Level 0 data are unprocessed instrument/payload data at full resolution. Any artifacts of the communication (e.g. synchronization frames, communication headers) of these data from the spacecraft to the ground station have been removed.
These data are the most raw format available, and are often not provided to endusers.
Level 1A data are reconstructed, unprocessed instrument data at full resolution, time-referenced and annotated with ancillary information including radiometric and geometric calibration coefficients and georeferencing parameters (e.g. platform ephemeris data) computed and appended but NOT applied to the Level 0 data.
Level 1B data are Level 1A data that have had instrument/radiometric calibrations applied
Level 2 data consist of derived geophysical variables at the same resolution as the source Level 1 data
Level 3 data are the derived geophysical variables binned/mapped to a uniform space/time grid scale.
Each Level 3 binned data product consists of the accumulated data for all L2 products, for the specified instrument and resolution, corresponding to a period of time (e.g. daily, 8 days, monthly, etc.) stored in a representation of a global, equal-area grid grid cells or "bins".
The Level 3 Standard Mapped Image (SMI) products are image representations of binned data products. This image is typically a scaled value (16bit), two-dimensional array of an Equidistant Cylindrical projection of the globe. The original SMI definition was a 9.2km (at the equator) resolution product, although other resolution maps can (and are) created.
Level 4 data are model output or results from analyses of lower level data (e.g., variables derived from multiple measurements). Ocean Primary Productivity is a good example of a Level 4 product.