Conventional SAR Imaging Research Articles

Target anisotropic scattering deduction model using multi-aspect SAR data

Typical targets in SAR observing scenes are characterized by anisotropy, which represents backscattering information changing with the observing angle. Anisotropic scattering can be used to study the structural characteristics of targets and the response differences between structures. In actual situations, due to flight conditions and other reasons, only partial observation information can be obtained, so it is important to use limited data to deduce remaining backscattering information for target analysis. In our article, the scattering deduction model based on matrix completion is established, including scattering characteristic analysis. The multi-aspect data is transferred from range and azimuth in conventional SAR image to pixel dimension and angle dimension for analysis. At the same time, the pixel similarity matrix is used as a feature in pixel dimension, and one-dimension back-amplitude scattering curve with angle is established as a feature matrix in angle dimension. In the process of modeling existing data, two corresponding decomposition matrices are generated with the help of collaborative filtering idea. Finally, a four-matrix combination for anisotropy deduction is formed. Ku-band multi-aspect data from autonomous experimental flights is used for algorithm. In the experiment, the scattering deduction of the remaining aspect sequence was carried out based on obtaining partial aspect sequence images, and the strong scattering regions of the target point and sub-aperture details were compared. The edge information and structure map of the target were reconstructed using scattering deduction results.

Microwave Photonic SAR High-Precision Imaging Based on Optimal Subaperture Division

Microwave photonic synthetic aperture radar (MWP-SAR) offers a larger signal bandwidth than conventional SAR, and its theoretical resolution can be improved to centimeter level. To achieve same order of magnitude resolution in azimuth direction, long synthetic aperture is always required, which results in extremely high requirements for the accuracy of imaging procedure. Compared with conventional SAR imaging algorithms, two major problems should be considered: 1) The scattering characteristics of target might vary from the frequency of signal and the angle of incidence, which seriously affects the coherence of received echo. 2) The imaging of MWP-SAR system is more sensitive to motion errors and therefore requires higher accuracy of motion compensation processing. To solve the above issues, a high-precision imaging method for MWP-SAR is proposed. First, based on the attribute scattering center(ASC) model, this paper analyzes the target-scattering characteristics with different frequencies and incident angles. Then, according to the influence of scattering phase on MWP-SAR imaging, an optimal sub-aperture division algorithm is proposed to guarantee the coherence of each sub-aperture data and better imaging results. Furthermore, to compensate for the effects of high-order motion errors, this paper proposes a motion error estimation algorithm based on sub-image registration, where the full aperture high-order motion errors can be divided into multiple linear components, and the flight trajectory of platform can be accurately reconstructed. Finally, a full-aperture time-domain high-precision imaging method is presented, and the effectiveness of the proposed formation is verified by both simulation and actual airborne MWP-SAR data processing.

DBF Processing in Range-Doppler Domain for MWE SAR Waveform Separation Based on Digital Array-Fed Reflector Antenna

With the rapid development of the multiple-input multiple-output synthetic aperture radar (MIMO SAR) system, the demands for miniaturization and high gain of antenna are increasing. The digital array-fed reflector antenna has such virtues so that it can play an important role in such system. However, the geometric models and signal models based on a reflector antenna are considerably different from the directly radiating planar antenna. The signal processing for the reflector antenna is more complex and difficult. As a result, the applications of the reflector antenna in SAR system are not as mature as those of the planar antenna. A combination of multidimensional waveform encoding (MWE) technique and digital beamforming (DBF) technology at the receiving end can greatly improve the MIMO SAR system performance, especially ambiguity suppression and waveform separation. This configuration can realize different radar functions and meet multidimensional observation requirements, such as the polarized SAR. Thus, this study combines digital array-fed reflector antenna and the DBF technique in the elevation direction for MWE SAR waveform separation. The echo models for the array-fed reflector antenna and the planar antenna are established based on short-time shift-orthogonal waveforms. In the models, a mismatch in steering vectors is inevitable if DBF processing is continuously performed traditionally in the azimuth-elevation two-dimensional time domain. This mismatch will worsen the waveform separation effect and the image quality. Therefore, we propose a DBF method which is processed in range-Doppler domain. The method enables waveform separation without ambiguity at the receiver. Then, the conventional SAR imaging methods are enabled, and we acquire an ideal SAR image. The simulation results for both point targets and distributed targets prove the effect and feasibility of the proposed DBF method.

Piecewise Constant Doppler Algorithm: Performance Analysis, Further Simplification, and Motion Compensation

The piecewise constant Doppler (PCD) algorithm is a novel radar imaging process recently proposed for the generalized continuous-wave synthetic aperture radar (GCW-SAR). This article presents a detailed theoretical analysis on the PCD algorithm's performance and proposes a further complexity-reduced PCD algorithm with motion compensation (MOCO) suitable for practical applications. First, the difference between conventional SAR imaging and PCD imaging, i.e., the zeroth-order versus the first-order slant range approximation, is revealed. Exact ambiguity function expressions of the PCD imaging in range and azimuth directions, respectively, are then derived. An error function of the PCD imaging as compared with the ideal matched filtering method is further defined and shown to be a function of an image quality factor, which can be used to quantify the PCD imaging performance. Finally, a faster and more flexible imaging process, called decimated PCD algorithm, is proposed, by which the image azimuth spacing can be easily extended, and hence, the computational complexity can be significantly reduced. The decimated PCD implementation incorporated with the MOCO is developed for practical GCW-SAR applications, and its imaging error lower-bounded by the PCD imaging error function is analyzed accordingly. Simulation and experimental results validate the theoretical analysis of the PCD imaging and show that the decimated PCD algorithm can achieve a high imaging quality at low cost.

Investigation of Azimuth Multichannel Reconstruction for Moving Targets in High Resolution Wide Swath SAR.

The azimuth multichannel imaging scheme with the large receive antenna divided into multiple sub-apertures usually leads to azimuth non-uniform sampling, and echoes from all azimuth channels should be reconstructed based on the signal model before conventional SAR imaging. Unfortunately, the multichannel signal model of a moving target is different from that of a fixed target. This paper analyzes the multichannel signal model of the moving target and the effect of the target velocity on azimuth multichannel reconstruction. Based on the multichannel signal mode of the moving target, a new multichannel signal reconstruction algorithm is proposed. Furthermore, the slant range velocity is estimated by computing signal energy distribution. Simulation results on point targets validate the proposed approach.

A New SAR–GMTI High-Accuracy Focusing and Relocation Method Using Instantaneous Interferometry

In this paper, for a multichannel synthetic aperture radar–ground moving target indication (SAR--GMTI) system, a new high-accuracy focusing and relocating method using instantaneous interferometry, i.e., carrying out interferometry operation in the azimuth time domain before focusing, is proposed. One of the key steps of this method is to perform instantaneous interferometry to get accurate equivalent cross-track velocity (ECV) estimation for cross-track motion compensation. After that, the signal from a moving target is concentrated in range, and along-track motion compensation becomes convenient. Motion compensation transforms a moving target into a stationary one; thus, the conventional SAR imaging algorithm can be applied to focus the moving target. Finally, a strategy for accurately relocating a moving target is presented. The processing results of simulated and measured data illustrate the effectiveness of the proposed method.

Improved Chirp Scaling Algorithm Based on Fractional Fourier Transform and Motion Compensation

Fractional fourier transform (FrFT) is a new tool of time-frequency analysis, and a good tool to compress chirp signal and to process non-stationary motion error. Aiming at the problem of the conventional SAR imaging algorithm based on FFT and two-step motion compensation, that of the effect of non-stationary error elimination is not obvious and the improvement of resolution is restricted, an improved Chirp Scaling imaging algorithm based on FrFT and motion compensation is put forward in this paper, which is expected to eliminate the influence of range motion error, so as to ob- tain high quality SAR images. Simulation results and imaging results of real SAR data show that the proposed algorithm can eliminate the influence of range motion error effectively. (the real SAR data provided by Institute of Electronics, Chi- nese Academy of Sciences).

Doppler-Related Distortions in TOPS SAR Images

A direct consequence of the TOPS acquisition geometry and the steering in azimuth of the antenna is the timevarying Doppler centroid within bursts. If this fact is not properly accommodated during SAR image formation, undesired distortions in both azimuth and range dimensions of the focused SAR images may appear. Azimuth distortions are caused by the local mismatch of both squint and topography. Range distortions arise from the inaccurate accommodation of the intrapulse motion of the platform, usually known as the stop-and-go approximation. Conventional spaceborne SAR image formation schemes will be, in general, unable to provide accurate TOPS SAR images. These distortions are discussed and evaluated for exemplary low-Earthorbit SAR scenarios. Compensation strategies are presented and validated with TerraSAR-X TOPS data. A discussion of the potential impact on the Sentinel-1 interferometric-wide-swath and extra-wide-swath modes (i.e., TOPS) is also given.

Speckle Reduction of PolSAR Images in Forest Regions Using Fast ICA Algorithm

Advancement in Polarimetric synthetic aperture radar technology (PolSAR) and its ability to capture images in different polarizations, facilitate accessing large volumes of information from a scene. These images as well as conventional SAR images are degraded by speckle noise. Processing and reducing the speckle of all the images separately is time consuming. So applying methods to simultaneously process different channels information can be very helpful. Several methods have been proposed for speckle reduction and each has advantages and disadvantages. In this paper, a despeckling approach based on fast independent component analysis (Fast ICA) algorithm is proposed for improving of the results when polarimetric channels are added. The results show the more input images to Fast ICA algorithm leads to better separation between signal components and the speckle. Analysis of the results for the ALOS (Advanced Land Observing Satellite) PALSAR polarimetric images showed that combination of polarimetric channels improved 37 % the ENL value in comparison with only using the HH (horizontal-horizontal)-, HV (horizontal-vertical)-, and VV (vertical-vertical)- polarized channels.

A novel modified Omega-K algorithm for circular trajectory scanning SAR imaging using series reversion

This article presents a modified Omega-K algorithm for circular trajectory scanning synthetic aperture radar (CTSSAR) imaging. Due to the curvature of circular trajectory, it is difficult to have access to the two-dimensional frequency spectrum for CTSSAR via the principle of stationary phase (POSP), as conventional SAR imaging methods RD and CS. Herein, the analytic point target spectrum is first derived by series reversion and the POSP, based on which a modified Omega-K algorithm is developed to focus data accurately. The accuracy can be controlled by keeping enough terms in the two series expansions so that a well-focused image can be achieved with a proper range approximation. After detailed analyses and experiments, the fourth-order approximation is proved to be the best choice. Furthermore, the computational efficiency is evaluated by comparing the given method with the back projection algorithm and other methods with different approximated orders. The proposed algorithm is verified to be the best one in terms of computational burden. A well-focused image is obtained by simulations, validating the feasibility of the proposed algorithm.

Ground moving target detection and location based on SAR images for distributed spaceborne SAR

This paper investigates data processing approaches to detect and locate ground moving targets using distributed spaceborne SAR systems with long cross-track baselines. In particular, it investigates the performance of ground moving target detection for two typical satellite formations: Cartwheel and Pendulum. An approach based on SAR images and a space-time adaptive processing (STAP) algorithm is proposed in order to overcome the effects of the ground terrain on the clutter suppression. The key idea of the approach is firstly to reduce the clutter degrees of freedom greatly by using conventional SAR imaging processing. Then the ground terrain clutter within each SAR pixel can be effectively cancelled by using the very limited spatial degrees of freedom. Finally, constant-false-alarm-rate (CFAR) techniques can be used to detect the remaining target SAR pixels after clutter cancellation. An approach to relocate the detected targets is also proposed, which is based on the estimation of the Doppler spectrum shifts of ground moving targets relative to the clutter Doppler spectrum. The proposed approaches in this paper have the advantages of simplicity and high efficiency.

Ocean wave imaging using an airborne single pass across-track interferometric SAR

An airborne single pass across-track interferometric synthetic aperture radar (InSAR) is used to image ocean waves. A theoretical model explaining the imaging mechanisms is developed, and simulations of the interferogram as well as the conventional SAR intensity image are presented for given ocean wave spectra. Distortions of digital elevation models (DEM) derived from InSAR data are explained by the motion of the sea surface. A Monte Carlo method based on forward simulations is used to estimate variance spectra of the distorted elevation models. It is shown that a straightforward estimation of wave height using the distorted InSAR elevation model is in good agreement with true wave height for low amplitude swell with about 10% error depending on propagation direction and coherence time. However, severe underestimation of wave height is found for wind seas propagating in flight direction. Forward simulations show that the distorted InSAR DEM is less dependent an the model chosen for the real aperture radar mechanism than conventional SAR images. Data acquired during an experiment over the North Sea by a high precision InSAR system are compared with simulations.

Simulation of ocean waves imaging by an along-track interferometric synthetic aperture radar

A two-dimensional (2D) model for describing the imaging of ocean waves by an along-track interferometric synthetic aperture radar (AT-INSAR) is derived. It includes the modulation of the normalized radar cross section by the long waves, velocity bunching, and azimuthal image smear due to orbital acceleration associated with long waves and due to the orbital velocity spread within the AT-INSAR resolution cell (parameterized by the scene coherence time). By applying the Monte-Carlo method, AT-INSAR amplitude and phase image spectra are calculated for different sea states and radar configurations. The Monte-Carlo simulations show that velocity bunching affects the AT-INSAR imaging mechanism of ocean waves, and that a unimodal ocean wave spectrum may be mapped into a bimodal AT-INSAR phase image spectrum due to an interference between the velocity term and the velocity bunching term in the AT-INSAR imaging model. It is shown that the AT-INSAR imaging mechanism of ocean waves depends on the ratio of the scene coherence time and the time separation between the observations by the two antennas. If this ratio is larger than one, the AT-INSAR phase image spectra are distorted. Furthermore, the simulations show that the AT-INSAR phase image spectrum is quite insensitive to the ocean wave-radar modulation transfer function. Comparing AT-INSAR with conventional SAR imaging of ocean waves, the authors find that the azimuthal cut-off in AT-INSAR phase image spectra is shifted toward higher wavenumbers than in conventional SAR image spectra. This implies that AT-INSAR can resolve shorter azimuthal wavenumbers than conventional SAR. Thus the authors conclude that AT-INSAR phase images are better suited for measuring ocean waves spectra than conventional SAR images.

An inverse method for the processing of synthetic aperture radar data

An inverse method for the processing of synthetic aperture radar signals is presented. The method is independent of the relative bandwidth of the radar signal and should thus work when, for instance, the frequency of the emitted radiation is of the same order as the bandwidth, i.e. in low frequency, high resolution applications. It is shown that the obtained relations reduce to the correlation integrals of conventional SAR image processing in the case that the relative bandwidth of the signal is small. Range attenuation and antenna illumination effects may be fully compensated for.