A Fully Automatic Autofocusing Algorithm for Post-processing ISAR Imaging based on Image Entropy Minimization

Abstract

Autofocus is a technique for improving inverse synthetic aperture radar (ISAR) imaging. In this paper, a novel autofocusing method is developed for high-resolution stepped-frequency ISAR. Non-uniform rotational motion is compensated through the proposed post-processing methodology. In this way, the computational cost of polar reformatting process can be circumvented. The proposed CPI-split autofocusing process results in well-focused ISAR images for high angular acceleration periods. Finally, ISAR image entropy dependencies are thoroughly examined through various simulation results, leading to an acceptable range of entropy values for the autofocusing process. Ill. 7, bibl. 9, tabl. 3 (in English; abstracts in English and Lithuanian).http://dx.doi.org/10.5755/j01.eee.110.4.305

Application of a Fully Automatic Autofocusing Algorithm for Post-Processing of Synthetic Aperture Radar Images based on Image Entropy Minimization

“…Subsequently in some specific CPI's, an oscillatory movement is induced to its position along the vertical z-axis, due to sea surface motion. The SAR images that are obtained for these CPI's are blurred due to the ship movement, which effect is removed by our proposed autofocusing algorithm [3]. The simulated ship geometry is shown in Fig.…”

“…It can be easily noticed that there is a change in the angle of observation as the SAR antenna moves along the flight path. The CPI-split autofocusing algorithm is employed in those CPI's which entropy values exceed a threshold that represents an acceptable SAR image quality [3]. The images with entropy values below the entropy threshold are called "focused" images, while the images with entropy values over the threshold are called "unfocused".…”

“…It is expected that the SAR images which correspond to those specific CPI's in which the ship movement is induced, will have greater entropy values than the SAR images corresponding to no ship movement. In this simulation scenario the value of the entropy threshold was set equal to 6.0 [3].…”

“…The SAR images for all CPI's are constructed from the raw data matrices through the traditional 'Range -Doppler' imaging technique, involving FFT processing in both range and Doppler directions [2]. In order to compare the quality of the SAR images obtained, the entropy value of each image is computed, which corresponds to the SAR image quality as a comparison criterion [3], [4].…”

“…The SAR image, in this case, is usually degraded by Manuscript received January 9, 2013; accepted April 10, 2013. defocus, distortion or displacement due to target movement. As moving targets are of great interest, the purpose of this paper is to apply the post processing CPI-split autofocusing algorithm [3] for the case of SAR geometry in order to get a focused SAR image of a moving target, as explained below.…”