Lever-arm and boresight correction, and field of view determination of a spectroradiometer mounted on an unmanned aircraft system

This study addresses the correction of lever-arm offset and boresight angle, and field of view (FOV) determination to enable accurate footprint determination of a spectroradiometer mounted on an unmanned aircraft system (UAS). To characterise the footprint, an accurate determination of the spectroradiometer position and orientation (pose) must be acquired with a global navigation satellite system (GNSS) and an inertial measurement unit (IMU). Accurate pose estimation requires an accurate lever-arm and boresight correction between the pose measuring sensors and the spectroradiometer. Similarly, the spectroradiometer FOV is required to determine the footprint size as a function of above ground level (AGL) flying height. The system used in this study consists of an IMU with dual-frequency and dual-antenna GNSS receiver, a machine vision camera, and a point measuring spectroradiometer (Ocean Optics QE Pro). The lever-arm offset was determined from a scaled 3D point cloud of the system, created using photos of the airframe and processed with the structure-from-motion (SfM) algorithm. The boresight angles were estimated with stationary experiments by computing the difference between the orientations of the IMU, the spectroradiometer, and the camera. The orientation of the spectroradiometer was determined by moving a spectrally distinct target into the FOV. The orientation of IMU was measured by averaging its readings during the stationary epoch, while SfM was employed as an independent technique to estimate the orientation of the camera. The footprint of the spectroradiometer for a combination of AGL height and Gershun tube aperture ring was determined experimentally, enabling computation of the effective FOV. In-flight validation of the lever-arm and boresight correction was performed by comparing the corrected pose of the co-mounted camera with the pose derived from SfM as the reference. Our experimental results demonstrate that controlled determination and correction of lever-arm and boresight increases the pose estimation accuracy and thereby supports the direct georeferencing of a UAS-mounted spectroradiometer point observation.