## 2 Materials and Methods
### Hardware and Physical Architecture
#### 2.1.1 Upper Package  
The upper package consists of a spectrometer and true-color (or red, green, and blue (RGB)) camera, two TIR sensors, a white reference panel with a custom panel housing, and an actuator. All of these components are mounted on a pan-tilt unit (PTU) (Figure 1, Table 1, Video S1). The spectrometer and RGB camera are housed in an enclosure.  
The spectrometer (ASD FieldSpec 4; Cambridge, MA, USA) measures electromagnetic radiation (EMR) that is reflected by land surfaces, such as plants and soils (Figure 1). EMR reflected by the land surface is collected across 2151 narrow contiguous spectral channels from a 350 to 2500 nm spectra range with 4 to 10 nm channel width (Table 1). The instrument uses a 1.5 m fiber optic cable having a 25 degree field of view (FOV). The spectrometer is housed in a securely sealed enclosure (ENC 14/16-MM-NC, Campbell Scientific Inc.; Logan, KY, USA) to be protected from elements. The optical fiber cable that is extended from the spectrometer is housed in a metal casing, and the tip of the cable is installed perpendicular to the ground to collect nadir-viewing measurement. The enclosure is placed on a steel plate secured to the top of a pan-tilt platform (PTU-D300, FLIR; Wilsonville, AL, USA) and mounted on the tower.  
The RGB camera (Axis Q1604; Lund, Sweden) housed in the enclosure is installed perpendicular to the land surface (Figure 1, Table 1). The camera captures true-color photos from the land surface around the tower via a small glass opening fabricated at the bottom of the enclosure. Because of the large FOV of the camera, each photo contains the FOV of the spectrometer and its vicinity. The RFB photos provide contextual information, such as soil exposure, plant condition, canopy closure, and shadowing, which aid in interpreting the spectrometer and TIR measurements. For example, RGB photos can be used to identify the cause of sudden changes in spectral reflectance and/or vegetation indices and flag those data. The photos may be useful for investigating the effects of soil exposure, leaf orientation, and shadow on optical and infrared measurements.  
Two TIR sensors (Apogee Instruments SI-111; Logan, KY, USA), downward-looking and upward-looking, are installed on the side of the metal casing of the fiber cable (Figure 1, Table 1). The downward-looking TIR sensor measures the radiant temperature of land surface consisting of plant canopies. This TIR sensor has approximately 22 degree FOV. The upward-looking sensor measures sky temperature that is indicative of cloudiness of the sky at a given time. Both sensors are connected to a data logger, and readings are recorded over the predefined time periods: 1 and 30 min, respectively.  
We developed a reference subsystem using the white reference panel (Labsphere spectralon target SRT-99-050; North Sutton, NH, USA), actuator (Progressive Automation PA 15-8-11; Arlington, TX, USA), and custom housing (Figure 1, Table 1). The 99% reflective Spectralon(r) panel extends only when reference data collection is needed. Movement of the reference subsystem is synchronized with  
Figure 1: EcoSpec system. (**a**) The system installed in a corn field (2016) powered by solar energy and (**b**) system components.  
movement of the PTU and the spectrometer's preparatory activities, such as parameter optimization (Video S1). The reference subsystem supports reliable optical and infrared reflectance data collection using a spectrometer by collecting white reference data and information on the sensor's internal noise more frequently than traditional settings (e.g., every 20-30 min).