## 4 Current Approaches to Open Challenges of F Estimations from Proximal to Airborne Scale
### Atmospheric Influences
#### 4.1.1 High-Altitude Sensing  
When measuring fluorescence using passive remote sensing techniques in the oxygen absorption regions from high-altitude airborne platforms, it is mandatory to compensate for atmospheric aerosol scattering and molecular absorption [120]. From the canopy, only the upwelling radiance, which is attenuated by its passage through the atmosphere, is typically measured at these altitudes (Figure 6). Thus, in order to estimate both \(\overline{E}\) and \(\overline{L}\) at the canopy level, the use of an atmospheric RTM is usually required. Generally, in order to characterize the atmospheric state to perform the atmospheric correction process, measurements related to the aerosol and water vapor content, as well as measurements related to the air temperature and pressure conditions, are normally acquired at the surface level. Thus, regardless of the selected atmospheric RTM, the model is commonly parametrized by making use of the ancillary atmospheric measurements acquired simultaneously to the airborne acquisitions (e.g., [120; 121]). In a past field campaign experiment [117], the researchers applied an oxygen transmittance compensation strategy as part of the FLD retrieval technique. In their work, an additional spectral correction factor of the transmittance was introduced by using non-fluorescence targets as a reference to compensate for the uncertainties associated with processing data that were mainly derived from assumptions made during the atmospheric modeling process. However, the use of non-fluorescence targets as a reference for calibration can also make the estimated fluorescence prone to errors in certain scenarios. The rationale behind this is that certain spectral artifacts (e.g., stray light) distort the absorption shape and depend on radiance levels. Therefore, radiometric calibration errors will impact bare soils and vegetated targets differently. Thus, in some cases, it may be preferable to use bare soil pixels only as a quality indicator to check the validity and quality of the applied radiometric and atmospheric correction process instead of using this data to derive a compensation factor [58]. As an alternative to the oxygen transmittance compensation applied in [115], any classical atmospheric correction technique based on the inversion of an RTM (as typically applied to data acquired from satellite platforms) can also be applied to correct airborne data. This way, all of the atmospheric transfer functions that account for the atmospheric scattering (the so-called path radiance) and the atmospheric backward reflection (the so-called spherical albedo) are determined together with the atmospheric transmittance (see [122] for more information regarding the approximations generally assumed in the atmospheric inversion formulation to estimate F). Figure 6: General system setups used to measure fluorescence at different vertical scales: tower, UAV, and airborne (from left to right). While the downwelling solar irradiance contribution is typically measured at the sensor level for systems mounted on towers and UAVs, at the airborne scale, only the upwelling canopy radiance is generally measured. Many RTM approaches are now available, such as the MODerate resolution atmospheric TRANsmission MODTRAN [113], the 6S code [123], and the LibRadtran RTM [124]. In general, the number of input parameters that need to be tuned will strongly depend on the flexibility of the selected model, as well as the ancillary information acquired that characterizes the atmospheric state. For instance, the majority of RTMs allow for configuring at least the viewing and illumination geometry, the aerosol presence, and the water vapor content. However, some RTMs, such as MODTRAN, allow the user to define a particular temperature vertical profile or parametrize the aerosol presence by defining a set of optical properties, such as the Aerosol Optical Thickness (AOT), the Angstrom exponent, the scattering asymmetry, the Single Scattering Albedo (SSA), or the aerosol vertical distribution.