Atmospheric Correction of Satellite Ocean-Color Imagery During the PACE Era

Abstract

The Plankton, Aerosol, Cloud, ocean Ecosystem (PACE) mission will carry into space the Ocean Color Instrument (OCI), a spectrometer measuring at 5 nm spectral resolution in the ultraviolet (UV) to near infrared (NIR) with additional spectral bands in the shortwave infrared (SWIR), and two multi-angle polarimeters that will overlap the OCI spectral range and spatial coverage, i. e., the Spectrometer for Planetary Exploration (SPEXone) and the Hyper-Angular Rainbow Polarimeter (HARP2). These instruments, especially when used in synergy, have great potential for improving estimates of water reflectance in the post Earth Observing System (EOS) era. Extending the topof-atmosphere (TOA) observations to the UV, where aerosol absorption is effective, adding spectral bands in the SWIR, where even the most turbid waters are black and sensitivity to the aerosol coarse mode is higher than at shorter wavelengths, and measuring in the oxygen A-band to estimate aerosol altitude will enable greater accuracy in atmospheric correction for ocean color science. The multi-angular and polarized measurements, sensitive to aerosol properties (e.g., size distribution, index of refraction), can further help to identify or constrain the aerosol model, or to retrieve directly water reflectance. Algorithms that exploit the new capabilities are presented, and their ability to improve accuracy is discussed. They embrace a modern, adapted heritage two-step algorithm and alternative schemes (deterministic, statistical) that aim at inverting the TOA signal in a single step. These schemes, by the nature of their construction, their robustness, their generalization properties, and their ability Frouin et al. Atmospheric Correction of Ocean-Color Imagery to associate uncertainties, are expected to become the new standard in the future. A strategy for atmospheric correction is presented that ensures continuity and consistency with past and present ocean-color missions while enabling full exploitation of the new dimensions and possibilities. Despite the major improvements anticipated with the PACE instruments, gaps/issues remain to be filled/tackled. They include dealing properly with whitecaps, taking into account Earth-curvature effects, correcting for adjacency effects, accounting for the coupling between scattering and absorption, modeling accurately water reflectance, and acquiring a sufficiently representative dataset of water reflectance in the UV to SWIR. Dedicated efforts, experimental and theoretical, are in order to gather the necessary information and rectify inadequacies. Ideas and solutions are put forward to address the unresolved issues. Thanks to its design and characteristics, the PACE mission will mark the beginning of a new era of unprecedented accuracy in ocean-color radiometry from space.

A unified approach to estimate land and water reflectances with uncertainties for coastal imaging spectroscopy

“…Rather than define such constraints explicitly using ad hoc heuristics, OE captures them automatically in the statistical properties of the combined model. OE is also distinct from other simultaneous open water retrieval methods (Wang et al, 2010) which apply mainly to multi-band data or limited water properties (Frouin et al, 2019), often disregard uncertainties, and do not generally work over land.…”

“…VSWIR spectra are sensitive to atmospheric effects ranging from extreme Rayleigh and Aerosol scattering in the ultraviolet, through pervasive water vapor effects throughout the near infrared and into deep water absorptions at 1480 and 1880 nm. Reviews are found in Thompson et al, 2018c for land andFrouin et al (2019) for open ocean. Most approaches are variations of multi-band sequential algorithms (Guanter et al, 2009).…”

Retrieving Aerosol Characteristics From the PACE Mission, Part 1: Ocean Color Instrument

“…We did not deliberate on the aerosol/land retrievals, but we did consider the possibilities of aerosol/ocean retrievals. These possibilities are discussed more fully in other papers appearing in this special issue (Chowdhary et al, 2019;Frouin et al, 2019;Remer et al, 2019).…”

On the Adequacy of Representing Water Reflectance by Semi-Analytical Models in Ocean Color Remote Sensing

“…The model parameters, i.e., [Chla] and bbs in MM01, and [Chla], fa, and fb in PR05, are obtained from the prescribed R rs at a set of wavelengths in the 400-900 nm spectral range using, as in POLYMER, the Nelder-Mead optimization scheme [24]. A simplex method is used to minimize the cost function f, which is defined as: f = (πŘ rs (λ i ) − πR rs (λ i )) 2 /norm, norm = 0.005 if πR rs (λ i ) < 0.005, otherwise norm = πŘ rs (λ i ), (1) whereŘ rs (λ i ) is the modeled remote sensing reflectance at wavelength λ i , and R rs (λ i ) is the Hydrolight simulated or field measured remote sensing reflectance at wavelength λ i . For the MM01 model, the first iteration simplex is defined by initial values log 10 ([Chla]) = −1 and bbs = 0, and initial steps 0.05 and 0.0005 m −1 .…”