## 5 Summary and Discussion
In contrast, constantly measuring fixed-sensor installations are very inflexible when it comes to measuring different places, but they remove the need for human operators. Table 1 summarizes F measurement considerations. Future research should investigate how different approaches and their advantages and disadvantages can be combined in a meaningful way to get the full F picture at various spatial and temporal scales. However, it is not only the technology and measurement setups that vary at different levels and scales. The review of different measurement protocols showed that differences also exist between F retrieval approaches. The most apparent difference between the leaf level and the canopy level is the ability to use a sharp-edge low-pass filter to cut off photosynthetically active light, as well as to measure both leaf upwelling and downwelling F radiation. This simplifies the retrieval of F, because it does not require disentangling the signal from the reflected or transmitted F radiance. With canopy-level measurements, the photon scattering and reabsorption further complicate the F estimation. Also, the different approaches have different temporal frequencies and continuities and are thus able to measure physiological processes to different extents. One of the open issues is the influence of the measurement geometry. It has been shown that measurement geometries influence radiance measurements and, consequently, also the retrieved biophysical parameters [106; 108]. Several studies [76; 82; 160] have reported that the anisotropy of F emissions is related to the canopy structure, and this relationship implies that the sensor viewing geometry is also important for F measurements. Thus, the comparability and standardization of nadir and oblique measurements (e.g., from towers) should be investigated, and common protocols for observations and instrumental setups should be established. Additionally, the atmosphere between the instrument and the observed canopy needs attention. While correction of an F signal recorded from less than 2 m above the top of the canopy does not seem necessary for either oxygen absorption or aerosol transmittance, even tower measurements require the correction of oxygen absorption. Some tower-based studies have already reported on such corrections [46; 47], while others have not. For airborne studies, atmospheric correction is mandatory [85; 87; 115; 158]. Nevertheless, further \begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Leaf-Level Leaf** & \begin{tabular}{c} **Canopy-Level Fixed** \\ **Cip** \\ \end{tabular} & \begin{tabular}{c} **Canopy-level** \\ **Ground/Tower** \\ **Installation** \\ \end{tabular} & \
\begin{tabular}{c} **Canopy-level** \\ **Low-Altitude** \\ **UAV Sensing** \\ \end{tabular} &
\begin{tabular}{c} **Canopy-Level** \\ **High-Altitude** \\ **Airborne Sensing** \\ \end{tabular} \\ \hline Spatial coverage & – & – & 0 & ++ \\ \hline Spatial resolution & - & – & ++ & ++ \\ \hline Temporal resolution & + & ++ & 0 & - \\ \hline Temporal frequency and continuity & + & ++ & – & – \\ \hline Setup effort & + & - & - & – \\ \hline Effort during a measurement campaign & + & ++ & - & – \\ \hline Flexibility in terms of campaign planning & + & – & 0 & - \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Summary table of F measurements at multiple levels and scales with small and high advantages (+, ++), neutral (no advantage or disadvantage) (0), and small and high disadvantages (-, -) for each aspect. research on atmospheric corrections should investigate the stability of F because of the large variety in measurement setups, places, and atmospheric conditions. Another critical point is the quality checks and quality flags present in the metadata. Not much has been reported on this topic.