## 4 Discussion  
### Accuracy of AIRS Irradiances  
The absolute accuracy of irradiance readings produced by AIRS is too complicated to estimate accurately. The results presented in Section 3.2.2 showed that, with tilt correction, the stability and linearity of the measurement within one flight were better than \(\pm\)2.5%. The experiments performed for this article do not allow exact numerical evaluation of the absolute accuracy of AIRS irradiances, however, based on the experiments and knowledge of the calibration process we can give our estimates on contributions of different error sources: First, the spectrometer has an accuracy of 1% (or better) due to its stability and linearity. Second, in normal use, the spectrometer diffuser optics are exposed to dust, dirt, and scratching. This can easily cause errors up to 5% before being noticed and cleaned. Third, the diffuser optics do not provide perfect Lambertian field-of-view; thus, the illumination with angular distributions other than which was used in calibration may not produce consistent readings. Fourth,  
Figure 7: Orthomosaics at the 844 nm spectral band produced using four different reflectance factor conversion methods (**a–d**). With (**c**,**d**), the continuous irradiance data was used to identify images taken during radical changes in illumination, allowing automatic removal of the partially clouded images from processing. For (**a**,**b**) such methodology was not feasible. the spectrometer and optics assembly were calibrated by transferring the radiometric calibration of an ASD FieldSpec Pro irradiance probe to it. The FieldSpec is calibrated by the manufacturer at NIST-traceable (National Institute of Standards and Technology, USA) accuracy of 2.5-3.5%, and we estimate that our transfer was done at an accuracy of approximately 3%. Fifth, in a controlled test, the AIRS tilt correction method was shown to work with an accuracy of 0.8% (RRMSE) in tilts up to the tilt angle of the photodiodes (10\({}^{\circ}\)), and an accuracy of 1.2\({}^{\circ}\) in tilts up to 15\({}^{\circ}\) (Section 3.1). In the flight test, the on-board measurement correlated with the on-ground measurement with an accuracy of \(\pm\)2.5%, even if these measurements were taken in different locations. Based on the propagation of error from these error sources, we estimate the overall absolute accuracy of the AIRS irradiances in operational conditions is to be in the order of \(\pm\)5%. The stability and linearity of our method was found to be roughly equivalent to the accuracy demonstrated by [PERSON] et al. (2018). Their method is based on measuring diffuse and direct components of the irradiance and correcting tilt in the direct component mathematically based on solar direction. With their solution, they were able to correct 90% of their flight data on tilts up to 10\({}^{\circ}\) to within 10 \(W/m^{2}\) of the reference value. As the total irradiance in their experiments varied between 350 and 1100 \(W/m^{2}\), this means their relative accuracy was between 0.9% and 2.9%. Their technical solution is based on broadband thermopile sensors, which are not directly suitable for determining the spectrum of the irradiance and thus not directly usable in the processing of optical remote sensing data. However, the principle of their measurement could also be implemented with optical irradiance sensors, which could allow the production of similar data as our method. As the technical requirements of our and their method are quite similar (require multiple irradiance sensors, and an IMU unit to measure the tilt of the system) and also the reported accuracies are in the same range, it is not possible yet to make definite conclusions on the superiority of either method. To make a direct comparison of the two methods, an optical version of their sensor should be prepared and evaluated against our method in similar conditions. The mathematics of AIRS tilt correction assumes that the irradiance changes linearly with the tilt angle.