Source: Diane Noserale, USGS

Access to consistent high-quality images to study changes on Earth’s surface is getting easier. The USGS Landsat standard (Level-1) product inventory is now structured by data quality and offers improved calibration. Data designated as Tier 1 provide the highest accuracy and can be reliably used to analyze changes to Earth’s surface over time.
“These actions are intended to facilitate discovery and data processing, allowing users more time for science,” said John Dwyer of the USGS EROS Data Center. “They’re based on our experience with users and are intended to maintain a well characterized data record and to improve the documentation of the data processing history.”
The new Landsat Collections structure provides ready access to high quality scenes, while allowing continuous improvement of the archive and access to all data as they are acquired. This will ensure consistent data quality through time and across instruments, with additional changes like metadata, enhanced quality information, and file names.
The collection definition includes three categories:

• Tier 1 scenes are calibrated to the highest geometric and radiometric quality to enable data stacking for time series analyses (better than 12m geodetic accuracy).
• Tier 2 data do not meet the Tier 1 criteria due to significant cloud cover, insufficient ground control, and other factors.
• The Real-Time Tier offers data immediately after acquisitions using estimated parameters that are reprocessed for inclusion into Tier 1 or Tier 2 when final parameters are available. Real-time data are key for rapid response to global disasters.
• “Pre-collection” data will remain available through the end of October, so scientists have time to publish results based on it.

Collection 1, now available for download from EarthExplorer, includes products from:

• Landsat 4-5 Thematic Mapper
• Landsat 7 Enhanced Thematic Mapper Plus
• Landsat 8 Operational Land Imager /Thermal Infrared Sensor products for the conterminous U.S.
• Landsat 8 processing for the rest of the world is expected to continue through June. Landsat 1-5 Multispectral Scanner data will be processed into Collection 1 after data quality analysis and processing level investigations are complete at the end of 2017.

Collection 2 is about two years out and will be generated using improved ground control points for the geometric processing, so the absolute geodetic accuracy will be much better.
More details about Landsat Collections effort, including the changes to data products, new files, new product identifiers, and EarthExplorer datasets can be found on the Landsat Collections webpage.
Further Reading:
+ USGS Landsat Collections