A Modified PGA for Spaceborne SAR Scintillation Compensation Based on the Weighted Maximum Likelihood Estimator and Data Division

Abstract

The P-band spaceborne synthetic aperture radar (SAR) is significantly affected by ionospheric scintillation. Although the traditional phase gradient autofocus (PGA) can estimate and compensate the scintillation phase of the single polarization SAR data, the quality of the refocused image will degrade at the scene edge. In this paper, a modified PGA based on the weighted maximum likelihood (WML) estimator and data division is proposed for full-scene image refocusing. First, a data division strategy for scintillation corrupted image is introduced, which is based on the prior scintillation information including its power spectral density (PSD) and autocorrelation function (ACF). Then, some pre-processing steps including data selection, circular shifting and windowing are performed for suppressing the noise and clutter. Finally, the scintillation phase is estimated by the WML estimator with fewer iterations, and after the scintillation phase of all data blocks are obtained, the distorted full-scene image is refocused. Using the generated wide-band ionospheric scintillation model (WBMOD) phase screen, numerical simulations based on point targets and scenes derived from real spaceborne SAR data are carried out. The evaluation results about resolution, peak side-lobe ratio (PSLR), integral side-lobe ratio (ISLR), correlation coefficient and image entropy demonstrate the effectiveness of the proposed algorithm.

Effective Vessel Recognition in High Resolution SAR Images Using Quantitative and Qualitative Training Data Enhancement From Target Velocity Phase Refocusing

“…Phase gradient was estimated and utilized to calibrate the phase error repeatedly, subsequently deriving the focused image chip after several iterations. Given the continuous enhancement of PGA, a modified PGA algorithm which utilized the weighted phase gradient estimation was utilized as a reference focusing algorithm [41]. Exploiting the normalized signal-to-clutter ratio weight, modified PGA enabled to intensify the contribution from significant targets, which accordingly enhanced the performance of phase refocusing compared to that of conventional PGA.…”

A Modified Range Doppler Algorithm for High-Squint SAR Data Imaging

“…The range compressed result after NLCS processing is depicted in Figure 4b, and the width of the range signal in the fast time domain is significantly reduced, which indicates that the window width of the correlation processing will be remarkably shortened for large squint angle cases. Additionally, Equation (32) shows that there is an additional range-variant cubic phase modulation of the rectangular window function in the range direction, which is also introduced by the residual cross-coupling. Different from the normal sinc function, the compressed range pulse r(τ, f η ; R 0 ) therefore has an asymmetric form as is shown in the partially enlarged view of Figure 4b, where obvious side-lobe energy asymmetry appears.…”

SAR Imaging of Swing Ships Based on Two-Dimensional Spatial Variance Correction

“…An appropriate idea is to segment the image into sub blocks and then perform PGA in each block. The existing spacedomain PGA method [9][10][11] uses a fixed-size segmentation strategy to segment the imaging result. For the swing ships, there are four problems with the fixed-size segmentation strategy: 1) For sub blocks where there is no ship, such as the sea surface near the ship, space-domain PGA still concentrates the energy to the strongest scatterer, which may cause the appearance of false targets.…”

“…5(a). The existing space-domain PGA method [9][10][11] estimates the gradient of the sea area and performs iterative optimization, which not only increases the computational burden, but also interferes with the phase estimation of the ship part. In the proposed SV-PGA, PGA is performed only in the area enclosed by the red line in Fig.…”

“…Mosaic PGA [9] is proposed to estimate and compensate for the spatially varying phase errors by applying PGA in the segmented along-track strips of the data. The subimage sizes of Mosaic PGA and other PGA based methods with data segmentation [10][11] are fixed. However, due to the existence of 2-D spatial variance, scatterers at different positions have different defocus degrees, and the fixed segmentation size cannot accurately contain all the energy of scatterers with different defocus degrees.…”