I.D. Longstaff

Università di Pisa, Pisa, Tuscany, Italy

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Publications (45)18.5 Total impact

  • J. E. Palmer, M. Martorella, I.D. Longstaff
    01/2011;
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    ABSTRACT: The use of multiple radar configurations can overcome some of the geometrical limitations that exist when obtaining radar images of a target using inverse synthetic aperture radar (ISAR) techniques. It is shown here how a particular bistatic configuration can produce three view angles and three ISAR images simultaneously. A new ISAR signal model is proposed and the applicability of employing existing monostatic ISAR techniques to bistatic configurations is analytically demonstrated. An analysis of the distortion introduced by the bistatic geometry to the ISAR image point spread function (PSF) is then carried out and the limits of the applicability of ISAR techniques (without the introduction of additional signal processing) are found and discussed. Simulations and proof of concept experimental data are also provided that support the theory.
    IEEE Transactions on Aerospace and Electronic Systems 08/2007; · 1.39 Impact Factor
  • B.J. Donnet, I.D. Longstaff
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    ABSTRACT: MIMO radar systems have been proposed elsewhere which utilise OFDM waveforms as the scene illuminator. This suggests an opportunity exists to code the OFDM radar waveform in such a way as to provide a communication link to broadcast the radar data to remote users. The benefit of this would arise from the dual use of the microwave band, addressing the problem where demand for bandwidth is exceeding capacity. This paper explores the technical issues associated with this idea and outlines some of the key features of such a system. We describe how OFDM waveforms can be applied to MIMO radar; and what constraints must be placed on the waveform to ensure robust operation for both radar and communication functions. A candidate system design is presented, along with basic analysis of the expected performance of both radar and communications functionality
    Radar Conference, 2006. EuRAD 2006. 3rd European; 10/2006
  • B.J. Donnet, I.D. Longstaff
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    ABSTRACT: MIMO techniques have been well studied for communications applications where they offer benefits in multi-path fading environments. This terminology is now appearing in the radar literature, but in several forms causing some confusion. Much of the existing work has focused on multistatic radars with sufficient spatial separation to de-correlate target RCS fluctuations, and where the data from the radars are combined incoherently. Another form of MIMO radar, closer to the communications technique, is where multiple orthogonal signals from the individual transmitter elements of a phased array are coherently combined on reception to form multiple beams. This paper presents a simple understanding of this within-aperture MIMO technique. We show how a filled array of "phantom" elements can be formed from relatively few physical elements. We explain the phantom element concept and extend it to the synthesis of various antenna array configurations, covering one, two and three dimensional geometries. The paper describes a range of waveform and signal processing options and discusses the implications of these radar capabilities
    Radar Conference, 2006. EuRAD 2006. 3rd European; 10/2006
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    ABSTRACT: The use of a monostatic radar configuration limits the ability of an inverse synthetic aperture radar (ISAR) system to image targets in certain geometries. By employing multistatic geometries this limitation may be overcome. This paper discusses the emulation of multistatic geometries, via sea surface multipath reflections, using a monostatic system. This application capitalises on the advantages provided by both monostatic and bistatic systems. The possibility of obtaining ISAR images using these emulated multistatic radar configurations is first theoretically discussed and then verified using experimental results.
    IEEE Transactions on Aerospace and Electronic Systems 11/2005; · 1.39 Impact Factor
  • K.H. Lai, I.D. Longstaff, G.D. Callaghan
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    ABSTRACT: The authors present a super-fast scanning (SFS) technique for phased array weather radar applications. The fast scanning feature of the SFS technique is described and its drawbacks identified. Techniques which combat these drawbacks are also presented. A concept design phased array radar system (CDPAR) is used as a benchmark to compare the performance of a conventional scanning phased array radar system with the SFS technique. It is shown that the SFS technique, in association with suitable waveform processing, can realise four times the scanning speed and achieve similar accuracy compared to the conventional phased array benchmark.
    IEE Proceedings - Radar Sonar and Navigation 11/2004; · 0.55 Impact Factor
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    A.P. Bradley, I.D. Longstaff
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    ABSTRACT: In This work we describe two related approaches to estimating the sample sizes required to statistically compare the performance of two classifiers: acceptable failure rates (AFR) and the area under the receiver operating characteristic (ROC) curve (AUC). In particular, we consider rare event detection problems, where the prior class probabilities are highly skewed, and measure performance at a specific operating point and for the whole ROC curve. It is shown that the use of AUC as a performance measure is preferable to AFR as it requires a smaller data set to demonstrate superiority of one classifier over another.
    Pattern Recognition, 2004. ICPR 2004. Proceedings of the 17th International Conference on; 09/2004
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    ABSTRACT: The 3D bistatic radar solution determines the coordinates of a target from bistatic radar measurements, where the transmitter and receiver are at different positions. This involves the solution of non-linear equations. The conventional approach is linearising these equations. In this paper, we propose a direct solution of the non-linear equations. This allows the evaluation of the multiple solutions, study of singularities and the design of geometric configurations which lead to unique solutions. The solution is demonstrated for satellite based transmitters which are used as sources of opportunity.
    Radar Conference, 2003. Proceedings of the International; 10/2003
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    J. Homer, I.D. Longstaff
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    ABSTRACT: Owing to the presence of relatively large amounts of noise, coregistration of synthetic aperture radar (SAR) images typically involves matching of local area based 'tie patches' rather than feature based 'tie points'. Some applications require small tie patch windows, e.g. stereo SAR. Small tie patch windows, however, can lead to poor estimates of the local co-registration parameters. A procedure to minimise the tie patch window size while retaining 'confidence' in the local co-registration result is proposed. Background theory to the procedure and the results of an experimental study are presented.
    Electronics Letters 02/2003; · 1.07 Impact Factor
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    Rupert Paget, I. Dennis Longstaff
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    ABSTRACT: This paper introduces a new classification scheme called “open-ended texture classification”. The standard approach for texture classification is to use a closed n-class classifier based on the Bayesian paradigm. These perform supervised classification, whereby all the texture classes have to be predefined. We propose a new texture classification scheme, one that does not require a complete set of predefined classes. Instead our texture classification scheme is based on a significance test. A texture is classified on the basis of whether or not its statistical properties are deemed to be from the same population of statistics as those that define a specific texture class. This new “open-ended texture classification” is considered potentially valuable in the practical application of terrain mapping of synthetic aperture radar (SAR) images
    11/2002;
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    J. Homer, I.D. Longstaff, Z. She, D. Gray
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    ABSTRACT: Spaceborne/airborne synthetic aperture radar (SAR) systems provide high resolution two-dimensional terrain imagery. The paper proposes a technique for combining multiple SAR images, acquired on flight paths slightly separated in the elevation direction, to generate high resolution three-dimensional imagery. The technique could be viewed as an extension to interferometric SAR (InSAR) in that it generates topographic imagery with an additional dimension of resolution. The 3-D multi-pass SAR imaging system is typically characterised by a relatively short ambiguity length in the elevation direction. To minimise the associated ambiguities we exploit the relative phase information within the set of images to track the terrain landscape. The SAR images are then coherently combined, via a nonuniform DFT, over a narrow (in elevation) volume centred on the 'dominant' terrain ground plane. The paper includes a detailed description of the technique, background theory, including achievable resolution, and the results of an experimental study
    IEE Proceedings - Radar Sonar and Navigation 03/2002; · 0.55 Impact Factor
  • B. Mojarrabi, J. Homer, K. Kubik, I.D. Longstaff
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    ABSTRACT: Signals from satellite systems like GPS, IRIDUM and Globalstar that are reflected from moving or stationary objects are utilised for their detection in a synthetic aperture bistatic radar system (SAR). The movements of the satellite and its position at different coordinates with respect to time can serve as a base for a synthetic aperture. This paper focuses on the development of a system based on the analysis of indirect signals, in particular GPS signals. The main concern in this study is the low power of the GPS signal at the earth's surface (typically about -160 dB). Due to the nature of the reflector/target, some power will be absorbed, and hence result in a further reduction in signal strength reflected by the target. However, the other equally important concern here is the ground and sea clutter, which is 18-22 dB stronger than the direct signal level. Here we present theoretical results of both the maximum detection range and land clutter contributions of the air target detection by bistatic SAR that utilizes the existing GPS satellite as the transmitting signal source.
    Geoscience and Remote Sensing Symposium, 2002. IGARSS '02. 2002 IEEE International; 02/2002
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    ABSTRACT: In this paper we describe the concept of a bistatic SAR system which involves a purpose built stationary ground based receiver but which utilizes existing or proposed low earth orbit satellite (LEOS) communication systems, such as Globalstar or ICO as the transmitting signal sources. In general, such a system has a 3-D configuration (in contrast to the 2-D bistatic SAR systems examined previously in the literature). We analyse this system. With earth observation applications in mind, we characterize quantitatively the 2-D resolving capability within the ground plane. This is demonstrated via a meaningful simulation.
    Geoscience and Remote Sensing Symposium, 2002. IGARSS '02. 2002 IEEE International; 02/2002
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    Z. She, D.A. Gray, R.E. Bogner, J. Homer, I.D. Longstaff
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    ABSTRACT: Three-dimensional (3D) synthetic aperture radar (SAR) imaging via multiple-pass processing is an extension of interferometric SAR imaging. It exploits more than two flight passes to achieve a desired resolution in elevation. In this paper, a novel approach is developed to reconstruct a 3D space-borne SAR image with multiple-pass processing. It involves image registration, phase correction and elevational imaging. An image model matching is developed for multiple image registration, an eigenvector method is proposed for the phase correction and the elevational imaging is conducted using a Fourier transform or a super-resolution method for enhancement of elevational resolution. 3D SAR images are obtained by processing simulated data and real data from the first European Remote Sensing satellite (ERS-1) with the proposed approaches.
    International Journal of Remote Sensing 01/2002; · 1.36 Impact Factor
  • Yong Wang, I.D. Longstaff, C.J. Leat, N.V. Shauley
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    ABSTRACT: A scheme is presented to incorporate a mixed potential integral equation (MPIE) using Michalski's “formulation C” with the method of moments (MoM) for analyzing the scattering of a plane wave from conducting planar objects buried in a dielectric half-space. The robust complex image method with a two-level approximation is used for the calculation of the Green's functions for the half-space. To further speed up the computation, an interpolation technique for filling the matrix is employed. While the induced current distributions on the object's surface are obtained in the frequency domain, the corresponding time domain responses are calculated via the inverse fast Fourier transform (FFT). The complex natural resonances of targets are then extracted from the late time response using the generalized pencil-of-function (GPOF) method. The authors investigate the pole trajectories as they vary the distance between strips and the depth and orientation of single, buried strips. The variation from the pole position of a single strip in a homogeneous dielectric medium was only a few percent for most of these parameter variations
    IEEE Transactions on Geoscience and Remote Sensing 07/2001; · 2.93 Impact Factor
  • Y. Wang, I.D. Longstaff, C.J. Leat
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    ABSTRACT: Synthetic aperture radar (SAR) images of resonant buried objects are modelled in the presence of ground surface clutter. The method of moments (MoM) is used to model scattered fields from a resonant buried conductor and clutter is modelled as a bivariant Gaussian distribution. A diffraction stack SAR imaging technique is applied to the ultra-wideband waveforms to give a bipolar signal image. A number of examples have been computed to illustrate the combined effects of SAR processing with resonant targets and clutter. SAR images of different targets show differences which may facilitate target identification. To maximise the peak signal-to-clutter ratio, an image correlation technique is applied and the results are shown
    IEE Proceedings - Radar Sonar and Navigation 07/2001; · 0.55 Impact Factor
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    I D Longstaff, B C Lovell
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    ABSTRACT: This paper presents a scheme for segmenting images on the basis of differences in localised measures of spatial texture. The scheme used was originally proposed by Wilson and Spann [1] but incorporates a new clustering algorithm which gives improved overall segmentation performance. The Wilson and Spann [1] algorithm uses a clustering algorithm which proved susceptible to initial input parameters and gave poor segmentation on our images. Our algorithm uses a modification of the Koontz, Narendra and Fukunaga [2] clustering algorithm. By linking the clustering to the resolution of the image, significant clusters were able to be realised, yielding a more robust segmentation scheme. The adaptation also resulted in a significant reduction in run-time. The paper is directed towards the problem of segmenting satellite synthetic aperture radar (SAR) images and we give comparisons of the techniques on SAR and other images.
    06/2001;
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    Rupert Paget, I. Dennis Longstaff
    12/2000;
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    I. D. Longstaff, B. Lopell
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    ABSTRACT: We present a nonparametric Markov H. andom Field model for classifying texture in images. This model can capture the characteristics of a wide variety of textures, varying from the highly structured to the stochastic. The power of our modelling technique is evident in that only a small training image is required, even when the training texture contains long range characteristics. We show how this model can be used for unsupervlsed segmentation and classification of images containing textures for which we have no prior knowledge of the constituent texture types. This technique can therefore be used to find a specific texture in a background of unknown textures.
    12/2000;
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    ABSTRACT: This paper describes a newly developed stepped-frequency ground penetrating radar SFGPR prototype operating across the 10–620 MHz frequency band. The SFGPR system has been developed to improve the maximum penetration depth Ž . capability of ground penetrating radar GPR applications without degradation of resolving power. The radar has the unique capability to employ a short gate at each frequency step that can suppress unwanted strong signals relative to weak reflections from deep targets. The system performance is approximately 175 dB in ungated mode and 170 dB in gated mode, with reference to a 10 ms integration time and a 10 dB signal-to-noise ratio. The current system is suitable for operation in a vehicle and incorporates a real time display similar to conventional impulse GPR. It is capable of recording 50 depth profiles per second. Subsurface images obtained with the gated SFGPR of a perched water table on a sand island, a culvert in high-loss ground, and a 500 lb bomb buried at 1.7 m deep are presented. A frequency compensation procedure is used to improve image clarity. The weakest confirmed underground target has a signal power 100 dB down on the transmitted power. Such a signal would be near the noise floor of a commercial impulse GPR. q 2000 Elsevier Science B.V. All rights reserved.
    Journal of Applied Geophysics 03/2000; 43:259-269. · 1.30 Impact Factor