Overview (History) of MODIS Data Collections
What are MODIS Data "Collections"?
A MODIS data "Collection" is basically a very broad MODIS data version.
A specific Collection of MODIS data generally stays "current" (is distributed) for approximately 3 to 4 years. Typically the previous Collection of MODIS data continues to be distributed while a new Collection is being processed. So in many instances two MODIS data Collections are in the public archive.
There have been six MODIS data Collections (or Versions) processed since MODIS/Terra was launched in early 2000. The Collection versions created thus far are 001, 003, 004, 005, 051, 006, and 061. It should be noted that Collection 051 and 061 were considered incremental updates, which is why it was tagged with a 051 and 061 (a surrogate for version 5.1 and 6.1).
During the processing of a Collection, an attempt is made to use the same version of the Science Algorithms or Program Executables (PGEs). However, typically a number of new PGE's (basically algorithm updates) are released during a Collection to fix bugs and perform enhancements (or make improvements) to the data. For bug fixes, if the bug is not serious, forward processing will continue with the new corrected PGE while old previously processed data (which was produced with an older version algorithm (PGE)) are left in the archive.
When more serious bugs are found or more profound fixes are implemented, a decision is sometimes made to reprocess the entire record with an "incremental" Collection (as was the case going from Collection 005 to Collection 051). When enough new & improved science algorithms are developed, the entire MODIS dataset (from launch) is reprocessed and then tagged & distributed as a new "full Collection" (for example going from Collection 004 to Collection 005). Preparation of going to a new Collection is a major effort that typically takes several years to prepare for and complete.
It should be noted that issues, anomalies, and problems in a particular Collection are noted on the Data Issues section of this website as they are discovered. Eventually a fix is implemented which incrementally rolls forward the PGE Version while the Collection number might remain the same.
One can always identify the Collection number for a particular HDF file as it's always included (as a 3 digit number) as part of the HDF filename (for example "006").
Terra Collection 6.1 reprocessing began on 28 September 2017 (began to be publicly released on 15 October 2017) and was completed on 7 December 2017. The Aqua Collection 6.1 reprocessing began on 3 January 2018 and was completed on 7 March 2018.
It should be noted that the parallel Collection 6.0 forward-stream processing ended on day 120 (year 2018) for both Terra and Aqua (30 April 2018) -- from that point forward only Collection 6.1 will be produced in the forward stream.