Yueting Zhang

Stationary-Wavelet-Based Despeckling of SAR Images Using Two-Sided Generalized Gamma Models

In this letter, a stationary-wavelet-based despeckling algorithm based on the two-sided generalized gamma distribution (GΓD) model is proposed. We first introduce the two-sided GΓD as a flexible and efficient model for the wavelet coefficients of logarithmically transformed synthetic aperture radar intensity or amplitude. The strength of the model is highlighted in terms of its fit to the data, its low computational cost, and the ease of parameter estimation. By empirical results, we then motivate the GΓD as model for the wavelet coefficients of the noise-free signal. The GΓD model parameters are estimated with moment methods, using both absolute central moments for the wavelet coefficients of the noisy signal and the noise. Finally, we exploit the prior information contained in the model by designing a Bayesian maximum a posteriori estimator for estimating the noise-free wavelet coefficients. Experimental results demonstrate the superiority of our method in terms of simultaneously reducing speckle and preserving structural details.

A Novel Approach for Shadow Enhancement in High-Resolution SAR Images Using the Height-Variant Phase Compensation Algorithm

In synthetic aperture radar (SAR) images, the edge of the shadow is blurred because the radar is moving while the data are collected. In this letter, this problem is expanded on by using the imaging formation perspective. First, an approximate method to represent the imaging quality of the boundary of the shadow region based on the quadratic phase errors (QPEs) is provided for the first time, which built up the relationship between the parameters of the shadow caster and the behavior of the shadow in the SAR image. We notice that the QPE is approximately a linear function of the height of the caster. Second, we deduced the height-dependent phases due to the synthetic aperture process to the raw data, and a novel algorithm called height-variant phase compensation (HVPC) on the complex SAR image data is proposed by compensating the unexpected phases in the azimuth to sharpen the shadow. Compared with the traditional approach called fixed-focus shadow enhancement (FFSE), HVPC removes twice as much of the QPE as FFSE approximately. Experiments on simulation and real data demonstrate the precision and the better effect on shadow enhancement of our work. It is expected that the work in this letter could be some help for the SAR image understanding and application.

SAR Imaging Simulation for Urban Structures Based on Analytical Models

In this letter, a novel synthetic aperture radar (SAR) imaging simulator is proposed based on analytical electromagnetic and geometric models for urban structures. The backscatter contributions are evaluated by an analytical electromagnetic model based on the Kirchhoff approach (KA) in either physics or geometrical optics approximations rather than specular and Lambertian models. In addition, the position vectors of object facets in the SAR imaging plane are evaluated by a closed-form analytical geometrical model based on the ray-tracing model. These models are expressed in terms of few and basic parameters. Compared with other numerical methods, they are helpful for improving the efficiency of the simulation, but more importantly, they are significant for direct understanding of and further interpreting the SAR image features. Some experiments validate the geometric and electromagnetic models and demonstrate the efficiency of the simulator.

Synthetic Aperture Radar Image Synthesis by Using Generative Adversarial Nets

Synthetic aperture radar (SAR) image simulators based on computer-aided drawing models play an important role in SAR applications, such as automatic target recognition and image interpretation. However, the accuracy of such simulators is due to geometric error and simplification in the electromagnetic calculation. In this letter, an end-to-end model was developed that could directly synthesize the desired images from the known image database. The model was based on generative adversarial nets (GANs), and its feasibility was validated by comparisons with real images and ray-tracing results. As a further step, the samples were synthesized at angles outside of the data set. However, the training process of GAN models was difficult, especially for SAR images which are usually affected by noise interference. The major failure modes were analyzed in experiments, and a clutter normalization method was proposed to ameliorate them. The results showed that the method improved the speed of convergence up to 10 times. The quality of the synthesized images was also improved.

An Improved Shape Contexts Based Ship Classification in SAR Images

In synthetic aperture radar (SAR) imagery, relating to maritime surveillance studies, the ship has always been the main focus of study. In this letter, a method of ship classification in SAR images is proposed to enhance classification accuracy. In the proposed method, to fully exploit the distinguishing characters of the ship targets, both topology and intensity of the scattering points of the ship are considered. The results of testing the proposed method on a data set of three types of ships, collected via a space-borne SAR sensor designed by the Institute of Electronics, Chinese Academy of Sciences (IECAS), establish that the proposed method is superior to several existing methods, including the original shape contexts method, traditional invariant moments and the recent approach.

SAR interferometrie phase filtering based on wavelet transform and local frequency estimation

A novel approach combining the local frequency estimation with wavelet transform is presented to reduce interferometric phase noise for InSAR. First, the maximum likelihood estimator is used to obtain the frequency range of the noisy interferogram. Then, the wavelet transform is employed to obtain the wavelet coefficients of the real and imaginary parts of the complex interferogram. For the wavelet coefficients within the estimated frequency range and that out of the range, the NeighShrink and VisuShrink methods are employed respectively to shrink them. As a result, the noise can be effectively filtered without the loss of detailed information of the interferogram based on the advantages of the two shrinkage methods. The performance of noise reduction and fringe preservation is verified by the experiments with real interferogram.

The Characteristics of the Multipath Scattering and the Application for Geometry Extraction in High-Resolution SAR Images

Due to the special principle of synthetic aperture radar (SAR), SAR images exhibit many special phenomena. This becomes more obvious on high-resolution (HR) SAR images, as much more details are contained. We firmly believe that these details are where the advantages of a SAR image lie on and they provide some key clues for building the framework to understand SAR images. It comes to our attention that many special details in SAR images are due to the mechanism of multipath (MP) scattering. Dominated by the synthetic aperture progress, MP scattering reveals many interesting properties in SAR images. In this paper, we first analyze the mechanism of MP scattering by using the ray perspective on typical structure units. The main characteristics of the MP scattering in the SAR image are presented, such as the offset, prolong, segment, and blurring. Then, the analysis is extended to typical structures with MP scattering, such as bridges, buildings, and oil tanks by using Terra-SAR images. Finally, we explore the utilization of characteristics of MP scattering on geometry extraction from HR SAR images. The results prove the validity and the precision of our analysis. Some advices are also provided on the applications of the characteristics of the MP scattering in the SAR image. It is expected that this paper is helpful for the SAR image understanding and application.

An imaging strategy for high-precision and wide-beam airborne SAR system

For the high-precision and long-aperture SAR, range shift errors are introduced by the residual errors after aperture-independent motion compensation, as well as the azimuth phase errors. Both the frequency domain methods and the time domain algorithms exist shortcoming when dealing with the problem, i.e. inaccuracy and huge computation, respectively. Limitations of these algorithms has been analyzed in this paper, and a strategy is proposed to balance the accuracy and computational cost.

Projection Shape Template-Based Ship Target Recognition in TerraSAR-X Images

Ship target recognition has always been a hot issue in the field of ocean surveillance. Due to the serious shortage of samples in ship target recognition for synthetic aperture radar (SAR) images, the template-based method is still one of the most effective ways to solve the problem. In this letter, we put forward a novel ship recognition method based on the projection shape template (PST), aiming at increasing both the accuracy and the robustness of the recognition. The PST of each category is calculated by projecting the 3-D model obtained from the two-view images of the target to the 2-D slant-plane image according to the SAR imaging model. Then, we propose a contour extraction method to detect the profile of ships, which served as the feature. Finally, the identity of the query ship is obtained through contour matching. Experimental results indicate that the proposed method is effective even when the number of samples is extremely small, consequently providing a promising way for the automatic interpretation of ship targets in the SAR images.

The shadow enhancement for targets with flat structures in SAR images

The edges of the shadow region are blurred in the SAR image due to the moving of the radar during data collection. This phenomenon becomes obvious in the High Resolution SAR images. Shadow enhancement is of great value for ATR especially when the scattering centers of the target itself are not clear. In this paper, an approach for shadow enhancement in the SAR images for targets with plat structures is presented. And experiments on the Mini-SAR data test the validity of the approach.