Reconstructing hyper-spectral downwelling irradiance from multi-spectral measurements

Jing Tan, R. Frouin, N. Haëntjens, Andrew Barnard, Emmanuel Boss, Paul Chamberlain, Matt Mazloff, Cristina Orrico
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Abstract

Checking the radiometric calibration of satellite hyper-spectral sensors such as the PACE Ocean Color Instrument (OCI) while they operate in orbit and evaluating remote sensing reflectance, the basic variable from which a variety of optical and biogeochemical ocean properties can be derived, requires measuring upwelling radiance just above the surface (Lw) and downwelling planar irradiance reaching the surface (Es). For this, the current HyperNav systems measure Lw at about 2 nm spectral resolution in the ultraviolet to near infrared, but Es in only four 10 nm wide spectral bands centered on 412, 489, 555, and 705 nm. In this study, the Es data acquired in these spectral bands in clear sky conditions are used to reconstruct via a multi-linear regression model the hyper-spectral Es signal at 0.5 nm resolution from 315 to 900 nm, the OCI spectral range, allowing an estimate of Es at the HyperNav, OCI, and other sensors’ resolutions. After correction of gaseous absorption and normalization by the top-of-atmosphere incident solar flux, the atmospheric diffuse transmittance is expressed as a linear combination of Es measured in those 4 spectral bands. Based on simulations for Sun zenith angles from 0 to 75° and a wide range of (i.e., expected) atmospheric, surface, and water conditions, the Es spectrum is reconstructed with a bias of less than 0.4% in magnitude and an RMS error (RMSE) ranging from 0% to 2.5%, depending on wavelength. The largest errors occur in spectral regions with strong gaseous absorption. In the presence of typical noise on Es measurements and uncertainties on the ancillary variables, the bias and RMSE become −2.5% and 7.0%, respectively. Using a General Additive Model with coefficients depending on Sun zenith angle and aerosol optical thickness at 550 nm improves statistical performance in the absence of noise, especially in the ultraviolet, but provides similar performance on noisy data, indicating more sensitivity to noise. Adding spectral bands in the ultraviolet, e.g., centered on 325, 340, and 380 nm, yields marginally more accurate results in the ultraviolet, due to uncertainties in the gaseous transmittance. Comparisons between the measured and reconstructed Es spectra acquired by the MOBY spectroradiometer show agreement within predicted uncertainties, i.e., biases less than 2% in magnitude and RMS differences less than 5%. Reconstruction can also be achieved accurately with other sets of spectral bands and extended to cloudy conditions since cloud optical properties, like aerosol properties, tend to vary regularly with wavelength. These results indicate that it is sufficient, for many scientific applications involving hyper-spectral Es, to measure Es in a few coarse spectral bands in the ultraviolet to near infrared and reconstruct the hyperspectral signal using the proposed multivariate linear modeling.
从多光谱测量重建超光谱下沉辐照度
检查卫星超光谱传感器(如 PACE 海洋色彩仪器(OCI))在轨运行时的辐射校准,并评估遥感反射率(可从中得出各种海洋光学和生物地球化学特性的基本变量),需要测量正上方表面的上涌辐射率(Lw)和到达表面的下沉平面辐照度(Es)。为此,目前的 HyperNav 系统以约 2 nm 的光谱分辨率测量紫外线到近红外的 Lw,但只测量以 412、489、555 和 705 nm 为中心的四个 10 nm 宽光谱带的 Es。在这项研究中,利用在晴空条件下获得的这些光谱波段的 Es 数据,通过一个多线性回归模型,重建了从 315 纳米到 900 纳米分辨率为 0.5 纳米的超光谱 Es 信号,即 OCI 光谱范围,从而可以估算出 HyperNav、OCI 和其他传感器分辨率下的 Es 信号。在对气体吸收进行校正并根据大气层顶入射太阳通量进行归一化处理后,大气漫反射透射率就可以表示为在这 4 个光谱波段测量到的 Es 的线性组合。根据从 0 到 75° 的太阳天顶角以及广泛的(即预期的)大气、地表和水条件的模拟,重建的 Es 光谱偏差小于 0.4%,均方根误差(RMSE)从 0% 到 2.5%(取决于波长)不等。最大的误差出现在有强烈气体吸收的光谱区域。如果存在 Es 测量的典型噪声和辅助变量的不确定性,偏差和均方根误差将分别为 -2.5% 和 7.0%。使用系数取决于太阳天顶角和 550 nm 处气溶胶光学厚度的通用加法模型,可改善无噪声时的统计性能,尤其是紫外线,但在噪声数据上的性能与此相似,表明对噪声更加敏感。由于气体透过率的不确定性,在紫外线中增加光谱带,例如以 325、340 和 380 nm 为中心的光谱带,可略微提高紫外线结果的准确性。MOBY 分光辐射计测量和重建的 Es 光谱之间的比较显示,两者在预测的不确定性范围内一致,即偏差小于 2%,均方根差异小于 5%。由于云的光学特性和气溶胶特性一样,往往随波长有规律地变化,因此使用其他光谱波段也能准确地进行重建,并可扩展到多云条件下。这些结果表明,对于许多涉及高光谱 Es 的科学应用来说,只需测量紫外线到近红外的几个粗光谱波段的 Es,并利用所提出的多元线性建模方法重建高光谱信号即可。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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