###### Abstract
Unfortunately, spectrometers are also usually larger, heavier, and more expensive than photodiodes. Thus, for RGB and multispectral sensors, photodiodes with filters can provide an excellent solution for irradiance reference. Additionally, as the spectral shape of natural illumination is predictable [29], a combination of just a few photodiodes with well-selected spectral filters could possibly provide practically the same information content as a full spectrometer does. The exact definition of irradiance is the radiant power received by a surface per unit area [2]. In the application of calibrating remote sensing images to reflectance factors, most commonly the surface is usually a horizontal plane located above the target and all its surroundings, i.e., top-of-canopy irradiance is used for the irradiance. To estimate this irradiance on target, by using a measurement on-board the UAV, includes some challenges. Firstly, the irradiance at UAV locations is not always the same as illumination in all parts of the image area. This is clearly visible near the edges of broken clouds, where it is common to have a full shadow at the UAV, while there is full sunshine at the ground target. This problem cannot be circumvented completely, but especially in low-altitude UAV images, it is often possible to flag and filter out most of the serious cases of variably illuminated images by detecting changes in the illumination. Secondly, the UAV irradiance sensor might not be perfectly horizontal as required by the reference. This has a large effect, especially in direct sunshine on a low elevation angle, where the error in reading can be many percent per degree tilted (see Section 3.1). As the small multirotor and fixed wing UAVs typically experience tilts of 5-10\({}^{\circ}\) in random directions, depending on flight maneuvers and wind gusts, an unstabilized irradiance sensor can observe errors in the order of tens of percents. Part of this error cannot be averaged out, as flight direction and wind cause systematic tilts to the UAV. Current state-of-the-art methods for improving quality of on-board irradiance measurements are to either use a stabilizing gimbal [30], to filter out the tilted data, or to apply a mathematical solar zenith angle correction to the tilted value. The solar zenith angle correction requires a knowledge of the direct and diffuse portions of the irradiance [31] as only the direct component should be corrected. Thus using this correction requires measurements of both a fully lit and shadowed components of the irradiance. In on-ground measurements using pyranometer stations, such direct and diffuse measurements are done routinely using moving shaders, but on-board unstable platforms such arrangements become tricky. [PERSON] et al. [31] presented in 2010, a method for measuring broadband shortwave irradiance onboard moving platforms using an \"SPN1 Sunshine Pyranometer\" (Delta-T Devices Ltd., London, UK). The SPN1 houses a set of seven thermopile irradiance sensors installed under a static shader that always fully shadows at least one of the sensors and fully exposes another one for direct sunlight. This setup allowed them to measure the direct and diffuse components onboard an airplane and correct the tilt effects using the tilt information in aircraft navigation data. In addition to the irradiance data, processing of aerial images is supported by the image positions and orientations measured by an integrated global navigation satellite system (GNSS) and an inertial measurement unit (IMU). Although with photogrammetric processing techniques, the image georeferencing can often be done using only the image data and ground control points (GCPs), the process gets more efficient and accurate if initial image positions and orientations are known. Although the highest georeferencing accuracy is usually reached with the help of GCPs, usingthem is not mandatory in all remote sensing applications if high-accuracy, real-time kinematic (RTK), or post-processed kinematic (PPK) camera positions are available.