# Near-Surface and High-Resolution Satellite Time Series for Detecting Crop Phenology
## 3 Results  
### Fine-Scale Sensor-Based Crop Phenological Characterization  
The fine-scale sensor-derived crop phenological characterizations facilitated direct comparisons with visually observed phenological events. With the corn site in the University of Illinois Energy Farm in 2017 as an example, the Beck phenological model fitted to GCC observations was shown in Figure 4. The weighting scheme in Beck diminished the impacts of negatively biased values. The RMSE of the Beck fitting was about 0.01. At this site, the crop emergence date through the visual observation was about the day of the year (DOY) 148 in 2017, with the consideration of a series of preceding and succeeding PhenoCam photos. The TRS- and DER-based SOS measures were about DOY 170 when the corn had developed several leaves on the ground. The CUR-based Greenup and Maturity estimates were about DOY 150 and DOY 189, respectively. As for the Gu-based method, the UD and SD phenological transition dates were about DOY 153 and DOY 184, respectively. Among the PhenoCam-retrieved phenological characteristics, the CUR-based Greenup and Gu-based UD measures were closer to the visually observed crop emergence date. The corresponding PhenoCam photos exhibited comparable crop status and landscape patterns upon comparison to the photo of crop emergence (Figure 4). The CUR-based Maturity and Gu-based SD estimates, on the other hand, approached the end of the corn vegetative stage when most of the plant leaves were developed. The crop maturity date by the visual comparison of a series of PhenoCam photos was about DOY 267 when about 50% of corn leaves turned yellow. Among the phenological transition date estimates, the TRS- and DER-based EOS estimates were about DOY 268, of which the crop growth status was comparable to the visually observed one. The CUR-based Senescence and Gu-derived DD dates were about DOY 245 and DOY 250, respectively. During these time periods, the crop was in the reproductive stage with the leave color starting to turn yellow. The CUR-based Dormancy and Gu-derived RD estimates were around DOY 290 and DOY 285, respectively. The corn was ready to be harvested at that time. Figure 4: The PhenoCam GCC observations fitted by the Beck double logistic model for the corn site of the University of Illinois Energy Farm in 2017. With the Beck fitted curve, four phenophase analysis methods (i.e., TRS-based, DER-based, CUR-based, and Gu-based methods) were employed to extract phenological metrics to be compared with corresponding visual observations. For this site-year, the visually observed crop emergence date was DOY 148, and the observed crop maturity date was DOY 267. The PhenoCam photos of the extracted metric and visual observation dates were also shown in this figure. The PlanetScope imagery was also analyzed for this specific site-year (Figure 5). About 80 PlanetScope images over the course of the year were downloaded and pre-processed. The Beck phenological model was fitted to pre-processed NDVI observations with the RMSE being 0.016. The phenological characteristics were also estimated to be compared with visually observed crop emergence and maturity dates. Similar to PhenoCam-derived measures, the CUR-based Greenup (DOY 154) and Gu-based UD (DOY 158) estimates were more aligned with the visually observed crop emergence date (DOY 148), upon comparison with TRS- and DER-based SOS estimates (about DOY 173). The TRS- and DER-based EOS estimates (about DOY 262) had better agreement with the visually observed crop maturity date (DOY 267), compared to the CUR-based Dormancy (DOY 291) and Gu-derived RD (DOY 287) estimates. The consistency between the PhenoCam and PlanetScope estimates indicated the potential of interpreting the fine-scale sensor-derived phenological characteristics in the context of visually observed crop phenological events.