scholarly journals Refocusing of Moving Ships in Squint SAR Images Based on Spectrum Orthogonalization

2021 ◽  
Vol 13 (14) ◽  
pp. 2807
Author(s):  
Xuyao Tong ◽  
Min Bao ◽  
Guangcai Sun ◽  
Liang Han ◽  
Yu Zhang ◽  
...  
Keyword(s):  
Phase Error ◽  
Bp Algorithm ◽  
Synthetic Aperture ◽  
Two Dimensional ◽  
Back Projection ◽  
Image Sharpness ◽  
Sar Images ◽  
Simulation Experiments ◽  
Geometric Deformation ◽  
Wavenumber Spectrum

Moving ship refocusing is challenging because the target motion parameters are unknown. Moreover, moving ships in squint synthetic aperture radar (SAR) images obtained by the back-projection (BP) algorithm usually suffer from geometric deformation and spectrum winding. Therefore, a spectrum-orthogonalization algorithm that refocuses moving ships in squint SAR images is presented. First, “squint minimization” is introduced to correct the spectrum by two spectrum compression functions: one to align the spectrum centers and another to translate the inclined spectrum into orthogonalized form. Then, the precise analytic function of the two-dimensional (2D) wavenumber spectrum is derived to obtain the phase error. Finally, motion compensation is performed in the two-dimensional wavenumber domain after the motion parameter is estimated by maximizing the image sharpness. This method has low computational complexity because it lacks interpolation and can be implemented by the inverse fast Fourier translation (IFFT) and fast Fourier translation (FFT). Processing results of simulation experiments and the GaoFen-3 squint SAR data validate the effectiveness of this method.

Bistatic Polar Format Algorithm Based on Wavenumber Vector

2015 ◽  
Vol 742 ◽  
pp. 335-339
Author(s):  
Chun Dong Qi ◽  
Shan Wei Shi ◽  
Kai Nan Peng ◽  
Han Yue Huang ◽  
Jia Chen Xu ◽  
...  
Keyword(s):  
Target Position ◽  
Bp Algorithm ◽  
Synthetic Aperture ◽  
Position Vector ◽  
Simulation Experiments ◽  
Imaging Geometry ◽  
Original Definition ◽  
Bistatic Synthetic Aperture Radar ◽  
Definition Of

The paper concentrates on bistatic polar format algorithm (PFA) for focusing the spotlight bistatic Synthetic aperture radar (SAR) data. Exploring the imaging geometry of spotlight mode, a wavenumber vector Kbis derived according to the original definition of wavenumber. The direction of the vector Kb corresponds to the path from the transmitter to the target in the scene to the receiver in the bistatic SAR configuration. Using the vector Kb, a point target position vector in the scene can be expressed as the same form as that of conventional monostatic spotlight SAR. Based on the vector Kb, the spotlight bistatic PFA is derived and is presented concisely. In the simulation experiments, the presented PFA and BP algorithm process the same area target respectively. The result of the experiments show the PFA mentioned in this paper is valid. The quality of the processing result achieved by the PFA is not worse than that by BP algorithm.

scholarly journals Small UAV-based SAR system using low-cost radar, position, and attitude sensors with onboard imaging capability

2021 ◽  
pp. 1-12
Author(s):  
Jan Svedin ◽  
Anders Bernland ◽  
Andreas Gustafsson ◽  
Eric Claar ◽  
John Luong
Keyword(s):  
Low Cost ◽  
Projection Algorithm ◽  
Synthetic Aperture ◽  
Back Projection ◽  
Data Link ◽  
Sar Images ◽  
Imaging Capability ◽  
Small Uav ◽  
Aerial Vehicle ◽  
High Resolution Images

Abstract This paper describes a small unmanned aerial vehicle (UAV)-based synthetic aperture radar (SAR) system using low-cost radar (5–6 GHz), position (GNSS/RTK) and attitude (IMU) sensors for the generation of high-resolution images. Measurements using straight as well as highly curved flight trajectories and varying flight speeds are presented, showing range and cross-range lobe-widths close to the theoretical limits. An analysis of the improvements obtained by the use of attitude angles (roll, pitch, and yaw), to correct for the relative offsets in antenna positions as the UAV moves, is included. A capability to generate SAR images onboard with the back-projection algorithm has been implemented using a GPU accelerated single-board computer. Generated images are transmitted to ground using a Wi-Fi data link.

scholarly journals Ground Moving Target Tracking and Refocusing Using Shadow in Video-SAR

2020 ◽  
Vol 12 (18) ◽  
pp. 3083 ◽  
Author(s):  
Xiaqing Yang ◽  
Jun Shi ◽  
Yuanyuan Zhou ◽  
Chen Wang ◽  
Yao Hu ◽  
...  
Keyword(s):  
Target Tracking ◽  
Bp Algorithm ◽  
Synthetic Aperture ◽  
Moving Target ◽  
Back Projection ◽  
Moving Targets ◽  
New Approach ◽  
Moving Target Tracking ◽  
Accuracy Requirement ◽  
Multiple Moving Targets

Stable and efficient ground moving target tracking and refocusing is a hard task in synthetic aperture radar (SAR) data processing. Since shadows in video-SAR indicate the actual positions of moving targets at different moments without any displacement, shadow-based methods provide a new approach for ground moving target processing. This paper constructs a novel framework to refocus ground moving targets by using shadows in video-SAR. To this end, an automatic-registered SAR video is first obtained using the video-SAR back-projection (v-BP) algorithm. The shadows of multiple moving targets are then tracked using a learning-based tracker, and the moving targets are ultimately refocused via a proposed moving target back-projection (m-BP) algorithm. With this framework, we can perform detecting, tracking, imaging for multiple moving targets integratedly, which significantly improves the ability of moving-target surveillance for SAR systems. Furthermore, a detailed explanation of the shadow of a moving target is presented herein. We find that the shadow of ground moving targets is affected by a target’s size, radar pitch angle, carrier frequency, synthetic aperture time, etc. With an elaborate system design, we can obtain a clear shadow of moving targets even in X or C band. By numerical experiments, we find that a deep network, such as SiamFc, can easily track shadows and precisely estimate the trajectories that meet the accuracy requirement of the trajectories for m-BP.

scholarly journals Robust Two-Dimensional Spatial-Variant Map-Drift Algorithm for UAV SAR Autofocusing

2019 ◽  
Vol 11 (3) ◽  
pp. 340 ◽  
Author(s):  
Guanyong Wang ◽  
Man Zhang ◽  
Yan Huang ◽  
Lei Zhang ◽  
Fengfei Wang
Keyword(s):  
Cross Correlation ◽  
Phase Error ◽  
Synthetic Aperture ◽  
Two Dimensional ◽  
Spatial Variance ◽  
Phase Errors ◽  
Residual Phase ◽  
Aerial Vehicle ◽  
Sar Imagery ◽  
The One

Autofocus has attracted wide attention for unmanned aerial vehicle (UAV) synthetic aperture radar (SAR) systems, because autofocus process is crucial and difficult when the phase error is spatially dependent on both range and azimuth directions. In this paper, a novel two-dimensional spatial-variant map-drift algorithm (2D-SVMDA) is developed to provide robust autofocusing performance for UAV SAR imagery. This proposed algorithm combines two enhanced map-drift kernels. On the one hand, based on the azimuth-dependent phase correction, a novel azimuth-variant map-drift algorithm (AVMDA) is established to model the residual phase error as a linear function in the azimuth direction. Then the model coefficients are efficiently estimated by a quadratic Newton optimization with modified maximum cross-correlation. On the other hand, by concatenating the existing range-dependent map-drift algorithm (RDMDA) and the proposed AVMDA in this paper, a phase autofocus procedure of 2D-SVMDA is finally established. The proposed 2D-SVMDA can handle spatial-variance problems induced by strong phase errors. Simulated and real measured data are employed to demonstrate that the proposed algorithm compensates both the range- and azimuth-variant phase errors effectively.

scholarly journals Refocusing Moving Ship Targets in SAR Images Based on Fast Minimum Entropy Phase Compensation

Sensors ◽  
2019 ◽  
Vol 19 (5) ◽  
pp. 1154 ◽  
Author(s):  
Xiangli Huang ◽  
Kefeng Ji ◽  
Xiangguang Leng ◽  
Ganggang Dong ◽  
Xiangwei Xing
Keyword(s):  
Synthetic Aperture Radar ◽  
Phase Error ◽  
Synthetic Aperture ◽  
Sar Image ◽  
Minimum Entropy ◽  
Sar Images ◽  
Phase Compensation ◽  
Translation Motion ◽  
Airborne Sar ◽  
Aperture Radar

Moving ship targets appear blurred and defocused in synthetic aperture radar (SAR) images due to the translation motion during the coherent processing. Motion compensation is required for refocusing moving ship targets in SAR scenes. A novel refocusing method for moving ship is developed in this paper. The method is exploiting inverse synthetic aperture radar (ISAR) technique to refocus the ship target in SAR image. Generally, most cases of refocusing are for raw echo data, not for SAR image. Taking into account the advantages of processing in SAR image, the processing data are SAR image rather than raw echo data in this paper. The ISAR processing is based on fast minimum entropy phase compensation method, an iterative approach to obtain the phase error. The proposed method has been tested using Spaceborne TerraSAR-X, Gaofeng-3 images and airborne SAR images of maritime targets.

scholarly journals A Novel Method for Refocusing Moving Ships in SAR Images via ISAR Technique

2021 ◽  
Vol 13 (14) ◽  
pp. 2738
Author(s):  
Xinlin Jia ◽  
Hongjun Song ◽  
Wenjing He
Keyword(s):  
Synthetic Aperture Radar ◽  
Phase Error ◽  
Three Dimensional ◽  
Simulated Data ◽  
Microwave Remote Sensing ◽  
Synthetic Aperture ◽  
Sar Images ◽  
Rectangular Window ◽  
Novel Method ◽  
Aperture Radar

As an active microwave remote sensing device, synthetic aperture radar (SAR) has been widely used in the field of marine surveillance. However, moving ships appear defocused in SAR images, which seriously affects the classification and identification of ships. Considering the three-dimensional (3-D) rotational motions (roll, pitch, and yaw) of the navigating ship, a novel method for refocusing moving ships in SAR images based on inverse synthetic aperture radar (ISAR) technique is proposed. First, a rectangular window is used to extract the defocused ship subimage. Next, the subimage is transformed into the ISAR equivalent echo domain, and the range migration and phase error caused by the identical movement of all ship scatterers are compensated. Then, the optimal imaging time can be selected by the maximum image contrast search method. Finally, the iterative adaptive approach (IAA) is used to obtain the image with high resolution. This method has satisfactory imaging performance in both azimuth resolution and image focus, and the amount of calculation is small due to the processing of subimages. Simulated data and Gaofen-3 real SAR data are used to verify the effectiveness of the proposed method.

scholarly journals Focusing of Ultrahigh Resolution Spaceborne Spotlight SAR on Curved Orbit

Electronics ◽  
2019 ◽  
Vol 8 (6) ◽  
pp. 628 ◽  
Author(s):  
Yulei Qian ◽  
Daiyin Zhu
Keyword(s):  
Synthetic Aperture ◽  
Two Dimensional ◽  
Sar Images ◽  
Ultrahigh Resolution ◽  
Orbital State ◽  
Dimensional Spectrum ◽  
Polynomial Expression ◽  
Range Migration ◽  
Value Decomposition ◽  
Azimuth Resolution

Aiming to acquire ultrahigh resolution images, algorithms for spaceborne spotlight synthetic aperture radar (SAR) imaging typically confront challenges of curved orbit and azimuth spectral aliasing. In order to conquer these difficulties, a method is proposed in this paper to obtain ultrahigh resolution spaceborne SAR images on a curved orbit, which is composed of the modified RMA (Range Migration Algorithm) and the modified deramping-based approach. The modified RMA is developed to deal with the effect introduced by a curved orbit and the modified deramping-based approach is utilized to handle the problem of azimuth spectral aliasing. In the modified RMA, the polynomial expression of SAR two-dimensional spectrum on a curved orbit is derived with fourth-order azimuth phase history model and series reversion. Then, the singular value decomposition (SVD) is applied to decompose the expression of SAR two-dimensional spectrum numerically in order to acquire coordinates for Stolt interpolation in the scenario of curved orbit. In addition, the modified deramping-based approach is derived by introducing orbital state vectors in order to accommodate the situation of curved orbit in the proposed method. Experiments are implemented on point target simulation in order to verify the effectiveness of the presented method. In experiments, the range and azimuth resolution can achieve 0.15 m and 0.14 m, with focused scene size of 3 km by 3 km.

A linearity test for thick specimens imaged by HVEM over a 180° angular range

1991 ◽  
Vol 49 ◽  
pp. 552-553
Author(s):  
Yu Liu
Keyword(s):  
Phase Contrast ◽  
Three Dimensional ◽  
Angular Range ◽  
Two Dimensional ◽  
Back Projection ◽  
Line Integral ◽  
Exponential Law ◽  
Transmission Electron ◽  
Absorption Law ◽  
Linearity Test

The image obtained in a transmission electron microscope is the two-dimensional projection of a three-dimensional (3D) object. The 3D reconstruction of the object can be calculated from a series of projections by back-projection, but this algorithm assumes that the image is linearly related to a line integral of the object function. However, there are two kinds of contrast in electron microscopy, scattering and phase contrast, of which only the latter is linear with the optical density (OD) in the micrograph. Therefore the OD can be used as a measure of the projection only for thin specimens where phase contrast dominates the image. For thick specimens, where scattering contrast predominates, an exponential absorption law holds, and a logarithm of OD must be used. However, for large thicknesses, the simple exponential law might break down due to multiple and inelastic scattering.

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