Why Does Radiometric Response Vary Between Images?

The PlanetScope constellation comprises satellites with diverse relative spectral responses (RSRs), necessitating an additional harmonization step for comparing data from different satellite generations. Additionally, PlanetScope surface reflectance (SR) data may exhibit variability due to various factors, mandating a normalization step alongside harmonization.

Directional reflectance effects remain uncorrected in PlanetScope SR data, leading to potential stray light presence in certain satellites. Moreover, SR corrections rely on radiative transfer models which are based on approximations that may have errors at low sun angles and other adverse conditions. SR corrections also rely on MODIS estimates of atmospheric optical thickness, collected at different times and spatial resolutions, introducing variability even within data of the same RSR.

Despite applying global harmonization corrections, variability persists between satellite generations, necessitating additional processing to rectify both factors before analysis. The scene-level normalization tool offers a method to approximate harmonization across varying RSRs and normalize data to mitigate variability, enhancing consistency, albeit potentially impacting absolute radiometric accuracy in some instances.

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