Improved On-Orbit MTF Measurement Method Based on Point Source Arrays

Abstract

The modulation transfer function (MTF) is a key characteristic used to assess the performance of optical remote sensing satellite sensors. MTF detection can directly measure a sensor’s two-dimensional (2D) point spread function (PSF); therefore, it has been applied to various high-resolution remote sensing satellites (e.g., Pleiades) using point sources. However, current point source methods mainly use 2D Gaussian functions to fit the discrete digital number (DN) of the point source on the image to extract the center of the point source and fit the PSF after encrypting multiple point sources; thus, noise robustness is poor and measurement accuracy varies widely. In this study, we developed a noise-resistant on-orbit MTF detection method based on the object space constraint among point source arrays. Utilizing object space constraint relationships among points in a point source array, a homography transformation model was established, enabling accurate extraction of sub-pixel coordinates for each point source response. Subsequently, aligning the luminosity distribution of all point sources concerning a reference point source, the encrypted PSF was obtained and then fitted to obtain the MTF. To validate the method, Gaofen-2 (GF-2) satellite images were used to conduct an in-orbit imaging experiment on the point source array of the Chinese Zhongwei remote sensing satellite calibration site. Compared with the Gaussian model methods, the proposed method yielded more accurate peak positions for each point source. Standard deviations of peak position constant ratios in along- and cross-track directions improved by 2.8 and 4.8 times, respectively. The root-mean-square error (RMSE) of the collinearity test results increased by 92%, and the noise resistance of the MTF curve improved by two times. Dynamic MTF values at the Nyquist frequency for the GF-2 panchromatic band in along- and cross-track directions were 0.0476 and 0.0705, respectively, and MTF values in different directions were well distinguished.

Analysis of the Slanted-Edge Measurement Method for the Modulation Transfer Function of Remote Sensing Cameras

“…Recognized as an indirect approach for measuring the MTF, the slanted-edge method has been successfully implemented in the in-orbit MTF measurement of satellites such as IKONOS [11,12], Orbview [13], GF-4 [14], and Quickbird [15]. The International Organization of Standards endorses this method as well (ISO12233:2023) [16], which involves analyzing a camera-captured slanted-edge target image to determine the edge spread function (ESF) [17], differentiating the ESF to acquire the line spread function (LSF), and then applying Fourier transform on the LSF and taking the modes to compute the MTF [18,19]. Offering a comprehensive evaluation at various spatial frequencies in a single instance and characterized by rapid detection, the slanted-edge method has garnered significant scholarly interest [20].…”