In-flight Geometric Calibration - an Experience with Cartosat-1 and Cartosat-2

The Cartosat-1 satellite was launched in May 2005 followed by Cartosat-2 in January 2007. Cartosat-1 is a stereo mission having twin cameras or two imaging sensors (Fore and Aft) with 2.5m resolutions while Cartosat-2 is a high-resolution satellite having single imaging sensor. The two cameras of Cartosat-1 provide systematic stereo coverage of the globe for mapping applications while Cartosat-2 has capability to provide scene specific spot imageries in paint-brush or spot or mulit-view modes for city/urban application needs. Both mapping and urban applications demand accuracy of data products to be within a few meters. One of the important activities during post-launch period of mission qualification stage is to assess the mission performance in terms of geometric quality and improve further using in-flight calibration exercises. The geometric quality or accuracy of data products is determined by the knowledge of precise imaging geometry, as well as the capability of the imaging model to use this information. The precise imaging geometry in its turn is established by the precise knowledge of (i) orbit, (ii) attitude, (iii) precise camera alignments with respect to the spacecraft and (iv) camera geometry. The imaging geometry is derived from measurements carried out on the spacecraft during the qualification stage. However it was found that (by experience from IRS series) there is a need to reestablish the imaging geometry from image data itself. Cartosat Data Products team had conducted study and specific exercises related to in-flight calibration of Cartosat-1 and Cartosat-2 imaging geometry model during initial period of three months. The data used for the in-flight calibration are a few ground control points and images for different cameras/strips for relative control point identification in the overlap area. This experiment called for estimation of image coordinates for the known ground coordinates of GCPs using photogrammetric collinearity condition based imaging model to compare with observed image positions of those points. Scan differences and pixel differences were used to statistically derive platform biases, focal length, camera alignment angles etc. On the other hand, presence of multiple imaging payloads (Cartosat-1) or multi-viewing of strips (Cartosat-2) and other sensors for measuring spacecraft orientation provide additional advantages, strengthening in-flight calibration exercises to make use of only imaging sensors as attitude sensors to derive pseudo parameters without resorting to any controls. The derived alignment angles and re-estimated camera parameters were incorporated in the software used for geometric correction of data products. Significant improvements in the location accuracy and internal distortion of Cartosat data products have been achieved after incorporating various geometry parameters determined from the imagery. Similar exercises were carried out for Cartosat-2 during January 2007 to April 2007. Experience of working with Cartosat-1 has helped in quickly developing imaging model for Cartosat-2. Different formulations and multiple observations are used for unambiguous resolution of disparity between predicted and observed image positions to derive platform biases. This paper describes the methodology and experimental details of exercises carried out during the initial phase of Cartosat-1 operations by which the imaging geometry for Cartosat-1 cameras was re-established. Also, details on the development of new approach using stereo imaging sensors with minimum or no control for Cartosat-1 are addressed. Results obtained for Cartosat-2 using in-flight calibration experiments are also covered in this paper. * Corresponding author.