users who are wanting to make use of the quality flag variables for any of the Level 2B datasets covering the following NASA Missions: QuikSCAT, ISS-RapidScat, OSCAT, and SeaWinds on ADEOS-II.

The policy adopted within the processing algorithms and software design is to flag values that are out of range or to indicate a non-nominal condition. Except where otherwise noted, a “1” or “set” bit indicates an error or abnormal condition, and a “0” or “cleared” bit indicates a normal condition. Some informational flags may have a number of set bits under normal conditions. Quality flag bits are set at the beginning of processing and are cleared when tests are performed and passed. If abnormal conditions terminate processing early, some bits may remain set. Since the processor may curtail subsequent operations for the wind vector cell that failed the test, those bit flags that normally would be tested in subsequent code also retain their initialized value. Thus, the order in which bit flags are processed determines whether their values are meaningful.

All flag variables for these missions utilize a bit wise convention, which means that the numerical flag value assigned to a particular flag variable represents a summation of all of the bits that have been set corresponding to a particular wind vector cell location. Thus, bit wise operations are required to extract a single bit flag value from the aggregate bit flag which contains a summation of multiple bit values (i.e., when multiple bits are set). Some flag variables utilize empty bits, which means those bits are skipped by default. For L2B datasets provided in netCDF format, the netCDF file metadata for a particular flag variable will serve as guidance for the users as to which bits are active as well as the respective meanings for those active bits.