## 5 Conclusions  
The UAS-based retrieval of SIF with non-imaging spectroradiometers is possible and has been demonstrated by a number of groups. Recent advances in the platform and instrument design contributed:  
1. The optical path bifurcation presented in the Piccolo-Doppio system for nearly simultaneous upwelling and downwelling measurements with two spectroradiometers, allowing synchronized VNIR and SIF measurements. 2. The implementation of a dual GNSS antenna system and IMU placed in the correct position, alongside the appropriate flight and sensor configurations reported in the AirSIF project, for the accurate pose characterization and footprint geolocation accuracies. 3. The development of the FloXPlane as a fixed wing UAS for very high altitude measurements, which will provide crucial information to understand the impact of large atmosphere columns on the retrieval of SIF. Despite many important advances achieved by [34; 36; 37], UAS-based SIF observations from non-imaging spectroradiometers still present uncertainties associated to the current sensor characteristics and the spatio-temporal mismatching between aerial and ground measurements, mostly caused by the footprint spatial extent and form and location dependency on the flying height and pose geometry. The latest complicated the proper robust validation of aerial UAS-SIF measurements. Consequently, more investigations are required addressing the accurate FOV size and location, along with the spatio-temporal matching of UAS-based and ground SIF measurements of the same targets. Complementary studies regarding the standardization of calibration methods and the characterization of spectroradiometers and data processing workflows are required. Open research questions like those related to the implementation of atmospheric correction, BRDF correction, and accurate surface elevation models should be addressed in the future. Conceptualization, J.Q.V., U.R., M.R., M.C. ; investigation, J.B., A.M.A., A.B., K.M., R.T.; writing--original draft preparation, J.Q.V. ; writing--review and editing, J.Q.V., J.B., A.M.A., T.J., B.S., M.R., M.C., T.K. ; supervision, U.R., O.M., B.S., M.R., M.C. ; project administration, M.R., M.C., D.S., T.K. ; funding acquisition, M.R., M.C, U.R., O.M., D.S. All authors have read and agreed to the published version of the manuscript. The research has received funding from the European Union's Horizon 2020 research and innovation program under the [PERSON] grant agreement no. 721995. This document was prepared within the Training on Remote Sensing for Ecosystem Modelling (TRuStEE) consortium. The \"AirFloX\" on board of the \"FloXPlane\" system (Section 3.3) is founded by the European Space Agency (ESA) project number 4000120771: Technical Assistance for the Development of Ground-based Systems for Long term measurements of Red and Far-red Sun Induced chlorophyll Fluorescence (DEFLOOX). The authors acknowledge the contribution of [PERSON] as collaborator in the development of the FloXPlane at JB Hyperspectral Devices; [PERSON], [PERSON], and [PERSON] at the University of Tasmania for the development of AirSIF; [PERSON] and [PERSON] at University of Edinburgh, Scotland, [PERSON] at RAL, UK and [PERSON] at University of Valencia, Spain, for their contribution to the Piccolo-Doppio project. We thank to [PERSON], [PERSON], and [PERSON], the first from the Environmental Remote Sensing and Spectroscopy Laboratory (SpecLab), Spanish National Research Council (CSIC), and the latest two from the Geo-information Science and Remote Sensing laboratory, at Wagening University and Research (WUR), for their support sharing information about the SpecLab and FluorSpec ongoing investigations, respectively. We also acknowledge the contribution of the Field Lab Campus Klein-Altendorf (CKA), University of Bonn, for their support enabling field experiments for the data collection of the AirSIF study case. The authors declare no conflict of interest. The following abbreviations are used in this manuscript: