Optical Orientation Determination for Airborne and Spaceborne Line Cameras

Kurzfassung

Airborne and spaceborne line cameras allow a very economic acquisition of high- resolution and wide swath images of the Earth. They generate two-dimensional images while rotating or translating, which causes a non-rigid image geometry. Non- uniform motion of the camera can negatively affect the image quality. Due to this, a key requirement for line cameras is that fast orientation changes have to be avoided or measured with a very high rate and precision. Camera stabilization is not always possible and can often only be achieved with very high effort. In such cases it is essential to measure the camera’s orientation accurately to ensure that the resulting imagery products can be geometrically corrected in a later processing step. Ade- quate high-end measurement systems are large and expensive and their angular and temporal resolution can be still too low for many possible applications. Due to these reasons there is great interest in approaches to support the orientation measurement by using optical information received through the main optics or telescope. In this thesis two different approaches are presented that allow the determination of a line camera’s orientation changes optically. In addition to this the determined orientation changes can be used to derive the precise absolute orientation of an unstabilized airborne line camera. One approach to determine fast orientation changes is based on small auxiliary frame image sensors, mounted on the focal plane of the main camera or behind separate optics. The optical flow on these sensors is determined by tracking suitable image features through a series of images. It is shown that this can be performed onboard and in real time using a standard CPU. From the optical flow the orientation changes of any remote sensing system can be derived. The second approach does not require any additional sensors to determine orien- tation changes. It relies only on the images of a line camera and a rough estimate of its trajectory by taking advantage of the typical eometry of multi-spectral line cameras. In a first step homologous points are detected within the distorted Images of different spectral bands. These points are used to calculate the orientation changes of the camera with a high temporal and angular resolution via bundle adjustment. Finally it is shown how the absolute exterior orientation of an airborne line camera can completely be derived from the optically determined orientation changes. The orientation changes are used to pre-correct the line images, in which homologous points can reliably be determined using an image feature detection and description algorithm. Together with position measurements these points are used to determine the precise absolute orientation via bundle adjustment of a block of overlapping line images.