## 1 Introduction
During the last years, HyPlant data has been the subject of a number of preliminary studies [21; 22; 23] related to the FLUorescence EXplorer (FLEX) mission of the European Space Agency (ESA) [24]. The first validated HyPlant maps were published in 2015, where contrasting SIF values among crops were found and attributed to differences in the photosynthetic apparatus activity [25]. Further studies reported the relation between HyPlant-derived SIF information and GPP [26]. Moreover, [PERSON] et al. [8] recently generated GPP maps over a forest area based on SIF retrieved from HyPlant imagery. They found significant correlations among SIF, GPP and absorbed photosynthetically active radiation in a range of R\({}^{2}\) = 0.43-0.46 (\(p<0.001\)). The interpretation and validation of space-borne and airborne SIF retrievals rely on the comparison with field measurements [27]. Yet, there is a scaling difference between these levels of observation that could possibly be addressed by unmanned aerial systems (UAS) [28] due to their intermediate sensor-surface distance. In a wide review of the existing remote sensing studies to retrieve top-of-canopy SIF, the authors of [29] commented about the lack of literature about UAS-based developments, and the lack of systematic protocols for data processing leading to uncertainties. [PERSON] et al. [30] simulated the effect of aggregated reflectance on satellite imagery by decreasing the pixel size of low altitude hyperspectral data. These authors found that the correlation between key physiological traits, specifically stomatal conductance and ChIF, was reduced from R\({}^{2}\) = 0.69 (\(p<0.01\)) to R\({}^{2}\) = 0.38 (\(p<0.05\)) when using the original UAS imagery and the decreased spatial resolution image, respectively. In a similar approach, in the study of [31] they flew up to 500 m above ground level (AGL) to simulate satellite-like conditions for SIF retrieval, but only preliminary results were presented without a deeper insight in the high altitude results. Contrary to UAS based imaging sensors, the technical feasibility for field measurements of non-imaging spectrometers [32], combined with their higher signal to noise ratio (SNR) and higher spectral and dynamic resolutions (allowing quantitative ChIF retrievals), as well as the reduced size and energy consumption encouraged employing these systems on UAS for SIF retrieval [33]. This concept has been in development for the past eight years, and notably in the last three to five years, it has been materialized in UAS models and prototypes leading to encouraging results. Nevertheless, further investigations are necessary until the point of a full operational UAS for SIF retrieval is reached. Thus, the aim of the present manuscript is to communicate the state of the art and future challenges in the development of UAS-based methods for SIF. The manuscript is organized in four sections. Section 2 presents an overview of previous efforts using UAS-mounted spectroradiometers for reflectance measurements. In Section 3, the main three projects for UAS-based retrieval of SIF are presented: 'Piccolo Doppio' [34; 35], 'AirSIF' [36], and 'FloXPlane' [37]. Section 4 is dedicated to a discussion about remaining challenges and open research questions.