The VNP09A1 Level-3 surface reflectance is an 8-day composite product produced for bands M1-M5, M7, M8, M10, and M11 that is projected onto a two-dimensional SIN grid and stored as 10km square tiles at 1 km resolution. It is a Level-3 composite of VNP09G1K.
Surface reflectance is obtained by adjusting top-of-atmosphere reflectance to compensate for atmospheric effects. Corrections are made for the effects of molecular gases, including ozone and water vapor, and for the effects of atmospheric aerosols. The inputs to the surface reflectance algorithm include top-of-atmosphere reflectance for the VIIRS visible bands (VNP02MOD, VNP02IMG), the VIIRS cloud mask and aerosol product (NPP-CMIP_L2), aerosol optical thickness (NPP_VAOTIP_L2, NPP_VAMIP_L2), and atmospheric data obtained from a reanalysis (surface pressure, atmospheric precipitable water, and ozone concentration).
Each product pixel contains the best possible L2G observation during an 8-day period as selected on the basis of high observation coverage, low view angle, absence of clouds or cloud shadow, and aerosol loading.
For each pixel, the compositing steps are:
- Observations from the same orbit are composited by observational coverage. Observations with the highest coverage are saved, and the rest discarded. This yields a list of one observation from each orbit.
- Each orbit's observation is then assigned a score, based upon whether it is flagged for cloud, cloud shadow, high aerosol or low aerosol, or contains high view angle or low solar zenith angle. The lowest score, 0, is assigned to observations with fill values for data. The remaining scores are:
- BAD data derived from a faulty or poorly corrected L1B pixel
- HIGHVIEW data with a high view angle (60 degrees or more)
- LOWSUN data with a high solar zenith angle (85 degrees or more)
- CLOUDY data flagged as cloudy or adjacent to cloud
- SHADOW data flagged as containing cloud shadow
- UNCORRECTED data flagged as uncorrected
- CLIMAEROSOL data flagged as containing the default level of aerosols
- HIGHAEROSOL data flagged as containing the highest level of aerosols
- SNOW data flagged as snow
- GOOD data which meets none of the above criteria
The observation with the highest score and the lowest view angle is selected for the VNP09A1.
All surface reflectance products are produced for daytime conditions only. The product is produced under all atmospheric conditions except for night and oceans.