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PGE Versions Primer

This PGE Versions section offers an overview summary of MODIS Atmosphere data products versions available for public distribution.  Provided on sub-pages in this section are the Program Executable (PGE) (aka. science algorithm) version used to produce the product for all data days since launch.

There are two ways MODIS Data is identified:  1.) The Collection version (which is a slowly varying number, that earmarks a broad-view separation in algorithm evolution); and 2.) the PGE Version (which is a more rapidly varying number, that shows smaller-scale, but often significant, changes in the algorithm used to produce the data).  There are two collections currently available for distribution: Collection 005/051 and Collection 006.  However the older Collection 005/051 may soon be removed from the archive.  Because of this, this section will focus on the PGE versions of only the later and more refined Collection 006 data.

The upshot here is that the PGE version can vary within a given Collection version; and it is important to become familiar with changes made to particular science algorithms during a given MODIS data collection.

Each PGE Version Product Page link provides a graphical view of: 1.) the latest (current) PGE Version of data available for download from LAADS; and 2.) a PGE history showing the evolution of data in the archive (that is, previously available versions:.  Also provided in the table below each PGE Version graphic is a brief summary of changes incorporated into each PGE version.   A key to assist in understanding how to interpret these PGE Version graphics is depicted below.

Data users can identify the maturity and quality of an HDF data product file by finding the PGE (program executable) version that was used to produce the file.  Since HDF files are self describing, the PGE Version used to process each HDF file is contained within the HDF file itself.    One way to ascertain this piece of information is by using the HDF utility ncdump -- the (UNIX) command for finding the PGE version that was used to create a particular HDF file is

ncdump -h *.hdf

This command dumps the header of the HDF file -- one then simply searches the output for the string PGEVERSION.

The PGE's associated with MODIS Atmosphere are: PGE03 (Cloud Mask & Profile), PGE04 (Aerosol & Water Vapor), PGE06 (Cloud), PGE83 (L2 Joint), PGE69 (L3 Tile), PGE56 (L3 Daily), PGE70 (L3 Eight Day), PGE57 (L3 Monthly), and PGE108 (Starry's L2 Global Mosaic Images).  Note that PGE69 (L3 Tile) is an interim product used to build the L3 Daily, so changes in PGE69 are tracked in PGE56 (L3 Daily).  Finally, PGE108 is used to make L2 Global Mosaic Images so changes are not tracked (since it's am imaging program only).