The USGS is making changes to manage the Landsat archive as a tiered Collection of data. A Collection will provide a stable environmental record. If significant radiometric or geometric changes are required, all data will be reprocessed and a new Collection will be released. The tiered structure clearly identifies the subset of the Landsat archive that meets radiometric and geometric criteria suitable for time series analysis and the creation of data stacks/cubes, while continuing to provide access to the entire Landsat archive.
Starting with Collection 1, every Landsat 4-5 (L4-5) Thematic Mapper (TM), Landsat 7 (L7) Enhanced Thematic Mapper Plus (ETM+), and Landsat 8 (L8) Operational Land Imager (OLI)/Thermal Infrared Sensor (TIRS) scene is assigned to a specific tier.
There are many changes being implemented to Landsat Level-1 data products to support this effort, including new Landsat Product Identifiers, “Collection” and “Tier” designations, metadata changes, and additional supporting files included with Level-1 data products.
In order to apply the changes required for Collection 1, it is necessary to reprocess the existing data held in the Landsat archive and create Collection 1 Level-1 data products. Details about the reprocessing times can be found here.
The Landsat Collections Web page contains the details about all aspects of the Collection 1 effort and includes sample data products.