On-orbit spatial resolution estimation of IRS: CARTOSAT-1 cameras with images of artificial and man-made targets - preliminary results

Abstract

This paper investigates the estimation of modulation transfer function (MTF) and point spread function (PSF) using onorbit data of the first dedicated cartographic mission of ISRO, namely, IRS-Cartosat-1. The Cartosat-1 was launched in May 2005 with a motivation to realize in-track stereo-pair imagery at a ground sampling distance of 2.5 m with one of its two cameras, AFT, kept to view a ground scene at -5° and the other, FORE, at +26° with respect to nadir. As with any high-resolution satellite imagery, several factors viz., stray light, optics aberrations, defocusing, satellite motion, atmospheric transmittance etc. can have a strong impact on the observed spatial quality of the Cartosat-1 imagery. These factors are cumulatively accounted by PSF or by the MTF in the spatial frequency domain. The MTF is, thus, of fundamental importance since it provides assessment of spatial response of the overall imaging performance of the system. In this paper, estimation of the PSF and MTF was carried out by capturing imagery over airport runway strip as well as artificial targets laid at two different locations within India. The method adapted here uses a sharp edge from two adjacent uniform dark and bright fields or targets. A super-resolved edge of sub-pixel resolution was constructed from the image edge slanted to satellite path to meet the basic requirement that the target width is much smaller than the spatial resolution width. From the preliminary results, the MTF for the FORE is found to be approximately lesser by about 2% with respect to AFT; this difference may be attributed to relatively a longer traverse of ground signal through the atmospheric column in the case of FORE camera.