Calibration of a Wide-angle Digital Camera System for near Real Time Scenarios

Near real time monitoring of natural disasters, mass events, and large traffic disasters with airborne SAR and optical sensors will be the focus of several projects in research and development at the German Aerospace Center (DLR) in the next years. For these projects, new airborne camera systems are applied and tested. An important part of the sensor suite plays the recently developed optical wide angle 3K camera system (3K = “3Kopf”), which consists of three non-metric off-the-shelf cameras (Canon EOS 1Ds Mark II, 16 MPixel). The cameras are aligned in an array with one camera looking in nadir direction and two in oblique sideward direction, which leads to an increased FOV of max 110°/ 31° in across track/flight direction. With this camera configuration, a high resolution, colour and wide-area monitoring task even at low flight altitudes, e.g. below the clouds, becomes feasible. The camera system is coupled to a GPS/IMU navigation system, which enables the direct georeferencing of the 3K optical images. The ability to acquire image sequences with up to 3Hz broadens the spectrum of possible applications in particular for traffic monitoring. In this paper, we present the concept of calibration and georeferencing which is adjusted to the requirements of a near real time monitoring task. The concept is based on straight forward georeferencing, using the GPS/IMU data to automatically estimate the not-measured boresight angles. To achieve this without measuring of ground control points (GCPs), we estimate on-the-fly boresight angles based on automatically matched 3-ray tie points in combination with GPS/IMU measurements. A prerequisite for obtaining robust results for the boresight angles is that the air plane attitude changes slightly during image taking; through these singular solutions can be avoided. Additionally, we assume known and fixed parameters of interior orientation. The determination of the interior orientation is performed ground based using a bundle adjustment of images from a calibration test field. The determination of the parameters of the interior orientation is repeated to check for their systematic changes in time. The proposed georeferencing and calibration concept was tested with images acquired during three flight campaigns in 2006. To evaluate the accuracy obtained by direct georeferencing using the proposed estimation procedure for the boresight angles without GCPs, the data are compared with the results of a bundle adjustment using GCPs and the GPS/IMU information. Summarizing, the RMSE of direct georeferencing with/without GCPs is 1.0m / 5.1m in position and 0.5m / 1.0m in height, at image scales of 1:20.000. The accuracy without GCPs is regarded as acceptable for near real time applications. Additionally, it is shown that the parameter of the interior orientation remain stable during three repetitive calibrations on a test field for all three cameras.