Solar-induced chlorophyll fluorescence (SIF) is a promising tool to estimate gross primary production (GPP), but the retrieval of SIF is commonly noisy and highly sensitive to various interference factors. Particularly, the retrieval of SIF in the red band (RSIF) is more challenging than in the far-red SIF (FRSIF) due to the weaker fluorescence signal and the weaker absorption depth of oxygen at the red band compared with the far-red band. A comprehensive evaluation of all factors will allow a reproducible interpretation of SIF signals and advance the estimation of GPP from SIF. Recent studies have assessed the sensitivity of SIF retrieval to sensor characteristics, retrieval methods, and hardware specifications. However, none of these studies have systematically investigated the directional retrieval error of SIF resulting from the mismatch between irradiance measured above the canopy and the true irradiance reaching the canopy components viewed by a sensor. This study illustrated the effect of mismatched irradiance on the retrieval of RSIF using the commonly used standard 3FLD method based on SCOPE model simulations. The retrieval accuracy was highest in the hotspot direction, but it decreased as the observation direction was away from the hotspot. The relative root mean square error (RRMSE) was generally higher than 20 % in the forward directions. To reduce the retrieval error due to the mismatch effect, we proposed a modified 3FLD method (MFLD) by calculating the true irradiance reaching the canopy in a given direction based on geometric optical theory. The results showed that MFLD clearly improved the retrieval accuracy for RSIF, especially in the forward directions where RRMSE decreased by 10 % in most cases. For example, the RRMSE was reduced from 19.26 % to 5.50 % after mitigating the mismatch between the measured and actual solar irradiance, when the solar zenith angle was 40° and viewing zenith angle was 30° in the forward solar principal plane. Even at the nadir observation, the RRMSE was also reduced from 12.84 % to 5.64 %. In summary, MFLD can effectively mitigate the irradiance mismatch effect on the retrieval of RSIF. These results will improve our interpretation of the relationship between GPP and RSIF at different observation directions.

中文翻译：

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减轻红波段太阳诱导叶绿素荧光的方向检索误差


太阳诱导叶绿素荧光 （SIF） 是估计总初级生产力 （GPP） 的一种很有前途的工具，但 SIF 的检索通常具有噪声并且对各种干扰因子高度敏感。特别是，由于与远红波段相比，红波段 （RSIF） 中 SIF 的检索比远红波段 （FRSIF） 更具挑战性，因为红波段的荧光信号较弱，并且红波段对氧的吸收深度较弱。对所有因素的全面评估将允许对 SIF 信号进行可重复的解释，并推进 SIF 对 GPP 的估计。最近的研究评估了 SIF 检索对传感器特性、检索方法和硬件规格的敏感性。然而，这些研究都没有系统地研究由于在冠层上方测量的辐照度与传感器观察到的到达冠层组件的真实辐照度之间的不匹配而导致的 SIF 定向检索误差。本研究使用基于 SCOPE 模型模拟的常用标准 3FLD 方法说明了失配辐照度对 RSIF 检索的影响。反演精度在热点方向最高，但随着观测方向远离热点，反演精度降低。相对均方根误差 （RRMSE） 在正向方向上通常高于 20 %。为了减少失配效应引起的检索误差，我们提出了一种改进的 3FLD 方法 （MFLD），该方法基于几何光学理论计算在给定方向上到达冠层的真实辐照度。结果表明，MFLD 明显提高了 RSIF 的检索准确性，尤其是在大多数情况下 RRMSE 下降了 10% 的正向。例如，RRMSE 从 19.26 % 降低到 5。在减轻测量和实际太阳辐照度之间的不匹配后，当太阳前方主平面的太阳天顶角为 40°，观测天顶角为 30°时，为平均值增加 50%。即使在最低点观测时，RRMSE 也从 12.84 % 降低到 5.64 %。综上所述，MFLD 可以有效减轻辐照度失配对 RSIF 检索的影响。这些结果将改进我们对不同观测方向下 GPP 和 RSIF 之间关系的解释。