Terrain Correction for VIIRS Imagery in Preparation for JPSS-2

VIIRS Imagery is now operationally available from both S-NPP (launched in 2011) and NOAA-20 /JPSS-1 (launched in 2017). Both satellites occupy a polar orbit at the same (1330) local time, but a half orbit difference, on opposite sides of the earth. This allows consecutive views of any location at VIIRS wavelengths, including the unique Day-Night-Band (DNB) and/or Near Constant Contrast (NCC) Imagery.

The two satellites/instruments provide two views at different scan angles, which can result in shifts in the geo-locations of surfaces at higher elevations as well as cloud features. This is because the imagery geo-locations are based on an ellipsoid Earth model and are not terrain corrected. That ellipsoid model works well for near-sea-level locations, but not for areas at higher elevations, such as much of the Western USA and Alaska, or any other higher-elevation location.

Terrain correction is not new to VIIRS, and is available for SDR Imagery, not for the EDR Imagery product that is remapped to eliminate bowtie deletions and overlapping pixels in the raw SDR imagery. This EDR Imagery is what is made available to NWS users in particular. Terrain Correction is a requirement for EDR Imagery from JPSS-2, so this is preparation for that launch, but Terrain Correction will also be applied to VIIRS Imagery from both S-NPP and JPSS-1. There, however, will be no reprocessing of older EDR Imagery prior to implementation.

Once implemented, Terrain Corrected geo-locations will replace ellipsoid geo-locations for EDRs, which has been the standard for MODIS Imagery without any issues for end users. The process, however, of implementing Terrain Correction is a long one, involving many steps before being placed into the operational VIIRS code. This implementation process allows VIIRS users to be informed and provide feedback about any potential issues that might arise because of the change.

The planned geo-location code changes are all internal, with no changes in output files, which simplifies the impacts on most users, with the exception of improved geo-locations. Details of the process will be presented, as well as a mention of future plans for other possible code changes once this current terrain-correction effort is implemented.

Disclaimer: The views, opinions, and findings contained in this article are those of the authors and should not be construed as an official National Oceanic and Atmospheric Administration (NOAA) or US Government position, policy, or decision.