# Near-Surface and High-Resolution Satellite Time Series for Detecting Crop Phenology
## 2 Materials and Methods  
### Study Sites and Data  
As the two most cultivated crops in the US, corn, and soybean are selected for phenological characterization in this study. Our study sites comprise major corn and soybean sites in the US of the North American PhenoCam network, with longitudes spanning from 100.93\({}^{\circ}\)W to 121.54\({}^{\circ}\)W, and latitudes from 38.11\({}^{\circ}\)N to 46.76\({}^{\circ}\)N (Figure 1). The study sites include the Kingman Farm Site (Strafford County, NH, USA), Kellogg Biological Station Site (Barry County, MI, USA), USDA-ARS Hawbecker Farm Site (Huntingdon County, PA, USA), Peat SSJ River Delta Bouldin Island Site (San Joaquin County, CA, USA), US-Ne1-3 Maize-Soybean Site (Saunders County, NE, USA), USDA Economic and Environmental Research Site (Prince George's County, MD, USA), Swan Lake Research Farm Site (Stevens County, MN, USA), ARS Morris Minnesota LTAR South Tower Site (Stevens County, MN, USA), Dryland Cropping System Site (Morton County, ND, USA), Rosemount Conventional AG Management Site (Dakota County, MN, USA), and University of Illinois Energy Farm Site (Champaign County, IL, USA) (Table 1). Those sites are selected with the careful assessment of the camera configuration (e.g., field of view and white balance), image availability and quality (e.g., illumination geometry and weather conditions), and site characteristics (e.g., cropping system and landscape homogeneity) [38; 63]. Figure 1: The geographic locations of the study sites. The near-surface PhenoCam imagery of all the sites for the years 2010-2019 was acquired from the PhenoCam server ([[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). The number of years for each site may vary, depending on the timing of site establishment. The PhenoCam imagery includes three bands (i.e., blue, green, and red), and provides the canopy-scale observation of crops, with each one taken about every 30 or 60 min throughout the year. In this study, only the images from 10:00 a.m. to 2:00 p.m. local time are considered to reduce the influence of illumination difference and atmospheric interference [44]. Most of the sites are type I or type II sites (Table 1). Type I sites utilize prescribed NetCam SC IR cameras (StarDot Technologies, Buena Park, CA, USA), and type II sites do not use the prescribed NetCam cameras. Yet for both site types, the cameras are generally deployed via a standardized phenological monitoring protocol with site personnel actively engaged in camera maintenance to ensure the image quality [38]. Specifically, the cameras are mounted to towers (or other secure points) taller than the crops of interest and are angled downward at 20-40\({}^{\circ}\) to obtain canopy views of the crops. The cameras are typically pointed north to minimize shadow and lens flare effects and are set to a fixed white balance to eliminate confounding variation in imaging sensor response. Some sites may contain several cameras, with each monitoring an agricultural field. For example, the US-Ne1-3 Maize-Soybean site has three cameras, which point to the US-Ne1 irrigated continuous maize field, the US-Ne2 irrigated maize-soyebean rotation field, and the US-Ne3 rainfed maize-soyebean rotation field, respectively. We then removed the site-years with images of relatively low quality or significant amount of missing data during the growing season to ensure the quality of PhenoCam observations. In total, there are 41 PhenoCam site-year observations for corn, and 16 PhenoCam site-year observations for soybean. Besides the PhenoCam imagery, we also acquired the PlanetScope surface reflectance (Version 2.0) time series of the study sites from 2017 to 2019. The PlanetScope imagery is under the near nadir-view and has four bands (i.e., blue, green, red, and near-infrared), with a 3.7 m average ground sample distance and daily revisit frequency.