Diurnal remote sensing of coastal/oceanic waters: a radiometric analysis for Geostationary Coastal and Air Pollution Events

摘要

Optical remote sensing systems aboard geostationary platforms can provide high-frequency observations of bio-optical properties in dynamical coastal/oceanic waters. From the end-user standpoint, it is recognized that the fidelity of daily science products relies heavily on the radiometric sensitivity/performance of the imaging system. This study aims to determine the theoretical detection limits for bio-optical properties observed diurnally from a geostationary orbit. The analysis is based upon coupled radiative transfer simulations and the minimum radiometric requirements defined for the GEOstationary Coastal and Air Pollution Events (GEO-CAPE) mission. The diurnal detection limits are found for the optically active constituents of water, including near-surface concentrations of chlorophyll-α (CHL) and total suspended solids (TSS), and the absorption of colored dissolved organic matter (α_(CDOM)). The diurnal top-of-atmosphere radiance (L_t) is modeled for several locations across the field of regard (FOR) to investigate the radiometric sensitivity at different imaging geometries. It is found that, in oceanic waters (CHL = 0.07 mg/m~3), detecting changes smaller than 0.01 mg/m~3 in CHL is feasible for all locations and hours except for late afternoon observations on the edge of the FOR. For more trophic/turbid waters (0.6 < CHL < 4.5), the proposed system is found sensitive to changes (in CHL) smaller than 0.1 mg/m~3 when the air mass fraction (AMF) is less than 5. For α_(CDOM)(440), detecting the changes larger than 0.02 m~(-1) (0.08 < α_(CDOM)(440) < 0.36) is found feasible for most of the imaging geometries. This is equivalent to AMF < 5. For TSS, changes on the order of ΔTSS = 0.1 g/m~3 (0.5 < TSS < 4.5) are detectable from early morning to late afternoon across the entire FOR. This study gives insights into the radiometric sensitivity of the GEO-CAPE mission in identifying the changes in bio-optical properties at top-of-atmosphere (TOA), which aids in a more lucid understanding of the uncertainties associated with the surface products.