Deriving anisotropic correction for upwelling radiance from PACE's multi-angle polarimetry

NASA's Plankton, Aerosol, Clouds, ocean Ecosystem (PACE) mission, launched on 8th February 2024, carries a hyperspectral radiometer, Ocean Color Instrument (OCI) and two multi-angle polarimeters, Hyper Angular Rainbow Polarimeter (HARP2) and Spectro-Polarimeter for Planetary Exploration one (SPEX-one). The simultaneous deployment of these sensors offers an unprecedented opportunity to derive more accurate bidirectional factors for correcting the Sun-sensor viewing dependence of the remote sensing reflectance derived from OCI. With a bidirectional remote sensing model based on quasi-single-scattering approximation to the radiative transfer equation, the angular shape of the volume scattering function (VSF) in backward directions, i.e., the χ factor for particles (χ_p) is derived from the multi-angle observation of HARP2. The derived χ_p is in turn used to drive the bidirectional remote sensing model to predict the bidirectional factor. Testing with prelaunch simulated HARP2 L1C data that includes uncertainties due to atmospheric correction, the proposed method can estimate bidirectional factor with an uncertainty <10 % at any three visible bands of HARP2. Because the proposed method estimates the χ_p directly from the multi-angle observation, it fully accounts for the natural variability of VSFs, which were assumed to confine within a limited range of variation in the earlier bidirectional correction models.