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Online ISSN 1751-9683 Print ISSN 1751-9675

IET Signal Processing

Volume 2, Issue 3, September 2008

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Volume 2, Issue 3

September 2008

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  • Signal processing techniques for ISAR and feature extraction

    • Editorial: Signal processing techniques for ISAR and feature extraction
      Recent developments in detection, imaging and classification for airborne maritime surveillance
      • Author(s): N. Bon ; G. Hajduch ; A. Khenchaf ; R. Garello ; J.-M. Quellec
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 192 –203
      • DOI:  10.1049/iet-spr:20070082
      • Type: Article
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      • p. 192 –203 (12)
        The role of maritime patrol missions is to monitor large oceanic areas. The authors focus on a complete signal-processing sequence from the primary detection of the targets to their classification or recognition from an airborne radar. Contrary to the classical approach in which detection, tracking, imaging and classification are considered separately, here the authors propose an integrated strategy based on a close cooperation among all of them. Recent developments in high range resolution target detection are presented and their integration in the complete system is discussed to limit false alarms. High-resolution ISAR imaging of ships is then tackled and associated with a feature extraction process and a support vector machine classifier. A set of real data is used to illustrate the imaging and classification results.
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      Translational rotational motion compensation: a single algorithm for different radar imaging applications
      • Author(s): A. Aprile ; D. Meledandri ; T.M. Pellizzeri ; A. Mauri
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 204 –215
      • DOI:  10.1049/iet-spr:20070080
      • Type: Article
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      • p. 204 –215 (12)
        A single algorithm for various radar imaging applications is presented. The idea behind the proposed approach is that any motion between radar and target can always be modelled as a composition of translational and rotational components. Novel techniques to estimate and compensate these two motion components are discussed and also validated by application to real radar data for different targets (ships, aircraft, ground). By the translational-rotational-motion-compensation algorithm, good results can be achieved, especially in cases characterised by evident variation of the rotation rate during the coherent processing time for imaging. This usually happens in aircraft, ship or ground mover inverse synthetic aperture radar (ISAR) imaging as well as in squinted or forward-looking SAR. In any case, the efficiency and the accuracy of the algorithm presented for translational motion compensation might be the initial step of alternative radar imaging processing. The simplicity of the proposed algorithms makes them suitable and convenient for real-time application even for compact and low-weight radar systems to be fitted on board unmanned aerial vehicles (UAV) or other airborne platforms. The proposed concept is the result of the authors' decennial experience devoted to achieve a single, simple, robust, reliable and efficient signal processing algorithm for several real-time radar imaging applications.
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      Detection of a manoeuvring air target in strong sea clutter via joint time–frequency representation
      • Author(s): Y. Zhang ; S. Qian ; T. Thayaparan
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 216 –222
      • DOI:  10.1049/iet-spr:20070047
      • Type: Article
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      • p. 216 –222 (7)
        Two new approaches for detecting a manoeuvring air target in strong sea clutter are proposed. Whereas one is based on the adaptive chirplet decomposition, the other is inspired by spectral subtraction. Unlike the other time–frequency transform-based methods, the approaches presented can both be used to compute a signal's time waveform with suppressed sea clutter. Experiments with real-world data samples show that both methods can effectively enhance the Doppler radar signals associated with a manoeuvring air target by more than 15 dB.
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      Motion compensation in ISAR imaging using the registration–restoration–fusion approach
      • Author(s): T. Thayaparan ; G. Lampropoulos ; L. Stankovic
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 223 –236
      • DOI:  10.1049/iet-spr:20070118
      • Type: Article
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      • p. 223 –236 (14)
        The distortion in the inverse synthetic aperture radar (ISAR) image of a target is a result of small time-varying perturbed motion experienced by the target during the image integration period and is attributed to a phase modulation effect of the radar return from the target. Large distortion in ISAR images of a moving target has been investigated and demonstrated under controlled experiments and simulation. Results from the analysis suggest that severe distortion is attributed to the phase modulation effect where a time-varying Doppler frequency provides the smearing mechanism. For applications of target identification, the registration–restoration–fusion method has been developed to refocus the distorted ISAR images. This method has been applied to both the experimental and simulated ISAR data. Results demonstrate that the registration–restoration–fusion motion compensation approach can improve the distorted ISAR image better than what can be achieved by conventional Fourier transform methods. This study also adds insight into the distortion mechanisms that affect the ISAR images of a target in motion.
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      SAR imaging of moving targets using polynomial Fourier transform
      Real-time motion compensation, image formation and image enhancement of moving targets in ISAR and SAR using S-method-based approach
      • Author(s): T. Thayaparan ; Lj. Stankovic ; C. Wernik ; M. Dakovic
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 247 –264
      • DOI:  10.1049/iet-spr:20070093
      • Type: Article
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      • p. 247 –264 (18)
        The commonly used technique for inverse synthetic aperture radar (ISAR)/synthetic aperture radar signal analysis is a two-dimensional Fourier transform (FT), which results in an image of the target's reflectivity mapped onto a range and cross-range plane. However, in cases where the line-of-sight projections of the target's point velocities change or there is uncompensated movement within the coherent integration time, the FT produces blurred images. For target recognition applications, mainly those in military surveillance and reconnaissance operations, a blurred ISAR image has to be refocused quickly so that it can be used for real-time target identification. Two standard techniques used for improvement of blurred ISAR images are motion compensation and the use of quadratic time–frequency representations. Both are computationally intensive. The authors present an effective quadratic time–frequency representation, the S-method. This approach performs better than the Fourier transform method by drastically improving images of fast manoeuvring targets and by increasing the SNR in both low and high noise environments. These advantages are a result of the S-method's ability to automatically compensate for quadratic and all even higher-order phase terms. Thus, targets with constant acceleration will undergo full motion compensation and their point scatterers will each be localised. It should be noted that the source of the quadratic term can come not only from acceleration, but also from non-uniform rotational motion and the cosine term in wide-angle imaging. The method is also computationally simple, requiring only slight modifications to the existing FT-based algorithm. The effectiveness of the S-method is demonstrated through application to simulated and experimental data sets.
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      Slope-based frame selection and scaling technique for ship ISAR imaging
      • Author(s): D. Pastina  and  C. Spina
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 265 –276
      • DOI:  10.1049/iet-spr:20070122
      • Type: Article
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      • p. 265 –276 (12)
        The authors address the problem of optimum imaging time selection and angular motion estimation for ship ISAR or hybrid SAR/ISAR imaging. The aim is to select proper imaging times and to estimate ship angular motion in order to obtain high-quality top view or side view scaled images of the ship, suitable for processing by classification/identification procedures. For this purpose, a new technique is proposed, able to select the time instants better suited for top or side view image formation and to estimate the rotation motion for image scaling. The technique is based on a novel model of the phase/Doppler frequency of the generic ship scatterer and operates entirely in the range-Doppler image domain (as obtained via Fourier transform) by using the slopes of some extracted linear features, thus without requiring coherent processing. The performance of the complete ISAR technique is analysed in depth; the results of the application of the proposed technique to both simulated and live ISAR data prove the effectiveness of the proposed approach.
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      Two-dimensional ISAR model and image reconstruction with stepped frequency-modulated signal
      • Author(s): A.V. Karakasiliotis ; A.D. Lazarov ; P.V. Frangos ; G. Boultadakis ; G. Kalognomos
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 277 –290
      • DOI:  10.1049/iet-spr:20070115
      • Type: Article
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      • p. 277 –290 (14)
        A two-dimensional (2-D) inverse synthetic aperture radar (ISAR) return signal model that employs stepped frequency (SF) modulation is developed. The geometry of the examined ISAR scenario is described by analytical geometrical equations. The target to be imaged is represented by a rectangular grid of point scatterers, moving along a rectilinear trajectory at constant speed, without any rotational motion. Thus, the inverse synthetic aperture results from the translational motion of the target for a short period of time. The process of ISAR signal modelling through coherent summation of the SF-modulated signals reflected from different point scatterers of the target is thoroughly described. Moreover, an efficient ISAR image reconstruction approach, including cross-correlation-based range compression and fast-Fourier-transform-based azimuth compression, is presented through analytical mathematical expressions. Numerical simulations are carried out for various SF ISAR scenarios and high-resolution ISAR images are obtained by applying the proposed ISAR image reconstruction approach. Simulation results (ISAR images and corresponding entropy values) indicate the validity of the proposed 2-D SF ISAR return signal model and the efficiency of the proposed imaging algorithms. Finally, a numerical simulation result is illustrated, which shows the comparison of the performance of the proposed ISAR image reconstruction algorithms based on SF and linear frequency modulation waveforms. It is shown that the two waveforms attain almost the same ISAR image resolution.
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      Doppler signatures of radar backscattering from objects with micro-motions
      • Author(s): V.C. Chen
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 291 –300
      • DOI:  10.1049/iet-spr:20070137
      • Type: Article
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      • p. 291 –300 (10)
        Radar backscattering from objects with micro-motions is subject to Doppler modulations that help determine dynamic properties of objects and provide useful information about the objects. Doppler modulations represented by joint time–frequency analysis provide useful time-varying Doppler characteristics and, thus, add additional time-dimension information to exploit motion characteristics. The author discusses how to simulate radar backscattering from objects with rigid body motions and objects with non-rigid body motions, and how to analyse, interpret and characterise Doppler signatures of objects that undergo these micro-motions. Precession heavy top and human locomotion are used as examples of rigid and non-rigid body motions, respectively. Radar micro-Doppler signatures derived from these motions illustrate the potential of the joint time–frequency analysis for exploiting kinetic and dynamic properties of objects.
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      Micro-Doppler analysis of wheels and pedestrians in ISAR imaging
      • Author(s): A. Ghaleb ; L. Vignaud ; J.M. Nicolas
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 301 –311
      • DOI:  10.1049/iet-spr:20070113
      • Type: Article
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      • p. 301 –311 (11)
        In radar imaging, it is well known that relative motion or deformations of parts of illuminated objects induce additional features in the Doppler frequency spectrum. These features are called micro-Doppler effect and appear as sidebands around the central Doppler frequency. They can provide valuable information about the structure of the moving parts and may be used for identification purposes. Previous papers have mostly focused on 1D micro-Doppler analysis. The authors propose to emphasise the analysis of such ‘non-stationary targets’ using a 2D imaging space, using both the micro-Doppler and a high-range resolution analysis. As in 2D-ISAR imaging, range separation enables to better discriminate the various effects caused by the time-varying reflectors. The study is focused on two different common examples: rotating wheels and human motion. With the help of micro-Doppler signature, information on the geometrical features of wheels (position, orientation) and on the gait of pedestrians can be extracted. Examples will be shown with simulated and experimental data.
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      Polarimetric ISAR autofocusing
      • Author(s): M. Martorella ; J. Palmer ; F. Berizzi ; B. Haywood ; B. Bates
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 312 –324
      • DOI:  10.1049/iet-spr:20070121
      • Type: Article
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      • p. 312 –324 (13)
        Interest in polarimetric inverse synthetic aperture radar (ISAR) systems has been growing because of their capability to provide extra information about the imaged target compared with single polarisation ISAR systems. Target classification and recognition can be improved by exploiting full polarisation ISAR images. Moreover, full polarisation can be exploited for improving image formation and in particular image autofocusing. Two novel image autofocusing techniques are defined by extending two well-known single polarisation autofocusing techniques, namely the image contrast-based autofocus and image entropy-based autofocus. A performance analysis is carried out by comparing the results obtained by using the extended and the original autofocusing techniques.
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      Manoeuvring target motion parameter estimation for ISAR image fusion
      • Author(s): Z. Li  and  R.M. Narayanan
      • Source: IET Signal Processing, Volume 2, Issue 3, p. 325 –334
      • DOI:  10.1049/iet-spr:20070111
      • Type: Article
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      • p. 325 –334 (10)
        In inverse synthetic aperture radar (ISAR) imaging applications, generating an image of a manoeuvring target is a challenge due to the tradeoff between the fine image resolution requirements and the temporal variability of the Doppler response. The time frequency transform (TFT) is a common method to solve this problem. Prior TFT-based research assumes that the ISAR observes the target from a single fixed aspect. Because of its manoeuvring characteristics, the target motion features are sometimes not extractable from the observing aspect, but become much more distinct from other aspects. ISAR image fusion is thus able to provide significant advantages, since multiple looks of the same target from different aspects will increase the available knowledge. After constructing the common projection plane and building up the physical connection between radars and the manoeuvring target, the parameters that describe the target motion are estimated for very short time duration within which the Doppler response can be assumed to be a constant. Simulation results confirm the effectiveness of the estimation procedure and thus the fusion method. The authors conclude that the ISAR system must be carefully configured in order to realise the full benefits of their image fusion technique for manoeuvring targets.
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