Using Multi-Angle Polarimetry to Derive Χ Factor and Improve BRDF Correction for PACE’s OCIPI: Xiaodong Zhang - University of Southern Mississippi
Co-PI: Deric Gray (Naval Research Lab)
Water leaving reflectance that an ocean color mission (such as PACE) measures is inherently bidirectional, i.e., it varies with the position of sun and the viewing direction. This variation, expressed as bidirectional reflectance distribution function (BRDF) is ultimately determined by the volume scattering function (VSF), and in particular its backward shape (called Χ factor). Previous studies investigating BRDF have used the VSFs that evidently underestimate the natural variability of Χ factor and could lead to errors that are greater than the 30% target of retrieving remote sensing reflectance from the PACE mission. On the other hand, the two multi-angle polarimeters (HARP2 and SPEXone) that the PACE will deploy along with the hyperspectral OCI sensor offer an opportunity to measure Χ factor. Taking advantage of this opportunity, we propose to develop a method using multi-angle polarimetry data to derive Χ factor and improve the BRDF correction for PACE's OCI remote sensing reflectance (rrs). The proposed work is based on insight and knowledge that we have gained during past decade in studying the natural variability of the volume scattering function and its impact on ocean color. Specifically, we propose to achieve three objectives. (1) Improve our understanding of natural variability of the Χ factor and its impact on bidirectional remote sensing reflectance. Because of the essential role played by the Χ factor in determining the BRDF effect, addressing this objective will allows us to develop a mechanistic understanding on the source of variability in BRDF. (2) Utilize Zaneveld's theoretical derivation that explicitly includes VSF in rrs formula to retrieve Χ factor from the multi-angle rrs observation. (3) Explore a multi-sensor capability to constrain the BRDF correction for the hyperspectral OCI using Χ factor retrieved from the multi-angle HARP2 and/or SPEXone. In additional to improving BRDF correction, our method will also retrieve Χ factor, which as an important inherent optical property, contains critical information on the composition and structure of the particle assemblages in the oceans. Therefore, it also has potential to derive additional biogeochemical products from the PACE mission.