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Radar, Sonar and Navigation, IEE Proceedings -

Issue 3 • Date 2 June 2003

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Displaying Results 1 - 14 of 14
  • PAMIR - a wideband phased array SAR/MTI system

    Page(s): 165 - 172
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1551 KB)  

    Air- and spaceborne imaging radar systems in forthcoming surveillance and reconnaissance tasks have to meet increasingly severe demands. The next generation of top-level synthetic aperture radar (SAR) systems will comprise, among others, high resolution and long-range imaging capabilities, highly sensitive ground moving target indication and a multitude of sophisticated operational modes. The variety of tasks can be fulfilled only by the use of a reconfigurable phased array antenna together with a comprehensive wideband system design and a multichannel capability. At FGAN a new experimental X-band radar has been conceived, which will possess in its final upgrade an electronically steerable phased array consisting of 16 autonomous and reconfigurable subapertures, five independent receive channels, and a total signal bandwidth of about 1.8 GHz. The sensor is called PAMIR (Phased Array Multifunctional Imaging Radar). It is envisaged to demonstrate SAR imaging at a very high resolution and for a long range. The fine resolution will also be achieved with inverse SAR (ISAR) imaging of ground moving objects. Furthermore, the number of receive channels will allow ground-moving target indication (GMTI) by space-time adaptive processing and single-pass interferometric SAR (IfSAR) with a very high 3-D resolution. In its current stage of extension PAMIR is operable with one receive channel and a mechanically steerable antenna array. The system design and the intended capabilities of PAMIR are described. Ground-based and airborne experimental results concerning high-resolution SAR and ISAR imaging are also presented. View full abstract»

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  • Analysis of multistatic configurations for spaceborne SAR interferometry

    Page(s): 87 - 96
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1143 KB)  

    Spaceborne bistatic and multistatic SAR configurations are an attractive approach to acquire along-track and cross-track interferograms on a global scale. An efficient realisation of such systems may be achieved by a set of low-cost, passive receivers onboard a constellation of microsatellites which simultaneously record the backscattered signals transmitted by a conventional spaceborne radar. The authors introduce several multistatic SAR configurations suitable for global single-pass interferometry and discuss their advantages and limitations. The achievable interferometric performance is analysed in detail, taking into account thermal noise, block adaptive quantisation, range and azimuth ambiguities, and geometric decorrelation for both surface and random volume scatterers. Based on the estimated interferometric phase errors, the relative height accuracies for three illuminators (PALSAR, ASAR, TerraSAR-X) are derived. The achievable height accuracies are of the order of 2 m for PALSAR and ASAR, and of the order of 1 m for TerraSAR-X. However, it turns out that for vegetated areas, volume decorrelation may become a limiting factor for configurations with large interferometric baselines. These restrictions can be overcome by making the interferometric configuration fully polarimetric and/or increasing the number of available baselines. View full abstract»

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  • Time-frequency analysis of vibrating targets in airborne SAR systems

    Page(s): 173 - 176
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (448 KB)  

    Vibrating targets may cause azimuth smearing in airborne synthetic aperture radar (SAR) images. Any motion of a reflector during the coherent integration time of a SAR image causes a phase modulation in the corresponding reflector phase history. For a sinusoidal motion, the phase modulation causes sidebands, often indistinguishable from stationary reflectors. The modulation is described in terms of a time-dependent Doppler frequency. Time-frequency methods are useful tools for such cases. Among the many possible time-frequency methods the authors chose the adaptive optimal kernel (AOK) method for the purposes of the present paper. With the AOK method they analysed an oscillating reflector within an airborne SAR image. The oscillation was clear and the oscillation parameters were estimated from the time-frequency distribution. The parameters agreed well with ground truth. In particular, the oscillation frequency agreed well, while the amplitude was less accurate, but still reasonable. View full abstract»

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  • Cross-calibration of interferometric SAR data

    Page(s): 177 - 183
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1446 KB)  

    Generation of digital elevation models from interferometric synthetic aperture radar (SAR) data is a well established technique. Achieving a high geometric fidelity calls for a calibration accounting for inaccurate navigation data and system parameters as well as system imperfections. Fully automated calibration techniques are preferable, especially for operational mapping. The author presents one such technique, called cross-calibration. Though developed for single-pass interferometry, it may be applicable to multi-pass interferometry, too. Cross-calibration requires stability during mapping, but not necessarily from map to map. It is based on natural distributed targets for which no a priori knowledge is needed. In particular, no DEM is required as in calibration techniques based on dedicated calibration scenes. To achieve absolute calibration, i.e. elimination of a constant elevation offset, a single ground control point is often needed. The paper presents the principles and mathematics of the cross-calibration technique and illustrates its successful application to EMISAR data. View full abstract»

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  • Innovative technologies for space-based radars

    Page(s): 104 - 112
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1003 KB)  

    In previous years technology developments for SAR front-end and for on-board SAR data processing have been performed at Astrium GmbH, Germany. Remarkable progress has been made in the respective demonstrator projects DESA, EUCLID RTP 9.7, and TOPAS. A summary of the major results and achievements is presented. View full abstract»

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  • Phase and amplitude histories model adapted to the spaceborne SAR survey

    Page(s): 184 - 192
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (797 KB)  

    A model of relative movement of a SAR platform and imaged footprint is proposed. The model has been specially adapted to conditions of a spaceborne SAR survey. The main difference of this model from that used before is that it takes into account the SAR platform movement and Earth rotation in addition to commonly used calculations of satellite motion in the disturbed Earth gravitation field. It is shown that SAR motion could be considered as linear, but with velocity derived from orbital motion parameters. The proposed model could be easily applied to analysis of SAR processing algorithms and the influence of the orbit parameter estimation errors on the resulting image quality. Use of the model simplifies the parameter estimation of the SAR sensors intended for planetary investigations. View full abstract»

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  • Multi-mode ENVISAT ASAR interferometry: techniques and preliminary results

    Page(s): 193 - 200
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (2785 KB)  

    The paper focuses on the interferometric capabilities of current and future SAR systems, like ENVISAT, RADARSAT-2, etc. The authors introduce a technique to obtain interferometric surveys by combining two images, coming from any SAR mode, together with a digital elevation mode (DEM). The DEM is exploited during the interferogram generation to provide, at one time, effective noise removal and compensation for the topographic-dependent fringes. The final product is a 'differential' (topography-compensated) interferogram to be used for monitoring over landslides, upswelling, etc. An innovative technique is also discussed to estimate local coherence maps, to be used for classification and change detection. Such a technique exploits windows whose shapes and sizes adapt locally to the target features, to attain at one time the highest resolution and the best statistical confidence. Finally, applications involving the various ENVISAT ASAR modes are discussed. View full abstract»

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  • Recent advances in extra-wide-band polarimetry, interferometry and polarimetric interferometry in synthetic aperture remote sensing and its applications

    Page(s): 113 - 124
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1647 KB)  

    The development of radar polarimetry and radar interferometry is advancing rapidly, and these novel radar technologies are revamping 'synthetic aperture radar imaging' decisively. The successive advancements are sketched eginning with the fundamental formulations and highlighting the salient points of these diverse remote sensing techniques. Whereas with radar polarimetry the textural fine-structure, target orientation and shape, symmetries and material constituents can be recovered with considerable improvements above that of standard 'amplitude-only polarisation radar'; with radar interferometry the spatial (in depth) structure can be explored. In 'polarimetric-interferometric synthetic aperture radar (POL-IN-SAR) imaging' it is possible to recover such co-registered textural plus spatial properties simultaneously. This includes the extraction of 'digital elevation maps (DEM)' from either 'fully polarimetric (scattering matrix)' or 'interferometric (dual antenna) SAR image data takes' with the additional benefit of obtaining co-registered three-dimensional 'POL-IN-DEM' information. Extra-wide-band POL-IN-SAR imaging - when applied to 'repeat-pass image overlay interferometry' - provides differential background validation and measurement, stress assessment, and environmental stress-change monitoring capabilities with hitherto unattained accuracy, which are essential tools for improved global biomass estimation and also for wetland assessment and monitoring. More recently, by applying multiple parallel repeat-pass EWB-POL-D(RP)-IN-SAR imaging along stacked (altitudinal) or displaced (horizontal) flight-lines will result in 'tomographic (multi-interferometric) polarimetric SAR stereo-imaging', including foliage- and ground-penetrating capabilities. In addition, various closely related topics of (i) acquiring additional and protecting existing spectral windows of the 'natural electromagnetic spectrum (NES)' pertinent to remote sensing; and (ii) mitigation against common 'radio frequency interference (RFI)' and intentional 'directive jamming of airborne and spaceborne POL-IN-SAR imaging platforms' are appraised. View full abstract»

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  • Three-stage inversion process for polarimetric SAR interferometry

    Page(s): 125 - 134
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1098 KB)  

    The authors provide a new geometrical approach for the inversion of a two-layer coherent scattering model, widely used for the interpretation of polarimetric interferometric SAR data. It has been shown in several recent publications that, by using interferograms in multiple polarisation channels, estimation of vegetation height, underlying ground topography and mean extinction is possible. Furthermore, this can be achieved with a single frequency sensor without the need for a separate reference DEM, other a priori information or the use of data-specific regression formulas. The authors first review the details of this approach and then develop a three-stage inversion procedure to illustrate the steps involved in parameter estimation. They then consider several possible sources of error in the inversion. In particular, they concentrate on the effects of vertical tree structure and on the effects of temporal decorrelation on inversion accuracy. It is shown that the former leads to errors, mainly in the extinction estimation, while the latter does not change the model structure but reduces the available parameter set and increases the variance of the parameter estimates. Finally, the new algorithm is applied to simulated vector coherent SAR data for a random canopy. View full abstract»

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  • Perspectives and visions for future SAR systems

    Page(s): 97 - 103
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (775 KB)  

    It is shown how far new SAR techniques, technologies and system principles, which at present are under consideration or under development, respectively, will fulfil future user requirements. Starting from the status quo, the expected development of future SAR techniques and technology is presented, leading to the vision of an autonomous, global reconnaissance and remote sensing system with integrated communication and positioning capability. This system will have central illuminators together with a synchronised fleet of both airborne and spaceborne receivers, which enable continuous availability with a nearly global coverage, and dedicated information transfer to specific users in real time. View full abstract»

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  • Polarisation orientation angle measurements of ocean internal waves and current fronts using polarimetric SAR

    Page(s): 135 - 143
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1322 KB)  

    Previous studies have shown that polarimetric synthetic aperture radar (POLSAR) data may be used to measure parameters such as bare surface roughness. Polarisation orientation angles, derived from POLSAR data, can be utilised to produce estimates of topography. Orientation angles have been used for SAR data compensation, in areas of rugged terrain, to ensure accurate estimation of geophysical parameters such as biomass and soil moisture. Orientation angles have also recently been investigated for use in measuring directional ocean wave spectra. Studies of these ocean features are conventionally done by measuring feature-related microwave backscatter intensity perturbations. In the work presented, a new approach is taken. Investigations have been made of changes in the polarisation orientation angle caused by ocean wave-current interaction features. The study uses airborne POLSAR data obtained in the New York Bight and in the Gulf Stream, east of Cape Hatteras. A model is used to determine the parametric dependencies of the orientation angle on internal wave current, wind-wave direction and wind-wave speed. An empirical relation is proposed to relate orientation angle perturbations to the underlying internal wave current strength. View full abstract»

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  • Ultra-wideband multistatic SAR for the detection and location of landmines

    Page(s): 158 - 164
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (751 KB)  

    Techniques for improving the signal to clutter ratio of an ultra-wideband SAR designed to detect small mine-like objects in the surface of the ground were investigated. In particular, images were collected using different bistatic antenna configurations in an attempt to decorrelate the clutter with respect to the targets. The images were converted to a reference depression angle, summed, and then converted to ground coordinates. The resulting target strengths were then compared with the amplitude distribution of the ground clutter to show the improvement obtained. While some improvement was demonstrated, this was for the relatively easy scenario of targets on the surface partially obscured by grass. Detection based on thresholding the raw RF signal (the bipolar response) rather than the envelope (baseband I2+Q2) was also considered to further enhance target-to-clutter ratios. View full abstract»

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  • Extension of the chirp scaling algorithm to 3-D near-field wideband radar imaging

    Page(s): 152 - 157
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (572 KB)  

    The authors' objective is to develop an efficient algorithm to focus two-dimensional and three-dimensional SAR images under the following extreme conditions: large coherent integration angles and high bandwidth-to-centre-frequency ratios. These conditions constitute a common situation in wideband high-resolution SAR configurations, as in anechoic chambers and in ground-based systems. The algorithm presented is an extension of the well known chirp scaling algorithm (CSA). First, the original 2-D CSA is generalised by extending the formulation from data obtained with a linear aperture to data measured by a planar synthetic aperture. Moreover, a compact generalised formula of the impulse response function in the three-dimensional frequency domain is presented. These extensions enable the development of a 3-D algorithm under the aforementioned conditions. Finally, the algorithm has been implemented and tested with satisfying results. View full abstract»

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  • Using topography statistics to help phase unwrapping

    Page(s): 144 - 151
    Save to Project icon | Click to expandQuick Abstract | PDF file iconPDF (1081 KB)  

    Conventional techniques approach phase unwrapping (PU) as an optimisation problem, where figures of merit like the total branch-cut length, the number of cuts, etc. are to be minimised. They disregard the properties of the field to be unwrapped: the topography, i.e. the DEM, projected in the SAR reference. The purpose of the paper is to fill this gap by providing statistics of the 'fringe maps'. The Woodward theorem is exploited to link the interferogram power spectrum density (PSD) with the probability density function (PDF) of the phase gradient that would result in a likely topography. A parametric model of the expected, unwrapped PG PDF is then derived by exploiting the fractal properties of topography. Its parameters can be accurately estimated given the wrapped PG. This model provides useful statistical information for phase unwrapping. It is then possible, for example, to estimate the number of residuals, to find the best azimuth presuming factor and to find the optimal interferogram range demodulation. Finally, the author exploits the second-order statistics of the PG field (as a fractal) to derive a suitable approximation for the expected length of the branch cuts. View full abstract»

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Aims & Scope

IEE Proceedings Radar, Sonar and Navigation covers the theory and practice of systems involving the processing of signals for radar, radio location, radio navigation and surveillance purposes.

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