The Influence of Signal to Noise Ratio of Legacy Airborne and Satellite Sensors for Simulating Next-Generation Coastal and Inland Water Products

Abstract

Presently, operational ocean color satellite sensors are designed with a legacy perspective for sampling the open ocean primarily in the visible domain, while high spatial resolution sensors such as Sentinel-2, Sentinel-3, and Landsat8 are increasingly used for observations of coastal and inland water quality. Next-generation satellites such as the NASA Plankton, Aerosol, Cloud and ocean Ecosystem (PACE) and Surface Biology and Geology (SBG) sensors are anticipated to increase spatial and/or spectral resolution. An important consideration is determining the minimum signal-to-noise ratio (SNR) needed to retrieve typical biogeochemical products, such as biomass, in aquatic systems, and whether legacy sensors can be used for algorithm development. Here, we evaluate SNR and remote-sensing reflectance (Rrs) uncertainty for representative bright and dim targets in coastal California, USA. The majority of existing sensors fail to meet proposed criteria. Despite these limitations, uncertainties in retrieved biomass as chlorophyll or normalized difference vegetation index (NDVI) remain well below a proposed threshold of 17.5%, suggesting that existing sensors can be used in coastal systems. Existing commercially available in-water and airborne instrument suites can exceed all proposed thresholds for SNR and Rrs uncertainty, providing a path forward for collection of calibration and validation data for future satellite missions.

Airborne Radiometry for Calibration, Validation, and Research in Oceanic, Coastal, and Inland Waters

“…Airborne flight planning over optically dark water targets provides unique challenges from the optically bright targets in terrestrial environments (Mustard et al, 2001;Kudela et al, 2019). Flight plans over water must take into consideration the following: (a) sensor field of view (FOV), integration, and data rate; (b) solar elevation and azimuth to optimize the observational geometry and minimize sun glint; (c) weather mitigation (less than 25% cloud cover); (d) calm wind conditions to simplify water surface roughness modeling, reduce whitecap effects, and facilitate in-water validation measurements; (e) flights ± 30 min of satellite overpass to capture dynamic changes in water features; (f) coordination with in situ validation teams (boat and targets); and (g) for flights including an airborne sun photometer, stacked flight transects at high and low altitudes for full column and intervening layer AOD retrievals.…”

“…The same instrument suite was used as for OCEANIA, but the C-AIR system was upgraded to use C-AERO. As described in Kudela et al (2019), the principal differences between the two embodiments are that the above-water C-AERO instrument suite has wavelengths spanning 320-1,640 nm (SWIR channels 1,020, 1,245, and 1,640 nm with 10, 15, and 30 nm bandwidths, respectively). This allows for atmospheric correction studies that emphasize long wavelengths.…”

“…The radiometric and ancillary data were processed using the Processing of Radiometric Observations of Seawater using Information Technologies (PROSIT) software package (Hooker et al, 2018b) to provide estimates of L W and R rs from above-and in-water measurements (Hooker et al, 2018c). As described in Kudela et al (2019), a full atmospheric correction is not applied given the low flight altitude for OCEANIA and C-HARRIER, but skylight reflectance is removed as per standard NASA abovewater reflectance protocols. For COAST, the HIS data were processed using both Tafkaa Tabular, hereafter referred to as Tafkaa, and the vector version of the Second Simulation of the Satellite Signal in the Solar Spectrum (6SV).…”

“…In coastal areas, the magnitude of the radiance signal in the visible (VIS) range (400-700 nm) is highly variable, ranging from very dark values in clear, deep water, as well as in water dominated by colored dissolved organic matter (CDOM) (e.g., Brezonik et al, 2015;Palmer et al, 2015). For example, typical albedo for deep ocean water is often assumed to be approximately 5% (Moses et al, 2012), but productive and turbid inland waters can easily exceed 25% or more (Kudela et al, 2019). Legacy and presently operational ocean color satellite sensors such as Sea-viewing Wide Field-of-View Sensor (SeaWiFS), Moderate Resolution Imaging Spectrometer (MODIS), Medium Resolution Imaging Spectrometer (MERIS), and the Visible Infrared Imaging Radiometer Suite (VIIRS) are optimally designed for openocean imagery.…”

“…They are calibrated for low spectral water-leaving radiances, L W (λ), and produce coarse spatial (km) and spectral resolution. While more recent sensors, such as the Ocean Land Color Instrument (OLCI), Operational Land Imager (OLI), and Multispectral Instrument (MSI), provide improved spatial and spectral resolution, they are not optimized for retrievals over inland waters (Kudela et al, 2019).…”