MPA: Meridian Photometer Array


Data Reduction

After initial corrections for nonlinearities and dark count, the data from a 30 second scan are compressed from eight to six channels by averaging the two 4861Å channels and the two 4861Å background channels (4800Å and 4935Å). The data then are sorted into the seventeen latitude bins, 0.5° (assuming an emission height of 110 km and centered on the 0.25° and 0.75° EDFL) for 4709Å, 4861Å and 5577Å, 1.0° (assuming an emission height of 250 km and centered on each degree EDFL) for 6300Å. Two scans, the one starting at the even minute and the one 30 seconds later, are averaged before transmission to the central data analysis network (DAN). Data is transmitted every two minutes, and each "message" contains a header, status block and two one minute data blocks (six seventeen-point scans).

Once a day, after the end of each universal day, data for that day are processed to Beta level. This consists of demultiplexing the raw data streams (Alpha data) and merging the photometer messages into one minute records. Each Beta record contains a header with time, lunar and cloud information and pointers to data present in the record. The remainder of the record contains the original header, status and data blocks for each platform if present. Header and status block are repeated for the odd minute (only one is available every two minutes). The header block is unchanged except for the addition of two cloud indices, which are based on the shape of the background channel scans. The status block is unchanged except for the addition of the average of the nightly calibration values (computed over the period 45 minutes after switchon until 45 mintes before switchoff to prevent contamination by twilight).

Since all data is stored in its original format (i.e. counts) the conversion to rayleighs, background subtraction and atmospheric corrections are performed "on the fly". I.e. these are embedded as code in the programs that read the data and produce the outputs. This allows for greater flexibility in developing and changing algorithms for processing the data.

Note added 8-JUL-1991 14:24:16.28.
The following are under review: Summary files are created by compressing the original data by a factor of ten. This data provides a snapshot of the following parameters every five minutes: * station status, * I(4861Å), I(5577Å), and I(6300Å) each degree of latitude for the eastern and western lines. In addition to this, a number of key parameters are produced for quick-search purposes: * station status and quality index, * average peak intensity over 5 min period for each meridian for I(4709Å), I(4861Å), I(5577Å), and I(6300Å), * intensity distribution index for each meridian.


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