# Near-Surface and High-Resolution Satellite Time Series for Detecting Crop Phenology
## 2 Materials and Methods
### Methodology
#### 2.2.3 Crop Phenological Transition Date Analysis  
With the Beck fitted time series, the phenological transition dates were then retrieved using four phenophase analysis methods, namely the time series threshold-based method (TRS-based method hereafter), the time series derivative-based method (DER-based method hereafter), the time series curvature-based method (CUR-based method hereafter), and the time series Gu-based method. For each of the methods, we retrieved relevant phenological metrics corresponding to the Start of greenness rising Season (i.e., SOS), as well as the metrics corresponding to the End of greenness falling Season (i.e., EOS). The retrieved metric-specific SOS would be compared with crop visual emergence observations, and the metric-specific EOS would be compared with crop visual maturity observations.  
The TRS-based method identified critical phenophases by extracting the dates when the Beck fitted curve reached 50% of the seasonal amplitude of vegetation index. Due to the difficulty in arbitrarily defining a universal threshold applicable to different vegetation indices or crops of varying biophysical characteristics [17], we only tested 50% of the amplitude, a widely used threshold in phenological studies, in both curve upward and downward directions to represent the TRS-based SOS and EOS in this study. The DER-basedmethod retrieved the phenological transition dates corresponding to the local extremes in the first derivative of the Beck fitted curve. The retrieved dates of maximum and minimum derivative values correspond to DER-based SOS and EOS, respectively. For the CUR-based method, it identified the local maxima/minima in the curvature change rate for the phenophase estimation [24]. During the greenness rising period, the CUR-based method extracted two phenological metrics, namely Greenup and Maturity, corresponding to the two local maxima in the change rate of the curvature. During the greenness falling period, it extracted another two phenological metrics, namely Senescence and Dormancy, corresponding to the two local minima in the curvature change rate. The Gu-based method extracted the phenological transition dates by leveraging the recovery, senescence, and boundary lines (i.e., base and plateau lines). The recovery line was a line tangent to the curve at the maximum point of the curve's first derivative. Similarly, the senescence line was a line tangent to the curve at the minimum point of the first derivative. The base and plateau lines were the horizontal lines with median dormant season and maximum vegetation index values, respectively. During the greenness rising period, the Gu-based method extracted the Upturn Date (UD) and Stabilization Date (SD) phenological metrics by intersecting the recovery line with the base and plateau lines, respectively. During the greenness falling period, it retrieved the Downturn Date (DD) and Recession Date (RD) metrics by intersecting the senescence line with the plateau and the baselines, respectively. More information about the Gu-based phenological extraction could be found in [PERSON], et al. [66].  
With the four phenophase analysis methods, a diverse range of metric-specific SOS and EOS characteristics of crop phenological transitioning could be retrieved. Specifically, the TRS-based SOS, DER-based SOS, CUR-based Greenup and Maturity, and Gu-based UD and SD metrics would be compared to the emerged stages of crops. The TRS-based EOS, DER-based EOS, CUR-based Senescence and Dormancy, and Gu-based DD and RD metrics would be compared to the mature stages of crops.