# Application of Reflectance Indices for Remote Sensing of Plants and Revealing Actions of Stressors
## 2 Optical Methods and Platforms of Remote Sensing of Plants
### Some Platforms for the Plant Remote Sensing  
The platforms of plant remote sensing can strongly differ; their type is dependent on the specific tasks of the remote sensing. In our review, we selected some groups of flying platforms: unmanned aircraft systems (UAS), including copters and drones, airplanes, and satellites. Manual measurements with handheld devices were also described because this \"classical\" method is also actively used for plant monitoring. Further, we used some criteria for the description of these systems of plant remote sensing, including the automatization of measurement, accessibility for researchers, possibility of construction of individual and complex of trajectory of measurements, and spatial and temporal resolution. The handheld devices, which include systems of multi- and hyperspectral measurements in single leaves or plants, are the most accessible variants for researchers. In many handheld devices (see, e.g., the devices by Photon System Instruments, Czech), the positions of the leaf, light source, and light detector are fixed; this means that the measurements of the spectral characteristics of the reflected light can be very accurate. It should be noted that artificial illumination or a combination of the artificial illumination and sunlight can be used with these handheld systems [52] (e.g., halogen lamps are optimal for hyperspectral measurements [44] or light-emitting diodes for multispectral measurements [17]); in contrast, most of the flying mobile platforms for plant remote sensing cannot support the artificial illumination of objects (only sunlight is used because the distances between the camera and objects are too large). However, the velocity and spatial and temporal resolution of these measurements (Figure 4) are strongly limited by the speed of manual measurements; any automatization of the trajectory of the measurements is absent [33]. The prospective group is UAS platforms, which includes copters and drones [23]. The automatization of plant remote sensing on the basis of UAS is very high for both drones and copters because a program of flight can be automatically formed in accordance with a program of measurements [33; 53]. The trajectories of multi- and hyperspectral measurements on the basis of copters can be constructed in wide ranges (the scanning of specific areas or point-by-point scanning of interesting parts can be used; the height of measurements can be controlled; the copter can hang over the area of interest and makes dependable observations; etc.) [53]. The resolutions of these systems vary from a few cm to tens of cm [33]; this property is interesting for the research of different spatial scales. An additional advantage of using the copter is their relatively low costs and accessibility for researchers [23]. However, using copters can be limited by the low loading capacity of copters, relatively slow speed, and short time of flight [23; 33]. These problems are not crucial for drones, which have a high speed and relatively high loading capacity [27]. However, drones are more expensive than copters and cannot be used for the accurate scanning of interesting areas (there are minimal speeds and heights of the flight of drones [27; 33]); as a result, increasing the total area of investigation and duration of the flight by using drones is accompanied with a decrease in the resolution of the measurements [33]. Using airplanes for plant remote sensing is similar to using  
Figure 4: Spatial (from leaf to ecosystem(s)) and temporal (from seconds to years) scales of plant remote sensing on basis of manual measurements using handheld devices and different flying platforms (copters, drones, airplanes, and satellites). drones, but their loading capacity and speed are even greater; the last point increases the spatial and temporal scale of the measurement and decreases the spatial resolution [53]. Additionally, airplanes are very expensive, and their application requires the participation of pilots (i.e., automatization is lower) [53].