Frequently Asked Questions

Data Basics

Q: What's the difference between the older "MODIS Standard" Atmosphere data and the newer "Continuity" Atmosphere data?

A: First, the older "MODIS Standard" Atmosphere data has a longer data record, extending from February 2000 (TERRA) and July 2002 (AQUA) through the present day.  The newer "Continuity" Atmosphere data has a shorter record (comparatively), beginning in March 2012 (which is the beginning of the VIIRS/SNPP record) and extending through the present day.  Second, the "MODIS Standard" Products were fine-tuned to spectral band data from the MODIS Instrument only (onboard two Platforms (TERRA & AQUA)). "Continuity" Products use a more general algorithm, which could be applied to multiple Instrument/Platform combinations -- which required limiting the spectral band input to a minimum set of common spectral channels. Due to this more general (and therefore more limited) spectral band range used in the "Continuity" algorithm, some parameters computed using the old "MODIS Standard" algorithm cannot be computed through the more spectral-band limited, but more instrument flexible, "Continuity" algorithm (for example separate Multi-Level and Single-Level Cloud Properties). That being said, since a single computational algorithm ( a "shared core") is used on all the various Instrument/Platform combinations in the "Continuity" paradigm, it allows for easier and better cross-instrument data comparisons (fewer degrees of freedom in the data allowing for more "level-playing field" comparisons). As of late 2018, only two Instrument/Platform combinations have been run through the "Continuity" algorithm: VIIRS/SNPP and MODIS/AQUA -- however this is expected to be extended to a number of additional combinations in the coming years. 

Q: What are Data Versions?

A: When new & much improved science algorithms are developed, the entire record (from launch) is reprocessed and then tagged & distributed as a new Data Version. During the processing of a particular Data Version, an attempt is made to use the same version of the Science Algorithms or Product Generation Executive (PGE). However, sometimes a bug is found in one or more of the PGEs in the middle of Data Version processing; and if the bug is not serious, processing will complete with the new corrected PGE. These anomalies and problems in processing are noted in the Data Issues section. One can always identify the Data Version number for a particular NetCDF4 file as it's always included (as a 3 digit number) as part of the filename. The initial Data Version for the "Continuity" Atmosphere Products will be Collection 001 and should be available for limited release in early 2019.