*2.2. Validation Datasets from the PROSAIL Model Simulation*

Canopy reflectance was simulated with the widely used PROSAIL model, which is a coupled model of the leaf reflectance model PROSPECT-5 [53] and canopy reflectance model SAILH [54,55]. In the PROSAIL model, homogeneous randomly distributed leaves are presumed to form a one-dimensional turbid medium [54], which is suitable for simulating the canopy reflectance of field crops. PROSPECT-5 simulates leaf reflectance and transmittance from 400 nm to 2500 nm as a function of six input parameters: *C*ab, the mesophyll structure parameter (*N*), carotenoid content (*C*car), brown pigment content (*C*brown), equivalent water thickness (*C*w), and dry matter content (*C*m). In addition to leaf optical properties, eight canopy structural parameters were used as inputs for PROSAIL: LAI, MTA (assuming an ellipsoidal distribution), solar zenith angle (*t*s), observer zenith angle (*t*o), relative azimuth angle (*ϕ*), soil reflectance, fraction of diffuse radiation (skyl) and hot spot size parameter. The PROSAIL model inputs, summarized in Table 1, were set in accordance with in-situ measurement conditions and scientific literature: *C*ab was set between 20 and 90 μg cm−2, in steps of 5 μg cm−2, *C*car was set to 20% of the *C*ab value based on the LOPEX93 dataset [56], *C*w was fixed to 0.001, *N* was fixed to 1.55—a mean value for various crops [57], *C*<sup>m</sup> was set to 0.005 g cm−2—the mean value of the six crop species [58–61], *C*brown was fixed to 0 assuming no senescent leaves during the measurements. LAI was set between 1 and 5 with a 0.1 interval, and MTA ranged from 20 to 70 with a 2-degree interval. Based on the conditions of airborne imaging spectroscopy data acquisition, the three illumination and view geometry parameters *t*s, *t*<sup>o</sup> and *ϕ* were set to 49.4◦, 9.0◦ and 90.0◦, respectively. The 6S atmosphere radiative transfer model was used to calculate the parameter skyl [62]. The hot spot parameter was fixed to 0.01 and the soil reference was measured using a handheld Analytical Spectral Devices spectroradiometer (ASD). In total, 15,990 canopy spectra between 400 nm and 1000 nm were simulated and resampled to satellite broadband reflectance.


**Table 1.** The variable settings of the PROSAIL model.

#### *2.3. Satellite Broadband Reflectance Simulations*

The airborne imaging spectroscopy data and PROSAIL model-simulated canopy reflectance in Visible to NIR spectral region (VNIR) were resampled to the broadband resolution of selected satellite sensors that had red edge channels: Sentinel-2, RapidEye, WorldView 2 and GaoFen-6. The MultiSpectral Instrument (MSI) of Sentinel-2 has 10 bands with three different spatial resolutions (10–60 m) in VNIR, including two red edge channels. RapidEye is a commercial Earth observation mission that offers high spatial resolution (6.5 m) imagery in five bands. The WorldView-2 satellite acquires very high spatial resolution (1.84 m) imagery in eight bands. The GaoFen-6 satellite, launched in 2018, has a multispectral sensor with 16 m spatial resolution in eight bands. The spectral response functions (SRFs, Figure A1 and Table A1) of the four multispectral instruments were used to convolve the modeled and measured narrow-band reflectance. The resampled four satellite broadband reflectance from the mean spectra of six crop species are presented in Figure 2.

**Figure 2.** Mean reflectance spectra of the six crop species used in the study: the four simulated satellite broadband spectra and AISA spectra.
