Extending aquatic spectral information with the first radiometric IR-B field observations

Abstract

Planetary radiometric observations enable remote sensing of biogeochemical parameters to describe spatiotemporal variability in aquatic ecosystems. For approximately the last half century, the science of aquatic radiometry has established a knowledge base using primarily, but not exclusively, visible wavelengths. Scientific subdisciplines supporting aquatic radiometry have evolved hardware, software, and procedures to maximize competency for exploiting visible wavelength information. This perspective culminates with the science requirement that visible spectral resolution must be continually increased to extract more information. Other sources of information, meanwhile, remain underexploited, particularly information from nonvisible wavelengths. Herein, absolute radiometry is used to evaluate spectral limits for deriving and exploiting aquatic data products, specifically the normalized water-leaving radiance, Γ(λ), and its derivative products. Radiometric observations presented herein are quality assured for individual wavebands, and spectral verification is conducted by analyzing celestial radiometric results, comparing agreement of above- and in-water observations at applicable wavelengths, and evaluating consistency with bio-optical models and optical theory. The results presented include the first absolute radiometric field observations of Γ(λ) within the IR-B spectral domain (i.e. spanning 1400–3000 nm), which indicate that IR-B signals confer greater and more variable flux than formerly ascribed. Black-pixel processing, a routine correction in satellite and in situ aquatic radiometry wherein a spectrum is offset corrected relative to a nonvisible waveband (often IR-B or a shorter legacy waveband) set to a null value, is shown to degrade aquatic spectra and derived biogeochemical parameters.

Expanded Signal to Noise Ratio Estimates for Validating Next-Generation Satellite Sensors in Oceanic, Coastal, and Inland Waters

“…At the other end of the spectrum, short-wave infrared bands are also underutilized. A recent analysis demonstrates that the technology used in C-AIR and C-AERO is fully capable of delivering absolute radiometric measurements of targets darker than sunlit waters (Figures 2 and 3A,D), and negating the common assumption of a "black pixel" in the NIR or SWIR region of the spectrum [51]. If the community of practice were to take advantage of those wavelengths, similar issues as for the UV bands become evident.…”

“…Figure 3 provides the corresponding SNR for the airborne and shipboard datasets included in this analysis. AVIRIS-NG exhibits higher SNR compared to AVIRIS-C, but still falls below a threshold of 20 for blue-UV (<~450 nm) and for wavelengths beyond 750 nm, although those bands are of less interest for ocean color (but see [51]). C-AIR is consistently above both SNR thresholds in the UV and visible bands, but also drops off for wavelengths beyond 750 nm, particularly for dark targets (e.g., S-MODE).…”