# Retrieval of Biophysical Crop Variables from Multi-Angular Canopy Spectroscopy
## Results  
### Impact of the Observer Zenith on Reflectance Spectra  
Analyzing all 3 \(\times\) 3 images separately would be time consuming and impractical. For this reason, the spatial dimension was partially sacrificed in favor of an additional temporal dimension by mosaicking all images of one growing season below each other. This principle is explained in Figure 3. The combination of several sub-images in one mosaic allows the application of algorithms for all observations at once and visualizes seasonal changes in reflectance.  
A strong dependency of the anisotropy factor (ANIF, Equation (1)) towards wavelength can be observed (see Figure 4). A striking anisotropic behavior of the canopy is evident in the short wave visible range for both illumination settings, with decreasing impact towards longer wavelengths. The experiment was repeated for three different growth stages: (1) Day of year (DOY) 128: stem elongation (flag leaf visible, but still rolled), (2) DOY 182: development of fruit (late milk) & (3) DOY 202: Ripening (fully ripe). Each of these three growth stages is represented by an average of all adjacent canopy spectral signatures of that specific date. For both forward and backscatter the ANIF is highest for phenological stage (1). This indicates that during earlier growth stages angular effects in the observer zenith have the strongest influence on the spectral signal. An ANIF of value 1.0 means identical reflectance for nadir and off-nadir observation. Reflectances of the fully ripe canopy (3) draw closest to this equilibrium line for both viewing directions. ANIF\({}_{\mathrm{fs}}\) marginally drops below 1.0 for the VIS-domain and from 1500 nm to 2500 nm at growth stage (2), due to lower reflectances in the off-nadir compared to nadir observations. For backscatter, ANIF\({}_{\mathrm{bs}}\) of (2) is situated mostly between (1) and (3). Lowest impact of angular variations can be assumed for the visible range for forward scatter and for the NIR plateau for backscatter observations. For green vegetation, there is a local minimum in ANIF\({}_{\mathrm{bs}}\) around 550 nm. On the other hand, maximum anisotropy is observed for shortest (450 nm) and longest (680 nm) visible wavelengths. ANIF\({}_{\mathrm{fs}}\) shows an anisotropic behavior that is exactly reverse to ANIF\({}_{\mathrm{bs}}\).  
In the special case of ANIF\({}_{\mathrm{fs/bs}}\), differences between negative and positive observation angles can be assessed directly. As expected, backscatter angles generally lead to higher reflectances. The 1.0 line is closest for the NIR plateau of green vegetation signatures. For all three settings, the senescent vegetation had a constant ANIF that was less sensitive to wavelength, but more sensitive to viewing direction.  
Figure 4: _Cont._  
Figure 3: Red-green-blue (RGB) composite imagery (left) and colored infrared (right) illustration of the spectral image mosaic (standard deviation stretch n = 3.0). Each of the stripes represents the same area of interest under a different observer zenith angle (OZA). OZA = \(-30^{\circ}\) is associated with forward scatter, OZA = \(0^{\circ}\) with nadir and OZA = \(+30^{\circ}\) with backscatter observations. The stripes are composed of 16 sub-images of 3 \(\times\) 3 pixels, each representing a different field date (nine in 2014 and seven in 2015), as indicated by the Julian day of year (DOY).