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

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-a (CHL) and total suspended solids (TSS), and the absorption of colored dissolved organic matter (aCDOM). The diurnal top-of-atmosphere radiance (Lt) 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/m3), detecting changes smaller than 0.01  mg/m3 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/m3 when the air mass fraction (AMF) is less than 5. For aCDOM(440), detecting the changes larger than 0.02  m(-1) (0.08<aCDOM(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/m3 (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.