The Advanced Baseline Imager (ABI) sensors on the Geostationary Operational Environment Satellite-R series (GOES-R) broaden the application of global vegetation monitoring due to their higher temporal (5–15 min) and appropriate spatial (0.5–1 km) resolution compared to previous geostationary and current polar-orbiting sensing systems. Notably, ABI Land Surface Phenology (LSP) quantification may be improved due to the greater availability of cloud-free observations as compared to those from legacy GOES satellite generations and from polar-orbiting sensors such as the Moderate Resolution Imaging Spectroradiometer (MODIS) and Visible Infrared Imaging Radiometer Suite (VIIRS). Geostationary satellites sense a location with a fixed view geometry but changing solar geometry and consequently capture pronounced temporal reflectance variations over anisotropic surfaces. These reflectance variations can be reduced by application of a Bidirectional Reflectance Distribution Function (BRDF) model to adjust or predict the reflectance for a new solar geometry and a fixed view geometry. Empirical and semi-empirical BRDF models perform less effectively when used to predict reflectance acquired at angles not found in the observations used to parameterize the model, or acquired under hot-spot sensing conditions when the solar and viewing directions coincide. Consequently, using a fixed solar geometry or even the geometry at local solar noon may introduce errors due to diurnal and seasonal variations in the position of the sun and the incidence of hot-spot sensing conditions. In this paper, a new solar geometry definition based on a Constant Scattering Angle (CSA) criterion is presented that, as we demonstrate, reduces the impacts of solar geometry changes on reflectance and derived vegetation indices used for LSP quantification. The CSA criterion is used with the Ross-Thick-Li-Sparse (RTLS) BRDF model applied to North America ABI surface reflectance data acquired by GOES-16 (1 January 2018 to 31 December 2020) and GOES-17 (1 January 2019 to 31 December 2020) to normalize solar geometry BRDF effects and generate 3-day two-band Enhanced Vegetation Index (EVI2) time series. Compared to the local solar noon geometry, the CSA criterion is shown to reduce solar geometry reflectance and EVI2 time series artifacts. Further, comparison with contemporaneous VIIRS NBAR (Nadir BRDF-Adjusted Reflectance) EVI2 time series is also presented to illustrate the efficacy of the CSA criterion. Finally, the CSA-adjusted EVI2 time series are shown to produce LSP results that agree well with PhenoCam-based observations, with no obvious systematic bias in onsets of vegetation maturity, senescence, and dormancy dates compared to about 10-day bias found with local solar noon adjusted EVI2 time series.

中文翻译：

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一种新的恒定散射角太阳几何定义，用于 GOES-R ABI 反射率时间序列的标准化，以支持地表物候研究


地球静止运行环境卫星 R 系列 (GOES-R) 上的高级基线成像仪 (ABI) 传感器由于其较高的时间（5-15 分钟）和适当的空间（0.5-1 公里）分辨率，拓宽了全球植被监测的应用与以前的地球静止和当前的极轨传感系统相比。值得注意的是，与传统 GOES 卫星以及中分辨率成像光谱仪 (MODIS) 和可见光等极轨传感器的观测相比，由于无云观测的可用性更高，ABI 地表物候 (LSP) 量化可能会得到改善。红外成像辐射计套件 (VIIRS)。对地静止卫星感测具有固定视图几何形状但改变太阳几何形状的位置，从而捕获各向异性表面上明显的时间反射率变化。通过应用双向反射分布函数 (BRDF) 模型来调整或预测新太阳几何形状和固定视图几何形状的反射率，可以减少这些反射率变化。当用于预测在用于参数化模型的观测中未找到的角度获取的反射率或在太阳方向与观察方向一致时在热点传感条件下获取的反射率时，经验和半经验 BRDF 模型的性能较差。因此，使用固定的太阳几何形状甚至当地太阳正午的几何形状可能会由于太阳位置的昼夜和季节变化以及热点感测条件的发生而引入误差。 在本文中，提出了一种基于恒定散射角（CSA）标准的新太阳几何定义，正如我们所证明的那样，它减少了太阳几何变化对反射率和用于 LSP 量化的衍生植被指数的影响。 CSA 准则与 Ross-Thick-Li-Sparse (RTLS) BRDF 模型结合使用，应用于 GOES-16（2018 年 1 月 1 日至 2020 年 12 月 31 日）和 GOES-17（2019 年 1 月 1 日至 12 月 31 日）获取的北美 ABI 表面反射率数据。 2020 年 12 月 31 日），以标准化太阳几何 BRDF 效应并生成 3 天两波段增强植被指数 (EVI2) 时间序列。与当地太阳正午几何相比，CSA 标准可以减少太阳几何反射率和 EVI2 时间序列伪影。此外，还提供了与同期 VIIRS NBAR（最低点 BRDF 调整反射率）EVI2 时间序列的比较，以说明 CSA 标准的有效性。最后，经 CSA 调整的 EVI2 时间序列显示产生的 LSP 结果与基于 PhenoCam 的观测结果非常吻合，与当地发现的约 10 天偏差相比，植被成熟、衰老和休眠日期的开始时间没有明显的系统偏差。太阳正午调整的 EVI2 时间序列。