Time series from optical sensors are frequently used to retrieve phenology information of forests. While SAR (synthetic aperture radar) sensors can potentially provide even denser time series than optical data, their potential to retrieve phenological information of forests is still underexplored. In addition, the backscatter information from SAR is frequently exploited in the same way (e.g., via dynamic thresholding) as optical data to retrieve phenological information. Sentinel-1 backscatter coefficients of VH (vertical–horizontal) and VV (vertical–vertical) polarizations and their ratio were retrieved for temperate deciduous broad-leaf and evergreen needle-leaf forests in Europe. Breakpoints and dynamic thresholds were retrieved in the locally smoothed time-series data and compared to reference data from PhenoCam and fluxtower networks. It was generally found that breakpoints outperform dynamic thresholds in both forest types in terms of root mean squared differences, bias and R2. Best results were achieved using breakpoints on the Sentinel-1 backscatter ratio with RMSEs of 18.4 days for the start of the season (SOS) and 14.0 days for the end of the season (EOS) compared to the 25% dynamic threshold of the seasonal amplitude in the reference data in deciduous broad-leaf forests. Substantially higher RMSE values of 56.7 days for SOS and 56.5 days for EOS were found in evergreen needle-leaf forests. This study suggests the potential of Sentinel-1 for the phenological retrieval of forests, in particular deciduous broad-leaf forests. This information could be used in combination with frequently used optical data to provide comprehensive phenological information on a large scale.

中文翻译：

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Sentinel-1 时间序列数据监测欧洲温带森林物候的潜力


来自光学传感器的时间序列经常用于检索森林的物候信息。虽然 SAR（合成孔径雷达）传感器可能提供比光学数据更密集的时间序列，但它们检索森林物候信息的潜力仍未得到充分开发。此外，来自 SAR 的反向散射信息经常以与光学数据相同的方式（例如，通过动态阈值）被利用来检索物候信息。检索了欧洲温带落叶阔叶林和常绿针叶林的 VH （垂直-水平）和 VV （垂直-垂直）极化的 Sentinel-1 反向散射系数及其比率。在本地平滑的时间序列数据中检索断点和动态阈值，并与来自 PhenoCam 和 fluxtower 网络的参考数据进行比较。通常发现，在两种森林类型中，断点在均方根差、偏差和 R2 方面都优于动态阈值。使用 Sentinel-1 反向散射比的断点取得了最佳结果，季节开始 （SOS） 的 RMSE 为 18.4 天，季节结束 （EOS） 的 RMSE 为 14.0 天，而落叶阔叶林参考数据中季节振幅的 25% 动态阈值。在常绿针叶林中发现 SOS 的 RMSE 值要高得多，分别为 56.7 天和 EOS 的 56.5 天。这项研究表明 Sentinel-1 在森林物候恢复方面的潜力，特别是落叶阔叶林。这些信息可以与常用的光学数据结合使用，以提供大规模的全面物候信息。