*2.2. Airplane-Based Hyperspectral Imaging*

Airborne hyperspectral imaging has been widely used to collect hyperspectral imagery for different monitoring purposes (e.g., for agriculture or forestry). The first hyperspectral sensor was an airborne visible/infrared imaging spectrometer (AVIRIS) that was developed and utilized in 1987 [84]. It collects spectral signals in 224 bands in the visible to SWIR range (Table 2). Researchers have applied AVIRIS data to help understand a wide range of agricultural features, such as investigating vegetation properties (e.g., yield, LAI, chlorophyll, and water content) [85–88], analyzing soil properties [89], evaluating crop health or identifying pest infestation [90–92], and mapping crop area or agricultural tillage practices [93,94].

Besides AVIRIS, the Compact Airborne Spectrographic Imager (CASI), Hyperspectral Mapper (HyMap), and AISA Eagle are also widely used airborne hyperspectral sensors (Table 2). For instance, CASI images have been used for estimating crop chlorophyll content [95], investigating crop cover fraction [96], classifying weeds [97], and delineating management zones [2]. The HyMap imagery has been applied to examining crop biophysical and biochemical variables (e.g., LAI, chlorophyll and water content) [98–100], detecting plant stress signals [101], and investigating the spatial patterns of SOC [102]. Regarding AISA Eagle imagery, Ryu et al. [35] and Cilia et al. [103] used this data for estimating crop nitrogen content, and Ambrus et al. [104] used it for estimating biomass.

Several other airborne hyperspectral sensors have also been used in previous studies. For instance, AVIS images were used for investigating a range of vegetation characteristics (e.g., biomass and chlorophyll) [105], Probe-1 hyperspectral images were used for investigating crop residues [106], RDACS-H4 hyperspectral images were used for detecting crop disease [34], AHS-160 hyperspectral sensor was used for mapping SOC [107], the SWIR Hyper Spectral Imaging (HSI) sensor was used for estimating soil moisture [108], the Pushbroom Hyperspectral Imager (PHI) was used for estimating winter wheat LAI [109], and airborne prism experiment (APEX) data were used for studying the relationship between SOC in croplands and the spectral signals [110].

Most of the aforementioned airborne hyperspectral images have been acquired by airplanes at medium to high altitude (e.g., 1–4 km altitude for CASI, 20 km for AVIRIS), and the acquired images generally having high to medium spatial resolution, such as 4 m for CASI imagery, 5 m for HyMap, and 20 m for AVIRIS [111–113]. Such spatial resolutions are appropriate for mapping many crop and soil features. However, image acquisition usually needs to be scheduled months or even years in advance, and flight missions are expensive [19]. Furthermore, for some specific applications, such as investigating species-level or community-level features (e.g., identification of weeds or early signal of crop disease), images with very high spatial resolutions (e.g., sub-meter) are preferred [114,115]. In addition, due to the unstable nature of airplanes as imaging platforms, a gimbal or high-accuracy inertial measurement unit (IMU) will be required to compensate for the orientation change of the airplanes or recording the orientation information for subsequent image correction, respectively. These factors limited the full application of airborne hyperspectral imaging in precision agriculture. Manned helicopters have also been used as platforms for hyperspectral imaging and investigation of vegetation features [27,116]. Helicopters have more flexible flight heights (e.g., 100 m–2 km) than airplanes and are capable of acquiring high-spatial-resolution images (e.g., sub-meter) over large areas. An aviation company with a manned helicopter is generally needed for the imaging task, which requires extra funding support and far advanced pre-scheduling.
