Calibration of Optical Satellite Sensors

Satellite cameras are calibrated before launch in detail and in general, but it cannot be guaranteed that the geometry is not changing during launch and caused by thermal influences and drying out effects in the orbit. Modern satellite imaging systems are based on CCD-line sensors. Because of the required high sampling rate, the length of used CCD-lines is limited. For reaching a sufficient swath width, some CCD-lines are combined to a longer virtual CCD-line. The images generated by the individual CCD-lines do overlap slightly and so they can be shifted in xand y-direction in relation to a chosen reference image just based on tie points. The geometry and geometric relation of the sub-scenes can be determined by a bundle orientation with a higher number of control points or a bundle block adjustment using a limited number of ground control points. Special additional parameters fitted to the image size are required for the calibration. The resulting virtual image has only negligible errors in areas with very large difference in height caused by the difference in the location of the projection centres. Colour images can be related to the joint panchromatic scenes just based on tie points. Pansharpened images may show small colour shifts in very mountainous areas and for moving objects, but it is also possible to respect digital elevation models for the optimal fit of the sub-scenes. The direct sensor orientation of the satellites has to be calibrated based on control points. Discrepancies in horizontal shift can only be separated from attitude discrepancies with a good three-dimensional control point distribution or with opposite scan directions. For such a calibration a program based on geometric reconstruction of the sensor orientation is required. The approximations of the scene orientation by 3D-affine transformation or direct linear transformation (DLT) cannot be used. These methods do have also disadvantages for standard sensor orientation. The image orientation by geometric reconstruction can be improved by self calibration with additional parameters for the analysis and compensation of remaining systematic effects for example caused by a not linear CCD-line but also a continuous change of the view direction. The determined sensor geometry can be used for the generation of sensor oriented rational polynomial coefficients, describing the sensor geometry by relations of polynomials of the ground coordinates X, Y and Z.