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GUVI: Understanding the Data Products
The GUVI Data Processing Payload Operations Center will routinely create scientific data products that are available for distribution via the web. In order for the data to be of use to scientists, industry and the public, rapid, efficient, and accurate operational algorithms must be developed into environmental parameters. Data from the GUVI instrument is procesed on the ground to generate data products at the different levels.
Level 1A is a "virtual" data product file in the sense that the data is not directly outputted to a data file. It consists of raw sensor data at full resolution. Level 1B is also a "virtual" data product file and it contains uncompressed instrument data, using a simple constrained maximum error compression algorithm that achieves modest compression factors, and has been calibrated to convert to unts of radiance within the specified "color" of the GUVI data (Rayleighs/color).
The Level 1C data products contain data directly measured by the sensors, such as photometer counts, uncertainties, light intensities and pointing information derived from the GUVI raw sensor data and the TIMED Satellite ephemeris data. The data are gridded into a GUVI based coordinate system consisting of the along orbit position of the sensor and the angle of the scan from the nadir position. This gridding eliminates overlap from scan to scan due to the large instantaneous field-of-view of GUVI (about 12 degrees) and involves coaddition to place the values on a uniform instrument shared grid. The data is then determined whether its from day, night, aurora or twilight pixels.
The Level 2B data products contain environmental parameters which are derived from the sensor data by scientific algorithms specific to day, night, and auroral regions. The data is averaged into 100 x 100 km2 resolution to form "super pixels" on the day and night and 25 x 25 km2 for the auroral products. The "super pixels" are then processed by the algorithms to yield geophysical parameters which are calculated from the radiance of the different colors.
The GUVI data product files are stored in full orbit increments according to the date and revolution number and data product level type. These files are in NetCDF which is a self describing platform independent binary format and NetCDF readers for the GUVI data products are available here. The file formats for the GUVI data product files are defined here in Word format.
In order to easily understand how to access and use the GUVI data products, a Word document is available here. A rich toolset exists both on the GUVI web site as well as by request for accessing the data, finding the data of interest and using the data. These are all described in this word document.