On-orbit modulation transfer-function estimation for bilsat imagers

BiLSAT, launched to its sun-synchronous orbit on 27.09.2003, is a 129 Kg small satellite carrying a panchromatic camera and a 4-band multispectral camera with Ground Sampling Distances (GSD) 12.6 m and 28 m, respectively. The multispectral camera is composed of 4 totally independent cameras. All five refractive optic cameras have 2048 by 2048 frame CCDs as sensors. The overall Modulation Transfer Functions (MTF) of the imaging systems are very important for characterization of imagers. In this study, the MTF of BiLSAT imagers are calculated using two different methods. The first method for image-based MTF determination uses sharp linear edges for estimating the Edge Spread Function (ESF), from which Point Spread Function (PSF) is obtained by differentiation. A man-made high-contrast almost linear structure is used as imaging target. For precise detection of the edges, a 2D edge model is fitted to manually picked linear edge and the root-mean-square (RMS) difference between the image and the model is minimized by gradient search. Then, a parametric curve fitting is performed using a new parametric model. The PSF and MTF are obtained after differentiation and Fourier transformation, respectively. Second method is based on comparing BiLSAT images to high-resolution IKONOS images whose PSF is already known. The blur function that minimizes the RMS difference between the registered images is estimated. Image registration, blur estimation and radiometric correction parameter estimation are performed simultaneously. The fact that the images are taken with 3 months time difference and from slightly different angles cause difficulties. Small and almost planar areas are chosen to avoid parallax effects. Finally, the results obtained by two different methods are compared and the results are discussed.

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