# Near-Surface and High-Resolution Satellite Time Series for Detecting Crop Phenology
## 5 Conclusions  
In this study, we assessed the near-surface PhenoCam and high-resolution PlanetScope time series in characterizing two critical crop phenological stages for a range of agricultural sites across the US. Among the transition date analysis methods, the CUR-based Greenup and Gu-based UD estimates from both PhenoCam and PlanetScope imagery showed good congruence with the visually observed crop emergence stage. The TRS- and DER-based EOS estimates reconciled well with visual crop maturity observations. The concordance among PlanetScope, PhenoCam, and visual phenology demonstrated the potential to interpret the fine-scale sensor-derived phenological characteristics in the context of physiologically well-characterized crop phenological events. This concordance could further facilitate the use of both fine-scale sensors in calibrating and validating phenological models, understanding the relationships between canopy phenology and agroecosystem processes (e.g., gross primary production and carbon-nutrient-water cycling), as well as paving the way to develop formal protocols for bridging ground-satellite phenological characterization. Overall, the intercomparison analysis in this study should increase our capability in retrieving the fine-scale phenological characteristics tied closely to the tangible crop growth stages on the ground and support an improved understanding of global agricultural production, particularly in heterogeneous and small-holder agricultural systems.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/rs14091957/s1](https://www.mdpi.com/article/10.3390/rs14091957/s1)]([https://www.mdpi.com/article/10.3390/rs14091957/s1](https://www.mdpi.com/article/10.3390/rs14091957/s1)), Figure S1: The accuracy statistics of comparisons between the PhenoCam GCC-based TRS-SOS and the visually observed emergence under different thresholds for the shared corn and soybean sites. Figure S2: The accuracy statistics of comparisons between the PlanetScope NDVI-based TRS-SOS and the visually observed emergence under different thresholds for the shared corn and soybean sites.  
Conceptualization, C.D. and G.L.; methodology, C.D. and G.L.; software, G.L.; validation, C.D. and G.L.; formal analysis, C.D. and G.L.; investigation, C.D. and G.L.; resources, C.D.; data curation, G.L.; writing--original draft preparation, C.D.; writing--review and editing, C.D. and G.L.; visualization, G.L.; supervision, C.D.; project administration, C.D.; funding acquisition, C.D. All authors have read and agreed to the published version of the manuscript.  
This research was funded by the National Science Foundation, grant number 2048068, and the National Aeronautics and Space Administration, grant number 80 NSSC21K0946.  
This study uses the PlanetScope imagery from the Planet Labs under the education and research program ([[https://www.planet.com/](https://www.planet.com/)]([https://www.planet.com/](https://www.planet.com/)); accessed on 10 June 2021), and the PhenoCam imagery that is publicly available from the North American PhenoCam network ([[https://phenocam.sr.unh.edu/webcam/](https://phenocam.sr.unh.edu/webcam/)]([https://phenocam.sr.unh.edu/webcam/](https://phenocam.sr.unh.edu/webcam/)); accessed on 15 November 2020).  
We gratefully acknowledge the North American PhenoCam network for providing the PhenoCam imagery, and the Planet Labs for providing the PlanetScope imagery.  
The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.