The edge method is a widely used way to assess the on-orbit Modulation Transfer Function (MTF). Since good quality is required for the edge, the higher the spatial resolution, the better the results are. In this case, an artificial target can be built and used to ensure a good edge quality. For moderate spatial resolutions, only natural targets are available. Hence the edge quality is unknown and generally rather poor. Improvements of the method have been researched in order to compensate for the poor quality of natural edges. This has been done through the use of symmetry and/or a transfer function model, which enables the elimination of noise. This has also been used for artificial target. In this case, the use of the model overcomes the incomplete sampling when the target is too small or gives the opportunity to assess the defocus of the sensor. This paper begins with a recall of the method followed by a presentation of the changes relying on transfer function parametric model. The transfer function model and the process corresponding to the changes are described. Applications of these changes for several satellites of the French spatial agency are presented: for SPOT 1, it enables to assess XS MTF with natural edges, for SPOT 5, it enables to use the Salon-de-Provence artificial target for MTF assessment in the HM mode, and for the foreseen Pleiades, it enables to estimate the defocus.

Improvement of the edge method for on-orbit MTF measurement

The integrating effect of the point spread function of a satellite remote sensing system significantly affects the signature from a single cover class if the surrounding cover is dissimilar and also reduces the spatial resolution of the output data. Digital SPOT P-Mode data acquired over linear ground edge and impulse features were analysed to determined the SPOT system's line spread function. This was integrated to give the normalised point spread function, and then inverted to derive an equation to estimate the true ground response by deconvolution of the observed response. The paper describes the procedures used in determining the point spread function, and applies the result to a number of urban features. A significant improvement in the image quality resulted.

Estimation of SPOT P-mode point spread function and derivation of a deconvolution filter

A sensor’s spatial resolution has traditionally been a difficult concept to define, but all would agree that it is inextricably linked to the Ground Sampling Distance (GSD) and Instantaneous Field of View (IFOV) of an imaging sensor system. As a measure of the geospatial quality of imagery, the Modulation Transfer Function (MTF) of the system is often used along with the signal-to-noise ratio (SNR). However, their calculation is not fully standardized. Further, consistent measurements and comparisons are often hard to obtain. Therefore, in the Infrared and Visible Optical Sensors (IVOS) subgroup of the Working Group on Calibration Validation (WGCV) of the Committee for Earth Observation Satellites (CEOS), a team from various countries and professional entities who are involved in MTF measurement was established to address the issue of on-orbit MTF measurements and comparisons. As a first step, a blind comparison of MTF measurements based on the slanted edge approach has been undertaken. A set of both artificial and actual satellite edge images was developed and a first comparison of processing results was generated. In all, seven organizations contributed to the experiment and several significant results were generated in 2016. No single participant produced the best results for all test images as measured by either the closest to the mean result, or closest to the truth for the synthetic test images. In addition, close estimates of the MTF value at Nyquist did not ensure the accuracy of other MTF values at other spatial frequencies. Some algorithm results showed that the accuracy of their estimates depended upon the type of MTF curve that was being analyzed. After the initial analysis, participants were allowed to modify their methodology and reprocess the test images since, in several cases, the results contained errors. Results from the second iteration, in 2017, verified that the anomalies in the experiment’s first iteration were due to errors in either coding or methodology, or both. One organization implemented a third trial to fix software errors. This emphasizes the importance of fully understanding both methodology and implementation, in order to ensure accurate and repeatable results. To extend this comparison study, a reference data set, which is composed of edge images and corresponding MTF curves, will be built. A broader audience will be able to access the edge images through the CEOS CalVal Portal (http://calvalportal.ceos.org/). This paper, which is associated with the reference data set, can serve as a new tool to either implement or check, or both, the MTF measurement that relies on the slanted edge method.

https://hal.archives-ouvertes.fr/hal-02055611/document

Comparison of MTF measurements using edge method: towards reference data set.

The world`s first geostationary ocean color imager (GOCI) is a three-mirror anastigmat optical system 140 mm in diameter. Designed for 500 m ground sampling distance, this paper deals with on-orbit modulation transfer function (MTF)measurement and analysis for GOCI. First, the knife-edge and point source methods were applied to the 8th band (865 nm) image measured April 5th, 2011. The target details used are the coastlines of the Korean peninsula and of Japan, and an island 400 meters in diameter. The resulting MTFs are 0.35 and 0.34 for the Korean East Coastline and Japanese West Coastline edge targets, respectively, and 0.38 for the island target. The daily and seasonal MTF variations at the Nyquist frequency were also checked, and the result is on average. From these results, we confirm that the GOCI on-orbit MTF performance satisfies the design requirements of 0.32 for 865 nm wavelength.

Initial On-Orbit Modulation Transfer Function Performance Analysis for Geostationary Ocean Color Imager

The precise edge locator to estimate landcover proportion in a mixel IFOV, of which algorithm is based on moment preserving principle, is tested in this research.Results of two computer experiments are as follows.1) The estimated landcover proportions were affected by the length of edges, noises, window sizes of the locators and PSFs of image.2) The average magnitude of errors of estimated landcover proportion in a mixel IFOV on an idealistic edge is less than 2% in any window size but the window size of 3×3. But, the magnitude of errors on a long and high contrasted straight edges on a digitized aerophotograph with much noise is around 20% or so.3) Some of improvements on the present algorithm to estimate landcover proportion in mixels on realistic images are necessary.

https://www.jstage.jst.go.jp/article/jsprs1975/33/2/33_2_28/_pdf

Estimation of Landcover Ratios in Mixel IFOVs

An endeavor was made to use Fourier transform techniques to estimate the line spread function (LSF) of the Thematic Mapper (TM) from processed data. Data near the boundary of small fields from TM scenes were sampled, fitted to a theoretical curve, lowpass filtered, and then deconvolved to obtain the LSF. Lowpass filtering improved the truncation effect and eliminated the discontinuity at both ends of the data sequence.The selection of the cutoff frequency and the order of a lowpass filter is based on a newly defined criterion which tests the distortion of the processed edge by comparing it with the original edge. A parameter of the LSF, the equivalent width, is used to compare these LSF's deconvolved from sampled edge data with those evaluated by the derivative techniques. The calculated equivalent widths using the Fourier transform techniques for infrared bands are very close to TM design specifications.

Fourier transform techniques for the evaluation of the Thematic Mapper line spread function

Atmospheric particulate blurring effects on Thematic Mapper data for areas with different particulate concentrations were studied through analysis of field edges. Edge gradient and Fourier techniques were utilized to determine the overall atmosphere‐sensor system line spread function (LSF). The sampled edge data were first fitted to a theoretical curve and then processed by a lowpass filter (LPF). Two parameters of the LSF, the equivalent width and the absolute equivalent width which is defined in this paper, were calculated for several sampling regions. In most cases, the results showed that a broader width (either equivalent or absolute equivalent) corresponds to higher particulate concentration and vice versa.

Guoping Zhou

Papers

Fourier transform techniques for the evaluation of the Thematic Mapper line spread function

Evaluation of atmospheric blurring effects via the line spread function