Abstract
The near-infrared (NIR) and shortwave infrared (SWIR)-based atmospheric correction algorithms are used in satellite ocean color data processing, with the SWIR-based algorithm particularly useful for turbid coastal and inland waters. In this study, we describe the NIR- and two SWIR-based on-orbit vicarious calibration approaches for satellite ocean color sensors, and compare results from these three on-orbit vicarious calibrations using satellite measurements from the Visible Infrared Imaging Radiometer Suite (VIIRS) onboard the Suomi National Polar-orbiting Partnership (SNPP). Vicarious calibration gains for VIIRS spectral bands are derived using the in situ normalized water-leaving radiance nLw(λ) spectra from the Marine Optical Buoy (MOBY) in waters off Hawaii. The SWIR vicarious gains are determined using VIIRS measurements from the South Pacific Gyre region, where waters are the clearest and generally stable. Specifically, vicarious gain sets for VIIRS spectral bands of 410, 443, 486, 551, and 671 nm derived from the NIR method using the NIR 745 and 862 nm bands, the SWIR method using the SWIR 1238 and 1601 nm bands, and the SWIR method using the SWIR 1238 and 2257 nm bands are (0.979954, 0.974892, 0.974685, 0.965832, 0.979042), (0.980344, 0.975344, 0.975357, 0.965531, 0.979518), and (0.980820, 0.975609, 0.975761, 0.965888, 0.978576), respectively. Thus, the NIR-based vicarious calibration gains are consistent with those from the two SWIR-based approaches with discrepancies mostly within ~0.05% from three data processing methods. In addition, the NIR vicarious gains (745 and 862 nm) derived from the two SWIR methods are (0.982065, 1.00001) and (0.981811, 1.00000), respectively, with the difference ~0.03% at the NIR 745 nm band. This is the fundamental basis for the NIR-SWIR combined atmospheric correction algorithm, which has been used to derive improved satellite ocean color products over open oceans and turbid coastal/inland waters. Therefore, a unified vicarious gain set for VIIRS bands M1-M8 and M10-M11 has been implemented in the VIIRS ocean color data processing. Using the unified vicarious gain set, VIIRS mission-long ocean color data have been successfully reprocessed using the NIR-, SWIR-, and NIR-SWIR-based atmospheric correction algorithms.
Highlights
Ocean color satellite sensors such as the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) [1], the Moderate Resolution Imaging Spectroradiometer (MODIS) on the Terra and Aqua satellites [2, 3], the Medium Resolution Imaging Spectrometer (MERIS) on the Envisat [4], and the Visible Infrared Imaging Radiometer Suite (VIIRS) on the Suomi National Polarorbiting Partnership (SNPP) [5, 6] measure the top-of-atmosphere (TOA) radiance Lt(λ) or reflectance ρt(λ), where ρt(λ) = π Lt(λ) / [F0(λ) cosθ0] with F0(λ) as the extraterrestrial solar irradiance [7] and θ0 as the solar-zenith angle, at multiple wavelengths from the visible to the near-infrared (NIR) and shortwave infrared (SWIR), which are designed for satellite ocean color remote sensing [8,9,10,11]
In the on-orbit vicarious calibration (VC) for satellite ocean color sensors, the quantities of each term on the righthand-side of Eq (1) are estimated and summed, and the sensor calibration is adjusted to yield a value of Lt(λ) that is in agreement with the estimation
Following the procedure outlined in the previous section, the SWIR VC gains at 1238 and 1601 nm were derived at the South Pacific Gyre (SPG) site
Summary
Because the atmosphere and ocean surface radiance contributions (first four terms in Eq (1)) can account for more than ~90% of the TOA radiance in the visible wavelengths [8,9,10], satellite-derived ocean color products such as normalized water-leaving radiance spectra nLw(λ) [8, 10], chlorophyll-a (Chl-a) concentration [28,29,30,31], water diffuse attenuation coefficient at the wavelength of 490 nm Kd(490) (or at photosynthetically available radiation (PAR) between 400 and 700 nm Kd(PAR)) [32,33,34,35], etc., are highly sensitive to the performance of the satellite sensor on-orbit calibration [36,37,38,39,40,41,42] This places highly stringent requirements on the sensor on-orbit radiometric calibration for ocean color remote sensing, in particular, for the short visible bands. A consistent and unified vicarious gain set can be employed for satellite ocean color data processing using different atmospheric correction algorithms for various water cases, e.g., the NIR-based [8], the two SWIR-based [9], and the NIR-SWIR combined algorithm [55] for VIIRS global ocean color data processing
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