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IN-FLIGHT GEOMETRIC CALIBRATION - AN EXPERIENCE WITH CARTOSAT-1 
AND CARTOSAT-2 
T. P. Srinivasan 1 *, B. Islam, Sanjay K. Singh, B. Gopala Krishna, P. K. Srivastava 
Space Applications Centre, Indian Space Research Organisation, Ahmedabad -380 015 (ISRO), India 
(tps, sac9270, sks, bgk, pradeep) @ sac.isro.gov.in 
Working Group WG 1/1 
KEYWORDS: Cartosat-1, Cartosat-2, GCP, collinearity condition, alignment angles, biases 
ABSTRACT: 
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 re 
establish 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. 
1. INTRODUCTION 
The Cartosat-1 satellite was launched on May 05, 2005 
followed by Cartosat-2 in January 10, 2007. Cartosat-1 is a 
stereo mission having twin cameras (Fore and Aft) with 2.5m 
resolutions while Cartosat-2 is a high-resolution satellite 
having single imaging sensor with ground resolution of around 
lm. 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 using step-stare mode 
of imaging, for city/urban application needs. Both mapping and 
urban applications demand geometric accuracy of data products 
to be within a few meters. 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 
turn is established by the knowledge of (i) orbit, (ii) attitude, 
(iii) precise camera alignments with respect to the spacecraft 
and (iv) camera geometry. The system level accuracy 
specifications for both Cartosat-1 and Cartosat-2 are around 
200m. For cartographic and urban applications, the above 
quoted accuracy is not adequate, which needs to be improved 
with or without control points. One of the important activities 
during post-launch period of Cartosat missions’ qualification 
stage was to assess the mission performance in terms of 
geometric quality and improve further by using in-flight 
calibration (Srivastava et al. 1997) exercises. 
* Corresponding author.