## 5 Conclusions  
The concept of FGI AIRS was shown to work well and to be a valuable accessory in UAV-based remote sensing by providing timing, orientation, position, and irradiance metadata for the aerial images. The real-time image positioning accuracy of the AIRS (26 cm) was found good enough for real-time georeferencing of the images using photogrammetric processing without GCPs in coarse resolution applications and for data preview purposes. For high resolution applications or for processing using only direct georeferencing, an upgrade on the GNSS/INS would be necessary.  
The AIRS irradiance sensor tilt correction method was shown to perform well and be a good alternative for using a stabilized gimbal. Thus, we recommend implementation of this method in all future UAV irradiance sensors if they are not to be installed on a stabilized gimbal. To implement the method on an irradiance sensor, only three photodiodes and an IMU must be included. For 1% (RRMSE) accuracy in typical low-solar elevations, the roll and pitch accuracy of the IMU should be better than 0.5\({}^{\circ}\), but preferably in the order of 0.1\({}^{\circ}\).  
In this study, we also tested different aerial image reflectance factor conversion methods. In our experiment with broken cloud conditions, the methods using irradiance from the on-ground spectrometer and from AIRS gave the best results, being both scientifically sensible and visually pleasing. In this weather, the reflectance reference panel method failed to produce good orthomosaic, as it was not able to correct for fast variations in illumination at all, leaving parts of the mosaic visibly dark and whole mosaics numerically unreliable. The simple on-board irradiance sensor without stabilization also failed miserably in producing good orthomosaic. Although the method was able to correct most of the shadowed areas to visibly pleasing images, the random tilting of the UAV caused irradiance data to be so noisy that the sunlit areas of the mosaic were visibly patchy and numerically unreliable.  
For UAV mapping flights covering only small areas (some hundreds of meters in range), we recommend usage of an on-ground spectrometer, the AIRS method, or a stabilized on-board irradiance sensor. If the atmospheric conditions are (near-)perfect, then the reflectance reference panel method can also be recommended. For mapping flights covering larger areas, the local methods usingon-ground spectrometer or panels are no longer suitable, and the only recommendable options are to use the AIRS method or a stabilized on-board irradiance sensor. Alternatively, the radiometric block adjustment method is capable of compensating for the radiometric differences due to the varying illumination conditions and can be used either with reflectance panels or an irradiance spectrometer. Usage of an unstabilized on-board irradiance sensor is not recommended for all use cases.  
Conceptualization, J.S., T.H., E.H.; methodology (FGI AIRS hardware), J.S., T.H.; software (image processing chain), R.A.O., J.S., L.M., N.V., R.N., E.H.; investigation, J.S., R.A.O., E.H.; writing-original draft preparation, J.S., R.A.O.; writing-review and editing, all authors; visualization, J.S., R.A.O.; supervision, E.H.; funding acquisition, E.H.  
This research was funded by Business Finland (DroneKnowledge project, grant No. 1617/31/2016) and the Academy of Finland (grant No. 305994).  
The authors declare no conflict of interest.