AquaFloat: A Near-Surface Hyperspectral Radiometer System for Improved Ocean Color Measurements within the UV-VIS-NIR Spectral Region

In P-I we demonstrated a low cost, easy to operate sea-surface-spectroradiometer system, AquaFloat, for measuring the underwater upwelling radiance at 3 adjustable depths close to the sea surface within the top 1 m for determination of water-leaving radiance remote-sensing reflectance just above the surface. A sensor for measuring the downwelling irradiance above the sea surface is also included. The sensors provide data at high spectral resolution over the UV-VIS-NIR and minimize the uncertainties of common approaches, e.g., free falling profilers that generally do not provide good data close to the surface and buoy-based systems with a single-depth radiometer placed at near-surface depth or multi-depth radiometers placed at depths of 1 m and deeper. The uncertainties associated with extrapolation of subsurface measurements taken at a single depth or depths of ~1 m and deeper to just below the surface are particularly significant in the red and NIR spectrum even in optically uniform water column owing to effects of inelastic Raman scattering by water molecules. By conducting time series measurements, AquaFloat reduces errors due to variations in underwater light field associated with the effects of sea surface waves, vertical changes in water inherent optical properties including the effects of intermittent bubble clouds, and changes in sky conditions, which can affect profiling measurements. The low-cost design, ease, and flexibility of use of AquaFloat offers an improved tool for routine work aimed at the development and validation of in-water ocean color algorithms. These features of AquaFloat also facilitate the use of multiple systems in special experiments to map horizontal variability in remote-sensing reflectance; subpixel variability within the GSD of a satellite sensor or various events in optically complex coastal waters as red-tide blooms and river plumes. AquaFloat is useful in vicarious calibration of current future satellite ocean color missions. The proposed instrument fills a need in ocean color measurements for more accurate in situ determinations of water-leaving radiance at high spectral resolution over a broad spectral range. Key features of our novel approach include (i) Near-surface measurements that account for the presence of vertical gradients in upwelling light attenuation within the extrapolation depth interval to just below the surface, and (ii) simultaneous, multi-depth time-series measurements to avoid short temporal variability due to surface wave effects, intermittent bubble clouds, and sky/cloud conditions. Existing traditional instruments that are used for determining the water-leaving radiance typically suffer from these deficiencies that are addressed in this new instrument. In addition, the instrument will provide a capability to measure the polarization state of underwater upwelling radiance at near-surface depths, from which the polarization state of water-leaving radiance can be determined. We plan to further develop, fabricate, and assemble the full operational AquaFloat system and enhance the AquaFloat capabilities, conduct tests at the SIO pier, and from a research vessel at sea. Some measurements will be conducted in parallel with ocean color satellite pass to allow us compare data.  We also plan to collaborate with other ocean-color, ocean-optics researchers and conduct measurements in lakes, rivers and estuaries.  These plus the publication of data and results in scientific journal is planned to support commercialization of the AquaFloat. Deliverables will include periodic progress reports, data and results of measurements, comparison to satellite remote sensing reflectance product, and the AquaFloat system.

AquaFloat supports NASA ocean color-related calibration/validation activities for universities, NASA centers, and government labs. AquaFloat is easy to deploy in deep ocean, littoral regions, estuaries, and inland water bodies. Current and future missions (e.g., MODIS, VIIRS/JPSS, PACe, HyspIRI, GEO-CAPE, OLCI/Sentinel, SGLI/GCOM, GOCI) involving ocean color measurements will benefit from AquaFloat's near-sea surface radiometry. AquaFloat benefits NASA Ocean Biology and Biogeochemistry programs supporting satellite ocean color missions. The AquaFloat sea surface radiometer will enhance the capabilities of existing marine radiometer sensors, improving applications of satellite and airborne ocean color remote sensing. This, in turn, will provide improvements in data for countries, states, municipalities, and research institutions to assess the conditions and trends within the aquatic environments and make informed decisions.

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