RADIOMETRIC CAMERA CALIBRATION OF THE BiLSAT SMALL SATELLITE: PRELIMINARY RESULTS J. Friedrich a, *, U. M. Leloğlu a , E. Tunalı a a TÜBİTAK BİLTEN, ODTU Campus, 06531 Ankara, Turkey - (jurgen.friedrich, leloglu, tunali)@bilten.metu.edu.tr Commission I, WG 6 KEY WORDS: Radiometric, Camera, Calibration, BiLSAT ABSTRACT: The studies on radiometric camera calibration of multispectral camera of the BiLSAT satellite is presented in this work. During the study, various dark images and flat-field images have been taken, and deviations from ideal has been investigated. Various sources of error have been studied and the non-homogeneous response of the CCD is explicitly modeled. Images have been corrected according to the models and the results have been discussed. 1. INTRODUCTION BiLSAT earth observing satellite has been built in the framework of a technology transfer program between SSTL, Guildford, UK and TÜBİTAK-BİLTEN, Ankara, Turkey. Since its launch on September 27 th , 2003, to its sun-synchronous orbit at 686 km high, it is being operated from the ground station in Ankara. BiLSAT has a panchromatic camera with 12,6 m ground sampling distance (GSD), and four separate cameras sensitive to red, green, blue and NIR bands, together forming a multispectral camera with 27,6 m GSD. The radiometric calibration of the cameras was not performed on the ground, so the parameters need to be estimated in-orbit. For that purpose, dark images have been obtained by imaging the Atlantic Ocean at night. Imaging has taken place in June 3 rd , 2004 at 23:14:44 UTC and 23:23:29 UTC. At both imaging attempts, 5 images have been taken with 5 second intervals. For the gain calculation of images, flat-field images needed. These images have been obtained by imaging snow plateaus in Antarctica (75S 120E). The imaging campaigns for flat field images have been conducted on December 25 th , 2004 at 16:07:03, January 3 rd , 2005 at 11:57:29 and January 16 th 2005 at 11:54:21. In this work, a study to determine relative radiometric camera calibration parameters of BiLSAT multi-spectral camera and the first results from the study are presented. For the absolute calibration of BiLSAT cameras, another study is being conducted for all Disaster Monitoring Constellation (DMC) satellites with the support of all satellite owners. This study is being conducted in the framework of an agreement between DMC Consortium and University of Arizona. In accordance with the agreement, test site supported by University of Arizona at Railroad Valley, Nevada, United States (38.5N 115.7E) is used and the imaging campaign took place during 3-11 July 2004. The top-of-the- atmosphere radiance values are not available yet, so the absolute calibration is not done. 2. CAMERA SYSTEM, ITS RADIOMETRIC BEHAVIOUR AND NOISE SOURCES BiLSAT carries three imagers on board. The imager named ÇOBAN is out of the scope of this study, so it will not be discussed. The other two imagers are multispectral imager and panchromatic imager. The radiometric calibration study has only been made for multispectral camera, so only this imager will be discussed. BiLSAT imagers are based on “Modular Camera” approach. The Modular Camera forms the basis of the area array camera of BiLSAT imagers. The Modular camera is a single channel unit, and is used in single configuration for a panchromatic camera, and as multiple units for a Multispectral imager. Multispectral camera is made of four physically different cameras. Multispectral imager was constructed by using four modular cameras with appropriate lenses and filters. In the modular camera, KAI 4000M CCDs were used. The specifications of the CCD are given in Table 1. * Corresponding author. 1