# Retrieval of Biophysical Crop Variables from Multi-Angular Canopy Spectroscopy
## 2 Materials and Methods
### In Situ Measurements  
#### 2.2.1 Spectral Data  
Spectral data were collected with an Analytical Spectral Devices Inc. (ASD) (Boulder, CO, USA) FieldSpec 3 Jr. Five separate measurements were carried out per ESU and per observation angle to \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Date** & **Spectral** & **Spectral** & **Crop** & **Date** & **Spectral** & **Spectral** & **Crop** \\  & **Nadir** & **Angles** & **Variables** & & **Nadir** & **Angles** & **Variables** \\ \hline
17 April 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 28 November 2014 & & & \(\surd\) \\
23 April 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 12 December 2014 & & \(\surd\) & \(\surd\) \\
30 April 2014 & & & & \(\surd\) & 19 March 2015 & & & \(\surd\) \\
6 May 2014 & & & & \(\surd\) & 10 April 2015 & & \(\surd\) & \(\surd\) \\
14 May 2014 & & & & \(\surd\) & 22 April 2015 & & \(\surd\) & \(\surd\) \\
9 May 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 5 May 2015 & & & \(\surd\) \\
26 May 2014 & & & & \(\surd\) & 8 May 2015 & & \(\surd\) & \(\surd\) \\
2 June 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 3 June 2015 & & \(\surd\) & \(\surd\) \\
6 June 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 12 June 2015 & & & \(\surd\) \\
18 June 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 1 July 2015 & & \(\surd\) & \(\surd\) \\
26 June 2014 & \(\surd\) & \(\surd\) & \(\surd\) & 10 July 2015 & & \(\surd\) & \(\surd\) \\
3 July 2014 & \(\surd\) & & & \(\surd\) & 16 July 2015 & & \(\surd\) & \(\surd\) \\
17 July 2014 & \(\surd\) & & & \(\surd\) & 21 July 2015 & & \(\surd\) & \(\surd\) \\
25 July 2014 & \(\surd\) & \(\surd\) & \(\surd\) & & & & \(\surd\) \\ Total observation & & & & & & & \\ number & 10 & 8 & 14 & & & 11 & 8 & 1 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Dates of field visits and corresponding availability of spectral observations nadir/angles as well as crop variable measurements, indicated with check marks. Crop variables were measured weekly for both growing cycles of 2014 (left) and 2014/2015 (right). The frequency of spectral observations was subjected to weather conditions, since reflectance measurements require a cloud-free sky. obtain representative values. Outliers were removed and the remaining spectra were averaged and subjected to further processing. The post-processing included splice correction, radiometric calibration to absolute reflectance values and smoothing with a moving Savitzky-Golay-filter [34]. Apart from nadir measurements, the canopy was also measured under observer zenith angles (OZA) of \(+30^{\circ}\) and \(-30^{\circ}\) regarding the solar plane: a sensor inclination towards the sun is defined as a positive OZA, whereas an inclination away from the sun is described as a negative OZA (Figure 1). Due to the _backscatter_ effect, spectra with positive OZA are noticeably brighter than nadir views or negative zenith angles, as they draw nearer to the spot of increased backscatter, also known as the hot spot [35]. The opposite direction shall accordingly be called cold spot or _forward scatter_ and usually leads to reduced reflectances and darker images. For the angular spectral measurements, a microphone stand was modified to hold the ASD glass fiber optic. The horizontal rod of the stand could be raised or lowered to adjust the viewing angle with help of an attached inclinometer. The observer azimuth angles (OAA) matched up with the row azimuth angle of the canopy stands (170\({}^{\circ}\) for field 517 and 150\({}^{\circ}\) for field 509). EnMAP will operate on a sun-synchronous orbit with 97.96\({}^{\circ}\) satellite inclination angle descending node [36] which corresponds to an OAA of 187.96\({}^{\circ}\). The angular effects measured in the presented campaign therefore are assumed to adequately represent the angular effects expected from future EnMAP data.