Annual time-series of the two satellites C-band SAR (Synthetic Aperture Radar) Sentinel-1A and 1B data over five years were used to characterize the phenological cycle of a temperate deciduous forest. Six phenological metrics of the start (SOS), middle (MOS) and end (EOS) of budburst and leaf expansion stage in spring, and the start (SOF), middle (MOF) and end (EOF) of leaf senescence in autumn were extracted using an asymmetric double sigmoid function (ADS) fitted to the time-series of the ratio (VV/VH) of backscattering at co-polarization VV (vertical–vertical) and at cross polarization VH (vertical-horizontal). Phenological metrics were also derived from other four vegetation proxies (Normalized Difference Vegetation Index NDVI time-series from Sentinel-2A and 2B images, and in situ measurements of NDVI measurements, Leaf Area Index LAI and litterfall temporal dynamics). These estimated phenological metrics were compared to phenological observations obtained by visual observations from the ground, achieved using binoculars by three inter-calibrated observers, on a bi-weekly basis during the budburst and weekly during the senescence. We observe a decrease in the backscattering coefficient (σ⁰) at VH cross polarization during the leaf development and the expansion phase in spring and an increase during the senescence phase, contrary to what is usually observed on various types of crops. In vertical polarization, σ⁰VV shows very little variation throughout the year. S-1 time-series of VV/VH ratio provide a good description of the seasonal vegetation cycle allowing the estimation of spring and autumn phenological metrics. Estimates provided by VV/VH of budburst dates using MOS criterion differ by approximately 8 days on average (mean average deviation) from phenological observations. During senescence phase, estimates using MOF criterion are later and deviate by about 20 days from phenological observations of leaf senescence while the differences are of the order of 2 to 4 days between the phenological observations and estimates based on in situ NDVI and LAI time-series, respectively. A deviation of about 7 days, comparable to that observed during budburst, is obtained between the estimates of senescence (MOF) from S-1 and those determined from the in situ monitoring of litterfall. While in spring, leaf emergence and expansion described by LAI or NDVI explain the increase of VV/VH (or the decrease of σ⁰VH), during senescence, S-1 VV/VH is decorrelated from LAI or NDVI and is better explained by litterfall temporal dynamics. This behavior resulted in a hysteresis phenomenon observed on the relationships between VV/VH and NDVI or LAI. For the same LAI or NDVI, the response of VV/VH is different depending on the phenological phase considered. This study shows the high potential offered by Sentinel-1 SAR C-band time-series for the detection of forest phenology, thus overcoming the limitations caused by cloud cover in optical remote sensing of vegetation phenology.

两颗卫星 C 波段 SAR（合成孔径雷达）Sentinel-1A 和 1B 五年内的年度时间序列数据用于表征温带落叶林的物候循环。春季发芽和叶片膨胀期的开始（SOS）、中期（MOS）和结束（EOS）以及秋季叶片衰老的开始（SOF）、中期（MOF）和结束（EOF）6个物候指标分别为使用不对称双 sigmoid 函数 (ADS) 提取，该函数适合于共极化 VV（垂直 - 垂直）和交叉极化 VH（垂直 - 水平）的反向散射的比率（VV / VH）的时间序列。物候指标也来自其他四个植被代理（来自 Sentinel-2A 和 2B 图像的归一化差异植被指数 NDVI 时间序列，以及 NDVI 测量的原位测量，叶面积指数 LAI 和凋落物时间动态）。这些估计的物候指标与从地面目视观察获得的物候观察进行比较，由三个相互校准的观察员使用双筒望远镜在萌芽期间每两周一次和衰老期间每周一次。我们观察到反向散射系数 (σ⁰ ) 在叶子发育和春季扩张阶段的 VH 交叉极化以及衰老阶段的增加，这与通常在各种作物上观察到的情况相反。在垂直极化中，σ ⁰VV 全年变化很小。VV/VH 比率的 S-1 时间序列很好地描述了季节性植被周期，从而可以估计春季和秋季的物候指标。使用 MOS 标准由 VV/VH 提供的萌芽日期估计值与物候观察平均相差约 8 天（平均偏差）。在衰老阶段，使用 MOF 标准的估计较晚，与叶片衰老的物候观察偏差约 20 天，而物候观察与基于原位 NDVI 和 LAI 时间序列的估计之间的差异约为 2 至 4 天， 分别。大约 7 天的偏差，与萌芽期间观察到的偏差相当，是在 S-1 的衰老 (MOF) 估计值和从凋落物的原位监测确定的估计值之间获得的。而在春季，LAI 或 NDVI 描述的叶片出现和扩张解释了 VV/VH 的增加（或 σ⁰ VH)，在衰老期间，S-1 VV/VH 与 LAI 或 NDVI 去相关，并且可以通过凋落物时间动态更好地解释。这种行为导致在 VV/VH 和 NDVI 或 LAI 之间的关系上观察到滞后现象。对于相同的 LAI 或 NDVI，VV/VH 的响应因所考虑的物候阶段而异。本研究显示了 Sentinel-1 SAR C 波段时间序列在检测森林物候方面的巨大潜力，从而克服了植被物候光学遥感中云量覆盖带来的局限性。