New ISAR imaging algorithm based on modified Wigner?Ville distribution

Inverse synthetic aperture radar (ISAR) imaging of air, space or ship targets with complex motion has attracted the attention of many researchers in the past decade. Complex motion of targets induce cross-range scatterer-variant quadratic phase terms, which will degrade the cross-range resolution and affect focusing quality. A new algorithm is proposed for the ISAR imaging of complex moving targets. First, conventional range alignment, phase compensation and range compression are performed over the raw phase history data such that each range bin can be modelled as the sum of several linear frequency modulation or chirp signals. Secondly, a modified- Wigner?Ville distribution (referred to as M-WVD) approach is proposed, which is based on a scale transform in the time-frequency distribution plane and can effectively suppress the troublesome cross-term interference associated with WVD via coherent integration. Finally, the azimuth ISAR image can be obtained via a simple maximisation projection from the two-dimensional accumulated plot to the azimuth dimension. Compared with existing WVDbased ISAR imaging algorithms, the proposed method has the following features: better cross-term interference reduction achieved at no resolution loss, computationally more efficient with no expensive two-dimensional parameter search, and higher signal processing gain because of coherent integration during the whole imaging time. Bothnumerical and experimental results are provided to demonstrate the performance of the proposed method.     

Inverse synthetic aperture radar imaging of ship target with complex motion

High-resolution inverse synthetic aperture radar (ISAR) imaging and recognition of ship target is very important for many applications. Although the principle of ISAR imaging of ship target on the sea is the same as that of flying target in the sky, the former usually has more complex motion (fluctuation with the oceanic waves) than the latter, which makes the motion compensation very difficult. However, the change in phase chirp rate caused by the complex motion of ships will deteriorate the azimuth focusing quality. In this paper, we first model the complex motion of ship target with cubic phase terms (parameterised on chirp rate and its change rate), then a new ISAR imaging method, referred to as TC-DechirpClean, is proposed, which estimates the chirp rate and the change rate of chirp rate of all scatters in the time? chirp distribution plane. Both numerical and experimental results are provided to demonstrate the performance of the proposed method.     

Discriminating real objects in radar imaging by exploiting the squared modulus of the continuous wavelet transform

New technique based on continuous wavelet transform (CWT) for classifying objects in synthetic aperture radar (SAR) imaging is presented. The CWT allows to analyse two-dimensional SAR images to highlight the frequency and angular behaviour of the scatterers. This technique allows to build a SAR hyperimage, that is, a four-dimensional data cube which represents for each spatial location (x, y) of the scatterer in the image, its frequency and angular energy behaviour. When analysing different targets, objects or areas in SAR images, it has been recently observed that some scatterers belonging to a same class of objects could have similar frequency and angular energy responses. The previous observations have motivated the determination to exploit these energy responses to discriminate these objects. This discrimination is performed by frequency and angular correlations between the response of a particular scatterer (measured) and those of all the scatterers in the SAR image. Some examples of discrimination from real SAR data are presented and show an interest of the method for target classification and recognition for SAR imaging     

自适应Chirplet信号分解用于ISAR目标三维转动检测

当ISAR目标作复杂的三维转动时,目标上各散射点的相位误差将与它们在目标上所处的位置有关,传统的相位聚焦方法难以采用统一的相位校正函数来进行补偿。为解决此问题,论文提出了一种基于自适应Chirplet信号分解的ISAR目标三维转动检测方法,该方法使用自适应Chirplet信号分解的快速算法来估计散射点子回波的相位信息,并根据两个散射点相位之间的非线性度来判断目标是否存在三维转动,从而只选择那些仅具有二维转动的数据段进行成像。仿真实验结果表明了它的有效性。     

Continuous wave ultrasonic tomography

As an object rotates with respect to a stationary ultrasonic beam, the scattering centers within the object return echoes that are Doppler-shifted in frequency by amounts depending on the velocities of the individual scatterers. The scattering centers that lie on a line of constant cross-range all have the same effective velocity in the direction pointing toward the transducer; therefore, the backscattered echo amplitude at any particular frequency is the line integral of the scattered radiation at the cross-range corresponding to that frequency. The amplitudes of the returned signals at other frequencies give the line integrals for the scatterers at the corresponding cross-ranges. The amplitude as a function of frequency can be interpreted as a tomographic projection. A continuum of the projections at different positions is generated while the object is rotating. A tomographic reconstruction algorithm can produce an image of the distribution of scattering centers in the insonified object from these projections. A microscanner was developed to investigate the approach of using continuous wave (CW) ultrasound for cross-sectional imaging. The resolution is limited by the target size and the ultrasonic wavelength.     

ISAR motion compensation using the burst derivative measure as a focal quality indicator

Inverse synthetic aperture radar (ISAR) is an imaging technique that shows great promise in classifying airborne targets in real-time under all weather conditions. The success of classifying targets using ISAR is predicated upon forming highly focused radar images of these target. Efforts to develop highly focused radar imaging computer software have been challenging, mainly because the imaging depends on and is affected by the motion of the target. Computationally intensive motion compensation algorithms have been developed to remove the unwanted degrading effects of target motion. Those particular motion compensation algorithms which require the use of a space domain focal quality indicator (e.g., entropy) to determine image sharpness as processing proceeds pay a severe computational penalty due to the large number of two-dimensional fast Fourier transforms (2D-FFTs) which must be computed. This is due to the fact that the actual processing of ISAR data is done primarily in the spatial frequency domain and not in the space domain where the final ISAR image is displayed. If a focal quality indicator could be developed to measure image sharpness in the spatial frequency domain, then the computational burden introduced by the numerous 2D-FFTs could be greatly relaxed. This article describes the use of a new focal quality indicator called the burst derivative measure for determining ISAR image sharpness in the spatial frequency domain. Tests have been performed on simulated as well as actual ISAR data using both the burst derivative measure and the entropy measure. Results indicate that the burst derivative measure, when used in conjunction with the entropy measure, can greatly reduce the number of 2D-FFTS presently required in these motion compensation algorithms.©1993 John Wiley & Sons Inc     

Quality enhancement of image generated with bistatic ground based noise waveform SAR

A new method for quality enhancement in a noise synthetic aperture radar (SAR) image and the first results of its application to the SAR image generated with the use of a bistatic Ka-band ground-based noise waveform SAR (GB NW-SAR) are presented. A SAR image generated with a noise SAR suffers from the masking effect which is tied to residual random fluctuations in noise radar response from bright scatterers in the scene. This is similar to the masking effect present in the deterministic waveform SAR when the signal sidelobes of echoes from bright scatterers may mask the main response from a weaker target. The procedure presented is a variation of the CLEAN algorithm. Knowing precisely the emitted signal and finding positions of the strongest scatterers one may model the echo signal originated from a selected scatterer. Extraction of the modelled signal from the received one reduces the residual fluctuations and makes it possible to clean the image and increase its dynamic range. The final image is constructed from the cleaned signal and the previously removed strongest scatterers. A theoretical background is provided to the proposed procedure and its application to enhance the SAR image using simulated data as well as data generated by the Ka-band bistatic GB NW-SAR is demonstrated. The GB NW-SAR, recently developed and tested in LNDES IRE NASU, may operate in CW and pulse random signal regimes for short range applications.     

Uniform rotational motion compensation for inverse synthetic aperture radar with non-cooperative targets

In the current scenario of high-range resolution radars, the rotational motion of the target usually generates migration through resolution cells (MTRC) in the inverse synthetic aperture radar (ISAR) images. A technique to correct the MTRC in the case of uniform rotation rate is proposed. An estimation of the rotation vector of the non-cooperative targets is not required. The approach compensates the rotational motion in two steps: the slant-range rotation compensation (SRRC) and the cross-range rotation compensation (CRRC). CRRC is based on an extension of phase difference (PD) and on a robust method to delete outliers. The technique is verified both with simulated and live data from a millimetre-wave linear frequency-modulated continuous wave radar. A comparison with a minimum entropy-based technique is also made.     

Automatic algorithm for inverse synthetic aperture radar images recognition and classification

The authors propose an algorithm for automatic aircraft categories that is models classification from inverse synthetic aperture radar (ISAR) images that use pulse reflection shape and Doppler shifts of parts of aircraft that are in any maneuver that introduces rotation to the target. The authors artificially generated five different categories of ISAR aircraft using computer simulations and tested these simulated ISAR aircraft images of the airplanes defined by size and shape that are flying in a prescribed holding pattern. The authors investigate in what parts of the holding pattern the ISAR reflections provide information that makes it possible to identify to which of the five categories an aircraft in the holding pattern belongs. The obtained results show that it is possible in most parts of the holding pattern to successfully classify the various aircraft targets.     

Multi-feature based automatic recognition of ship targets in ISAR

This paper deals with the problem of non-cooperative target recognition. Specifically, the aim is the automatic recognition of ship targets from inverse synthetic aperture radar (ISAR) images. For this purpose a new two-step multi-feature based technique is proposed; this technique uses a number of features extracted from the ship radar image and matches these features with those extracted from the images obtained by properly projecting the target models of the classification library. Both cases of a priori known or unknown ship aspect angles are considered: the knowledge of the ship aspect (as available from tracking data) allows the selection of the candidate models on the basis of the matching between the ship and the model length, thus resulting in a performance improvement. Moreover, both single- and multi-frame-based processing techniques are proposed in order to assess the performance improvement achievable when an increasing number of ISAR images are involved in the decision; the fusion strategy adopted for the exploitation of the information from the multiple images is also described. The performance of the overall proposed technique is deeply investigated against simulated data. Results of its application to a set of live ISAR images of a ship target are also provided showing the effectiveness of the proposed approach.     

Nonlinear holographic imaging of phase errors

Experimental measurements and computer simulations of nonlinear holographic imaging of phase errors in laser beams are presented. The computer models are found to accurately predict the results of the experiments. Comparison with similar results by use of amplitude scatterers reveals that the image location (along the propagation path) is the same for phase and amplitude scatterers. However, the intensity and fluence of the image of a phase scatterer are significantly larger, indicating that phase objects pose a larger damage threat to optical components.     

Approach for single channel SAR ground moving target imaging and motion parameter estimation

In recent years, ground moving target imaging in synthetic aperture radar (SAR) has attracted the attention of many researchers all over the world. A novel approach is proposed for the ground moving target imaging and motion parameter estimation using single channel SAR. First, a second-order generalised keystone formatting method is used to compensate for the range curvature. Secondly, the estimated slope of the target echo's envelope is used for the range walk compensation. Thirdly, Doppler parameters of moving targets obtained via spectral analysis are used for the imaging and positioning of ground moving targets. Finally, motion parameters of moving targets can be estimated on the basis of the relationship between Doppler and motion parameters. Both numerical and experimental results are provided to demonstrate the performance of the proposed approach     

Analysis and extraction of stepped frequency radar signature for micro-motion structure

Radar signature produced by micro-motion structures contains movement and structure information which is useful for radar target recognition. For stepped frequency (SF) radar, the Doppler modulation is coupled with the stepped carrier frequency. It shifts and smears the high-resolution range profile and makes it difficult to analyse and extract the micro-motion information from the range profile directly. The authors propose to comprehend the SF radar as a special pulse Doppler radar, and the range profile as a Doppler profile for the convenience of motion analysis. The signature of moving targets for SF radar is analysed from this point of view and the equivalent instantaneous Doppler frequency (EIDF) is introduced. Then, a typical micro-motion, rotation, is taken as an example. The sinusoidal vibration of the peaks in the range profile sequence is explained in detail, especially the relationship between the sinusoid parameters and the rotation parameters. An approach to extract the rotation parameters from the range profile or EIDF spectrum sequence is proposed based on the Hough transform. Simulated and experimental results validate the theoretical analysis and the feature extraction method.     

Computer simulation study of nonlinear imaging properties for two phase-typed scatterers

Nonlinear imaging of two phase-typed scatterers in super-Gaussian laser beams is modeled and its properties obtained by computer simulation are presented. The formations of hot images and second-order hot images are verified. It is found that the in-beam locations of hot images correspond to those of the scatterers, but that there can be only one second-order hot image, which is at the middle point between the in-beam locations of the scatterers. Interestingly, the image intensity can be suppressed and it increases in an oscillating manner with some regularity as the distance between the scatterers increases. Moreover, one more scatterer makes the effect of the B integral and that of the phase shift caused by scatterers quite different from the predictions for single-scatterer case. The variation trend of hot image intensity with the B integral is not in agreement with that described by the analytical result for the single-scatterer case. The variation of phase shift caused by the scatterers can result in two peaks in the variation of hot image intensity with it, and the phase shift corresponding to the larger peak is approximately half the predicted result for the single-scatterer case. These results indicate that the number of scatterers has a significant influence on nonlinear imaging.     

Land surface deformation parameter estimation using persistent scatterer interferometry approach in an underground metal mining environment

Persistent scatterer interferometry (PSI) is a major advancement in radar interferometry for detecting and monitoring land deformation. PSI is the most advanced class of differential interferometric synthetic aperture radar (DInSAR) techniques. The technique conquers the main drawbacks of the conventional DInSAR technique by identifying radar targets having stable backscattering characteristics in time. These targets are termed as persistent scatterers (PSs). The higher the number of PSs for a study area the higher the accuracy of the results will be, which is most common for deformation analysis in urban areas. However, for non-urban or highly de-correlated areas, PSs density collapses significantly, which needs to increase for optimal results. For this purpose, partially coherent/distributed scatterers (DSs) are being exploited in addition to the PSs. The field surface of this study is one of the copper-rich mining belts in India, which consists of two major underground metal mines. Scatterer characterziation of the field surface under study suggests that most of the scatterers are DSs and very few scatterers under the influence of the mining zone are PSs. In addition to this, a preliminary investigation of deformation characteristics of the field surface under study reveals that the spatial extent of deformation is small/localized along with slow and non-linear deformation. Keeping in view scatterer and deformation characteristics of the field surface under study, in this research paper, a Quasi-Persistent Scatterer based PSI approach has been applied using high-resolution TerraSAR-X interferometric data stack (10 images) to generate deformation time series and deformation velocity. Furthermore, results obtained from the applied PSI approach and ground-based observations (using GNSS) have shown good agreement with each other, in the order of ?5.20?mm/year (LOS) and ?5.38?mm/year (subsiding), respectively.     

平飞斜视模式双站合成孔径雷达数据处理算法

研究了发射和接收分离的双站合成孔径雷达成像算法.主要分析了双站合成孔径雷达在平飞斜视工作模式下的方位分辨率,以及二次距离徙动校正判据,研究了利用非线性Chirp Scaling(NLCS)的方法,来消除同一距离单元上的不同方位调频斜率带来的影响,并且推导出了平飞斜视模式下的方位匹配滤波函数的频域表示,通过点目标成像仿真验证了该方法的正确性.     

Range-instantaneous Doppler imaging of inverse synthetic aperture sonar

Because the existing range-Doppler algorithm in inverse synthetic aperture sonar (ISAS) is based on target model of uniform motion, it may be invalidated for maneuvering targets due to the time-varying changes of both individual scatter′s Doppler and imaging projection plane. To resolve the problem, a new range-instantaneous Doppler imaging method is proposed for imaging maneuvering targets based on time-frequency analysis. The proposed approach is verified using real underwater acoustic data.     

Angle density functions for active sensor imaging

A new target density function (TDF) is proposed for active sensor imaging. The TDF, called the angle density function, is studied by utilising the angular distribution of targets at a fixed range. Active sensor imaging based on the angle density function is achieved using a phased array radar system. The phased array system is arranged for stationary radar-stationary target configuration. The imaging algorithm is applied for the whole target area. Instead of pointwise imaging, an approach including the whole target area globally is developed. An advantage of the technique is the use of standard Fourier-based analysis. This makes it possible to use of simple functions for global radar imaging. Although the imaging is accomplished by way of the phased array radars, beamforming is not necessary with the proposed technique.     

Influence of size parameter and refractive index of the scatterer on polarization-gated optical imaging through turbid media

The influence of incident polarized light, refractive index, and size parameter of the scatterer on achievable resolution and contrast (image quality) of polarization-gated transillumination imaging in turbid media is reported here. Differential polarization detection led to significant improvement of image quality of an object embedded in a medium of small-sized scatterers (diameter Dor=lambda,g>or=0.7), the improvement in image quality was less pronounced using either linear or circular polarization gating when the refractive index of the scatterer was high (ns=1.59), but for a lower value of refractive index (ns=1.37), image quality improved with the differential circular polarization gating. We offer a plausible explanation for these observations.