Recently the applications of remotely sensed Solar-Induced chlorophyll Fluorescence (SIF) in the study of photosynthesis, stress conditions, and gross primary production have increased significantly. The full SIF spectrum spans over a spectral region of 650 ∼ 850 nm with two characteristic peaks around 685 nm and 740 nm. Over recent decades, many retrieval algorithms have been developed to estimate SIF at Top-Of-Canopy (TOC) using in-situ measurements of solar irradiance and canopy radiance spectra. Although the majority of retrieval methods retrieve SIF at a narrow spectral window, there exists a potential for retrieval of SIF in the full emission spectrum. Moreover, solar irradiance and canopy radiance spectra should ideally be measured at the same time but are usually measured sequentially with a certain time lag, raising potential errors in SIF retrieval. In this study, an enhanced retrieval algorithm of the full SIF spectrum at TOC is proposed. The proposed algorithm attempts to minimize the errors owing to time mismatch in measurements of solar irradiance and canopy radiance spectra. As an improvement to the previous algorithm (advanced Fluorescence Spectrum Reconstruction, aFSR), this proposed algorithm (aFSR-SVE) models the SIF-free contribution with principal components using the singular value decomposition technique. The optimal parameter settings in the forward model were determined for the experimental data collected by spectrometers used in the study. Firstly, the proposed algorithm was used to reconstruct full SIF spectrum for simulated data. The results were compared with known reference SIF values. After achieving satisfying results from simulated data, the proposed algorithm was compared with retrievals from established algorithms using experimental data. The results show improved SIF retrieval accuracy, without the need to simultaneously measure solar irradiance and canopy radiance spectra. The retrieval values comply with the results of previous algorithms in terms of spectral shape, diurnal trend, and temporal variations. The induced errors in SIF retrievals due to non-simultaneous measurements of solar irradiance and canopy radiance spectra were also investigated and the proposed algorithm was found to be less prone to such errors. Hence, the proposed algorithm is an improvement in reconstructing the full SIF spectrum with near-ground measurements. With the help of the proposed algorithm, field measurements using sequential systems and automated measurements of multiple targets can be performed effectively as it relaxes the requirement of concurrent measurement of solar irradiance and canopy radiance spectra. For future work, the applicability of this method can be investigated under more variable illumination conditions, like high cirrus clouds, passing clouds or persistent thin clouds.

中文翻译：

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一种用于近地测量的完整 SIF 光谱重建的增强方法


最近，遥感太阳诱导叶绿素荧光 （SIF） 在光合作用、胁迫条件和总初级生产研究中的应用显着增加。完整的 SIF 光谱跨越 650 ∼ 850 nm 的光谱区域，在 685 nm 和 740 nm 附近有两个特征峰。近几十年来，已经开发了许多检索算法，使用太阳辐照度和冠层辐射光谱的原位测量来估计冠层顶部 （TOC） 的 SIF。尽管大多数检索方法在较窄的光谱窗口检索 SIF，但有可能在整个发射光谱中检索 SIF。此外，理想情况下，太阳辐照度和冠层辐射光谱应同时测量，但通常是按顺序测量的，具有一定的时间滞后，从而在 SIF 检索中增加潜在误差。在本研究中，提出了一种在 TOC 处全 SIF 谱的增强检索算法。所提出的算法试图最大限度地减少太阳辐照度和冠层辐射光谱测量中由于时间不匹配引起的误差。作为对先前算法（高级荧光光谱重建，aFSR）的改进，该算法 （aFSR-SVE） 使用奇异值分解技术对主成分的无 SIF 贡献进行建模。正向模型中的最佳参数设置是根据研究中使用的光谱仪收集的实验数据确定的。首先，利用所提算法对模拟数据进行全SIF谱重构;将结果与已知的参考 SIF 值进行比较。 在从模拟数据中获得令人满意的结果后，将所提出的算法与使用实验数据的已建立算法的检索进行了比较。结果表明，SIF 检索精度提高，无需同时测量太阳辐照度和冠层辐射光谱。检索值在频谱形状、昼夜趋势和时间变化方面与先前算法的结果一致。还研究了由于非同时测量太阳辐照度和冠层辐射光谱而导致的 SIF 检索中的诱发误差，发现所提出的算法不易出现此类误差。因此，所提出的算法是通过近地面测量重建完整 SIF 频谱的改进。在所提出的算法的帮助下，可以有效地使用顺序系统进行现场测量和多个目标的自动测量，因为它放宽了同时测量太阳辐照度和冠层辐射光谱的要求。对于未来的工作，可以在更多变的照明条件下研究这种方法的适用性，例如高卷云、经过云或持续的薄云。